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NATL INST OF STANDARDS & TECH R.I.C.

A111 00988509

/NBS monograph QC100 .U556 V12;1960 C.1 NBS-PUB-C 1S59

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/

il.S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS



UXITED STATES DEPARTMENT OF COMMERCE • Frederick H. Mueller, 5^cr^^ary NATIONAL BUREAU OF STANDARDS • A. V. Astin, Director

Stabilization of Free Radicals

at Low Temperatures

Summary of the NBS Program

Edited by

Arnold M. Bass and H. P. Broida

National Bureau of Standards Monograph 12

Issued August 1, 1960

For sale by the Superintendent of DocumeDts, U.S. Government Printing OflBce Washington 25, D.C. - Price $1.50
Katol Bureau of Stsrrdards 2 9 1960

Co p\f 2
Foreword

The three-year Free Radicals Research Program, which was

initiated at the National Bureau of Standards in September 1956

under Department of Defense sponsorship was terminated, as planned, on October 1, 1959. The status of the field of radical trapping and

the accomplishments of the NBS-DOD program were reviewed and

summarized in a group of seven papers presented at the Fourth

International Symposium on Free Radical Stabilization in Washington, D.C., August 31 through September 2, 1959. These seven

papers form the basis for this volume, which is the final report to the

sponsors of the National Bureau of Standards Free Radicals Research Program. A cross-referenced bibliography of papers published

as a result of the research completed during this three year period,

and a summary account of the 1959 Symposium are also included in

this report.

Although the first paper in this volume describes the general

aspects of the technical management of the program, it is important to emphasize that much of the success and productivity of this program

stemmed from the s^^mpathetic understanding of its basic viewpoint by

both the Department of Defense and the National Bureau of Standards.

This basic tenet was that competent senior scientists provided with

adequate research facilities, having freedom to choose their re- search activities, and relieved of administrative distractions, would

achieve maximum productivity. Mr. Norman L. Klein of the

Office of the Chief of Ordnance, Department of the Army, and Dr.

R. D. Huntoon, Deputy Director, National Bureau of Standards,

who had the ultimate responsibility for the program, were receptive

to undertaking the research program in this way. It was only

with their support, encouragement, and assistance that the research

could be undertaken as it was, and the important roles that they

played in the development and direction of this program are gratefully acknowledged. In addition, it is a pleasure to acknowledge

the generous support of some twenty companies for their cooperation

and participation in this collaborative enterprise.

There are many others who have contributed to the work of the last three years but the greatest credit for successful accomplishment must go to the research scientists. It has been their

enthusiasm, their ideas, and their hard labor that has produced the

results. However, without a smoothly running administrative

services group and supporting staff, it would not have been possible

for the scientific staff to have been so productive. Thus a special

acknowledgement must be made for the competent and continuing

assistance of Dr. Arnold M. Bass, under whose direction the administrative aspects were accomplished.

H. P. Broida

Technical Director Free Radicals Research Program.

m
Contents

Page

Foreword . in

1. Organization and rationale of the NBS free radicals program,

J. W. Moyer 1 2. Qualitative comments on the physical and chemical processes in trapped radical systems, C. M. Herzfeld and M. D. Scheer 5

3. Experimental aspects of the National Bureau of Standards Free

Radicals Program, M. D. Scheer 7

4. A survey of theoretical work on trapped radicals at the National Bureau of Standards, C. M. Herzfeld 21

5. Low-temperature chemistry, R. Klein 37

6. Methods of production of trapped radicals and properties of radical- trapping solids, A. Thomas 55

7. Identity and concentrations of trapped radicals, R. W. Zwanzig 67

8. Interactions between trapped species and the matrix, O. Schnepp__ 77

9. Summary of the Fourth International Symposium on Free Radical

Stabilization, M. W. Windsor 89

10. Bibliography of the free radicals research program 101 Author index 109 NBS Technical News Bulletins 110
1. Organization and Rationale of the NBS Free

Radicals Program

James W. 'Moyer ^

In response to the advice of technical committees, the Department

of Defense, in micl-1956, institiitecl a national program to accelerate

the rate of learning on the formation and properties of free radicals.

In addition to increasing the total contract effort in fields related to

free radicals, it was decided to set up a central research effort for a 3-year period, for the pm-pose of pm'suing new programs with maximum effectiveness and to disseminate widely the information and

techniques. In particular, it was desired that there be as broad an

industrial base as possible. By this means, if a striking advance

should be made, the industrial laboratories would be in a '^cocked"

position, ready to shoot at development or semiproduction targets

envisioned as possible followups.

The National Bureau of Standards was asked to carry out the

'^central laboratory" project theme by virtue of its research management capability, the interest shown there through some of the pioneer

experiments, and the ''neutral" atmosphere favorable to industrial

participation. The program was launched in September 1956.

A program staffed by 30 to 35 senior scientists was conceived. In

addition to the Bureau staff members and the enthusiastic associates

from universities, about half the scientists were sent by industrial

laboratories, their salary, travel and living expenses borne by their

home offices. Thus, the project was actually supported by Defense

and b}" Industr}'. Table 1 shows the companies participating during

the 3-year period.

Each industrial participant signed an agreement to work on the

Bureau Project as a 'Tree Radicals Research Guest" abiding by the

Bureau's administrative procedures and agreeing to share information

and research results conceived while at the Bureau. Ideas generated

after leaving the Bureau were left for his company to capitalize on, thus providing additional incentive for prompt participation. This

unique arrangement between Government and industry resulted in

widespread interest not only in the technical area of the project but

as a model for possible future cooperative research investigations of national importance as well. The base of support for the project was, of course, spread more

widely than U.S. Government and U.S. industry. In addition to

scientists from three non-U. S. industries, much manpower came from

universities and other establishments, here and abroad. These are shown in table 2. Table 3 is a detailed breakdown of manpower as the program developed showing NBS employees, industrial, university

staff, and consultant affiliations.

1 Consultant from General Electric Company, Santa Barbara, California.

1
Table 1. Industrial participation in the NBS free radicals research program

Company Scientist Time at NBS

British Oxygen Res. and Dev. Ltd

California Research Corp

Callery Chemical Co

Columbia-Southern Chemical Corp

Convair Dow Chemical of Canada, Ltd

Ethyl Corporation General Electric Co Grumman Aircraft Engineering Corp Humble Oil and Refining Co

Koppers Co

Monsanto Chemical Co

Olin-Mathieson Chemical Corp . Shell Research Ltd

Sinclair Research Laboratories Space Technology Laboratories Standard Oil, Indiana. Union Carbide Corp

U.S. Industrial Chemicals Co

Wyandotte Chemicals Corp

Combustion and Explosives Research, Inc

General Electric Co

General Motors Corp

Westinghouse Electric Corp

Senior Research StafI

J. G. Waller

B. J. Fontana

J. S. Hashman

D. Kimpton

T. D. Ling

R. N. Barfield

J. T. Clarke M. D. Scheer

T. Baurer

J. L. Franklin

E. L. Pascal /R. A. Ruehrwein

U. W. Edwards

R. Klein

A. Thomas

A. Chatenever M. W. Windsor

A. Zletz F. I. Scherber

(A. Leifer

II. Mador

L. E. Kuentzel

Industrial Consultants

/B. Lewis

IG. von Elbe

fJ. W. Moyer

\H. C. Mattraw

R. C. Herman

T. G. Castle

4/59- 9/59 1/57- 8/58

1/57- 1/58 3/59- 5/59 9/57-12/57

1/57-11/58

6/57- 8/58 5/57- 7/58

1/58- 6/59 4/57- 7/58 3/57- 6/58 11/56- 8/57 2/57- 2/58

1/57-10/59 1/59-10/59 6/58-10/59

3/58- 9/59 3/57- 3/58 2/58- 5/59 9/57-10/59

5/57-10/59 12/56- 1/58

5/58- 7/59 5/58- 7/59 10/56-10/59

10/58-10/59

10/56- 7/59 10/56- 7/59 Table 2. Nonindustrial participation in the NBS free radicals research program

Organization Scientist Time at NBS

Catholic University of America

Centre National de la Recherche Scientifique Imperial College of Science and Technology.

Israel Institute of Technology

Universite Laval

Universite de Lyon

Wayne University

Brandeis University

Catholic University of America

University of Maryland

Senior Research Staff 'K. Dressier

.V. Griffing A. M. Pilon G. J, Minkoff

O. Schnepp

K. B. Harvey

M, Peyron

B. Donn

University Consultants

S. Golden

F. O. Rice

E. Montroll

11/57-10/59 2/57- 7/59 10/56-12/56

1/57- 2/58 9/58-10/59

10/56- 1/58 3/57-10/58

12/56- 7/59

12/56- 9/59 1/57- 7/59 4/57- 7/58 Table 3. Personnel summary—NBS Free Radicals Research Program 10156-10159 NBS research staff Non-NBS research staff Supporting staff Date Total

Senior Assistants Industrial University Consultants NBS

10/1/58 -2 (1/2) 1 (1) 0 0 2 5 (4) 9 (5) 1/1/57 11 (9) 7 (7) 3 (21/2) 4 (1/2) 6 6 (5) 33 (25) 7/1/57 15 (11) 18 (16) 16 (13) 9 (5) 7 9 (7) 64 (54) 2/1/58 16 (12) 20 (16) 13 (12) 7 (5) 6 10 (8) 68 (52) 7/1/58- 15 (11) 21 (13) 12 (10) 7 (3) 8 10 (7) 69 (50) 2/1/59 [ 19 (15) 19 (15) 8 (7) 6 (3) 8 (8) 11 (8) 68 48)

7/1/59 12 (8%) 12 (lOM) 7 (6) 3 {2M) 3 10 (8) 44 (35) Total number

of people 25 37 23 10 8 13

a Figures in parentheses indicate man-years.
To administer the NBS program, a special Free Radicals Section was set up in the Heat Division of the Bureau, with Dr. H. P. Broida

as Chief of the Section, and Dr. A. M. Bass as Assistant Chief. Dr.

Broida gave overall technical direction to the project, while Dr. Bass

handled the administrative end. Bureau management cooperated to launch the program quickly by permitting speedups in normal

procedures for the preparation of laboratory and office space, for the

acquisition of material, and by creating niches in existing Bureau

sections for many of the research guests. This was done with a minimum interference to other established research programs.

One important ingredient of the program rationale was the freedom

of choice of research work on the part of each senior scientist. Participants were first chosen for their interest in the broad field and their

established research competence, after which they were essentially

unrestricted as to their experimental or theoretical approach. This

procedure has become so rare generally that it seems worthwhile to examine how it worked.

The freedom given researchers contributed, of course, to a very high

morale in the project. This was further bolstered by laboratory

arrangements for working odd hours, nights, and Sundays. With

such freedom, however, goes responsibility both on the part of the

scientist and the management, to alter courses of action which, after a suitable period, appear to be nonproductive. Since all initial approaches were not equally effective (hidden difficulties soon came to

light!) it was inevitable that some regrouping should occur. Such

adjustments continued from the first 6 months almost to the program's

termination.

In a research atmosphere devoted to freedom of choice, it is also the responsibility of the management to provide as many factors as possible in the environment to capitalize on the larger number of fresh

ideas which accrue. To this end "cross-fertilization" techniques were

instituted. Weekly seminars, partly with Bureau researchers, partly

with outside guests, were instituted. A bimonthly Newsletter was

edited and distributed by Mrs. Floyd Mauer. This Newsletter

contained preliminary knowledge of research progress at the Bureau

as well as heartening and informative items from many of the other

laboratories throughout the world, contributed on a voluntary basis. The Newsletter circulation rose from 90 to 550, and its success was due

to able and prompt editing as well as adherence to the printed policy

of not permitting quoting or copying of such preliminary informal

material.

Travel to attend meetings and to other laboratories was greatly

encouraged and found to be especially stimulating. The Defense

Department greatly assisted in much of this travel, especially for several of the project personnel to attend the 1958 Free Radicals

Symposium in Sheffield, England. The Defense Department also gave special assistance to the travel requirements for many scientists

from abroad to work or attend meetings at the Bureau, thus providing

further stimulus.

Several of the industrial scientists carried their interests and new

techniques home with them, starting up or enriching their firm's free radicals research. Out of the one hundred or more research papers to evolve, many are authored jointly by Government-industry teams. The scientific content of the program is reported below. But it may be said here that a greatly increased understanding of low3
temperature phenomena and techniques was one of the byproducts of the project. Liquid heUum handhng has now become routine, and

because of the project, commercial Dewars of many special designs

are now available at reasonable cost. Advances in the spectroscopic

instrumentation art have been made, and X-ray and electron diffraction have been carried out on low temperature specimens. Liquid

hydrogen handling techniques have also been improved, as well as means for production of intermediate low temperatures. As the

program progressed, more extensive use was made of the powerful new

electron-spin-resonance techniques, although there is much more to be

gained in this application than the project time permitted.

That the program did not result in a so-called ' 'breakthrough" for new rocket propellants should disappoint only the space merchants.

The remainder of the scientific fraternity is grateful for a vigorous

novel approach to team research which resulted in a flowering of the

state of the art in a field of universal interest and importance.

Washington 25, D.C., November 16, 1959.

4
2. Qualitative Comments on the Physical and

Chemical Processes in Trapped Radical Systems

C. M. Herzfeld and M. D. Scheer

It is clear from a qualitative consideration of the phenomena in- volved in trapped radical research that several general types of processes occur, either in a sequence or concurrent!}^. One may list these types of processes in the order in which they occur, and so obtain

a fairly clear qualitative picture of the events which take place.

First of all, free radicals must be formed. In the gas phase this may be done by electron impact in an electric discharge, or by heating

the gas to high enough temperature to dissociate the molecules, or by

optical methods such as photoh^sis. In the solid, free radicals may

be produced by electric discharges, by photolj'sis, or by bombardment

with nuclear particles or electrons. In many of these processes

ions are formed as well as radicals (which are, by definition, electri- cally neutral). Xot much is known about the importance of these

ions in the processes we discuss, and om* attention will be focussed

primarily on the molecules and on the radicals produced in the ways

which have been mentioned.

When the gas containing molecules and radicals (and perhaps inert

diluent gases) hits a cold surface (in this work usually below 20° K),

the gas condenses. Molecules, radicals, and inerts are all set down in a complex mixture. ^lany processes contribute to this result. The

individual species hit the surface, some stick, others are reflected, some radicals react on the surface, some species wander over the

surface and some diffuse through the solid. There may be gradients

of concentration, temperature, mean energy, etc., within the gas and

within the solid. The whole phenomenon and the structure of the

resulting solid are certainly very complicated.

Radicals which are in excited states when they hit the solid, either

radiate, or transmit their energy to the lattice. They may radiate

from the surface or from the interior of the solid. The energy levels and transition probabilities of species in the solid are perturbed by

their neighbors in the solid in ways which may be characteristic of the neighbors. Radicals (and molecules and inerts) will absorb

radiation incident upon the solid, and the former will also often

exhibit paramagnetic resonance absorption.

The manner in which radicals are trapped plays a considerable role in some processes. The trapping may involve just a ''freezing in"

at some position in the lattices (either in a substitutional or in an

interstitial site), where the radicals are prevented from recombining

simply by isolation from each other. Trapping may also be accomplished b}^ formation of loosely bound molecules (not ordinarily

stable) between radicals and molecules or radicals and diluents. The

short range forces between all species will have important effects on

these trapping mechanisms.

Whenever one has a system with more than one component, the

question arises whether this system has stable thermodynamic phases.

Also the equations of state of free radical systems may be significant

in some cases. 5
Free radicals in solids ma}^ recombine and liberate energy. An

increase in local temperature and in radical diffusion rates occurs, so that other radicals arc liberated which in turn may react, and so on. Such radical chains may or may not branch. It is clear that branching processes lead to an instability in the system (explosions) because

the number of radicals produced is greater than the number destroyed.

Diffusion controlled flames may also occur in such systems. In

addition, free radicals may react with the solid to produce radicals

not originalh^ present. These may either be stabilized or react further to produce other radicals without branching in the radical

chains. If a continuous production of radicals is maintained, profound changes in the chemical nature of the original solid result. It

is evident that the kinetics of this latter type of process depends

strongly on rates of radical diffusion and the activation energies of the radical-molecule reactions involved.

Finally, after some time, the system returns to its equilibrium state. The presentation of the results of the program at the National

Bureau of Standards will follow this sequence of events approximately.

In this way a rather detailed picture of the processes which occur can

be built up, and one can then judge readily what parts of the whole

problem are reasonably well understood, and where the largest gaps

in our knowledge are. Washington 25, D.C., December 18, 1959.

6
3. Experimental Aspects of the National Bureau of

Standards Free Radicals Program

Milton D. Scheer

1. Radical Production Techniques

1.1. Production in the Gas Phase and Depositon on a Cold Surface

a. Electric Discharge

The most widely used method of generating gaseous free radicals at NBS has been the coupling of microwave power at 2,450 Mc to a quartz tube through which the gas under investigation is flowing at pressures in the region of 0.1 to 10 mm Hg [1] ^ (fig. 1). No significant

WAVEGUIDE

RESONATOR

Figure 1. A typical liquid helium Dewar used for the study cf the condensed

products of a microwave discharge through N2.

effect of microwave frequency on the production of radicals has been

found [2]. Usually about 10 to 100 w of microwave power is applied

to the coupling resonator. The concentration of atoms and radicals

in the gas stream flowing out of the discharge plasma is relatively

independent of the power applied to the resonator. A careful stud}^

by T. Shaw [2] has shown that the fraction of H2 dissociated to hydrogen atoms in a 3,000 Mc discharge increases somewhat with

1 Figures in brackets indicate the literature references at the end of this paper. 7
increasing power and decreasing flow rate. The main advantage of using microwave power larger than 100 w is that a stable discharge can be maintained at higher pressures and larger flow rates. The dissociation of polyatomic gases into radicals in an electric discharge is largeh^ caused by energetic electron-molecule and ionmolecule collisions. Most of the electrons are formed as a result of

the ionizing collisions which occur in the discharge plasma. In order

to maintain the discharge, the rate of electron production must balance

the rate of electron loss occurring at the walls of the discharge tube

[3]. The question whether ions and electrons are important species at appreciable distances outside of the discharge zone is largely unanswered even though for simple gases like N2 and O2 the radiating species on the downstream side of the discharge zone are unaffected by large

external electric or magnetic flelds [4]. Microwave measurements of the free electron density in the afterglow region of discharged nitrogen

have shown the presence of about 10^ electrons/cm^ which corresponds

to about one electron for every 10^ nitrogen atoms present [5]. The

influence of these charged particles on the process of radical collection on a cold surface is unknown at present. Further, electronically and

vibrationally excited species [18] are also present in the downstream

side of the discharge, thereby complicating even further the process of radical stabilization on a cold surface. Thus, while this method of

generating gaseous radicals can in some instances be made quite

efficient (up to 90% for the dissociation of H2), there is considerable

uncertainty in regard to the exact nature of the gas which is finally

deposited upon the cold surface.

The condensed products of discharged nitrogen have been extensively investigated at NBS in the 1° to 35° K region. The demonstration that ground state nitrogen atoms can be stabilized in such

a solid is made quite conclusive in view of the spin resonance spectra

obtained by many investigators. [8, 15, 17] (fig. 2). Interpretation

Figure 2, ESR spectrum of N atoms in an N2 matrix at 4° K. 8
of the cold solid emission spectra has indicated that stabilized, excited

X atoms interact with at least one neighboring N2 molecule [6, 7,

9, 10, 11, 12] (fig. 3). Estimates obtained from magnetic susceptibility and electron spin resonance measurements have shown that the

X atom concentration in the 4° K solid is of the order of 0.1 percent.

When the concentration of atoms exceeds this value of about 1 part

per 1,000, the solid becomes unstable, and phenomena closely akin to explosions can take place in this low-temperature environment

[13, 14]. The radiation emitted during these ^'explosions" and during

warm up of the solid is undoubtedh' due largely to the recombination

of the nitrogen atoms. However, the details of the kinetics of

this process are far from clear at the present time. The appearance

of intense Vegard-Kaplan bands (^S„-^1S+) has been tentatively

attributed to X atom recombination in the solid. It is interesting to note however, that the first positive system of nitrogen {^Ug-^^Zt) is

A bands _ 1 ^ ^

1 ^

4500 5000 5500 6500

ANGSTROMS

FIGU.^F. 3, Emission spectrum in visible region of nitrogen irradiated by 10 kv

electrons at 1^.2° K.

9
not present to any appreciable extent in these flashes while it is one of the principal spectral features of the gaseous afterglow where N atom

recombination occurs with a rate constant of 5X 10^^ cm^ mole~^

sec-i [48].

b. Thermal Dissociation

The method of thermal dissociation is capable of yielding a well

defined stream of molecular fragments provided that thermodynamic

equilibration is achieved in the furnace source and recombination

processes are minimized in the effluent gas. Complete knowledge of

the identity and concentration of the species in the heated source is only possible if the complete set of thermodynamic properties is known for the system being investigated. The requirement is readily

met for some of the simple diatomic molecules such as H2, O2, N2, and

the halogens, but for more complex systems complete thermodynamic

information is generall}^ lacking. In order to maintain the welldefined gas composition downstream of the furnace source, recombination processes must be avoided. This can be done in either one of

two ways. Either molecular collisions are eliminated by collimating

the effluent stream into a molecular beam, or by high dilution with some inert gas. The former is easily the most certain of achieving this

objective, but it suffers the disadvantage that obtainable beam intensities are small ('^lO^^ particles/cm^/sec) so that only very small

quantities can be deposited on a cold surface per unit time.

There have been several systems constructed at NBS in which

thermal dissociation has been used to obtain molecular fragments in

the gas. Two such pieces of apparatus using the molecular beam

technique were designed, one for examining the deposition of I atoms

on a cold glass surface, (fig. 4), the other for investigating the surface

15 cm 24 cm Refrigerant

Thermocouple ^"^"^ ^p=ldrnmHg

well ^

> Iodine

Crystals Pumps

Colibroted' Needle Valve

Pumps

•n-heptone

p=IO mm Hg

1 y Monochromotor 450-2000mju

Tungsten Lamp Light Source Figure 4. Schematic diagram of molecular beam apparatus.

10
migration of H atoms on a tungsten surface by means of the field emission microscope. The dissociation of hydrogen on an incandescent tungston ribbon has been used to explore radical producing

reactions in cold solids [19, 20]. In attempts to produce a solid containing unstable molecular fragments, organic compounds liave

been pyrolyzed in a furnace mounted near a 4° K surface [21]. Evidence for the stabilization of solid free radicals produced in this fashion is rather meager. When a beam of thermally dissociated

iodine impinges upon a 77° or 20° K glass surface, the predominant

process is atom recombination on the surface. The pyrolysis of azoethane (highly diluted with argon) near a 4°K surface has resulted

in the formation of A^-butane after warm up of the cold solid. In- frared spectra taken of the 4° K solid, however, did not show the

presence of A^-butane o: the ethyl radical for that matter. The

irradiation of solid olefins at 77° K with hydrogen atoms has yielded

large conversions to the alkane (-^90 %), and the radical dimer

('^10 However, an extensive search for stabihzed alkyl radicals

with an electron spin resonance spectrometer failed to yield positive

results. It appears therefore, that efficient radical-radical recombination occurs in the cold solid under these conditions. The absence

of a spin resonance signal indicates that less than 1 out of every 10*^ to

10^ H atoms which react yields a stabilized free alkyl radical.

c. Gaseous Photolysis and Radiolysis

Of the possible techniques for generating radicals in the gas, the

least promising and hence the least explored is gaseous photolysis

(ultraviolet irradiation) or radiolysis (X- or y- irradiation) prior

to desposition. Since low-pressure gases in general are rather trans- parent either to ultraviolet or ionizing radiation, the generation of detectable radical concentrations is not possible unless very intense

radiation sources are used. The possibility of rapidly pulsing a highintensity fiash lamp and depositing a molecular beam of the photolytic fragments has suggested itself to one of the NBS investigators

[22], but shielding the cold deposit from the intense radiation source

(in order to avoid ambiguities in interpretation) is a formidable

experimental problem. In any case no serious attempts have been

made at NBS to collect radicals generated in this fashion.

d. Deposition Process The condensation of a gas upon a cold surface is one of the least understood of the processes which occur during stabilization of radicals

initially produced in the gas phase. When a molecule strikes a surface

it may rebound back into the gas after a single collision with the surface, it may become bound to a surface molecule for a time which is large compared to a single oscillation about the bond made before it escapes back into the gas, or it may migrate at random over the sur- face and coalesce in a process of nucleation with other such particles

and become a permanent part of the deposit. The extent to which

each of these occurs is not known for any particular system. A review

paper [23] by S. Wexler on the deposition of atomic beams shows what

measurements have been made of overall condensation coefficients defined as the fraction of the total number of particles striking a sur- face which remain deposited. In general, the lower the surface temperature the more efficient is the deposition process for a given species.

11
Measurements made at NBS of the condensation coefficient of argon on a 4° K argon surface yielded a value of 0.6 [24] while the condensation of iodine on a 77° K glass surface occurred with nearly prefect

efficiency [25]. It is seen, therefore, that not only the surface temperature but the material being deposited and the nature of the substrate surface are of significance. In any case, a complete understanding of the stabilization of gaseous radicals in the solid state must

be preceded by a more detailed insight into the deposition process. As an example of the complex phenomena occurring during deposition,

M. Windsor at NBS has observed large temperature fluctuations in

solid hydrogen and deuterium during deposition on a liquid helium

cooled surface. Similar observations have been reported by B. Fontana [13] for the deposition of discharged nitrogen. Such observations strongly suggest the existence of large temperature gradients in

such solids as they are formed by condensation from the gas. The

effect of these gradients on the rate of radical recombination and the

production of radical trapping sites is little understood at the present

time.

1.2. Production of Radicals Directly in the Solid

a. Electron Bombardment and 7-ray Irradiation

Electron bombardment and 7-irradiation are probably the most

widely used techniques for generating radicals in a cold solid. Irradiation of nitrogen deposited on a 4° K surface by 10 to 50 kev electrons

has produced essentially the same emission spectra as that shown by

the condensed products of an electric discharge through nitrogen

[26, 27] (fig. 5). Bombardment by Co^° 7-rays of nitrogen, hydrogen,

and methane condensed at 4° K has yielded solids which exhibit spin

H

ii

f -- H

V

C B R, R2 O

Figure 5. Simplified vertical cross section of cryostat for electron diffraction experiments.

C—chamber; B—copper block; Ri and R2—radiation shields; O—openings; Vi—liquid He vessel; V2—

liquid N2 vessel; H—horizontal sliding mechanism; V—vertical adjustment screws. 12
resonance spectra of the N, H, and CH3 radicals [17, 28] (fig. 6). Qualitative examination of the warmup glows from 7-irradiated nitrogen has shown the same spectral features observed in the other methods

of generating N atoms in a solid nitrogen matrix [29]. In general it has been found that the irradiation of cold soHds with ionizing radiation has been one of the more successful methods of producing stabilized molecular fragments, although the concentrations obtained rarely

exceed 1 part per 1,000.

POLYETHYLENE SACKFiGURE 6. Helium Dewar used in electron spin resonance experiments.

b. Ultraviolet Irradiation

Ingram et al. [30, 31] have produced organic radicals in solids at 77°-

110° K by photolyzing solid mixtures of H2O2 and various alcohols

and hydrocarbons. These radicals have been tentatively identified b}^ means of the hyperfine structure shown by the ESR spectra but no

positive identification has been made.

Attempts to produce radicals by this method have been made at NBS. Conventional absorption spectroscopy was used for radical

detection instead of the more sensitive ESR technique. Were these

attempts successful (i. e., if the number produced were large enough to be detected), they would have provided independent evidence for

specific radical identification. A typical case is the one reported by

539369—60 2 13
H. Brown at the Sheffield meeting m September 1958, in which CH3I

was photolyzed at high dilution in an argon matrix at 20° and 4° K.

Infrared absorption of the solid indicated that considerable reaction

occurred, but the CH3 radical was not detected. Jen et al. [32] per- formed an almost identical experiment but used ESR for detection.

These investigators found the quartet spectrum at g=2 which has been

attributed to the hyperfine interaction of the unpaired electron with

the three protons in methyl. In essence, then, this method of generating radicals suffers from the same difficulty which appears as a recurrent theme throughout these investigations, namely that the

concentrations produced are too low to be detected by any method

except the most sensitive.

c. H-atcm Irradiation and Subsequent Reaction in the Gold Solid

The possibility of providing two independent methods of radical

identification for the same system presented itself when it was dis- covered that H atoms can react extensively with cold solids [19, 20,

33]. The addition of H atoms to solid olefins undoubtedly involves

the primary formation of an alkyl radical [19, 20]. The presence of di-isopropyl and 3,4 dimethyl hexane in the warmed-up products of the hydrogenations of propylene and 1-butene indicated that H atom

addition occurs at the terminal carbon and that secondary alkyl

radicals are formed. The observed reaction was clean in the sense that complicating processes such as atomic cracking (CH4 was not

observed as a product) did not occur. Hence, if unpaired electron

spins were detected, they could be unambiguously attributed to the

secondary alkyl radical. As has been mentioned previously, an

extensive search using an ESK spectrometer, failed to reveal the

presence of these radicals. Consequently, while this technique has

provided sound chemical evidence for the formation of a specific

radical species, a surprising amount of chemical reactivity reduced the

concentration beyond the detectable limits of the sensitive spin resonance technique.

Radical Storage

2.1. Low-Temperature Technique (1° to 100° K)

a. Refrigerants

Inextricably tied to attempts at free radical stabilization is the use of refrigerants for the 1° to 100° K temperature range. The

liquids used at NBS are helium, hvdrogen, nitrogen, and oxygen with

normal boiling points at 4.2°, 20.3°, 77.4°, and 90.2° K, respectively.

Lowering the pressure over these boiling liquids makes possible the

attainment of temperatures somewhat lower than the normal boiling

points. For example, by suitably pumping on liquid nitrogen, temperatures as low as 55° K can be obtained. Similarly a temperature

of 1.2° K can be reached by pumping the vapor in equilibrium with

liquid helium down to a pressure of several hundred microns.

Liquid hydrogen and helium require somewhat specialized techniques in so far as their storage and transfer from one Dewar to another is concerned. Mr. A. Stober and his several assistants at NBS have not only made these liquids continuously available, but

have simplified things for the individual investigator by providing

the service of transferring the refrigerant from a storage to the experi14
mental Dewar. In practice, after 24 hours advance notice, at tlie time designated for an experiment, the refrigerant and an appropriate

transfer siphon are brought to the laboratory, the transfer made, and

the experiment begun.

b. Dewars

The design of appropriate Dewar systems for the storing of lowboiling liquids has been a necessary activit}^ for most of the investigators in the Free Radicals Research Program. The requirement of minimal heat leak from the surroundings must sometimes be compromised by the types of experiments that are to be performed. The

design details of some of the more successful glass Dewars which have

been used for optical studies is summarized by Schoen, Kuentzel, and

Broida [34]. The metal Dewar used in the electron diffraction

studies (fig. 5) is typical of the metal systems constructed at NBS.

2.2. Matrix Isolation

The technique of dispersing molecular fragments in a large excess

of an inert solid phase has been extensively studied by Pimentel and

his coworkers [35, 36]. These authors conclude that at matrix to radical ratios >100, xenon, argon, and nitrogen provide suitable

trapping sites at 20°K and below, while CCI4, CO2, and cj^clohexane

are fairly effective in the neighborhood of the boiling point of nitrogen

(77° K). This technique of matrix isolation has been widely used at NBS. Among the most interesting features of the solids containing

trapped radicals are the matrix-radical interactions shown by their

emission, absorption, and electron spin resonance spectra [7, 12, 17,

37]. Particular attention has been paid to the shifts in energy level from the free gas values, for such species as 'Ni^D^^S) and 0(^S-^'^D)

trapped in nitrogen and inert gas matrices. A complete understanding of these phenomena has not been achieved at the time of this

\\Titing. However, the wealth of experimental information obtained

should stimulate the theorist to address himself to these intriguingproblems in solid state ph^^sics. A start in this direction has been

taken by C. Herzfeld [6, 7] who has shown that a nitrogen atom

interacting with the cr^^stalline field and loosely bound to a neighboring N2 molecule can qualitatively account for the sharp a lines (^D^'^S nitrogen atom transition) observed in glowing, solid nitrogen.

A similar treatment [38] for oxygen atoms trapped in solid nitrogen

accounts for the more diffuse /3 lines (^S-^'^D oxygen atom transition)

which usually accompanies the a group in these emission spectra.

The ^P-^^D transition of atomic nitrogen has also been observed in

the near infrared emission spectrum of excited solid nitrogen [12]. A shift of about 40cm~^ from the free gas value was observed and

satellite lines on either side of the ^P^^D and W^^S transitions are separated from the main lines by about 2,300 cm~^ This value

is close to the vibrational frequency of gaseous N2. Also, the ex- pected isotope shifts have been observed with W^. Thus, the ex- istence of a loose N and N2 complex is strongly indicated and the

interaction between N and N2 in a solid nitrogen matrix appears

to be a mutual one. While these interpretations are by no means

final, something like a complete understanding of this system may

not be long in coming.

15
2.3. Stability of Stored Free Radicals

Once it has been established that radicals are present in a cold solid,

the question of concentration stability naturally arises. There have

been several experiments performed at NBS which have yielded information on this point [28]. Nitrogen atoms (^8), generated in a solid

nitrogen matrix at 4° K by Co^^ 7-irradiation, were observed with an ESR spectrometer. There was no detectable decrease in signal

strength after the solid had been stored at 4° K for 20 hr. Hydrogen

atoms i^S) generated, measured and stored in a similar fashion showed, on the other hand, a distinct decrease('^50 %) in concentration. The

7-irradiation of CH4 at 4° K resulted in a solid which gave the spin resonance spectra associated with CH3 and H. After warming this

solid to about 20° K and then cooling back to 4° K, it was noted that

the ESR signal for H decayed drasticall}' , while the CH3 signal re- mained virtually intact. It appears then that hydrogen atoms diffuse

and recombine in solid hydrogen and solid methane in the 4° to 20° K

temperature range, while CH3 and N are stable at 4° K in solid methane

and nitrogen, respectively. In view of these results, it is not surprising that hydrogen atoms were found to diffuse readily through solid

l\ydrocarbons at 77° K [20]. Livingston and coworkers [39,40] on the

other hand, have successfullv trapped H atoms in aqueous solid solutions of HCIO4, H2SO4 and H3PO4 at 77° K. This is indeed remarkable in light of the above observations.

An interesting experiment concerned with the stability of N

atoms during evaporation from one cold surface to another has been

performed at NBS [41]. The products from a 2,450 Mc electrodeless discharge though nitrogen were collected on a liquid hydrogen

cooled surface. The usual glow phenomena associated with the excited

solid nitrogen were observed. Another glass surface, downstream

from the 20° K one, was maintained at 4° K by liquid helium. When

the liquid hydrogen was evaporated, it was found that the condensate

formed on the 4° K surface emitted the a lines associated with the

\S atomic nitrogen transition. In contrast to this experience are the unsuccessful attempts which have been made at NBS to detect N

atoms mass spectrometrically in the gas evaporating from the surface

of excited solid nitrogen [42]. Once again it appears that the detection

and indentification of radical species requires extreme sensitivity, in this case the ready detectability of the light emitted by nitrogen

atoms.

3. Properties of Stabilized Free Radicals and Free

Radical Containing Solids

There have been a number of methods used at NBS for investigation

of the properties of solids at low temperatures with and without free radicals present. The types of studies which have been made are characterized in the following list. 1. Spectroscopy (infrared, visible, ultraviolet, and vacuum ultraviolet)

2. Electron spin resonance and magnetic susceptibility

3. Mass spectroscopy

4. Calorimetric studies

16
5. Measurements on condensed gases at low temperatures

a. X-ray and electron diffraction

b. Index of refraction

c. Thermal conductivity

d. Dielectric and relaxation measurements.

In addition to the spectroscopic results mentioned previously, some

interesting results have been obtained at NBS in the region of the

vacuum ultraviolet (> 1200 A) [43]. In this spectral range, scattering

is a serious experimental problem. Fortunately, high absorption

coefficients are usually encountered so that very thin films can be used,

thereby minimizmg the scattered light. Solid ammonia, diluted 300

to 1 with argon at 4° K was photolyzed by radiation in 1200 to 2000 A

region. Absorption was observed due to XH whose spectral features were similar to those reported by Robinson and McCarty [44] for the

products condensed at 4° K from an electrodeless discharge though a

1 : 25 hydrazine-argon mixture. In addition to XH, absorption due to XH2 was also observed in the vacuum ultraviolet photolysis of solid

ammonia at 4° K. Warmup of the photolyzed XH3 argon mixture to 35° K showed the disappearance of XH2 but not the XH. The reason

for this behavior is not understood at present. Experiments with the

rare gases have shown the resonance lines of xenon (1305 and 1485 A)

and krypton (1220 A) can be observed in absorption through thin films

of these gases condensed at 4° K. The frequencies at whinh these lines occur in the solid are only slightly different ('^100 cm~^) from the free atom values, contrary to a recent theoretical prediction [45]. Electron spin resonance [17, 28] and magnetic susceptibility measurements [13, 46] have been employed at XBS to detect, identify and

measure free radical concentrations. The radicals examined in

this fashion have been X, H, and CH3 trapped in various solid matrices at 4° K [17, 28], the blue deposit obtained by condensing the

dissociation products of HX3 at 77° K [47], and the various nonequilibrium, colored forms of sulfur which can be frozen at 77° K from the

vapor [47]. As a result of these investigations, the unpaired electron

spins in the condensed dissociation products of HX3 [55] are now

believed to be due to F-centers in the solid [56], while the various

colored forms of sulfur have been shown to be paramagnetic as a

result of the presence of unpaired electron spins in polymeric chains

of sulfur atoms [57]. In addition to the attempts to detect X atoms evaporating from an

excited solid nitrogen surface, mass spectrometry at XBS has been

used for such studies as the gaseous recombination kinetics of X atoms

[48] and the reaction of X atoms with hydrocarbons in the gas phase

[49]. Of particular interest to investigators using the microwave dis- charge technique has been a determination of the X atom concentration in the afterglow region [48]. This number turns out to be

between 1 and 2 percent, thereby providing an upper limit to the

concentration of atoms m condensed nitrogen prepared from such

a source. Measurements of the energy liberated by various discharged gases

condensed at 4° K have been made [16, 50] at XBS. The results obtamed mdicate that as much as several percent of trapped energetic species are present in the solids produced in this fashion. These

estimates are probably high since the calorimeters used undoubtedly

had heat leaks of micertain magnitude. A precision low temperature

calorimeter has been designed at XBS [51] and at the time of this 17
writing, is almost completely built. It is hoped that this device

will yield more quantitative values for the energy trapped in solids

condensed from gases containing free radicals.

Important contributions in the elucidation of the structure of con- densed gases at 4°, 20°, and 77° K have been made by the X-ray dif- fraction group at NBS. The possibility of obtaining information

about the number and type of crystallographic sites available for

free radical trapping is an attractive feature of this type of investigation. The materials which have been studied in some detail are argon, Ng, NHg, H2O, NH3, O2, alcohols, and BsHg [52, 53]. In

general, it was found that slow rates of gaseous deposition resulted

in solids with a considerably less ordered structure than that obtained during rapid deposition rates. The use of an electric dis- charge in the gas prior to deposition resulted in increased order in the solid, and when amorphous solids were heated an exothermic

annealing to the stable crystalline configuration generally occurred.

Certainly part of the released energy observed in the warm up of

excited solids must be attributed to these crystallographic changes.

Some interesting results have been obtained at NBS in an electron

diffraction study of thin solid nitrogen films at 4° and 20° K [54]. For extremely thin films at 4° K, the observed diffraction patterns

indicate the presence of randomly oriented fine crystallites (<20 A).

As the amount of deposited material is increased, larger crystals are formed which exhibit stacking faults in the direction of the 111 plane.

For nitrogen deposited at 20° K, only large crystals O200 A) were

observed. Intense electron bombardment of the 4° nitrogen deposit also resulted in a more orderly crystalline structure. In any

case, both the X-ray and electron diffraction observations are consistent with the view that more orderly solids are obtained when

sufficient energy is available to heat the deposit to a temperature

region where annealing of the solid can take place. It seems to be

immaterial whether this energy is supplied by the heat of condensation, electric discharge of the gas prior to deposition, or electron

bombardment of the solid. Index of refraction measurements of condensed gases at 4° K have

been made [58]. No significant difference in the refractive index was

observed when the gases to be condensed were first passed through an

electric discharge. This is consistent with the fact that radical

concentrations generally do not exceed 0.1 percent. With the ellipsometer used for these measurements it was also possible to make

rather precise estimates of film thickness (±5 A). The application

of this technique to the measurement of condensation coefficients

would be of great value.

Fmally, measurement of the thermal conductivity [59] and di- electric constants [60] of condensed gases have been made. Again,

the presence of stabilized radicals was not detected, demonstrating

once more the very low concentrations attainable in these cold solids. The intriguing notion of avoiding atom recombination in the solid by

depositing a polarized beam of H atoms (the two components being

separated in a Stern-Gerlach experiment) has suggested itself to several investigators [61, 62, 63]. However, the results of both

theoretical [62] and experimental investigations [61, 63] have not

been promising so far.

18
4. Conclusions

The possibility of stabilizing sizeable concentrations of energetic

free radicals now seems quite remote. However, the development of techniques for studying and understanding the behavior of matter in nonequilibrium situations at low temperatures is of unquestioned

scientific importance. It is apparent that many incompletely understood phenomena have been uncovered in these investigations.

Instead of closmg this discussion with a list of NBS accomplishments,

the most significant of which have or will appear in the literature,

the author prefers to list what he considers the major areas of ignorance. They are:

1. The interaction of condensable gases with low-temperature sur- faces (the deposition process).

2. The kinetic details of the chemical processes which can occur in

solids below 100° K (low-temperature chemistry).

3. The general problem of the interaction of trapped nonequilibrium

species (free radicals being a special case) with its surrounding

matrix.

4. The kinetics of the crystallographic changes which have been

observed to occur in solids deposited at extremely low temperatures,

and finally

5. The positive identification of some of the free radicals generated

by 7-irradiation of organic substances of varying complexity.

In particular it would be extremely significant to show whether

the present identifications based on the hyperfine structure of ESE, spectra are correct.

5. References

[1] A. M. Bass and H. P. Broida, Phys. Rev. 101, 1740 (1956).

[2] T. M. Shaw in Formation and trapping of free radicals, ed. A. M. Bass and

H. P. Broida (Academic Press, Inc., Xew York, N.Y., 1960).

[3] H, S. W. Massey and E. H. S. Burhop, Electronic and ionic impact phenomena (Oxford University Press. London, 1952).

[4] R. L. McCarthy, J. Chem. Phvs. 22, 1360 (1954).

[5] J. M. Benson, J. Appl. Phvs. 23, 575 (1952).

[6] C. M. Herzfeld and H. P. Broida, Phvs. Rev. 101, 606 (1956).

[7] C. M. Herzfeld, Phvs. Rev. 107, 12,39 (1957).

[8] T. Cole, J. T. Harding, J. R. Pellam, and D. M. Yost, J. Chem. Phvs. 27, 593 (1957).

[9] H. P. Broida and M. Pevron, J. phys. 19, 480 (1958).

[lOj H. P. Broida and ^l. PeVron, ibid, 18, 593 (1957).

[11] H. P. Broida and M. PeVron, J. Chem. Phvs. 30, 139 (1959).

[12] M. Pevron, E. Horl, H.'^Brown. and H. P. Broida, J. Chem. Phys. 30, 1304

(1959).

[13] B. J. Fontana, J. Chem. Phvs. 31, 148 (1959).

[14] J. L. Jackson, J. Chem. Phvs. 31, 154 (1959).

[15] S. X. Foner, C. K. Jen, E. C. Cochran, and V. A. Bowers, J. Chem. Phys.

28, 351 (1958).

[16] H. P. Broida and O. S. Lutes, J. Chem. Phys. 24, 484 (1956).

[17] L. A. Wall, D. Brown, and R. E. Florin, J. Chem. Phys. 30, 602 (1959).

[18] ^Ug X2 molecules are present in the afterglow of discharged nitrogen, since the

1st positive bands of X2 are readily observed; also vibrationally excited 12+^X2 (i'=l) are present as shown bv K. Dressier, J. Chem. Phys. 30, 1621 (19.59) and F. Kaufman and J. Kelso, J. Chem. Phvs. 28, 510 (1958).

[19] R. Klein and M. D. Scheer, J. Am. Chem. Soc. 80, 1007 (1958).

[20] R. Klein and M. D. Scheer, J. Phys. Chem. 62, 1011 (1958).

[21] Such an apparatus has been constructed by L. Kuentzel and applied to these pyrolytic studies by Clarke, Comeford, and Gould at the XBS.

19
[22] L. J. Schoen—private discussion.

[23] S. Wexler, Rev. Modern Phys. 30, 402 (1958).

[24] F. A. Mauer, S. N. Foner, and L. H. Bolz, J. Chem. Phys. 31, 546 (1959).

[25] M. D. Scheer (unpublished results).

[26] E. Horl, J. Mol. Spect. 3, 425 (1959).

[27] L. Schoen and R. Rebbert, J. Mol. Spect. 3, 417 (1959).

[28] L. A. Wall, D. Brown, and R. E. Florin, J. Phys. Chem. 63, 1762 (1959).

[29] M. Peyron and H. P. Broida (unpublished results).

[30] J. F. Gibson, D. J. E. Ingram, M. C. R. Symons, and M. G. Townsend,

Trans. Faraday Soc. 53, 914 (1957).

[31] D. J. E. Ingram and M. C. R. SymonSj Faraday society discussions, (Sheffield University, Sept. 1958).

[32] C. K. Jen, S. N. Foner, E, L. Cochran, and V. A. Bowers (in press).

[33] M. D. Scheer and R. Klein, J. Chem. Phys. 31, 278 (1959).

[34] L. J. Schoen, L. E. Kuentzel, and H. P. Broida, Rev. Sci. Instr. 39, 633

(1958).

[35] E. D. Becker and G. C. Pimentel, J. Chem. Phvs. 35, 224 (1956).

[36] E. D. Becker, G. C. Pimentel, and M. Van Thiel, J. Chem. Phys. 26, 145

(1957) . [37] L. J. Schoen and R. E. Rebbert, J. Mol. Spect. 3, 417 (1959).

[38] H. Goldberg and C. M. Herzfeld, Bui. Am. Phvs. Soc. Series II, 3, 180

(1958) . [39] R. Livingston, H. Zeldes, and E. H. Tavlor, Phys. Rev. 94, 725 (1954).

[40] R. Livingston, H. Zeldes, and E. H. Taylor, Disc. Faraday Soc, No. 19, 166

(1955).

[41] H. P. Broida and M. Peyron, J. Chem. Phys. 28, 725 (1958).

[42] J. T. Herron and V. H. Dibeler, J. Chem. Phys. 31, 1662 (1959).

[43] K. Dressier and O. Schnepp, to be published.

[44] G. W. Robinson and M. McCarty, Jr., J. Chem. Phys. 38, 349 (1958).

[45] R. S. Knox, University of Rochester Ph. D. Thesis (1958).

[46] In addition to the results of Dr. B. J. Fontana, more precise measurements

of the magnetic susceptibility of the frozen products of discharged nitrogen

are being made by Dr. C. Zahn of the Atomic Physics Section at NBS.

[47] These materials were supplied by Prof. F. O. Rice of Catholic University.

[48] J. T. Herron, J. L. Franklin, P. Bradt, and V. A. Dibeler, J. Chem. Phvs.

39, 230 (1958).

[49] J. L. Franklin, J. T. Herron, and P. Bradt, Can. J. Chem. 37, 579 (1959).

[50] G. J. Minkoff, F. Scherber, and J. Gallagher, J. Chem. Phys. 30, 753 (1959).

[51] G. T. Furukawa and D. Ditmars of the NBS Thermodynamics Section.

[52] L. H. Bolz, F. A. Mauer, and H. S. Reiser, J. Chem. Phys. 30, 349 (1959).

[53] L. H. Bolz, M. E. Boyd, F. A. Mauer, and H. S. Reiser, Acta. Cryst. 13, 247

(1959) . [54] E. Horl, Bui. Am. Phys. Soc. B9, 221 (April 20, 1959).

[55] F. O. Rice and M. Freamo, J. Am. Chem. Soc. 73, 5529 (1951).

[56] W. Gager and F. 0. Rice, J. Chem. Phys. 31, 564 (1959).

[57] Experiments performed by Dr. H. Radford of the Cryogenic Physics Section at NBS and Prof. F. O. Rice of Catholic University.

[58] J. Kruger and W. J. Ambs, J. Opt. Soc. Am. 49, 1195 (1959).

[59] R. J. Corruccini and H. M. Roder, NBS Staff, Boulder Colorado; J. Clarke,

guest scientist at NBS; and R. Gorden, NBS staff: and A. Chatenever,

guest scientist at NBS.

[60] N. L. Brown, NBS staff. [61] W. L. Fite, General Atomic Division of the General Dynamics Corp.

[62] H. L. Mayer and M. Johnson, Aeronutronic Systems, Inc., personal

communication

[63] M, W. Windsor, guest scientist at NBS.

Washington 25, D.C., December 18, 1959.

20
4. A Survey of Theoretical Work on Trapped

Radicals at the National Bureau of Standards

C. M. Herzfeld

1. Introduction

This discussion of the theoretical work done at NBS will be organized on the basis of the c^iialitative discussion given above as an introduction. In this way we hope to give the best rounded picture of the

work, and also to arrive at reliable conclusions about the processes

involved. In general, the details of the calculations will not be

given here. For these the reader is referred to the published work.

We shall concentrate instead on the assumptions, results, and conclusions of the theories.

2. Production and Deposition of Radicals

The first qualitative step is the production of free radicals in the

gas phase or in the solid. No theoretical work has been done on this phase of the problem.

The second qualitative step is the condensation of the gas phase

onto a solid. It is clear that this process is a very complicated one, and that it has several aspects. The first has to do with the reactions

in a flowing gas, and with the composition of the gas just before the

impact on the solid. Xo theoretical work has been done on this. Rather, all calculations have assumed that a gas of given composition (mole fraction [R] of radical B, mole fraction [i?2] of molecule

R2, and mole fraction [M] of inert molecules or atoms) interacts with

the surface. Also nearly all calculations ignore the complex behavior

expected of a streaming gas undergoing chemical reactions.

A summary of most of the processes which are important in the

deposition is given in table 1. This table is used below to characterize some of the theories. It also gives a graphic indication of the

complexity of the problem.

Two types of theories have been constructed to deal with the

deposition problem. The first type, essentially statistical, studies

the deposition by studying the number of ways in which radicals,

molecules and inert species can be laid down together. This is done

using various assumptions about the system. In this way upper limits

of concentrations of trapped radicals can be estimated, and their

dependence on the various parameters of the problem studied. In

addition considerable insight into the statistical features of the

process can be obtained. The second type of theory studies the

detailed mechanisms of the processes involved in deposition. From

it qualitative or semiquantitative estimates of '^sticking probabilities" can be obtained, as weU as insight into the dynamic features of the

deposition process.

2.1. Statistical Theories

The first of the statistical theories is clue to Jackson and Alontroll

[1].^ The theory is based on the following model (i.e., basic assumptions):

1 Figures in brackets indicate the literature references at the end of this paper. 21
Table 1. The processes in the deposition: a schematic summary

The species: i2=atoin, free radical i?2= stable molecule M=inert atom or molecule

Subscripts: G = gas, <S= surface, 7=interior of solid Rate Constants: k-i is the rate constant for the inverse process of constant ki. The Processes

1. Rg^Rs ki

2. R2G^R2s ki

3. Mg->Ms ks

4. 2Rg-^R2g ki

5. 2Rs-^Ris h

6. 2i?/->i?2/ h

7. R on surface Di

8. /?2 on surface Di

9. Mon surface Dg

10. R surface-^interior

11. i?2 surface^interior Dn

12. Af surface^interior JD12 13. R in interior Biz

14. i?2 in interior Du

15. Min interior D15 One starts with a mixture of R, R2, and M of known composition

in the gas. The gas is deposited in a random way, and each R combines randomly with another R which is a nearest neighbor. Two

R which are nearest neighbors are not stable and must combine. All

energetic or diffusion effects are ignored.

The problem is thus chiefly a combinatorial one, and concentrates on possible reactions in the solid, once all species are set down in a random manner. This amounts to ignoring processes 1 to 4 and 7

to 15 in table 1, and effectively eliminates competition of various

processes. Three cases are treated in detail. First, the result of depositing a linear chain of R is investigated, and the final concentration [R] in the chain is calculated exactly. This is done by considering all possible configurations of the chain which do not have 2 R in adja- cent positions. The result is the following:

/=(2xi +3)-\ (1) where / is the fraction of R in an infinitely long chain, and a::i = 1.3247 a number which is obtained in the solution of the combinatorial

problem. The maximum concentration of R is then given by /, and

turns out to be 17.7 percent.

The second case they considered is a one-dimensional chain which

contains the species R and Min random positions. An exact solution

of this problem is also obtained. The fraction of trapped radicals is given by eq (3.6) of the reference. It predicts a maximum concentration oi' R of about 19 percent, when the initial beam which is deposited consists of about 45 percent inert atoms. A similar treat- ment is given for a beam containing R, R2, and M. Here the R2 is inert but occupies two lattice sites. The third case discussed is the three-dimensional condensate

obtained, both for the case starting only with R in the gas, and starting

A. Deposition

B. Reaction

C. Diffusion

22
with R and M in the gas. This case is solved only approximately.

Various lattice types are considered, which differ principally in the

number of nearest neighbors of each lattice position. The results are summarized in figure 1. Maximum concentrations of trapped R are seen to lie between 10 and 15 percent. Similar results are obtained

when the effect of diluents is considered.

The second statistical theory is due to Golden [2]. Golden considered five different types of situations. They all have the following

features in common: The gas is assumed to consist of R, R2, and M

in known proportions. All chemical reaction rates are considered

negligibly small compared to A*4 through of table 1. Recombination

is assumed to result from nearest neighbor interactions in the con- densed phase. Stabilization is assumed to occur only by isolation of the R by R2 or M. Diffusion processes are neglected. The problem

is treated as a statistical one. The models considered by Golden are the following:

1. The Densely Packed Bulk Condensation Model. This model

assumes that all sites in the solid are filled, and each atom or molecule

is assumed to take up one site each. Effectively, radicals near each

other are assumed to react with one another before they condense.

For a given composition of the mixture, the number of ways of arranging this mixture are calculated. The calculation is subjected to the

boundary condition that no two R may occupy neighboring sites.

1 \ \ \ 1 r

2 4 6 8 10 12 14 16 18 20

COORDINATION NUMBER Z Figure 1. Variation of free-radical concentration, with coordinate number z of

stabilized matrix.

(Fig. 6 of Jackson and Montrol [1].) 23
The calculation also takes into account explicitly the statistical weights of the states of radicals, molecules, and inerts. The results appear as a complicated equation (eq (10) and fig. 5 of [2]) and imply

that concentrations of trapped radicals of up to 7 percent may be

expected.

2. The Diffusely Packed Bulk Condensation Model. In this model

it is assumed that all species first condense, one to a site, and then

recombine if they can. Vacancies arise from these recombinations.

The vacancies are assumed not to contribute to diffusion (which is obviously a doubtful assumption). The results are given in eq (14)

and figure 6 of [2], and predict from 3 to over 18 percent of trapped

radicals, depending on the number of nearest neighbors and the

initial concentration of radicals in the gas.

3. The Surface Condensation Model. This model assumes that each

condensed layer comes to some statistically stable state before the

next layer can deposit. This is a good assumption if the number of

collisions of species with the surface is small. Each layer is assumed

formed in a way consistent with model 1 above. The results lead one

to expect even higher concentrations of trapped radical than model 2. 4. Nonideal Condensation Model. This model assumes that species

may hit the surface, do not always stick, but evaporate again and

react within coming gas in the gas phase. The behavior of this model can be related readily to that of the densely packed bulk condensation

model, for which the calculations are given. The results are given in figure 2. As can be seen, the incomplete condensation reduces the

percentage of trapped radicals very significantly.

5. Clustering Model. Here it is assumed that some clusters of species will be formed in the gas phase, and that the clusters will condense.

The model considers that a whole range of sizes of clusters are formed.

.07 1 1 1 1 1 II 1 I

1

LUM .06

_l

STAB

ION (

.05 LOO/"^

MOLE

FRACTI .04

ICAL .03

-RADI

FREE- .02

.01 1 1 1 1 1 1 II 1 1

Z M2

a = 0.1

1 1 1 1 1 1. L

.01 .1 1.0 CONDENSING FRACTION , f Figure 2. Corn-position of the densely packed hulk stabilized condensed phase as a

function of the condensing fraction f. Curves are labeled by mole fraction of free radicals in the gas. (Fig. 9 of Golden [2].) 24
"While the results obtained are not complete, it does not seem that

this phenomenon will affect greatly the expected concentration of trapped radicals.

Golden discusses two shortcomings of his treatment. First, no- where are the energetics of the system introduced into the calculation,

and second, diffusion is ignored throughout. It appears to the present

author that both features, if taken into account, would lower the ex- pected concentrations of free radicals.

The statistical models of Jackson and Montroll and of Golden have

several features in common. They neglect energy considerations and

diffusion, hence are likely to give high estimates of trapped radical

coDcentrations. The calculations are of combinatorial type and are

either rigorous, or contain sensible mathematical approximations.

The vai'ious models can be characterized by their assumptions about

relative rates of various important processes. This characterization

is summarized in table 2, The statistical theories have the advantage

of being logically straightforward, but the disadvantage that they

ignore important processes, and further that they are not very easy to see through intuitively. The results, however, are clear cut, and are quite useful for the predicting of general trends, if the limitations of

the theories are kept firml}^ in mind.

Table 2. Relations of rate constants assumed by statistical models

Reference

[1] Jackson and MontrolL

[2] Golden

A;i to A-3 ^ ko, k(, Model 1. » ^-1, k;, k^ Model 2. /ci to A;3 » /.'o, A-6 Model 3. ki to fcs « ko, /C6<< ki Model 4. ki^kito ki. Model 5. Same as model 1, with clustering

D? to Z»i5=0 Di to Z)i5=0

Z>7 to Dio=0

X»7 to i>1.5=0 D? to Dio=0

2>7 to i)i5=0

2.2. Kinetic Theories

The only attempt so far at a discussion of the details of the process

of deposition has been carried out by Zwanzig [3]. He attempts to calculate the probabilit}" of adsorption of an atom on a solid. He

chooses a very simple model: The solid is represented by a semiinfinite chain of atoms connected by nearest neighbor interactions.

The lattice is initially at rest (effectively: T=0°K), and the energy

transfer is calculated for a collision of an atom with this chain at rest. The precise point is the following: In previous theories of this phe- nomenon it has seemed that it is difficult for an atom to transfer appreciable amounts of energy to the chain. Certain approximate

quantum mechanical calculations had indicated that processes in which

the lattice absorbs more than one phonon are unlikely. This would

mean a low-sticking probability, i.e., the atom would hit and nearly

always rebound again. Zwanzig has examined this feature with considerable care. He calculates the critical kinetic energy of the in- coming atom, i.e., that kinetic energy above which the particle is always reflected. It turns out that this critical kinetic energy depends

strongl}" on the ratio /z==mass of incoming atom/mass of atom in chain.

Exact solutions can be given for )U=1 and ji, and a good asymptotic

solution for small /i. The mass effect is important. For /x=l large

25
amounts of energy are transferred effectively. (This corresponds to the many-phonon prosesses, which were previously thought to be

unimportant.) For less energy is transferred. The largest

fraction of the energy is transferred when the original energy is low.

It is clear that this theory has several limitations. The most

serious are the one-dimensional model of the solid, and the classical

treatment of forces. In spite of these the theory should predict the mass effect reasonably well. A higher (2- or 3-) dimensional solid

will make transfer of energy easier, but probably not change the qualitative features, and only for hydrogen are the quantum effects likely to dominate. The theory should be particularly good for the prediction of relative deposition rates of various noble gases on crystals

of noble gases.

3. Emission and Absorption of Energy

When the mixture of gas containing R, R2, and M condenses, radiation is sometimes emitted. This arises either because excited species are deposited, which radiate during or after deposition, or because

species react, such as 2R^R2, and produce highly excited new species.

When the deposit "settles down" one can often observe absorption

of energy. Three broad theoretical problems have been investigated.

First, historically, the emission of radiation from nitrogen deposits has

attracted considerable effort. Second, the paramagnetic resonance

absorption of hydrogen deposits has been studied. And third, the

effects of the solid on the absorption spectra of molecules have been

investigated. We shall take up each of these broad inquiries, and see what conclusions they have led to. 3.1. Emission of Radiation from Nitrogen Deposits

Very early in this work, before the Free Radicals Program was ini- tiated, a semiquantitative interpretation of some features of the very

complex light emission from nitrogen deposits was given by Herzfeld

and Broida [4]. The general point of view was adopted that in the

deposition of nitrogen which has passed through an electric discharge,

N atoms are deposited, these radiate, and then recombine giving excited N2 molecules which also radiate. In this manner the a lines were

interpreted as arising from the highly forbidden transition '^D-^^S in N atoms, the 0 lines were ascribed to transition between high-lying

levels in N atoms, and the ^-bands were assigned to the transition

to of newly formed N2 molecules. The arguments were qualitative. It is now clear that the a lines do indeed arise from a transition like that proposed, though the situation is known to be vastly more complex than thought at the time. The j3 lines and ^-bands

belong to transitions of oxygen atoms and molecules, respectively.

Later an attempt was made by Herzfeld to analyze the structure

of the a-lines in detail [5]. He applied crystal field theory to the

analysis of the spectrum. As before, the a lines are assumed to arise

from the transition ^D-^'^S. The effects of the solid on the positions

of the levels appear through off-diagonal matrix elements of the crystal

field, while the transition probabilities are affected by a combination

of crystal field and spin-orbit coupling. (See figure 3.) By assuming

the values of two crystal field parameters, and of one atomic parameter,

the positions of all five spectral lines of the a group which were known then

26
2p^ 3p 2d

2p2 3s

SO

2p'

2P

97

86

83

28

20

FiGUKE 3. Schematic summary of levels involved in the a-line spectrum;. SO represents mixing through spin-orbit coupling; T'l through hnear potential terms; through quadratic

potential terms. IS umbers on the right give energy in units of 1,000 cm-i. (Fig. 2 of Herzfeld [5].) were fitted quite well. (See figure 4.) Also relative transition probabilities were predicted which agreed well with the measured relative

intensities. A numerical model was constructed to relate the crystal

field parameters needed to fit the data to the geometry of the trapped

N atom. Because of the short range forces m solid N2, and because of the low symmetry of the trapping site required to fit the data, it was

argued that each X atom was perturbed by, and attached loosely to, one X2 molecule only. On the basis of a point charge model some esti- mates of the configuration were given. The theory had one major

intrinsic flaw. To obtain agreement between calculated and observed

energy levels, a wholly mireasonable value of rj, the spin-other-orbit

couplmg constant of the trapped atom had to be assumed.

It is now knowm that the details of this analysis are wrong. First of

all, eight or perhaps more lines have been observed in the a group.

This implies [5] that more than one trapping site is available to the

X atoms. Second, the mtensities have been shown to be very poor

indicators of the transition probabilities. ^Mien they are compared [6] with measured half-lives, they prove without a doubt that there exist wide variations in the populations of the excited states. The general

ideas of the theory continue to be applicable, as shown by the analyses

of spectra discussed m the preceding paper, but the details of the cal- culations must be modified.

In the deposition of ''excited" nitrogen gas, some oxygen is apparently present. It has been shown (see the preceding paper) that the

j8 lines observed in nitrogen deposits arise from transitions in trapped

oxygen atoms. Goldberg and Herzfeld [7] have applied a crystal field analysis to the proposed transition, namely ^S-^^D, and have found

27
-100

0.2 0.4

Figure 4. Comparison of theoretical energy levels with early observed levels for

the a-lines. The theoretical levels are plotted versus -a'^jW for |<r2|=230 cm-i, and 77=3.46 cm-i. Circles indicate experimental values. Newer data are yet to be interpreted. (Fig. 3 of Herzfeld [5].) reasonable agreement between experiment and theory. Again it is assumed that each O atom which radiates is held in loose complex w^ith one N2 molecule. Similar techniques have been applied to the groundstate, ^P, of trapped O atoms. This calculation has shown that the

electronic ground state of trapped O atoms is very likely nondegenerate,

and that the next higher level lies several cm~^ above the ground state. This result explains the absence of paramagnetic resonance absorption

by trapped O atoms.

Recently Griffing has investigated the a lines from a different

point of view [8]. She assumes that an N atom replaces an N2 molecule

in the lattice, and then moves to readjust the lattice in such a way as

to minimize its energy. Several nonequivalent motions of this type

can be found, which give rise to enough levels to explain the a group.

So far the results obtained are only qualitative, but the approach is a very promising one and is complementary to crystal field theory.

3.2. Paramagnetic Resonance Absorption

When atoms with an odd number of electrons are trapped in a solid,

they ar6 very likely to exhibit paramagnetic resonance absorption.

Griffing, Jackson, and Ransil [9] have examined in detail the theoretical paramagnetic resonance absorption spectrum of the molecule H3.

Their motive was to see what this spectrum would look like, to make an

experimental search for this molecule easier. Also, at about the same

28
time, the resonance absorption of hydrogen deposits was being studied

b}' Jen, Foner, et al. at the Apphed Physics Laboratory. They observed, among many other spectra which they interpreted, a spectrum

which defied interpretation at the time, and which was conjectured by

some to arise from H3. (The spectrum has since been interpreted by

Jen and Foner as being due to tlie radical SiHg.)

The method of calculation was the following: First, the best wave

functions for a linear H3 molecule with equidistant H atoms were

calculated. This was done in the LCAO-MO-SCF-CI scheme.

(This means that Linear Combinations of Atomic Orbitals were used to construct Molecular Orbitals, that a set of these molecular orbitals was picked which give a Self-Consistent Field for each electron to move in, and that Configuration Interaction was taken into account to some extent. For details of the method the reader is referred to the

reference, and the literature cited there.) Second, once the best

functions, and the total energy, were available, these functions were

used to calculate the hyperfine coupling constants, i.e., the magnitude

of the interactions of the electrons with the magnetic moments of the

nuclei. Last, the energy levels of the molecule in an external magnetic

field are calculated.

In the first part of the calculation, the effective atomic number Z

of the atomic wave functions, and the equilibrium separation Bab of the atoms are varied, until the lowest value of the energy of the molecule is found. Z=L10 and i?a^,= 1.90 atomic units give the lowest

energy. In the second part, the Fermi interaction between the elec- trons and the nuclear magnetic moments was calculated. The dipoledipole magnetic interaction between the electrons and the nuclei was

ignored, because it was shown to be smaller than the contact interaction. Third, the energy of the molecule in the external magnetic

field was obtained with second-order perturbation theory. The energ}^

levels in the absence of this field were also calculated.

The results of the calculation are the following : The calculated total energy was correct to about 4 percent, the calculated binding energy was 65 percent of the correct value. The predicted resonance spec- trum is quite different from any that have been observed so far. It consists of two doublets near the free electron resonance, and two more

doublets, one on each side of the free electron resonance, very far away

from it. (This separation of the two far doublets is greater than the

H hyperfine splitting.) The spacing of the central doublets is quite

sensitive to the contribution of configuration interaction to the wave

function. This part of the spectrum is shown in figure 5. A calculation of the resonance spectrum of H2" was also carried out

for comparison.

The conclusion of the calculation was that no H3 had been observed

so far. The only other discussion of resonance absorption carried out in the NBS program is that of Goldberg and Herzfeld for trapped O

atoms, which was discussed above [7]. 3.3. Molecular Spectra of Trapped Species

The effect of the solid on the spectra of molecules has been investigated by Zwanzig [10]. He has treated two problems. First, the

shift and broadening of the spectrum of a molecule in a lattice composed of the same molecules, and second, the shift of the spectrum of a molecule in a lattice of inert atoms.

539369—60 3 ' 29
9350 -

1

1 1 1 1 1 1

9300 -

9250 -

^ha=l23l.5

E5-E12

9200 hg = 600 ^ ^

-

9150 ^^^^^

-

9100 - <"

^/''^hQ =600, 1,231.5 / 8050 \ / "\^ / ^ ^ hQ = 600 -

9000

8950

1 1.. 1 1 1 1

E4-E,3

1 0 . -50 -100 -150 -200 -250 -300

Figure 5. Theoretical spectrum for H3, as a function, of two hyperfine parameters, ha and hb. Only the four central lines are shown. The sohd lines belong to the calculated value of ha = l,2Sl Mc,

Aj = -227.2 Mc. (Fig. 1 of [9].) The first calculation is based on the interaction of the dispersion

dipoles of the molecules. (A molecule may have a momentary dipole

due to fluctuations in its electron distribution, this dipole induces a

dipole in a neighbor, these dipoles interact, and so forth.) The various

moments of the spectrum are calculated. The zeroth moment gives

the total absorption, and is not affected by the interaction. The first moment gives a small shift, due to electrostatic perturbations. The

second moment gives the widths of the lines, it arises from fiuctuations

in the total potential which are due to phonons or imperfections. A

numerical estimate of line width for anthracene crystals gives the

correct order of magnitude.

The second problem arose from an observed slight shift in the

Vegard-Kaplan system of the emission spectrum of N2 trapped in ar- gon. Zwanzig estimates the shift in the ground state of N2 in argon.

Three approaches are investigated. Simple dispersion theory is used

with the polarizibility of solid argon calculated from the index of re- fraction, and interactions between N2 and A calculated from LennardJones potentials. The result depends on the frequencies for the N2

vibration, hence on the vibrational quantum number of N2. This

calculation gives the correct magnitude (approximately 5 cm~^) and

direction (to the red) of the shift. Two other effects were studied,

both gave results several orders of magnitude too small. They were

the force arising from quadrupole-induced-dipole interactions, and

second order dispersion theory.

30
In summary, a fairly large amount of work has been done on the

theory of the spectra. This work has shown beyond doubt that some phenomena are explained in a simple and straightforward way by

assuming that active species are trapped in the solid. Furthermore,

this work has solved a large number of special problems. The precise

nature of the solutions will undoubtedly change as more details are observed, but the general picture seems very firmly established.

4. Trapping Forces and Structures

A number of significant facts about trapping forces have been dis- covered in the work under consideration. It has become fairly clear

that the trapping of atoms in molecular lattices is a quite complex

phenomenon. It is evident that loose complex formation with molecules occurs often, at least in the case of N trapped in N2 and O trapped

in N2. The theoretical evidence was discussed above, but the most

strikmg evidence comes from experiment, particularly from the a! and

^' line groups.

Another method to investigate trapping forces has been used by

Jackson and Castle [12]. They investigated the following question:

Are there any positions in or on an N2 lattice where an N3 molecule

could rotate, either freely or in a hindered manner? Experimental

evidence for such behavior exists, in portions of the a" group of spectral lines. Jackson and Castle calculated the energy levels of molecules on the surface of the crystal, and also studied the long-range

interaction potential between N and N2. So far their results are

inconclusive.

As pomted out above, crj^stal field theory has been a useful tool in the interpretation of spectra of trapped radicals. The theory has,

however, the basic flaw that it is difficult to correlate the '^fitted" values of the parameters of the theory with geometrical models. An

attempt has therefore been made [13] to investigate the nature of

crystal field theory itself, using essentially molecular orbital techniques. The model studied consists of an excited H atom (in a 2p

state) and an unexcited H2 molecule. The interaction energy is cal- culated in two ways. First a modified molecular orbital type treat- ment is used, then a crystal field type, and the two compared. The

procedure sheds considerable light on crystal field theory, and may

provide a good (though complicated) calculational scheme for the

crystal field parameters.

Calculations of properties of two triatomic systems are being done

by Bovd [14]. The method is molecular orbital theory in both cases (LCAO-MO-SCF-CI method). One molecule studied is linear N3

with equidistant N atoms. The range of p (interatomic distance between 2 N atoms) is ro<p<2ro, where is the interatomic distance

for the N2 groundstate. The second molecule being studied is IIO2.

The geometry is that of a right triangle, with assumed r(0—H) as in H2O and r(0—0)^=^1.3 A, which is typical for superoxides. These

calculations should give a qualitative or semiquantitative idea of such

things as binding energies, energy of excited electronic states, and of

energy as function of geometry. From these a rough idea of the spectra can be obtained. In another investigation, Boyd has collaborated [15] in working

out some cr^^stal structures from X-ray diffraction. These studies

31
centered on establishing the structure of the a phase of N2 (which is stable at low temperatures). There has been some controversy

about this structure in the past. The present work established the

space group of the a phase as Pa3, thus showing that Vegard's early

result was incorrect. Furthermore, electron density maps were

prepared [16] of the a phase. Several questions still remain unsettled

in this problem such as precise N—N distances.

5. Thermodynamics of Radicals

It is of interest to inquire whether radicals exhibit unusual thermodynamic properties, either when free or trapped. Clearly, in a system in equilibrium the concentration of free radicals can be calculated from thermodynamic data. For light radicals of the type

under consideration here, it is obvious that at equilibrium at low

temperatures the concentation of such radicals will be minute. The

point is, however, that it may be possible to prevent the system from

coming to equilibrium, and thus amounts of radicals larger than

equilibrium may be kept for some time. This was the philosophy of the statistical trapping papers of section 3. A quite different approach was the attempt by Jackson and Pollack [17] to investigate the

equation of state of H atoms. Of course the H atoms would normally

recombine, but if they could be kept from doing so, say, by collect- ing atoms in a very strong external magnetic field with their spins

parallel, then they will recombine only at some slow rate (yet un- known) and there is a real point in discussing the thermodynamics

of such an assembly. They apply de Boer's theory [18] of condensed

permanent gases (which is a type of corresponding states theory) to H, D, and T atoms, which are assumed to be kept in parallel spin

states. From a minimum of experimental data they obtain energies,

densities, and critical temperatures for liquids supposed formed from

these atoms. The application of the theory to these atoms is not

fully worked out. If some means can be found to collect spin oriented

atoms, this method for predicting their properties may become useful.

6. Disappearance of Trapped Radicals

From qualitative observations it is obvious that trapped radicals

disappear by at least two distinct mechanisms. These observations

are most clear-cut in the case of nitrogen deposits. Here one has

known from direct observation from the earliest experiments, that

several types of flashes occur during deposition or during warmup.

One type of flash seems to propagate so rapidly that it appears to cover an area of several square millimeters instantaneously. A

second type of flash propagates slowly enough that the movement

of a definite front can be discerned. This has been called by many a

cold flame. Recent thermal and ma2:netic studies by Fontana (see

the previous paper for reference) indicate these flashes seem to be

intimately ' connected with some critical concentration of trapped

radicals. This has led Jackson [19] to work out an elegant treatment

of stability in trapped radical systems, and of the ways in which

unstable systems disappear. Recently Reed and Herzfeld [20] have

given a treatment of the speed of the slowly moving 'Aflame". Thus

32
two aspects of the disappearance of trapped radicals can now be

understood.

Jackson's work consists of three major parts. The first part uses quahtative arguments to obtain a simple expression for a critical concentration, of trapped radicals, which, if exceeded, gives

unstable systems which decay explosively. The second part consists

of a detailed analysis of discrete sequences of radical reactions, with an analysis of the circumstances under which steady states can exist. The third part involves a study of the partial differential equations

for the average values of concentrations. Remarkably, all three

treatments give the same criterion for stability.

The fu'st, qualitative, theory goes as follows. One assumes: (1)

A lattice of Bo, with R distributed at random; (2) the R are in traps,

sufficiently deep that at 4° K no diffusion of B takes place; and (3)

all traps are assumed alike, with a given trapping energy Wt. Now,

when 2B Bj the released energy is assumed to heat a volume V of

the lattice, and to free the B in this volume. The probability that an B so freed combines with another B is estimated, and it is assumed

that this probability is zero except near the place where the B

comes to rest again. If the average number of secondary recombinations is equal to one, i.e., if every B produced in a recombination

itself recombines itself again, a steady state exists. This gives

the following equation for the critical concentration:

where n,. is the critical concentration of B, M the concentration of B2, a a number of order unity chosen to give a reasonable average specific

heat of the solid, k the Boltzman constant, Tf the threshold temperature for release of radicals, Tq the initial temperature of the deposit, ^ is related to the probability of capture of an B, and is of the order of the

coordination number of the B in the lattice, and W is the energy re- leased in each recombination. If one assumes reasonable values for the parameters one obtains ric^O.l percent. A maximum estimate

(straining all plausibility) gives r(c~4.7 percent, a similarly unlikely

minimum estimate gives 77^ — 0.2 percent. This qualitative theory

has a number of obvious flaws. It treats the heat release macroscopically, all else microscopically. Further, all the released energy does

not go into the lattice, some is radiated. Also, the estimated probability of recombination may be too low. Xone of these are likely to

aft'ect the final result greatly, except that the low estimate of the recombination probability leads to a predicted value of Uc which is too high.

Several important conclusions can be drawn from this qualitative

theory directly. First, and most miportant, the maximum concentrations of trapped B likely to be stable are about 1 percent. Second,

there is little point to cool the lattice much below 4° K. Third, the

value of ric does not depend greatly on the substance used.

The second approach used by Jackson is a detailed analysis of the

structure of radical chain, and its dependence on the parameters of the problem. In particular the probabilit}' of terminating chains is studied. It can be shown that when n<C^nc all chains terminate, and

the s^'stem as a whole is stable. When n =nc, nonterminating chains

exist, which lead to explosions. The expression for Uc obtained is the same as above.

33
The third method involves writing down partial differentialjequations for the averages of the variables. These equations give the same

criterion for stability which was obtained before.

Several special calculations are carried out by Jackson. The concentration of R in an irradiated solid is calculated as a function of time,

and a saturation curve obtained (see fig. 6). This should describe

irradiation experiments quite well. Another special calculation gives

the effect of size on stability. As in nuclear pile theory, the smaller

the sample, the larger the stable concentration, because then some

radicals escape without producing secondaries. Finally, another value

of 7ic=0-13 percent was obtained from the theory using late experiments.

Reed and Herzfeld [20] have given a semiquantitive theory of the

speed of a diffusion controlled slow flame through the deposit containing trapped N atoms. These slow flashes have a speed of propagation of approximatel}^ 2 cm/sec. The theory is simple. The flame

front heats up the material ahead of it, releases trapped R which re- combine, this heats up material ahead of them, and so forth. Simple

calculations using fairly reasonable approximations show that the

speed of propagation v of the flame is of the form

V— (2e~^ In 2)2 {Km)^ 7i=mole fraction of radicals.

where K is the thermal diffusivity of the solid N2, and vis a, frequency

characteristic of the jumping rate of species in the lattice, and hence

related to the diffusion rate. Using a measured value [21] of K, and

an upper limit of v obtained from the width of the hyperfine lines of N

atoms in paramagnetic resonance absorption, an upper limit of y of 3 cm/sec is obtained. This agrees very well with the observed speed,

considering the crudeness of the theory.

These theories account satisfactorily for different aspects of the dis- appearance of radicals, but the relation between the two theories is not

clear.

Figure 6. Theoretical radical concentration n as a function of time t in an

irradiation experiment.

Us is the saturation concentration, K the number of E produced per cubic centimeter per second and

fi=^nclM. (Fig. 2 of Jaclison [19].) 34
7. Conclusion

We shall attempt to summarize here both the results and the

remaming problems. While much has been accomplished, it is probably true to say that there are now more questions than there were

three years ago, and more questions now than there are answers. Of course, many special results have been obtained; these have been

discussed in the preceding pages. But, more importantly, the

theoretical work was usually done in close contact with the experimental work, has stimulated it and was stimulated by it. In this way it has had indirect effects which are impossible to demonstrate,

but which are quite evident to the members of the Program.

Several new fields have been opened up or significantly expanded.

To this writer the most important seem to be two: (1) The theory of chemical condensation, of condensation of chemically reacting gases. This field is of unusual complexity because of the interactions of chemical, solid state, h^^droch^namic, and statistical effects. The

field has considerable importance in many scientific and engineering

applications. Only a beginning has been made, mainly on the statistical parts of the problem. (2) The theory of photochemistry of

solids. Much experimental work has been done elsewhere, and some

theory also, but in the efforts described above one can discern the

beginnings of systematic theoretical approaches to the interaction

between photochemical and solid state effects. A field in which existing theoretical techniques have been applied

to new systems and circumstances is the theor}^ of spectra of solids. While few new theoretical ideas have emerged from tfiis, the range of applicability of existing theories has been extended significantly.

What are some of the outstanding problems which still require

attention? Perhaps the most important of these is the ver}^

fundamental one of describing in detail the interactions between two

reacting species. This is a venerable, unsolved problem of chemical

physics and a good deal of thought goes into it these days. Here the

problem would be modified by the presence of sohd state effects. The two new fields of ''chemical condensation" and ''photochemistry

of solids" need considerable theoretical development before they can

be said to be established.

Furthermore a host of special problems need attention, such as interpretation of spectra, calculation of energ}^ levels of unusual species,

and so forth. 8. References

[1] J. L. Jackson and E. W. Montroll, J. Chem. Phvs. 28, 1101 (1958).

[2] S. Golden, J. Chem. Phys. 29, 61 (1958).

[3] R. Zwanzig, personal cummunication.

[4] C. AI. Herzfeld and H. P. Broida, Phvs. Rev. 101, 606 (1956).

[5] C. M. Herzfeld, Phys. Rev. 107, 1239 (1957).

[6] CM. Herzfeld, unpublished.

[7] H. Goldberg and C. M. Herzfeld, Bull. Am. Phys. Soc, Series II, 3, 180, 1958. H. Goldberg, Phvsics Ph. D. Thesis, Universitv of Marviand, 1959 (to be published).

[8J V. Griffing, personal communication.

[9] Griffing, Jackson, and Ransil, J. Chem. Phys. 30, 1066 (1959).

[10] R. Zwanzig, personal communication.

[11] V. Griffing, personal communication.

[12] J. L. Jackson and B. Castle, personal communication.

[13] H. Goldberg and C. M. Herzfeld, to be published.

35
[14] M. E. Boyd, personal communication.

[15] Bolz, Boyd, Mauer, and Peiser, Acta Cryst. 13, 247 (1959).

[16] Bolz, Boyd, Mauer, and Peiser, unpublished.

[17] J. L. Jackson and G. Pollack, personal communication.

[18] J. de Boer, Physica 14, 109, 149, 520 (1948).

[19] J. L.Jackson, J. Chem. Phys. 30, 154, 31, 722 (1959). J. chirn. phys. 56, 771 (1959).

[20] S. G. Reed and C. M. Herzfeld, J. Chem. Phys. 33, 1 (1960).

[21] H, Roder, NBS, Boulder, personal communication.

Washington, October 14, 1959.

36
5. Low-Temperature Chemistry

Ralph Klein 1

1. Introduction

The interest in free radical stabilization in recent years has stimulated investigation of chemical reactions at low temperatures and

emphasized that because of the many unique problems and techniques low-temperature chemistry can be considered a distmct

discipline. This review of low-temperature chemistry will be limited

to chemical reactions that occur at temperatures below 150° K, an

arbitrary upper temperature limit. No attempt has been made to be

exhaustive; the major emphasis has been on work done within the

National Bureau of Standards Free Radicals Research Program.

Whether a chemical reaction can occur at a given temperature at a conveniently measurable rate depends, among other things, on the

activation energy through the Arrhenius relationship. It is quite

evident that low-temperature chemical reactions will be characterized

by low-activation energies, in general less than 5 kcal. This encompasses radical-molecule, radical-radical, ion-molecule, ion-ion, certain

molecule-molecule reactions, and some catalytic reactions. Usually

these will be reactions occurring either in the crystalline or the glassy

state, although the liquid and gaseous states are not excluded. In

distinction to the high temperature regions, problems of diffusion,

both thermal and particulate, assume great importance. Chemical

reactions become simpler since secondary reactions are usually absent.

Although the number of reactions that can be investigated is limited,

the development and application of optical spectroscopy, electron spin resonance, field emission microscopy, and other techniques to the low

temperature region, have greatly increased the scope within which

an understanding of low-temperature reactivity can be developed.

The preparation of systems capable of reacting at low temperatures,

and the control and measurement of parameters such as temperature,

rates, and concentration often present formidable experimental difficulties. Experiments with radical reactions require preliminary

preparation such as high-temperature pyrolysis or electric discharge

followed by a rapid, low-temperature condensation. Other methods

include photolysis, 7-irradiation, X-ray, electron, ion, or free radical

bombardment of solids. Many of these techniques have been used

successfully, often with interesting results. The series of investigations of Geib and Harteck in 1932 and 1933

[1, 2, 3]^ were among the first in low-temperature chemistry. Their

experiments consisted m mixing a gas that had passed through a glow

discharge, and hence contained free radical species, with another gas

in a reaction vessel at liquid air temperature. Condensable products,

if formed, froze out on the walls and could be subsequently analyzed

by conventional methods after warmup. In this way they observed

that atomic hydrogen reacted with oxygen, nitric oxide, sulfur dioxide,

ethylene, and benzene. They reported what they considered to be a

' Guest scientist from Olin-Mathieson Chemical Co., New Haven, Conn., now NBS staff.

2 Figures in brackets indicate the literature references at the end of this paper. 37
H

new form of hydrogen peroxide, O—O, from the results of their

experiments with H atoms and molecular oxygen.

2. Rapid Condensation of Reacting Gases

Ruehrwein, Edwards, and Hashman [4, 5, 6] in recent articles, have

refined the technique of Geib and Harteck in that the gases could be

condensed at liquid helium temperature. Their reaction vessel is shown in figure 1. Edwards and Hashman [6] investigated the

reaction of H atoms with oxygen, condensing the reaction products on a liquid helium cooled surface. A solid was obtained which

evolved oxygen at about 160° K and left a residue of water and hydrogen peroxide when finally warmed to room temperature. Similar

results were obtained when the experiments were done either with

water discharge products or H atoms with ozone at 77° K. It appeared that the oxygen evolving solid was a gel-like molecular

complex of O2 and H2O2 and/or H2O. A solid of similar characteristics could be prepared by condensing oxygen with hydrogen peroxide

at 4.2° K. The radicals HO2 and the hypothetical H2O3 and H2O4

were not involved to any appreciable extent, nor was it necessary to invoke any structurally new species, according to these authors.

The interpretation of the Geib-Harteck experiments, as extended

by Ruehrwein, Edwards, and Hashman, is uncertain in some respects.

The temperature and physical state under which the free radicals

produced in the discharge tube react are not defined. There are

several nonmutually-exclusive possibilities: (1) reaction in the gas

phase; (2) reaction in the solid during deposition; (3) reaction in the

solid during warmup. It seems probable, however, that at least part of the reaction occurs at temperatures well below 150° K.

Ruehrwein and Hashman [5] studied the formation of ozone from

atomic oxygen at low temperatures. The deposit of discharged

oxygen condensed on a 4.2° K surface was initially faint blue. The

color became more intense upon warming through the temperature

range 13° to 30° K. As noted, this is not necessarily an indication of chemical reaction in the solid. Oxygen undergoes a phase transition at 23.7° K, and this may account for the change in appearance of the

deposit. Indeed, Harvey and Bass [7], showed that the concentration

of ozone present in the solid as deposited showed no increase on warming.

Ruehrwein, Hashman, and Edwards [5] examined the products of several gases that had been passed through a discharge and condensed

at 4.2° K. Table 1 is a summary of the results. They also found

that molecular oxygen forms complexes with several solids such as hydrogen peroxide, water, and nitrogen dioxide. These complexes

appear to be stable at the boiling point of oxygen.

Free radicals may be prepared by thermal decomposition reactions.

Rice and coworkers [8, 9] have carried out several interesting experiments in which gases, after being pyrolyzed at high temperatures, were rapidly condensed on liquid nitrogen cooled surfaces. Chemical

reaction could often be observed in these frozen solids well below 150° K

38
Table 1. Observed products from discharged gases after condensation and

evaporation

Gas discharged Principal stable products

O2 O3 H2O H2O2, H2 XO2 N2, O3 NH3 N2, H2 CO C, CO2 CO2 CO, O3 SO2 O2, s

39
For example, Rice and Scherber [8] passed hydrazine through a furnace at 850° C and condensed the products on a hquid nitrogen

cooled finger. The condensed material was yellow in color. It decomposed exothermally with disappearance of the color at —178° C

to give N2. Rice and Grelecki [9] observed that the green solid,

condensed at — 196° C from the products of discharged dimethylamine,

became lighter on standing. Since a temperature dependent H2 evolution accompanied the process, they were able to measure an activation

energy for the process of 1.3 kcal. It was assumed, as in similar

experiments, that free radical species in the solid react to give the

observed products.

Films of condensed pyrolysis products of azo-ethane are being

studied by Comeford [10] at the National Bureau of Standards.,

Azo-ethane (plus argon as a diluent) is passed through a furnace in the

region 200° to 500° C. The products are condensed on a 4.2° K cesium

iodide window 1 to 2 cm distant from the exit of the furnace. Figure

2 shows the experimental arrangement. [11] Infrared spectra of the

films formed under various flow and temperature conditions are recorded.

The gases from the film after warmup are collected and analyzed

with a mass spectrometer. Ethane, ethylene, butane, and methane

have been identified both by infrared and mass spectrometric analysis.

From indications that under some conditions the amount of butane

in the products, as determined by mass spectrometry of the gas

phase, had increased over that found by infrared anal3^sis of the

condensed gases at 4.2° K, it may be inferred that ethyl radicals were

stabilized in the condensed phase. Unambiguous evidence of reaction in the solid from differences in the infrared spectra as the film is warmed above 4.2° K has not as yet been obtained.

Kuentzel [12], also employing the apparatus as shown in figure 2, has been studying the ethylene oxide pyrolysis products condensed on

a 4.2° surface. Infrared spectral features differing from that of

ethylene oxide or other possible stable reaction products have been

observed. The intensity of these features is stronger the shorter the

distance between the furnace exit and the condensing surface.

3. Atomic Beam Deposition and Surface Processes

In an attempt to stabilize iodine atoms, Scheer [13] condensed an

iodine atom beam on a nitrogen cooled Pyrex glass surface figure 3. The condensed iodine was observed by measuring the intensity of the

I2 absorption bands during buildup of the deposit. No difference

could be detected between the condensed film formed by an atomic

iodine beam and that of a molecular beam on a per-number basis. This is shown in figure 4. It was estimated that for a pure I2 deposit, a change in absorption corresponding to a 3 percent difference in film thickness would be detectable. Even when ?i-heptane was made to impinge on the condensing surface with the iodine in a

ratio of 250 to 1, no change of the deposit of the iodine atom beam

from that of the molecular beam was observed. Since the iodine

atom recombination in the solid must be diffusion controlled, sufficient mobility for atom-atom encounters must occur during the

deposition process, for once the beam is deposited, no boundary spreading of the condensed spot can be detected even after several hours.

Surface diffusion processes may be examined in some detail with the

40
41
15 cm 24 cm Refrigeront

Furnace I (20-1000°) I Pocked with '^"T^ Quartz Rod

n-heptone

Tungsten Lomp Light Source

Monochromator 450-2000m;^ Figure 3. Iodine atomic beam apparatue,

X

500yu

530

Beam Intensity = 6.3 X 10

particles cm2 min. Beam Constitution = o 99.9% I2

X 99.9% I Deposit Temperatures 77*»K

1.2 1.6 X 10'

0.4 0.8

porticl es cm^ Figure 4. Adsorption characteristics of condensed films of iodine atoyns and

molecules. field emission microscope. The migration of carbon monoxide [14] was investigated in a field emission tube operating at liquid helium

temperature. The tungsten surface upon which the carbon monoxide

was condensed, could be maintained at any desired temperature above

that of liquid helium [15]. Three temperature regions for migration were observed. This is shown in figure 5. The most mobile molecules are those bound by van der Waal's forces. Thus, the mobility of carbon monoxide over carbon monoxide takes place at a conveniently

observable rate at temperatures lower than 20° K. On the other hand,

42
CLEAN TUNGSTEN SHADOWED SPREAD SPREADING OF VAN DER WAALS' LAYER BELOW S'O" K

MIGRATION OF "WEAKLY" CHEMISORBED LAYER ABOVE 200° K ^ V

5 SECONDS 15 SECONDS 95 SECONOa

MIGRATION OF " STRONGLY" CHEMISORBED LAYER ABOVE 835" K Figure 5. Some characteristic surface phenomena of carbon monoxide adsorbed on

tungsten observed icith the field emission microscope.

mobility in the chemisorbed layer commences only at 200° K for the more weakly bomid surface state and at 650° K for the more strongly

bound one. These studies serve to emphasize the complexities of diffusion controlled reactions, of which radical-radical reactions are

typical.

It is feasible to extend field emission microscope techniques to the

investigation of chemical reactions at low temperatures. An atomic

hydrogen beam source has been set up in conjunction with a field emission microscope, figure 6. Surface reactions between an initially

43
Figure 6. Apparatus for the study of hydrogen atoms in the field emission

microscope.

A, The field emission microscope, attached to the atomic beam apparatus. In operation it is sm-rounded by liquid helium; B, The furnace for dissociating hydrogen. deposited layer of a suitable reactant, such as oxygen and hydrogen

atoms, may then be studied in the low-temperature region.

4. Photolytic Techniques

Photolysis of solids at low temperatures is sometimes useful in initiating reactions below 150° K. A. Leifer [17, 18] has deposited

hydroxylamine hydrochloride and hydrazine hydrochloride on CsBr

and NaCl disks by sublimation. The films so obtained were irradiated

with 1849 A radiation at 77° K for 1 to 3 days. Infrared examination

showed that decomposition was considerable. The major product was

identified as NH4CI in each case. It is conjectured that the initial photolytic act is the breaking of the N—N bond in the NH2—NH3CI

or the N—O bond in OH—NH3CI to give radicals. -NHtCl- is then presumed to abstract hydrogen to give the observed NH4CI.

Schnepp and Dressier photolyzed 0.3 mol percent ammonia in an

argon matrix at 4.2° K below 2000 A. Both NH and NH2 were observed by absorption spectroscopy. NH2 disappears close to 20° K,

whereas NH is still observable up to 36° K. Photolysis at 20° K is approximately five times less efficient than at 4.2° K. NH and NH2

44
formation both increase at first and then level off, an indication of the

independence of their formation processes. A valuable contribution to low temperature chemistry has been

made by J. R. McNesby [19] at the National Bureau of Standards,

by photolysis of liquids in the region of 80° K. Oxygen, in solution in liquid nitrogen or in liquid carbon monoxide was photolyzed with

predominantly 1849 A radiation. Ozone and nitrous oxide were found in the former and CO2 and some ozone in the latter system.

The products may be accounted for on the basis of the following

reactions:

O2—^20PP) (1) 1849 A

M+0eP)+02 ^Oa+M (2)

M+ N2 +O(1 D) ^NaO +M (4) M+CO+OCZ>) ^CO,-fM. (5)

As noted by McNesby, if ''hot atom" effects can be shown to be absent,

this method promises to be a valuable tool in the study of certain

elementar}^ reactions.

There is a low-temperature reaction between copper and oxygen from

a discharge. Krugar and Ambs [16] prepared a sphere of pure singlecrystal copper. Tank oxj^gen, discharged at 2,450 Mc and condensed on this sphere at 4.2° K, reacted with it as evidenced by pitting of the

copper surface, figure 7. X-ray diffraction analysis showed that the

reaction product was Cu(N03)2. When high-purity oxygen (99.998%) was used, no pitting was observed, but a 20 A film, presumably the

oxide, was found on the copper. The mechanism of the reaction has

not been determined, but the application of quantitative techniques

to observations of film formation promises to extend the understanding

of this interesting reaction. These film thicknesses are being accurately measured with an ellipsometer so that the buildup of the copper

oxide formed on the copper with condensed oxygen from a discharge can be followed.

5. Gas- Solid Interactions

A new and extremely promising technique in low-temperature

chemistry resulted from the finding that hydrogen atoms, produced

in the gas phase diffuse into and react with a variety of condensed

solids at low temperatures [20, 21, 22, 23]. The experimental method

is the exposure of solid films of substances such as olefins, oxygen, or halogens to hydrogen atoms produced by the dissociation of hydrogen

on a hot tungsten ribbon. (Reference [21] gives experimental details.)

The reaction can be followed by measurement of pressure decrease of

the hydrogen or spectroscopic analysis of the films, in addition to the

analysis of the warmed-up products. This method is valuable because

a variety of low temperature reactions can now be studied quantitatively. The temperature and physical state of the reactants are 539369—60 45
Figure 7. Copper spherical single crystal pitted by bornbardment with oxygen atoms.

specified, the extent of the reaction in time is determinable, and

variables such as the radical flux can be fixed. Examples of some of the results may be given. Olefins differ in their reactivity at 77° K. Olefins having no terminal double bonds,

such as butene-2, do not react. Propylene, butene-1, and 3 methyl

butene-1 react quite readily. Hexene-1 shows no reaction. The

differences are ascribable to differences in activation energy or pre- exponential factor for the primary step of H atom addition, for

the diffusion of H atoms through solid hydrocarbons at liquid nitrogen temperatures occurs quite readily and is not rate controlling.

The rate of reaction is directly proportional to the concentration

of olefin in the solid, dilution being effected initially by admixture

with the corresponding alkane (fig. 8). Inert hydrocarbon films

up to 1 fjL can be deposited over the reactive olefin film before a diminution in rate is observed.

From experiments with the addition of deuterium atoms to solid

propylene at 77° K, the occurrence of doubly deuterated propane in the products suggested that propyl radicals might build up to a measurable concentration in the solid. There are two paths for the

formation of the saturated alkane; one is the consecutive addition of

two atoms, and the second is a disproportionation of two alkyl radicals

to give the original olefin and the saturated alkane. By far the

greater proportion of the alkane formed in the latter case should be

the singly deuterated variety, however. Consecutive H atom addition implies at least a transient existence of propyl radicals. Therefore, films of solid propylene at 77° K, exposed to H atoms for varying

lengths of time, were carefully examined with an electron paramagnetic

resonance spectrometer [24]. No signal was ever obtained. The

steady state concentration of radical species must be below the limit

46
0 .01 .02 .03 .04 .05 .06 .07 ,08 .09 1.0 mol fraction of propene Figure 8. Rate of hydrogen uptake hy solid propene at 77° K as a function of concentration. Propane is the diluent. of detectability of the instrument, or 10^^ spins. This corresponded

to a concentration of less than one free radical in 10" molecules in these

experiments. Two other compounds, of the aromatic type, toluene

and unsymmetrical diphenyl ethylene, also gave negative results. Both added H atoms at 77° K. In spite of the possibility of increased

radical stability by resonance, free radical buildup in the solid did

not occur. The general picture that emerges is that hydrogen atoms, diffusing

readily through the film, react with the olefin and with the radical

formed. The rate is linearly proportional to the concentration of both H atoms and olefins. The reaction is temperature dependent.

It is interesting to examine the temperature distribution in the solid in order to determine the seriousness of the departure from isothermal

conditions during the reaction. A hydrogen atom adds to an olefin with a heat release of about 35 kcal/mole. It is assumed that the olefin exists as an infinite isotropic continuous medium initially at 77° K.

A point source of heat Q p c calories, where p and c are the density and

specific heat of the medium, is applied to the origin at ^=0. The

temperature as a function of the distance from the origin is 47
has its maximum value [25] of

The r for t^T= 10°, which in the case of propylene would be the melting

point if the initial temperature were 77° K, is calculated to be approximately 18 A. The relaxation time is of the order of 10~^^ sec for these small radii, however. The assumption of isothermal conditions

does not appear to be seriously in error. Application of the ordinary

heat conduction equation shows that even for the production of heat

of 1 cal/cm^ sec, corresponding to the maximum rates of hydrogen

pickup observed, significant temperature gradients cannot be maintained in olefin films 0.1 /x thick.

A descriptive model follows for the reaction of atoms introduced at the surface of a low-temperature solid which can react with the free radical atomic species, expressed in terms of the reaction of hydrogen

atoms with olefins. The observable is the rate of pressure decrease

in the gas phase. If a la^w of solid of thickness /, is considered,

dt 2 ^\uo\)m]dx.

This expresses the rate of hydrogen uptake in terms of the reaction

between H atoms and the olefin (01) arising from H atom addition

to the olefin. A is the area of the film. The factor )^ arises from

stoichiometry. Hydrogen atoms diffuse readily through these films so that the concentration at any level X may be described by the

diffusion equation including the appropriate chemical reaction term:

D where K=h{0\).

It is assumed that the reaction does not occur to such an extent that

there is an appreciable change in the olefin concentration. The H

atom recombination term, i^'H^ is not included, for the inert layer

experiment shows that in the film thicknesses and H atom concentrations used in these experiments, the recombination reaction is negligible. Under the boundary conditions

H=Ho at x=0

^^^=0 at x=l

ox

there is obtained

H=0 at ^=0

Vi^i^ Ho tan h yj^ I neglecting the transient term. For relatively thick films, this is essentially constant and equal to 48
On the other hand, for thin films

so that the rate should be proportional to film thickness. Appropriate rate measm'ements can be interpreted in terms of a rate constant for the elementary addition reaction and a diffusion coefficient for volume diffusion of hydrogen atoms in the solid. Besides addition of an H atom to a double bond, another lowtemperature reaction was found to be hydrogen abstraction from

an alkyl radical by hydrogen atoms [26]. Since abstraction of a secondary hydrogen is favored, the result of the process is a double

bond isomerization in which there is a double bond shift from a

internal position. Butene-1 is isomerized to transbutene-2 at 77° K,

for example,

Butene-2 does not react at 77° and hence accumulates in a film of

solid butene-1 exposed to H atoms. If a film of butene-1 is reacted

with D atoms under the same experimental conditions, HD can be

observed to increase in the gas phase so that the given reaction is established. It is interesting to note that although butene-1 and 3, methyl, butene-1 give HD with D atoms, propylene or alkanes in- cluding those with tertiary hydrogens do not. The absence of HD

formation in the case of propylene implies first, that abstraction of a

primar}" hydrogen does not occur, and second, that abstraction of a secondary hydrogen attached to a carbon with an unpaired electron

does not take place. The energy requirement is probably too high

for the first. A low-temperature H atom addition reaction, observed at 20° K, is that between H atoms and solid oxygen [27]. The reaction is ef- fected in an apparatus as shown in figure 9, using liquid hydrogen as the refrigerant. Reaction was followed by observing the pressure

decrease of the hydrogen in the vessel, and also by mass spectrometric analysis of the warmed-up products. Traces of hydrogen

peroxide were detected by a yellow color with TiCl4 solution. The

reaction undoubtedly occurs by way of an initial addition of a hydrogen atom to an oxygen molecule to give the radical HO2, but

because HO2 can react further with H atoms,

it is not likely that HO2 can be stabilized under these conditions.

C—C—C=C + H

I I . I -C—C—C—C—

C— + H2.

I I I

HO2+H 20H

OH+H H2O

49
Figure 9. Apparatus for the investigation of the hydrogen atom-solid oxygen

reaction at 20° K.

6. Special Techniques

An apparatus, figure 10, has been built by Thomas [28] to study

reactions resulting from solids bombarded with free radicals. This

consists of a highly pohshed, rapidly rotating stainless steel cylinder

containing liquid helium. At one point gases such as oxygen or

olefins are deposited in molecular beam fashion. Diametrically

opposite, the film is bombarded, again in molecular flow, with free radicals.

A method of inducing chemical reactions in systems below 150° K is electron bombardment of the condensed solid [29]. The apparatus is shown in figure 11. Mixtures of N^^N^^—N^^N^^ were exposed to an

electron beam of 15 kv at current levels of 5 to lO/x amp. The ex- tent of reaction was determined with a mass spectrometer by the

N^^N^^ formed. The radiochemical G value based on the exchange

process at 4.2° K is about 0.2. At 20° K this drops to 0.04. The

formation of N^^N^^ results from atom recombination as well as

possible excited molecule or ion reactions.

Gamma irradiation of solids at low temperature with a cobalt-60 source has been used in preparing stabihzed free radical systems

[30]. Nitrogen, hydrogen, deuterium, and methane have been

irradiated, and in each case free radicals were found to be present

using the ' electron paramagnetic resonance spectrometer. It was

observed that in the case of hydrogen, a decrease in the hydrogen

atom concentration occurred on storage. This would indicate that

H atoms are somewhat mobile in a molecular hydrogen matrix even

at 4.2° K. Gamma irradiated methane at 4.2° K was interesting in 50
Figure 10. Rotating Dewar for the study of radical molecule reactions at low temperatures

that an electron paramagnetic resonance spectrum of both CH3 and

H were obtained. Momentary warming of the sample to 20° K and

recooling to 4.2° K resulted in the disappearance of H but no diminution in CH3. This result is difficult to interpret unless either a nonzero activation energy or a large steric factor applies to the H+CH3

recombination at 20° K.

Only brief mention is made here of the extensive studies of photolysis in rigid solvents at low temperatures. An excellent review

paper by Linschitz [31] summarizes work done in photo-dissociation,

excited states, photo-reduction and photo-ionization processes. Many catalytic processes can occur in the low-temperature region.

The classic experiment of Bonhoeffer and Harteck [32] on the conversion of ortho- to para-hydrogen on charcoal at liquid air temperature is one of the earliest examples. Gould, Bleakney, and Taylor

[33] showed that Cr203 and Ni catalyze the hydrogen-deuterium

exchange even at —190° C, while Eley [34] found that a tungsten

51
I I Figure 11. Apparatus for investigating reactions at liquid helium temperatures

induced by electron homhardment of condensed solids.

surface was effective for both the ortho-para conversion and the hydrogen-deuterium exchange at 77° K.

Although low-temperature chemistry is in an early state of development, recent activity indicates that considerable further progress

may be expected. This includes the elucidation of elementary

reactions and an experimental approach to concomitant problems of

diffusion, energy exchange, and free radical stabilization.

7. References

[1] K. H. Geib, P. Harteck, Ber. 65B, 1551 (1932).

[2] P. Harteck, Ber. 66A, 423 (1933).

[3] K. H. Geib and P. Harteck, Ber. 66B, 1815 (1933).

[4] R. A. Ruehrwein and J. S. Hashman, J. Chem. Phys. 30, 823 (1959).

[5] R. A. Ruehrwein, J. S. Hashman, and J. W. Edwards, Chemical reactions of free radicals at low temperatures, in press.

[6] J. W. Edwards and J. S. Hashman, Reactions of oxygen and atomic hydrogen, in process.

[7] K. B. Harvey and A. M. Bass, J. Mol. Spectroscopy 3, 405 (1958).

[8] F. O. Rice and F. Scherber, J. Am.. Chem. Soc. 77, 291 (1955).

[9] F. O. Rice and C. Grelecki, J. Phys. Chem. 61, 824 (1957).

[10] J. Comeford, personal communication.

[11] Designed by L. Kuentzel.

[12] L. Kuentzel, Informal report at Fourth Free Radical International Symposium Stabihzation (Washington, D.C., Sept. 1959). 52
[13] M. D. Scheer, The formation and stablization of free radicals II, Informal

and private communication.

[14] R. Klein, Adsorption, migration, and desorption of carbon monoxide on tungsten, J. Chem. Phvs. 31, 1306 (1959).

[15] R. Klein and J. A. Simpson, Rev. Sci. Instr. 29, 770 (1958).

[16] J. Kruger and W. Ambs, The formation and stabilization of free radicals II, Informal communication.

[17] A. Leifer, The sublimation of hydroxylamine hydrochloride, to be published.

[18] A. Leifer, The photolysis of hydroxylamine hydrochloride and hydrazine

monohydrochloride, to be published.

[19] J. R. AlcXesbv, Photolvsis of oxvgen in solution in liquid nitrogen and in Hquid carbon monoxide at 1849 A, J. Chem. Phvs. 31, 283 (1959).

[20] R. Klein and M. D. Scheer, J. Am. Chem. Soc. 80^ 1007 (1958).

[21] R. Klein and M. D. Scheer, J. Phvs. Chem. 62, 1011 (1958).

[22] R. Klein and M. D. Scheer, J. Chem. Phys. 31, 278 (1959).

[23] R. Klein, M. D. Scheer, and J. Waller, unpublished results.

[24] M. D. Scheer, J. Fine, and R. Klein, unpublished results.

[25] H. S. Carslaw and J. C. Jaeger, Conduction of heat in solids, p. 256 (Oxford

Universitv Press, 1959).

[26] M. D. Scheer and R. Klein, J. Phvs. Chem. 63, 1517 (1959).

[27] R. Klein and M. D. Scheer, J. Chem. Phys. 31, 278 (1959).

[28] A. Thomas, private communication.

[29] R. Klein, and E. Horl, J. Chem. Phys. 33, 307 (1960).

[30] L. A. Wall, D. W. Brown, and R. E. Florin, J. Phys. Chem. 63, 1762 (1959).

[31] H. Linschitz, The study of photochemical processes by the rigid solvent technique. Faradav Societv, Informal discussion on Free Radical Stabili- zation, p. '37 (Sept'. 4, 1958).

[32] K. F. Bonhoeffer and P. Harteck, Z. Phvs Chem. B4, 113 (1929).

[33] A. J. Gould, W. Bleaknev, and H. S. Tavlor, J. Chem. Phys. 3, 362 (1934).

[34] D. D. Eley, Proc. Roy. Soc. 178A, 452 (i941).

Washington 25, D.C., December 9, 1959.

53

6. Methods of Production of Trapped Radicals and

Properties of Radical-Trapping Solids

Alun Thomas ^

1. Introduction

Numerous reviews describing methods of producing trapped radicals

have appeared within the last few years. In particular, two excellent

reviews have been written very recently: The first by B. A. Thrush

[1]^ deals with radicals formed in the gas phase before being trapped;

and the second by G. C. Pimentel [2] deals with production of trapped

radicals within solids. These reviews describe the subjects very

comprehensively and so recently that there is little point in covering

exactly the same ground here. Instead, since the burden of complete

description and historical introduction has been shouldered by these

previous authors, the opportunity is taken of concentrating upon the

principles underlying the trapping of radicals rather than on details of the work of individuals. Emphasis is placed on the mechanism of trapping, and work in the field is described only insofar as it throws

light on the processes involved.

2. Trapping of Radicals—the Trapped State

In the first place what is meant by a ''trapped radical"? All that

is meant in this paper is a radical which is stored in such manner that

it cannot recombine with another radical even though in other circumstances it would recombine with liberation of much energy.

Trapping, then, is the storage of radicals such that they do not

recombine over an indefinitely long time.

2.1. Recombination of Radicals

Radicals recombine with each other at a rate which, like that of

all bimolecular chemical reactions, is the product of three quantities:

(1) The number of collisions between radicals per second; (2) the

fraction of such collisions which possess some minimum amount of energy; and f3) the probability that fruitful reaction will result from

a collision of sufficient energy.

For complete inhibition of recombination, i.e., for successful trapping, at least one of these terms must be reduced to zero.

a. Prevention of Collisions Between Radicals

Collisions between radicals in solids can be prevented by isolating

radicals within a matrix of inert material such that diffusion of the

radicals within this matrix is reduced to a negligible amount. This

point has been amply made by Whittle, Dows, and Pimentel [3], Norman and Porter [4], and other authors.

It is not enough merely to contain the radicals within solids, for

dift'usion within solids, particularly of small radicals, can take place

quite rapidly. Klein and Scheer [5] have shown that dift'usion of

1 Guest scientist from Shell Research, Ltd., Chester, England.

2 Figures in brackets indicate the literature references at the end of this paper. 55
hydrogen atoms in thin films of soHd olefins at 77^ K is not even a

rate-controlling process in the hydrogenation of the olefins. (However, when films of inert hydrocarbons of 10~* mm thickness, and

greater, are placed between the reactive olefin and the source of atoms,

then some inhibition of reaction rate takes place. The experimental

method thas holds promise for providing badly needed information on diffiision coefficients of radicals in solids.)

Many authors have described experiments where radicals have

recombined at temperatures well below the softening point of the

matrices in which they were embedded. For example by making

electron spin resonance (ESR) measurements Livingston, Zeldes,

Weinberger, and Holmberg [6] detected h3^drogen atoms in 7-irradiated

samples of solidified mineral acids at low temperature. The ESR

signal attributed to these atoms disappeared when the solids were warmed to temperatures which were still well below their softeningpoints.

There is some evidence to show that diffusion of radicals is completely prevented when they are trapped, but such evidence is re- stricted to large complex radicals. Lewis and Bigeleisen [7] photolyzed various substituted benzidenes with plane polarized ultraviolet

light and showed that the colored radicals formed absorbed polarized

light most strongly when the plane of polarization was parallel to that of the exciting light. The radicals were thus shown to be held

very rigidly in their environment. Furthermore Ghosh and Whiffen

[8] have shown that radicals formed by irradiation of a single crystal

of glycine are precisely alined (and therefore rigidly held) within the

crystal. It may be worth pointing out, however, that such experiments do not conclusively show that prevention of diffusion is an

essential prerequisite for trapping. For such a demonstration we need

a convincing correlation between rates of diffusion and lifetimes of radicals in a matrix.

b. Energy of Activation of Recombination of Radicals

The literature abounds with statements that no energy of activation

is needed for recombination of radicals. Thus Hinshelwood [9] states ''For a reaction between two atoms such as CI or Br no activation can be required." Or Semenov: [10] ''Processes of recombination of free radicals proceed in the majority of cases with no activation energy."

Or Steacie: [11] . . in general there is no activation energy for re- actions in which two atoms or free radicals combine." But these

statements should be taken in their context which is discussions of gas

phase reactions at and above room temperature where activation

energies of say 0.25 kcal/mole would be wholly negligible. On the

other hand at 4.2° K and below, 0.25 kcal/mole would present a for- midable energy barrier to reaction and it is essential to give careful

consideration to whether in fact there may not be some small activation energy for radical-radical reactions. Even if no activation energy

is needed for gas phase recombination, would the same hold true for

radicals diffusing in a solid? There is very little experimental evidence on the subject, which is not surprising for the experimental techniques

for such an investigation would be extremely difficult. The question

is unanswered but is vital to the whole field of low-temperature

chemistry.

56
c. Probability of Recombination When Collision with Requisite Energy of Activation Takes Place Kadical-radical reactions in the gas phase are affected by two factors. One of these is the necessity for a third body in the recombination of atoms and very simple radicals to prevent the molecule formed

on collision from immediately redissociating [12]. Complex radicals do not need this third body for they possess many internal degrees

of freedom which can accommodate the excess energy sufficiently long

for it to be removed in a single subsequent collision. (Reference will be made again to this third body effect w^hen discussing the trapping

of radicals from gases.) The other factor is a true steric effect which

appears to be small for sunple radicals, but which msij become large

for complex species.

Both these factors are drastically altered in solids. There are always thhd bodies present so that recombination of simple radicals are no longer hindered through lack of an energy receptor, but on the

other hand, true steric effects may become very much more important.

For example one may consider the recombination of methyl radicals.

The steric factor for recombination in the gas phase has been estimated

from experimental results to be about 10-^ or higher [13]. One can

reasonably miagine that the radicals, which have a planar or nearly

planar configuration, may need to face each other in order to react. This they may do in the gas phase where molecular rotations are unhindered with a probability of 1 in 10 or higher. (This assumes that

the factor is purely and literally a ''steric" one.) If now the radicals are diffusing in a solid lattice it seems reasonable to picture them as much more likely to move edgeways through the lattice, than in a line perpendicular to their plane shape. This will be so if molecular rotation about an axis through the plane of the molecule is hindered.

Thus collisions between two methyl radicals in solids may take place

exclusively at the edges of the planes; and reaction may become far

less probable than it was in the gas phase. There is some evidence in the work of Wall, Brown, and Florin [14] that even hydrogen atoms

may not react very efficiently with methyl radicals in solid methane.

An attempt at reducing the probability of reaction betw^een radicals

by alining the spins of the unpaired electrons so that they are all parallel to one another has been made independently by Fite [15] and

Windsor [16]. They produced molecular beams of partially dissociated hydrogen and by means of ver}^ strong inhomogeneous magnetic

fields split these beams into three separate beams: one of undissociated

molecules; one of atoms with electron spins alined in one direction;

and one of atoms with electron spins alined in the other direction. If these beams are condensed separately, and if the alinement of spins can

be preserved during the condensation process and in the resulting solid,

high concentrations of trapped atoms might be achieved. However,

results so far reported by Fite, indicate that few atoms survive the

condensation process without recombining.

3. Generation of Free Radicals

The chemical processes in which free radicals are formed are very

similar in both solids and gases. It is thus convenient first to describe

these processes generally and then to examine features peculiar to the

different phases.

The only way in which radicals (excluding triplet states of molecules) can be initially produced from stable molecules is by dissocia57
tion. Subsequently these radicals may decompose, or may add on to saturated molecules, or may abstract part of a saturated molecule

leaving the remainder as a free radical. The primary act however is dissociation, which results from the acquisition by a molecule of suffi- cient energy to break one of the bonds holding the molecule together.

This energy can be obtained either b}^ collision, or by interaction with

radiation.

3.1. Dissociation Resulting from Collision

a. Collision with Molecules—Thermal Decomposition

Much of the early work on free radicals, especially the extensive

investigations of F. O. Rice and his coworkers, [17] was directed at understanding the thermal decomposition of organic compounds.

Methyl, and to a lesser extent ethyl, radicals were shown to be products

of thermal decomposition. More recently Scheer [18] has worked with

atomic iodine produced from molecular iodine in a furnace, and Robinson [19] has used nitrogen dioxide prepared essentially by pyrolyzing

dinitrogen tetroxide. Hydrogen atoms are also conveniently prepared

by thermal decomposition on a tungsten filament, a well-known method that has recently been used by Klein and Scheer. [5] Klein [20] has also constructed a high-temperature furnace from which hydrogen

atoms emerge in known concentration in a molecular beam. The

method can be employed reasonably where appreciable dissociation

takes place at easily attainable temperatures. It has been naturallv

used mainly, if not exclusively, with gases for the simple reason that

at the temperatures necessary for dissociation the species are gaseous.

b. Collisions with Electrons and Ions—Electrical Methods

Whereas dissociation by the use of thermal energy is a process that

has been restricted to gases, dissociation by the use of electrical energy

has been applied to solids as well. In the gas, electrical discharges are generally used. Microwave

discharges, other high frequency electrodeless discharges, and dis- charges between electrodes all produce somewhat similar results,

though some methods work better in some ranges of pressure or in some gases than others. It has been shown [21] that discharges between electrodes can result in contamination by the electrode material,

but in many applications this may not be a serious drawback. Electrical discharges are very convenient experimentally but they result in a wide variety of labile products, namely ions and radicals in various

excited states and also excited molecules. The exact process b}^ which

radicals are produced is not at all clear, but it must involve collisions with charged species moving at high velocities which have been

achieved by acceleration in a steep potential gradient. Furthermore

it is not possible to estimate (as it is for thermal dissociation) the

concentration of radicals produced.

Processes of controlled electron bombardment of gases do not seem,

to have been much used in work on trapping of radicals, though they

are used Very extensively in mass spectrometers, and there is precise

information on the nature of the charged molecular fragments

produced.

Discharges have also been applied to solids and again seem to be

somewhat indiscriminate with the further complication that the actual

58
locality where the discharge strikes is by no means clearly defined.

Kebbert and Schoen [22] have applied high d-c potentials from a sharp pointed electrode in a high vacuum to solidified mixtures of oxygen in various matrices. Considerable light was emitted during

the ensuing passage of current, and the emission was characterized

as similar in many respects to that emitted during condensation of gases which had passed through discharges (Peyron and Broida [23]).

This emission has been attributed partly to excited molecules and

partty to excited atoms. Vegard [24] used well-defined beams of ions or electrons in his classical work on the emission produced by the

impingement of these beams on solid nitrogen and rare gases. McLennan and Shrum [25], and more recently Alger and Anderson [26], and also Horl [27] have controlled and refined the bombardment by

electrons of solid species. Electron bombardment of solids appears to be capable of greater control than other discharges since both the ener- gy of the electrons and the area of bombardment can be well defined.

3.2. Interaction with Radiation

Ultraviolet light, X-rays, and T-rays have all been used to dissociate molecules in the solid phase, but onh' ultraviolet light has found

much application with gaseous molecules.

Photolysis is a very discriminating process: b}^ the use of suitable

wavelengths of irradiating light and suitable absorbing substances, the

nature of the dissociating process and the identit}^ of the subsequent

fragments may be fairly well known. In fact Porter, Chilton, and

Strachan [28] have photolyzed in rigid solvents groups of various compounds, chosen deliberately, and found b}" absorption spectroscopy

that the same radical was produced from all members of the same

group.

Radiolysis, i.e., the absorption of X-rays and 7-rays, would be ex- pected to break bonds indiscriminately because the quanta of energy

absorbed are very large. In practice however, radiolysis may be

remarkably specific in its effect. Porter, Chilton, and Strachan [28] report that irradiation of triphenyl methane in solid aliphatic solvents

results in dissociation of the solute in an amount greater than could be

explained without postulating transfer to the solute of energy initially

absorbed b}^ the solvent. Thus though the radiation may have been

absorbed indiscriminately, the products finally formed were far more

specific.

X-rays, used by Alger and Anderson [26], and 7-rays used by various

workers [29, 6, 13] have been very successfully used for the production

of well defined radicals at very low temperatures. In all these cases, the detection of radicals was by ESR spectroscopy which is sensitive

only to compounds with unpaired electrons. Thus, provided the

number of radical species was small, it did not matter how complex was the mixture of stable products of irradiation.

Vacuum, ultraviolet photolysis is a promising new method for radical

production. It is more universally applicable than photolysis at longer wavelengths for there are many more substances which absorb

in the vacuum ultraviolet than do in the visible and near ultraviolet.

Xelson and Ramsay [21] have recently produced a variety of free radicals by such photolysis of gases, and Dressier and Schnepp [30] have succeeded in trapping NH and NH2 by photolyzing ammonia

in solid argon. 59
3.3. Secondary Radical Formation

So far, we have discussed dissociation, which is the primary process

for forming radicals. Other radicals may be formed in secondary

reactions. A radical may decompose to form a small radical and a

stable molecule ; or in reverse a radical may add on to a stable molecule

to form a larger radical; or again, a radical may abstract a portion of

another molecule to satisfy its own free valence, and leave the remaining fragment of the molecule as a new free radical. These types of

reactions in the gas phase have received considerable discussion

Steacie [12], Thrush [1], and we shall return to them in a later section.

4. Trapping Radicals from Gases

Most free radicals have extremely short life times in the gaseous

phase: recombination takes place so rapidly that appreciable concentrations of radicals can have but a very transient existence. Exceptions to this generalization are: (a) Radicals which can exist in

thermal equilibrium with their dimers, e.g. NO2, I, and certain reso- nance-stabilized aromatic radicals; (b) very simple radicals which

require three body collisions for recombination, such as H, N, and O;

and (c) radicals in a molecular beam where they are largely isolated

from collision. It is only with these exceptions that there is well-founded hope of trapping appreciable quantities of radicals out of the gaseous phase.

Further difficulties arise during condensation, for this process has

requirements which are somewhat incompatible. On the one hand all the translational energy of the radical must be lost, which can only be

through collisions, and on the other, recombination during collisions

must be prevented.

Furthermore there may not be a direct transition from gaseous

phase to solid phase. Even though the liquid phase may not inter- vene, there is likely to be considerable migration of molecules and

radicals over the surface of the solid with great frequency of collision with other migrating and fixed molecules, all being effectively threebody" collisions. Thus even if the radicals arrive unscathed into a condensed phase, they stand a good chance of recombining before they

are finally trapped.

The optimum conditions for trapping would seem to be as follows:

(1) Radicals must lose their translational energy in as few collisions as possible and should stick without migrating around the surface; and

(2) radicals should not alight on other radicals, but on inert molecules,

and should then be protected from following radicals by a surrounding

sheath of inert molecules.

The best illustration of the principles discussed here is provided in the work of Robinson and McCarty [3] who succeeded in trapping

diatomic and even triatomic radicals from the gas phase. The initially

stable molecules were mixed with a large excess of inert gas and then

passed through a discharge. The products emerged into an uncoUimated molecular beam and then impinged on a very cold surface. The

rate of deposition was kept very small so that thermal gradients

through the deposit would be small, and the outer surface would re- main cold. Thus migration on the surface would be kept to a minimum. By such means were prepared an impressive array of radicals

60
which were identified by ultraviolet and visible absorption spectroscopy.

Unless the radicals are monatomic and require third bodies for re- combination it seems essential to provide great dilution or molecular

beam conditions, or both, to enable them to survive transport to the

cold surface, and subsequent condensation. There may be very

little chance even so of trapping polyatomic radicals greater than, say, ethyl by such a scheme. Attempts to trap large reactive radicals

produced during thermal decomposition of organic materials have so

far been unsuccessful.

Comeford and Gould [32] passed azoethane in argon at low pressure

through a furnace, and rapidly condensed the products of pyrolysis on

a cesium iodide window cooled by liquid helium. Infrared absorption

spectra indicated the presence of ethylene and ethane in the solid film

but no azoethane, no -butane, and no bands attributable to alkyl

radicals. The gases obtained on evaporating the condensate nevertheless contained an appreciable quantity of 7?-butane which would

certainly have been detected in the solid film if it had existed there. This evidence has been taken to indicate the presence in the film of

labile species, the exact nature of which remains to be discovered.

5. Trapping Radicals that are Generated in Solids

5.1. Cage Effect

When radicals are produced within the structure of solid material

the complexities that relate to trapping from gases disappear, and are replaced by a somewhat simpler problem. This problem is simply

that dissociation leads to two free radicals, and not just one. Whereas

in the gas phase the two radicals fly apart freely, in the solid phase

they are more constrained to remain together, and stand a chance of recombining immediately. This is known as the cage effect, and was

proposed by Franck and Rabinowitch [33] to explain some very low

quantum yields during photolysis. Whether this cage effect is significant in a particular process will depend on the ease with which the

radicals can move apart after they are formed. This in turn would seem to depend on: (a) The surplus kinetic energy possessed by the

two fragments after separation; and (b) the diffusion coefficients of the fragments. Thus if one fragment is a hydrogen atom it may be

very easily lost. Lewis and Lipkin [34] were nevertheless able to pre- pare diphenyl nitrogen radicals, which are large and unlikely to diffuse

rapidly, by photolysis of tetraphenyl hydrazine in a rigid solvent.

Linschitz [35] has pointed out that in this case simple changes in rela- tive orientation of the fragments could result in stabilization. Porter

and his coworkers [36] report little evidence of a cage effect in their

work. Pimentel [2] however in a lengthy discussion of the subject

points out evidence demonstrating a substantial cage effect and suggests that little decomposition occurs during photolysis unless the

fragment trapped is very stable, or the other fragment reacts with the

matrix.

Even if the matrix allows effective dissociation, it is still necessary

that it should trap one of the radicals formed. Leifer [37] has photolyzed crystalline hydrazine hydrochloride and hydroxylamine hydrochloride at 77° K up to the point of complete decomposition, and at 20° K to a lesser extent, yet infrared absorption spectroscopy has

shown no sign of intermediate products or labile species, and ESR

measurements have indicated only a trace of free radical content.

5393G9—GO 5 61
5.2. Secondary Reactions

If the solid matrix is itself reactive, then secondary radicals may be

formed by reaction between primary radicals and the matrix material.

Gibson, Ingram, Symons, and Townsend [38] have exploited this possibility by freezing dilute solutions of hydrogen peroxide in various

solvents and photolyzing the solids. Hydroxyl radicals are first formed which then react with the solvent (mainly alcohols) abstracting hydrogen to form water, and leaving a solvent radical. The technique seems very versatile, and greatly extends the range of radicals

that can be produced by photolysis.

Klein and Scheer [5] have reacted frozen olefins with atomic hydrogen. Analysis of the products demonstrated conclusively that alkyl

radicals had been produced, even though none was stabilized. In the

circumstances of their experiments where hydrogen atoms were bombarding a thin film of olefin it was likely that hydrogenation would go

to completion: a hydrogen atom would react with an alkyl radical in

preference to an unattacked olefin because of the lower activation

energy that would be needed. Thus the alkyl radicals would not

survive.

Thomas [39] is attempting to overcome this difficulty in the reaction of gaseous radicals with solid surfaces by laying down a very thin

layer (less than a monolayer) of atomic hydrogen upon a thicker layer

of a solid reactant. The atomic hydrogen layer is then covered with

another thick layer of the reactant, and the process repeated, building

up a ''multideck sandwich." This is achieved by depositing the two

reactants on diametrically opposite sides of a swiftly rotating cold

cylinder.

6. Desirable Properties of Radical-Trapping Solids

Some of the desirable properties of the medium in which free radicals are trapped are common to both trapping radicals from gases and

trapping radicals in situ. They are suggested by considerations of the

nature of the trapped state, and of the purpose of the experiments.

Other desirable properties are peculiar to the method of trapping.

6.1. Low Temperature

The most important physical property is that the solid should be

at low temperature. Although examples exist of radicals trapped at normal temperatures (e.g., by the occlusion of radicals in polymers

and even here the lower the temperature the better the trapping)

most work on species which have a great tendence to recombine has

been carried out at or below the temperature of liquid nitrogen.

Diffusion is slower, and activation energies are harder to come by.

This is perhaps obvious, but none-the-less worth stating.

6.2. Structure of Matrix and Rates of Diffusion

The principal function of the matrix is to keep radicals apart, and

this means essentially to restrict the diffusion of radicals. There is unfortunately very little information available on rates of diffusion in

molecular crystals. Pimentel [2] has surveyed the temperatures at which diffusion has become observable in rigid systems (as detected

by the onset of chemical reactions between previously trapped react- 62
ants) and has proposed that diffusion becomes appreciable when the

temperature exceeds about one half the temperature at the meltingpoint. He bases his figures partly on surveying some very varied

experiments and partly on analogy with the work of Xachtrieb [40] on

self diffusion in metals. Thus a high melting point appears to be

desirable.

Such a simple relationship of diffusion coefficients to the melting

point of matrices might be expected in the case of ver}^ large complex

radicals. If these radicals are large compared with the lattice dimensions of the matrix crystal, then their movement will depend mainly on

the viscosity of the medium, and thus will be related to the melting

point. For very small radicals however, we have to consider more

carefully the manner in which they are held in the solid. (Pimentel

excludes small radicals from his main discussion and suggests their

difTusion might take place at temperatures nearer one tenth of the

mxatrix melting point.)

6.3. Trapping Sites

It may be helpful here to summarize some of the suggestions that

have been made concerning the nature of trapping sites. It appears

possible usefully to distinguish between the following types of sites, even though some of them may overlap: (a) Lattice sites, where the

radical replaces one of the molecules in the crystal lattice of the matrix;

(b) interstitial sites; (c) at dislocations; and (d) in holes which are left in amorphous materials like glasses. The forces involved in trapping

so far are of the van der Waals type, but we may add another category

of: (e) as portions of loosely bound chemical species.

A knowledge of the crystal structure of matrices is of very great

value in attempting to decide what sort of sites are occupied by radicals. Bolz, ^^auer, and Peiser [41] have investigated the crystal structure of condensed gases at very low temperatures by an X-ray

diffraction technique. They examined gases condensed both before

and after passing through discharges and also studied how the structure

of tlie solids changed on warming. Their conclusions very briefly were that most gases condensed at low rates showed some long range

order. On warming, annealing and further crystallization took place

with evolution of heat. The degree of disorder was less when the gases

had passed through a discharge, an effect attributed to local heating

and annealing following recombination of radicals on the surface. Solids in which hydrogen bonding takes place were most lacking in long range order, and would thus contain most faults. This work

provides a sound background to understanding tlie structure of con- densed gases and is now being complemented by electron diffraction

studies by Horl [42]. A further example of the importance of the structure of a matrix is the fact that diffusion rates along one of the main axes of a crystal may

be 10^ times greater than those along another axis of the same crvstal

(Jost [4.3]). [ Livingston [6] and his colleagues have shown that 20-fold higher

concentrations of H atoms can be stabilized in glass-like solids than in highly crystalline ones of the same chemical composition. High concentrations of radicals might result here either from increased accommodation or from reduced mobility i.e., the total number of sites available for trapping may be greater than in a crystalline solid, or the

.5.39.369—60 6 63
rates of diffusion of the trapped species may be less. Another way

of putting it is that both the number of sites and the magnitude of the

trapping bond can influence the concentration of species trapped.

Measurements of diffusion coefficients in molecular crystals are not easy and there is a great dearth of information on the subject. A

different type of measurement concerned with the rigidit}^ of molecules

in a solid is the ease with which dipoles can follow a changing electrical

field. N. Brown [44] has developed equipment to make measurements

of the dipole moments, and variation in power loss with frequency of an

alternating electric field across solids at low temperatures. Results

for solid ammonia over a range of temperatures show that the dipoles

cannot follow a frequency as low as 40 cps which is the present low

frequency limit of the equipment. Extension of the work to much

lower frequencies would be of great interest.

6.4. Formation of Loose Compounds

If a free radical can be induced to form a loose bond with a stable

molecule, then that bond might present a formidable barrier to diffusion and recombination of the radical. A bond of only 1 kcal energy

would more than suffice. Perhaps the most discussed examples of

such loose compounds are the H3 and N3 molecules which have been

theoretically postulated. Experimental evidence [48] has recently

been obtained to support the postulate of an N3 (or preferably N2— N)

molecule.

6.5. Thermal Conductivity

The liberation of energy during condensation of gases and recombination of radicals on surfaces needs to be quickly dissipated by the

matrix, which therefore should have for preference a high thermal conductivity. Little information has been published concerning the thermal conductivity of solidified gases at low temperature, but recently

Roder [45] and Chatenever [46] have been making measurements

independently, and with different types of equipment.

Comparison of results indicates that the structure of the solid (not

unnaturally) greatly influences the conductivity, a dense deposit being

the most conductive.

6.6. Properties Related to Purpose of Experiments

Selection of a matrix is very much conditioned by the purpose of an

experiment. Thus if measurements are to be made of properties of the radicals, then the matrix should interfere as little as possible.

For example, if absorption spectroscopy is to be used to examine the

radicals then the matrix should neither absorb nor scatter (which is less easy to achieve) the radiation used (Becker and Pimentel [49]).

Further, Robinson and McCarty [31] have required for their purposes that the matrix should not perturb the electron states of the

trapped radical. They state it is also preferable if the matrix falls into a well ordered lattice or reproducible arrangement surrounding

the radicals since random perturbations would broaden the lines of the absorption spectra which they wish to observe.

6.7. Properties Related to Trapping Radicals from Gases

Robinson and McCarty [31] have stated that the matrix must be

stable in the conditions in which the radicals are produced, and must

64
be inert to the radicals in the gaseous phase as well as in the solid. Becker and Pimentel [49] have also stipulated that the medium

should have a suitable volatility for ease of handling, while Thrush [1] has recommended good thermal conductivity of the solid for rapid

cooling of the gas on the surface of already-formed deposit.

Condensing molecules need to lose their energy in as few collisions as possible, and it is consequently of great interest to examine what

determines how much energy is lost in a collision between a gaseous

molecule and a surface. Zwanzig [47] has given some attention to

this problem and concludes that for most efficient transfer of energy:

(1) The mass of the gaseous molecule should not be much less than

that of the molecules of the solid; and (2) the velocity of the alighting particle should not exceed the velocity of sound in the solid. It immediately follows that for optimum energy transfer the matrix

should generally be of low molecular weight, and should propagate

sound wav^es with great velocity.

6.8. Properties Related to Trapping of Radicals Generated in Situ

The properties needed for trapping in situ are: (1) The solid must

be unaffected by the process producing radicals, e.g., it must be trans- parent to the photolyzing radiation; (2) it should not scatter the

incident radiation; and (3) there should be little ''cage effect". As discussed earlier, Pimentel [2] suggested that the matrix could

with advantage react chemically with one of the products of primary

dissociation. In conclusion, table 1 is presented in which many of the desirable

properties of a matrix are brought together. The lack of precision in defining the most important qualities illustrates the great deficiencies

in our knowledge, and underlines the directions in which future work

might profitably be undertaken.

Table 1. Summary of desirable properties of solid matrices for trapping radicals

1. The matrix should be at low temperature.

2. It should be inert to the method of generation of radical.

3. It should be chemically inert to the radicals trapped.

4. It should not interfere with the method of analysis for the radicals, nor with measurements to be made of properties of the radicals.

5. Its structure should accommodate high concentrations of radicals—(a "glassy"

structure seems to be favorable).

6. It should greatly impede diffusion of radicals by:

(a) providing closed-in sites (again a glassy structure may be favorable) or

(b) forming a strong localized attachment to the radical.

7. It should have a high thermal conductivity (here a compact dense structure would be favorable)

8. It should have a high melting point.

9. For trapping from gases the matrix material should:

(a) be of low molecular weight, and

(b) transmit sound with great velocity.

10. For trapping radicals from solids, it should allow the dissociating molecule to

part, but should prevent recombination.

65
7. References

B. A. Thrush in, The formation and trapping of free radicals, ed. A. ^I. Bass

and H. P. Broida (Academic Press Inc., N.Y., N.Y. 1960), chapt. 2. G. C. Pimentel, ibid, chapter 4. E. ^Yhittle, D. A. Dows, and G. C. Pimentel, J. Chem.. Phvs. 22, 1943 (1954).

I. Xorm.an and G. Porter, Nature 174, 508 (1954).

R. Klein and M. D. Scheer, J. Phys. Chem.. 62, 1011 (1958).

R. Livingston, H. Zeldes, A. Weinberger, and R. Holmberg, Faraday Society

Discussions, p. 26 (Sheffield University, September 1958).

G. X. Le^yis and J. Bigeleisen, J. Am. Chem.. Soc. 65, 520 (1943).

D. K. Ghosh and D. H. Whiffen, Faraday society discussions, p. 32 (Sheffield

University, September 1958\

C. N. Hinshelwood, Kinetics of chemical change, p. 58 (Oxford University

Press, London, 1940).

X. X"^. Semenov, Some problems in chemical kinetics and reactivity, 1, 176

(Princeton University Press, Princeton, N.Y., 1958).

E. W. R. Steacie, Atomic and free radical reactions, I, 72 (Reinhold Publishing Corp., X.Y., X.Y., 1954).

Ibid. p. 16.

G. B. Kistiakowsky and E. K. Roberts, J. Chem. Phvs. 21, 1637 (1953\

L. A. Wall, D. W. Brown, and R. E. Florin, J. Phvs. Chem. 63, 1762 (1959);

J. Chem. Phys. 30, 602 (1959).

W. L. Fife, private communication.

M. W. Windsor, unpublished work at XBS.

F. O. Rice, Trans. Faraday Soc. 30, 152 (1934).

M. D. Scheer, unpublished work at X^BS.

G. W. Robinson. M. McCartv, and M. C. Keeltv, J. Chem. Phys. 27, 972

(1957).

R. Klein, unpubhshed work at X^BS.

L. S. Xelson and D. A. Ramsay, J. Chem. Phys. 25, 373 (1956).

R. Rebbert and L. J. Schoen, J. Mol. Spect. 3, 417 (1959V

M. Peyron and H. P. Broida, J. Chem. Phys. 30, 139 (1959).

L. Vegard, Compt. Rend. 182, 211 (1926).

J. C. McLennan and G. M. Shrum, Proc. Roy. Soc. A106, 138 (1924).

R. S. Alger and T. H. Anderson, J. Chem. Phys. 30, 695 (1959).

E. M. Horl, J. Mol. Spect. 3, 425 (1959).

H. T. J. Chilton, G. Porter, and E. E. Strachan, ibid. p. 11.

M. S. Matheson, ibid, p. 45.

K. Dressier and O. Schnepp, unpublished work at NBS.

G. W. Robinson and M. McCartv, Faraday society discussions, p. 58 (Sheffield University, Sept. 1958).

J. Comeford, unpublished work at X^BS.

J. Frank and E. Rabinowitch, Trans. Faraday Soc. 30, 129 (1934).

G. X. Lewis and D. Lipkin, J. Am.. Chem. Soc. 64, 2801 (1942).

H. Linschitz, Faraday society discussions, p. 37 (Sheffield University, Sept.

1958).

I. Xorman and G. Porter, Proc. Roy. Soc. A230, 399 (1955).

A. Leifer, unpublished work at NBS.

J. F. Gibson, D. J. E. Ingram, 'M. C. R. Symons, and M. G. Townsend,

Trans. Faraday Soc. 53, 914 (1957).

A. Thomas, unpublished work at X"BS.

X. H. Xachtrieb and G. S. Handler, J. Chem. Phys. 23, 1187 (1955).

H. S. Reiser in The formation and trapping of free radicals, ed. A. M. Bass

and H. P. Broida (Academic Press, Inc., X^ew York, X'.Y., 1960).

E. M. Horl, unpubhshed work at X"BS.

W. Jost, Diffusion in solids, liquids, and gases, p. 6 (Academic Press, Inc.,

X.Y., X.Y., 1952).

X. Brown, unpublished work at XBS.

H. M. Roder, unpublished wcrk at Ts'BS Boulder Laboratories.

A. Chatenever, unpublished work at X'BS.

R. Zwanzig, unpublished work at X^BS.

M. Peyron, E. M. Horl, H. W. Brown, and H. P. Broida, J. Chem. Phvs. 30,

1304 (1959).

[49] E. D. Becker and G. C. Pimentel, J. Chem. Phys. 25, 224 (1956,\

Washington 25, D.C., January 15, 1960.

66
7. Identity and Concentrations of Trapped Radicals

Robert W. Zwanzig

1. Introduction

Many methods of identifying trapped reactive species and measuring

their concentrations have been proposed. Some of these are standard

ones, for example optical spectroscopy and electron spin resonance,

and have been studied extensively at NBS and elsewhere. Other

more unconventional methods have been investigated in exploratory

work mainly at NBS. These methods all have good and bad features;

some have turned out to be of little value, but a few have given useful

information. In this survey a number of these methods will be

discussed. Major emphasis will be put on the utility of the method,

rather than on the results which have been obtained so far. Since

research which has been done in other laboratories is described

extensively in the scientific literature, this review shall be confined,

for illustrations, mainly to work done at NBS. Since other surveys

in this volum.e contain extensive bibliographies, no specific literature

references will be given here.

Before the various methods are discussed, a brief outline of experimental results obtained during the NBS Free Radical Research

Program will be presented.

The trapped radicals which have been identified w^ith reasonable

certainty at NBS are N (or perhaps N3), H, O, NH, NHo, CH3, HNO.

Other radicals for which there is some partial evidence, but not enough

for positive identification are NH3+, C2H5, CN, HO2, CI. Aside

from species which we are accustomed to call radicals, certain other

interesting reactive or unstable species have been studied: for example,

^'red" magnesium and excited electronic states of N2 and O2.

Experimental methods of estimating the concentrations of these

species are not in satisfactory agreement with each other. The

results of electron spin resonance, which presently appear to be the

most reliable, indicate a maximum concentration which is of the order

of tenths of a (mole) percent. The highest estimate, and also the most

suspect, comes from calorimetric studies: this is about 4 percent.

After the experimental methods have been discussed, some

theoretical work on concentrations will be mentioned. Several

theories have been proposed which predict upper limits to the

concentration of radicals that can be trapped. These predictions are

in rough agreement with experiment. By judicious choice of a theory

one can get more than ten percent or less than a tenth of a percent.

2. General Comments

The experimental techniques which will be discussed are

optical spectroscopy

electron spin resonance

magnetic susceptibility

mass spectrometry

67
calorimetry

X-ray diffraction

electron diffraction

dielectric constant

index of refraction.

This list shall be gone through to indicate, wherever appropriate,

the physical basis for each method, with comments on its utility.

But first some general comments, relevant to almost all of these

methods, should be made.

Any method of identification is in principle also a method for

determinmg concentration. For example, an extreme low^er limit on

concentration is always given when the sensitivity of the method is known. Unfortunately, some methods are so sensitive that this

lower limit is far from realistic. Emission spectroscopy is an example

of this. Some methods of identification are used very often for analysis;

for example, absorption spectroscopy. But here one suffers from

standardization troubles. Unless the absorption transition probabilities are known, measurements of absolute absorption intensity are

useless. As far as the writer is aware, no one has measured concentrations of the simpler trapped radicals in this way.

Many methods of measuring concentration are useless unless only one reactive species is present, and its identity is known. This is characteristic of magnetic susceptibility studies, and an extreme case

is the calorimetric measurement of "heat of recombination".

The effect of the matrix can cause serious troubles in the process

of identification. A question which often causes arguments is: what is meant by the ''identity" of a species which has been trapped in a matrix? This comes up, for example, in discussions of the difference

between a loosely bound N+N2 "molecule" and an N "atom", both

contained in a molecular nitrogen matrix. I think that the best answer is the obvious one, that if the effect of the matrix is a small

perturbation of the properties of the isolated species ("small" means

that only the lowest order in quantum mechanical perturbation theor}^

is needed) , then the identity of the isolated species is preserved. If the

effect of the matrix is too large to be handled by simple perturbation

theory, then the species has not been properly defined, and at least part of the environment must be combined with the original species

to make a more complex species.

When a species has been trapped, one should generally expect that

its properties will shift in numerical value, and that qualitatively

new features can appear. This is particularly true in spectroscopy,

but it should be watched for in other techniques. A typical question here is: how much of a shift in frequency is consistent with

preservation of identity? In the green glow from the condensed

nitrogen discharge, a variety of features appears. It is tempting to

settle the conflict between different possible assignments of the

spectral lines by picking that assignment which gives the smallest

shifts. In the absence of a workable theory of line shifts this is an

uncomfortable basis for decision. Another problem in spectroscopy

is that transitions which are forbidden to the isolated species can become allowed as a result of interaction with the matrix.

These preceding points, 1. identification can imply concentration;

68
2 . identification is generally required for measurement of concentration

3. the matrix can have an effect on '^identity", should be kept in mind when a particular experimental method is proposed.

3. Optical Spectroscopy

A substantial amount of eft'ort, both at NBS and elsewhere, has

gone into the study of the absorption and emission spectra of trapped

species. The entire spectrum from the infrared to the vacuum ultraviolet has been spanned. At XBS, infrared spectra have been studied

by Harvey, H. Brown (absorption and emission), and Mador, Leifer,

Comeford. and Gould (absorption). Visible emission spectra were

investigated by Bass, Broida, Horl, Peyron, Rebbert, and Schoen.

Vacuum ultraviolet absorption has been studied by Dressier and

Schnepp.

The physical basis of spectroscopic analysis is too well known for further discussion here, but the limitations inherent in this technique

deserve discussion. I have already pointed out that the major stumbling-block in measurements of concentration is the lack of information

about transition probabilities (or /-values). If the trapped species is in weak interaction with its surroundings, and if the transition is strongly allowed in the dipole approximation, one would expect that

the /-value does not dift'er much from the gas phase—but this conjecture has to be tested. Also, it is hard enough to get /-values for the

gas phase. If the interaction with the matrix is strong, or the transition weakly allowed in the gas, then anything can happen.

Optical spectroscopy has been used mainly for identification. If a comparison spectrum of the species in the gas phase is available, and

if only a very few, spectrally separated species appear to be present,

then identification rests on the reasonableness of frequency shifts. McCarty and Robinson have done outstanding work of this kind.

They have supported their identifications by an experimental investigation of the effect on frequency shift of a change in matrix. They

have also developed a crude theory of shifts based on empirically

known strengths of ground state intermolecular forces. At NBS,

Schoen has made a systematic study of the matrix effect, but his data

have not yet been theoretically analyzed. There is experimental

evidence for shifts not only in the electronic term values but also, in the

case of diatomic molecules, in the vibrational frequency and anharmonicity. Each quantum state of a molecule can interact somewhat

differently with the environment. This should be a powerful tool for the study of intermolecular forces. Bass, Broida, Brown, Horl, Peyron, Rebbert, and Schoen at NBS

have investigated the emission spectra from nitrogen which has been

passed through a discharge and then condensed at 4° K, or else con- densed and then activated by various methods. Their biggest problem

is to disentangle the many possible overlapping molecular and atomic

transitions, complicated by matrix effects. Here one must depend

very strongly on the assumption that shifts are small and systematic.

Some of the trouble due to overlapping spectra of different species can be avoided by careful elimination of unwanted material. This is not always easy— for example, Schoen has obtained t^^pical, though

weak, nitrogen and oxygen emission spectra from discharged argon

containing 10 parts per million of impurity. In many ways it is better

to use isotopic substitution; this has been done in the nitrogen problem,

69
and one can be sure that certain lines in the nitrogen spectrum are indeed due to species containing nitrogen. But this device has

hmited utiHty.

In summary, optical spectroscopy is not presently useful for absolute

determinations of concentration. Its utility for identification is greatest (and in fact it is probably the best method) when a comparison

spectrum is available. Until a workable theory of line shifts is developed, the results of optical spectroscopy depend on the assumption

that frequency shifts are reasonably small.

4. Electron Spin Resonance

Electron spin resonance (ESR) has become a very popular and quite

successful technique for identifying trapped radicals. It has also provided the most reliable data presently available on concentrations

of the simpler trapped species. Many laboratories in this country and

abroad are using ESR; at NBS it has been used by D. Brown, Florin,

Gager, Radford, Wall, and Scheer.

The basic physical effect which is observed by ESR is the interaction of an unpaired electron spin with an external magnetic field. (More precisely, one measures the absorption of microwave energy

accompanying transitions between quantum states of the electron in the magnetic field.) Therefore the observation of an ESR signal is evidence that unpaired electrons are present in the sample. By itself, this does not mean that radicals are present. However, there are

finer details associated with this gross effect, due to the extra magnetic

fields that occur when certain nuclei with magnetic moments are present. In ESR one observes the nuclear spin environment of an

unpaired electron spin: this is the basis for identification. If the

electron belongs to a molecule, the nuclear spins within the molecule

give the biggest environmental contribution, but there are also effects

clue to the surrounding matrix. If the electron spin belongs, for

example, to a nitrogen atom, the fine structure is due entirely to the

surroundings. (This is an example of the definition of identity"

which was introduced earlier. The species is properly identified only

when the effects of the surrounding matrix are small.)

The nuclear spin environment is seen in two ways. First, it gives

rise to a splitting of the free electron ESR spectral line into a number of

lines. Second, these lines have different intensities. If the nuclear

spin environment is specified, the number of lines and their relative

intensities can be predicted theoretically and compared with experiment. Unfortunately the lines are not sharp and their shapes are not

precisely known, so that the integrated or total intensities may be hard

to get accurately. Another complication, of an experimental sort, is that lines which are broad and weak can be submerged in background

noise. Because of this the number of lines can be incorrectly reported.

In spite of these difficulties, ESR measurements in several laboratories have given reasonably conclusive evidence that, for example,

hydrogen atoms can be trapped in solid hydrogen, nitrogen atoms in solid nitrogen, and many larger radicals in various rigid matrices.

ESR provides also a method for measuring concentrations of electron

spins associated with particular nuclear spin environments. The most

serious complication in these measurements appears to be the rather

routine one of calibration. Before too long this trouble ought to be

eliminated.

70
At NBS, D. Brown, Florin, and Wall have investigated the concentrations of hydrogen and nitrogen atoms which can be stabilized in a matrix: The atoms are produced by gamma-irradiation. Their re- sults indicate an upper limit to the concentration of the order of a few

tenths of a percent. As far as the writer knows, no one has observed

appreciably higher percentages of atoms by ESR.

In summary, ESR is a valuable method of observing the nuclear

spin environment of unpaired electrons, and hence radicals. In many

cases the interpretation of the spectrum is straightforward, and qualitative observations are enough. But interpretations based on relative or absolute intensities are not so safe. More work needs to be done on

the shapes of ESR lines in radical-matrix systems, and on the calibration of the equipment at low temperatures.

5. Magnetic Susceptibility

Another method of studying trapped radicals, somewhat like ESR,

is measurement of paramagnetic susceptibility. The main work in this field has been done by Barfield, Fontana, and Zahn at NBS.

The magnetic susceptibility of a system is due to its total average

magnetic dipole moment. In typical cases this magnetic moment

comes from unpaired electron spins. If radicals are present, and there

are no other sources of electron spin, this technique can be used to measure concentrations. In the present case it cannot conveniently

be used to identify the species present. The susceptibility is directly

proportional to the number of spins and inversely proportional to the

temperature.

While absolute measurements of the magnetic susceptibility of a system are possible, there are serious experimental difficulties. In systems which have been studied at NBS, the contribution to the

susceptibility from radicals is comparable with the contribution from

the Pyrex glass of the sample cell. Also, the temperature of the sys- tem must be known—any error in temperature is equivalent to an

error in concentration.

Fontana has performed some interesting experiments using this technique. He deposited the products of a nitrogen discharge on a helium-cooled surface, and measured the susceptibility and temperature simultaneously as functions of time. He could therefore follow

the rate of increase of the number of trapped spins under various

conditions, and compare this with the rate of deposit. Also, the gas

flow could be stopped, so that the stability of the deposit could be

observed. Fontana found that when the gaseous discharge contained more than 1 percent of atoms, no spins could be observed in the deposit. He inferred that the atoms were recombining '^instantaneously". But when the concentration of atoms in the gas was of the order

of hundredths of a percent, the deposit contained about that many

spins and remained fairly stable. Concentrations of the order of a few tenths of a percent could be stabilized for as much as five minutes,

but this was the limit of stability at these concentrations. Under

these conditions he observed spontaneous ''explosions", in which the

temperature rose and the spins disappeared rapidly. Of course it is difficult to control and measure temperatures during the deposition of

a hot gas, so these results may be quantitatively in error. To sum up, the technique of paramagnetic susceptibility is capable

71
of giving useful information about concentrations of trapped radicals,

but cannot be used for identification in these systems. Great precision

is needed, and the temperature must be known with confidence. This

method does not appear to be as useful as ESR, especially if the ESR

calibration problems can be eliminated.

6. Mass Spectrometry

Mass spectrometry has been used at XBS by Franklin, Herron,

Dibeler, and Bradt in an interesting attempt to measure the concentration of trapped nitrogen atoms. A gaseous nitrogen discharge was condensed on a helium-cooled surface, under conditions where it was believed that nitrogen atoms were trapped. The deposit was

then allowed to warmup, and the evolved gas was fed into a mass

spectrometer. A search was made for nitrogen atoms, but none were

observed. The sensitivity of the measurement was such that a hundredth of a percent of those atoms which were believed to be trapped

could have been detected. Of course this does not prove that no atoms were present in the solid, since they might have recombined

during the evaporation. A positive result would have been more

encouraging; but it appears that mass spectrometry is not a valuable

technique for measuring concentrations of trapped atoms.

7. Calorimetry

In manv fields of chemistry and physics substantial understanding

of the properties of materials is gained from thermodynamic measurements. There are, however, both conceptual and practical difficulties in appljdng calorimetry to svstems of reactive species trapped in matrices. The practical difficulties will no doubt be overcome, but

we shall have to live with the conceptual ones. One of the most basic of thermod^mamic quantites is heat capacity.

A measured amount of heat is added to a s^^stem and the consequent

increase in temperature is measured. But will such measurements

have meaning or utility when trapped teactire species are present?

The conceptual difficulty here is that the system is not in thermodynamic equilibrium. Species are trapped because their coefficients of diffusion in the matrix are small. When heat is added the temperature increases, the rate of diffusion increases, and the species begin to

react. We are dealing with a nonequilibrium process. The specific

heat which is measured in this way will generally depend on the rate at which heat is added. Such measurements will, of course, still be

interesting; but they are likely to give more information about the

kinetics of recombination than about thermodynamic properties.

A conceptually safer kind of calorimetric measurement, which appears to have great value, is that of integral heats of reaction. This

should give information about concentrations of trapped species. In

a typical experiment, a known amount of heat is allowed to leak into

the system. After a while the matrix is warm enough that species can approach each other and react. The evolved heat warms the matrix

further, more species react, and so on. Finally the system settles down, and the only subsequent increase in temperature is due to the

initial heat leak. If the molar heat of reaction has been determined in some other way, if the specific heat of the pure matrix is known, and

72
if it is assumed that these quantities do not change appreciably when

the species is trapped in the matrix, then the initial concentration of reactive species can be obtained.

This kind of experiment was first done by Broida and Lutes. The}^

studied the deposit formed when the products of a nitrogen discharge

are condensed on a helium-cooled surface. They could not control the

external heat leak (which in fact varies as the vapor pressure of the

system changes), so the}- made similar measurements on the deposit

formed from undischarged nitrogen, and used this to determine the

external heat input. They estimated that the nitrogen atom content

of the matrix was about 0.2 percent.

Minkoff, Scherber, and Gallegher repeated these measurements,

after making substantial changes in the calorimeter. These investigators obtained concentrations as high as 4 percent. But considerable

doubt may be cast on their results. For example, the nitrogen atom

content of the gas in the discharge was determined by entirely different

methods to be typically only about 1 or 2 percent. Minkoff and

Scherber also observed, using the same equipment, that substantial

amounts of heat were released when the condensed products of an

argon or ki\vpton discharge were warmed up. Either they were trapping excited atomic states, or some entirely different effect is responsible

for their results. Also, Scherber and Fine observed a large evolution

of heat when condensed undischarged hydrogen was warmed up; and

Schoen and Rebbert, by an independent method, observed corresponding thermal anomalies when undischarged h^^clrogen was deposited.

Either something ver}' strange is happening when these gases are condensed on a cold surface or else the experimental technique is not

well-suited to this kind of measurement.

Although the calorimetric measurements which have been made are rather discouraging, the idea of determining concentrations from heats

of reaction is still very attractive, and more work should be done.

8. X-ray and Electron Diffraction

In this section the techniques of X-ray and electron diffraction

will be discussed. Peiser, Bolz, and Mauer have done low-temperature

X-ray diffraction studies on a number of the solids which are popular

as matrices; and Horl has studied electron diffraction on solid nitrogen.

In both X-ray and electron diffraction one observes the Fourier

transform of the electric charge density in the solid. It should therefore be possible to detect and to measure the concentrations of trapped

species. However, it is not likely to be practical unless the trapped

species has a considerably higher molecular weight than the matrix

molecules.

X-ray diffraction patterns have been obtained for deposits of various condensed gases (argon, nitrogen, oxygen, etc.), both dis- charged and undischarged. The major difference is that in general

the discharged gases produce a more ordered structure. But there is no indication of the presence of atoms in the deposited solids. It appears to be impossible, using the experimental techniques now

available, to identify or measure concentrations of the simpler trapped

species in matrices.

Horl has obtained electron diffraction patterns of thin films of nitrogen. Electron bombardment of the solid is known to produce

73
atoms, as shown, for example, by emission spectra. However, the

electron diffraction patterns themselves do not show the presence of

atoms.

9. Dielectric Properties

N. Brown has investigated the possibility of measuring the dielectric

constants of deposited materials, with the hope of observing and determining the concentrations of radicals with electric dipole moments.

Dielectric measurements at very low temperatures were found to be

unusually difficult. No evidence was observed of any change when a Tesla coil was discharged through solid ammonia.

J. Kruger has studied the index of refraction of thin films of deposited material at low temperatures. In comparisons of discharged and

undischarged gases, no systematic differences have been observed. In

order to use this method for absolute determination of concentration, one would have to know the polarizability of the trapped species: this

is a serious obstacle to precise work, since polarizabilities cannot be

estimated reliably for any but the simplest substances.

10. Theoretical Studies

Some theoretical work on trapping of reactive species will be described in this section. Several models have been studied which lead

to upper limits on the concentration of species which can be trapped.

It is convenient to divide these into static and dynamical theories.

The static models developed by Golden and by Montroll and Jackson

depend strongly on the notion of isolation. They assume that if two

radicals (e.g., N atoms) are nearest neighbors in a lattice, they will immediately react to give an uninteresting stable species (e.g., N2 molecules). They also assume that, aside from this nearest neighbor prohibition, the arrangement of radicals and diluent molecules on the

lattice is random. The Golden and Alontroll-Jackson calculations

differ mainly in the details of the statistical analysis. These theories

predict maximum concentrations of the order of 10 percent.

The assumption that nearest neighbor radicals will react is certainly

plausible. However, it may be too restrictive. (For example,

crystalline diphenylpicrylhydrazyl, the most common radical for cali- bration of ESR equipment, is perfectly stable.) If radicals having

parallel spins are nearest neighbors, they will not react unless one of

the spins is reversed; and one needs a time dependent perturbation to do this. At very low temperatures the lattice is essentially at rest, and spin-lattice interactions may be quite weak. In this case the radicals can remain unreactive for a long time. At NBS, Windsor per- formed experiments on the deposition of a beam of spin-alined hydrogen atoms, to see whether the spins would indeed remain parallel in the deposit. Results were inconclusive.

Jackson has proposed a dynamical theory dealing with concentrations of trapped radicals. This theory predicts conditions under which

a trapped radical system will be unstable and "explode". In fact his

theory shows strong similarities to conventional explosion theory. It

is assumed that radicals will occasionally react, as a result of slow

diffusion or some external influence. (Therefore the system is not

intrinsically stable, though its decay may have a very long lifetime.)

74
The heat of combination warms the hittice in the neigliborhood of the

reacting radicals. This heat is dissipated by thermal conduction. (It

is conceivable that some other mechanism, for example a shock wave, removes the energy from the reaction region. This point has not been

mvestigated. ) Jackson also assumes that there is a critical temperature, such that radicals in a colder region remain trapped, while radicals in a warmer region become ''free". By solving the heat conduction problem, one can show that the temperature will rise above the

critical temperature only inside a certain volume. If "free" radicals

react immediately a chain reaction will ensue whenever the concentration is high enough that more than two radicals are within this volume. By making reasonable guesses about numerical parameters

involved in the theory. Jackson obtained critical concentrations of the

order of tenths of a percent for nitrogen atoms in solid nitrogen.

Any dynamical theory must remain largely speculative until experimental information is available about mechanisms and rates of reaction

of radicals in matrices.

Washixgtox 25. D.C.. December 9, 1959.

75

8. Interactions Between Trapped Species and the

Matrix

O. Schnepp ^

1. Nuclear Degrees of Freedom

The degrees of freedom of a radical or molecule trapped in a matrix

are obviously of great importance in the study of such systems. We

shall discuss the interactions which determine the extent of translational, vibrational, and rotational freedom in this section.

1.1. Translation

On the basis of available information, there is a striking correlation

between the ease of diffusion of a trapped species and the lattice energy of the matrix. By the use of the electron spin resonance

technique it has been demonstrated that hydrogen atoms can be

stabilized in a matrix of solid hydrogen at 4.2° K,i.e., the diffusion

of hydrogen atoms is severely impaired under these conditions [20, 21].

Similar observations have been made on atomic hydrogen trapped in solid methane, [21, 43]. For this system it was observed that the resonance spectrum ascribed to atomic hydrogen disappears on slight

warming, i.e., below 20° K, indicating that diffusion becomes appreciable. On the other hand, hydrogen atoms liave been trapped at 77° K in solid hydrogen acids, concentrated and diluted with ice

[45, 23, 22], and in pure ice [26, 40]. In such matrices diffusion only

becomes important near 100° K. Similar observations have been

made for hydrogen atoms trapped in ammonia [7]. The same matrix will, however, limit the diffusion of different

species to a different extent. For example, both hydrogen atoms and

methyl radicals have been produced in a methane matrix at 4.2° K

but atomic hydrogen disappears on slight warming whereas methyl

radicals were observed to survive the same treatment [43]. In another

experiment, Smaller and Matheson [39] report that the resonance

signal due to methyl in methane decays in a few minutes at 77° K,

indicating that the radical has a finite stability at that temperature.

A much larger radical, benzyl, has been stabilized at 77° K by Norman

and Porter [28] in a glass containing alcohol and ether in addition to a liydrocarbon.

The interactions determining the diffusion rates and activation

energies can probably be divided into two parts. The first type is the interaction between the trapped species and the matrix molecules.

This is presumably mostly of the van der Waals type in the cases discussed above. It is reasonable that the interaction between a methyl radical and methane molecules should be greater than that

between hydrogen atoms and methane, for the same reason that the

sublimation energy of solid hydrogen is much smaller than that of

solid methane. On the other hand, the comparative stability of h}^-

drogen atoms in ice and in ammonia cannot be understood on the

basis of such interactions. It seems tliat here the binding of the

matrix material itself is decisive, leading to a correlation of stability

' On leave from Israel Institute of Technology, Haifa, Israel.

2 Figures in brackets indicate the references at the end of this paper. 77
of radicals with the binding energy of the matrix hittice. Thus the

strongly hydrogen bonded solids may form more rigid cages from

which the radical cannot escape at low temperatures. This ''cage

effect" is then the second type of interaction referred to. There does

not seem to be a valid alternate explanation of this stability of free hydrogen atoms in hydrogen bonded media. It does not seem rea- sonable to invoke hydrogen bonding between free hydrogen atoms

and the matrix molecules, as the presently accepted picture of such

interactions requires the hydrogen atom involved to be one of the

participants in a polar bond.

1.2. Vibration

There is some published evidence concerning the extent of the

interactions betw^een trapped species and the matrix as observed hi connection with the vibrational degrees of freedom of radicals and

molecules. There are two distinct sources of information, the first being from vibrational spectra in the infrared region of the spectrum

and the second from the vibrational structure of electronic band

systems in the visible and ultraviolet. The latter method of investigation has the advantage of potentially giving information on the

deviations of the vibrational quanta from the free molecule values over a wide range of quantum numbers in the ground and excited

states. Infrared investigations have shown that tlie first vibrational spacing

is not appreciably affected by the matrix materials studied, i.e., rare gases and nitrogen, the spectra of the trapped species coinciding with

those of the gaseous molecules within a few cm~^ [2, 42]. This observation is not unexpected, since the bond energies are very large

compared to the binding energies of these solids and presumably the

binding energies of the trapped species to the matrix, and since

the vibrational amplitudes are small compared to lattice spacings.

Robinson and McCarty [35, 36, 33] have observed the absorption

spectrum of NH2 betweeji 7900 A and 3700 A, and have compared it with the spectrum of the radical in the gaseous state investigated by

Ramsay [31, 32]. Twenty quanta of the bending vibration in the

excited electronic state of the trapped radical in solid argon have been

recorded. The first 10 quanta follow the free molecule pattern very

closely but the higher vibrational quanta show a tendency to crowd

together in the matrix, as compared to the gas, such that the matrix

shift is reported to be 2.5 cm~^ to the red for the band corresponding

to vibrational quantum number 18, whereas the shift was to the blue

by 26.3 cm~^ for quantum number 6. The matrix effect on the vi- bration is here certainly a small one and it is the more difficult to evaluate its significance due to the complexity of the situation in NH2

introduced by the Renner effect [12]. This term refers to severe anomalies in the vibrational structure due to strong interactions

between the bending vibration and electronic motion.

Recently Dressier and Schnepp [13] have observed the SchumanRunge bands of oxygen, condensed in matrices of nitrogen and of argon, in the spectral range between 1900 A and 1700 A. Discrete

but broad bands have been observed between vibrational quantum

numbers 4 and 15 in the excited electronic state, follow^ed by a con- tinuum. This system is of particular interest since the equilibrium

internuclear distance changes very much from ground state to excited

78
state, namel}^ from 1.21 A to 1.60 A [17]. The vibrational intervals

show a slight increase over those of the free molecule. The bands at

first seem to converge toward a dissociation limit as in the gas, but

about 2,000 cm~^ before this limit the bands stop converging and even seem to diverge. This observation is interpreted by the authors as the result of the repulsive forces acting on the oxygen at large internuclear distances due to the surrounding matrix "cage". It is to be

noted that the amplitude of the molecular vibrations at this point is twice the ground state mternuclear equilibrium distance.

Robinson and McCarty [35, 34] have also reported several vibrational intervals in the electronic spectrum of HNO in an argon matrix.

Here the vibrational quanta agree with the gas phase values to within

a wavenumber unit.

The Herzberg system of oxygen has been studied by Broida and

Pe3^ron [4] and by Schoen and Broida [38] and the Vegard-Kaplan

system of nitrogen has been studied by Broida and Peyron [5] and by

Schoen and Broida [38] all in emission. These authors have observed

long progressions of the ground state vibrations of the molecules and

have found the deviations from the free molecule vibrational

parameters to be small.

1.3. Rotation

During the last year direct evidence for the occurrence of free or

near-free rotation of molecules and radicals in matrices at low temperatures has been presented by various authors. Fine structure in the vibrational absorption bands of water in argon at 20° K has been

interpreted by Catalano and Milligan [6] as indicating rotational degrees of freedom. Robinson and McCarty's investigation [35, 36] of the electronic absorption spectra of NH2 and of NH in argon at 4.2° K

has produced excellent evidence that these radicals rotate quite freely

in the matrix. They have shown that most of the fine structure of the vibronic bands can be interpreted very satisfactorily if it is as- sumed that the rotational constants are identical to those observed

for the gaseous species. It should also be pointed out that the close similarity between the vibronic structure of the trapped species and

that of the gaseous radical can be interpreted as implying that rotation

about the axis of smallest moment of inertia is quite free in the matrix.

This rotational angular momentum is a factor in the theory of the

Renner effect which is the basis of the interpretation of the observed

vibronic structure [12]. The recent work by Hexter and Milligan [27],

principally on ammonia in matrices, lends further support to the as- sumption that small molecules have considerable freedom to rotate in solid matrices.

Very recently McConnell [25] has pointed out that the intensity

relations among the hyperfine components in the electron spin reso- nance studies of the NH2 radical reported by Foner et al. [15] can be

interpreted as evidence for free rotation. Foner and coworkers found

that the spectrum consists of a hyperfine triplet due to the N^* nuclear

spin of 1 with every component split into a further triplet due to interaction with the nuclear spin of the 2 protons. The components all have the same intensity. There are two ways of coupling the spins

of the protons to give ortho- and para-NHg with the proton spins

parallel or antiparallel. The para-species cannot reach the lowest

rotational state (Oq), being constrained to remain in the l_i rotational

state at low temperatures. This latter level lies 21.11 cm~^ above the

79
Oo rotational state for the free molecule. If only the ortho-species is present in the matrix at low temperatures the observed intensity re- lation among the hyperfine structure lines of the spin resonance spectrum can be explained. For this situation to be realized, there must

be free conversion between ortho- and para-molecules, such that thermal equilibrium can be established witli considerable speed at 4.2° K.

The absorption studies of the electronic spectrum of NH2 indicate

that this is indeed the case [35]. McConnell then goes on to argue

that the intensity relations in the observed spin resonance spectrum

are evidence for the presence of ortho-NH2 molecules only, i^ssuming

then thermal equilibrium, this indicates that the l_i state is appreciably higher lying than the Oq state or in other words close to the free molecule level, indicating that free rotation exists. If the rotation were seriously hindered in the matrix, the l_i level would be expected

to lie much closer to the Oq level, allowing appreciable population of

the higher state and therefore an appreciable concentration of psraspecies. In this case the proton triplets would approach an intensity

distribution of 1 : 2 : 1 . It is thus suggested that such intensity rela- tions in spin resonance spectra are indicative of the degree of hindering

of rotation coupled with information on the ease of interconversion

between different nuclear spin species.

2. Electronic Interactions

We shall now turn our attention to the effects of the interactions

between trapped species and matrix on the electronic transitions of

the former. The most intensively studied cases are those of the emission spectra of nitrogen and oxygen atoms in discharged nitrogen deposited on a cold surface at 4.2° K. The studies of these effects are due to Herzfeld and Broida [18] and to Herzfeld [19]. It is demonstrated in these theoretical studies and interpretations of the experimental observations that the highly forbidden atomic transition ^ D-

^ S oi the nitrogen atom is affected in three ways by the crystal field of the matrix. The ^ D levels are shifted to lower energies by 50 to 100 cm~^ relative to the ground state in the solid and they are split by

the cubic crystal field. Furthermore, the radiative lifetime of the excited state is reduced by the solid interactions by a factor of 10^ from

the free atom value to about 15 sec in the matrix. The emission

spectrum attributed by Broida and Peyron [3] to the transition ^ S-

^ D2 of the oxygen atom has been shown [16] to be consistent with the

predicted crystal field splitting of this transition into three components, due to the splitting of the D atomic state in the cubic field of

the matrix which allows a degree of degeneracy of at most 3. In recent months spectroscopic investigation of several metal atoms

in rare gas matrices have been reported. McCarty and Robinson [24] have observed the absorption spectra of mercury and sodium ; Dressier

[11] has extended the observations on mercury to the 1849 A line; and Schnepp [37] has studied manganese and magnesium. In all cases the electronic transitions were between S ground states and P excited

states, although the multiplicities vary. In all cases, the observed

spectra fall close to the free atom spectra and can therefore be cor- related with these. It is also true that in all cases groups of three

broad lines were observed, although for magnesium and manganese

weaker pairs of lines have been observed in addition. It is therefore

80
indicated to conclude that the atoms are located at asymmetric sites in the solid, such that the orbital degeneracy of the excited states is removed wholl}^ or partially. Alternately, the Jahn-Teller effect may

be invoked to explain the removal of the degeneracy [24]. Quantitative theoretical analysis is needed before a decision can be made as to which of the two possible interpretations is valid. The observed

crystal field splittings are of the order of several hundred wavenumber

units and the line widths vary from 20 to 70 cm~^

Due to the low degree of degeneracy encountered with the electronic

states of polyatomic radicals and molecules studied in matrices, crystal

field splittings are not observed in these cases. The matrix effects manifest themselves, however, in the observed shifts of the electronic

spectra. Robinson and McCarty [35] have discussed the shifts observed for a series of trapped species, namely CH, NH, OH, C2,

NH2, HNO, NO2, H2CO, HCO, and NCO. In the majority of cases a red shift is observed, the largest being that recorded for OH

amounting to 395 cm~^ Blue shifts have been observed by these

authors for NH2 (28 cm"^), NO2 (55 to 70 cm"^), for H2CO (215 cm-^)

and possibly for NCO (70 cm~^). The interpretation of these shifts

is very difficult due to the smallness of the effect, amounting in most

cases to only a small fraction of the lattice energy of the matrix.

It therefore becomes very difficult to divide the observed shift between

ground and excited state contributions. Moreover, present day

theory of the electronic states of molecules is not capable of predicting

with sufficient accuracy small changes in the polarizability of the

molecules and radicals accompanying an electronic transition.

Robinson and McCarty probably correctly ascribe major importance

to this change in connection with the matrix shift problem. They

also show that for the same species in different matrices the shift increases with the polarizability of the matrix atoms or molecules.

Thus the shift for NH in argon is 195 cm~^ while it is 263 cm~^ in krypton. The shift for HNO is reported to be 9 cm~^ in neon, 54 cm~^

in argon, and 68 cm~^ in nitrogen. They point out that ''the diatomic

species (experience) for the most part large red shifts which corre- spond to an mcrease of polarizability in the excited state in accordance

with the Tr<—a type of transitions involved." They attribute the

considerable blue shift observed for formaldehyde to the marked

decrease of the dipole moment in the excited state. This gives rise to stronger binding with the matrix in the ground state than in the

excited state. Recently A'IcCarty and Robinson [24] have presented a quantitative

treatment of matrix shifts. They use a Lennarcl-Jones potential

function to represent the interaction between the trapped species and

the matrix atoms or molecules and empirically determine the differ- ences in the parameters between ground and excited states. In this

way they are able to predict the shift in a third matrix if the data for

two matrices are known. Their predictions agree very well with the

experimental results in most cases. This work is a very important

contribution since it will inevitably further work in the field. On the

other hand, the authors point out that their model does not seem to apply to sodium atoms in rare gas matrices. It may be added that

the recent results of Dressier [10] for xenon atoms in krypton and argon

probably also cannot be accounted for by their scheme. Dressier

observed shifts to higher energies of 5,000 cm~^ and more for this

system. It is certainly correct that the difference in polarizability

5.39369—60 7 81
between the two states will affect the London dispersion forces which

play the major role in van der Waals binding. On the other hand,

the shifting of charge density from the periphery of a molecule to the

interior or vice versa will in many cases seriously affect the energy

difference between two electronic states. Such effects have been

demonstrated as being mainly responsible for the solvent shifts

encountered in cases where hydrogen bonding occurs [30]. In such

cases red shifts or blue shifts are observed, depending on the direction

of the charge shifts which accompany the electronic transition.

Similar considerations should apply in the condensed matrix case but

it does not seem clear so far what the criteria are which determine

if an attractive or a repulsive effect is to be expected for the electronic

interactions between trapped species and the matrix.

3. Electron Spin Interactions

The electron spin resonance spectra of a number of trapped radicals at low temperatures have been reported. Of these the best identified are hydrogen, [7, 21, 22, 23, 45, 26], deuterium, [20, 45, 26], nitrogen,

[7,

8,"^ 14, 21, 43, 44], methyl, [21, 39, 43], and NH2, [15]. Since the

spectra of the last two are not known in the gas phase, it is difficult to compare and draw conclusions as to the matrix interaction but the

three atomic species have been the subjects of a number of theoretical

analyses which give information on this point. A table of hyperfine

structure values and ^-factors of radicals in different matrices is given

by Jen et al. [21], and is reproduced here with some additions in table 1. Table 1. g-fadors and hyperine coupling constants A of radicals in various matrices

Matrix QJ Deviationof A from

free value

2.002256(24) 2.00230 (8) 2.00220 (8) 2.00207 (8)

2. 0022

2. 0024

2. 0024

Mc/sec

1420. 40573(5)

(20)

(40)

(32)

2.00244 (8) 2.00215 (3) 2.00202 (8) 2.00200 (8) 2.00203 (8)

1417. 11 1413. 82 1411.09

1407 1415

1423

327. 3843

326. 60 (23)

10. 45 (2) 11. 45 (8)

12. 08 (12)

13. 54 (22)

-0.23

-0. 46 -0. 66 -0.9

-0. 35 +0.2

-0. 24 +9.6

+15.6

+29.5

2.00266 (8) 2.00203 (8) 2.00242 (8)

65. 07 (13)

64. 64 (20)

64. 39 (18)

a Numbers in parentheses following values of and A indicate the last digit accuracies of these quantities where available. 3.1. ^-factors

The ^-factors are all in agreement with those obtained from atomic

beam experiments within close limits. However, there are very

small variations and Jen et al. note that gj decreases with increasing

binding energy of the matrix.

82
3.2. Hyperfine Structure

The hyperfiiie structure factors observed for trapped hydrogen and

deuterium atoms deviate from the atomic beam value by less than

1 percent except for the case of ice matrices as reported by Smaller

et al. [26, 40]. For the most part, the deviations from the free atom

values are negative, phosphoric acid matrix being an exception in this

respect, the deviation there amounting to -t-0.2 percent. Very

recently Adrian [1] has reported a theoretical calculation of the effect of van der Waals interactions and overlap forces on the hyperfine

splittmg constant of hydrogen atoms in hydrogen and argon matrices.

His results are in good qualitative agreement with the observations.

For nitrogen atoms trapped in inert matrices, the relative deviations

of the hyperfine structure constant from the atomic beam value is larger, ranging from 9.6 percent in hydrogen to 29.5 percent in methane, but the absolute deviations are actually smaller. Jen et al. [21] discuss and compare these effects for hydrogen and nitrogen

atoms and give a very satisf^dng qualitative picture on which a

successful congelation and perhaps future theoretical work can be

based. These authors stipulate that the reason for the negative

deviations of the hyperfine structure constant for hydrogen, as opposed to the positive de\'iations observed for nitrogen atoms

can be understood in terms of the dift'erence in the electron density

of the free electrons near the nucleus. In the case of hydrogen, the

unpaired electron is in a Ls-orbital which has high density near the

nucleus and therefore has a large h.f.s. constant. Any perturbation which would cause admixture of a higher p-orbital would

tend to reduce this electron density since a pure p-orbital has zero density at the origin. As a result, the h.f.s. constant is here re- duced by an external perturbation and the percentage effect will be

small since the free atom value is very large. Conversely, in the case of the nitrogen atom, the unpaired electrons are in nearly pure ^-orbitals and the observed h.f.s. constant for the free atom is very small

since it is onh' different from zero in as much as the unpaired elec- trons are not pure ^-orbitals but have slight .s-character. External

perturbations would be expected to increase the admixture of higher

S'-orbitals thus increasing the ''spin density" at the nucleus and

with it the h.f.s. constant. In this case the eft'ect will be large, per- centage-wise since the atomic beam value is very small.

Smaller. Matheson, and Yasaitis [26, 40] have reported observation of an electron spin resonance spectrum characteristic of hydrogen atoms after irradiating ice with 7-radiation. This spectrum

consisted of the expected doublet except that the hyperfine separation

between them was only }U that of the free atom. They repeated the

experiment with D2O and observed a triplet, again having a similarly reduced separation. This observation is the only one which

de\'iates from the pattern discussed so far. They propose a model to explain the giTat reduction in hyperfine splitting constant based on

the introduction of an effective dielectric constant. Livingston,

Zeldes. and Taylor [23] tried to repeat the experiment and failed. They

note, however, that their work on concentrated and dilute acids

leads them to believe that in the limit of pure ice, the h.f.s. constant

should still be close to the free atom value. These authors seem

justified in their criticism of the model proposed by Smaller et al. to explain their anomalous results and until further work is done the

83
observations must be considered as unexplained. The intensity

distribution reported by them in their deuterium spectrum is also

anomalous. They report a distribution in the triplet of 1 :2:1 whereas

a ratio of 1:1:1 is expected and observed by other authors, although

other observers [21] have reported a more intense center line due to a

signal obtained from 7-irradiated quartz.

3.3. Fine Structure

The most elegant chapter to date on the subject of external field perturbations on trapped free radicals has been written by Cole and

McConnell [9]. They have treated the crystal field perturbation of a nitrogen atom in a molecular nitrogen crystalline matrix. In zero

field they find that the electron spin components of the ^Sy^ ground

state of the nitrogen atom are split by the crystal field by an amount

equal to 2D where D is the zero field splitting constant. The two components of electron spin of }^ and are then further split by the magnetic field into two components each. The energies of the four states are

also dependent on the angle between the magnetic field and the crystal axes and the anal^^sis of the problem shows that one of the three

expected transitions (s2=}^<-^— )0 is independent of this angle

whereas the other two (s2=^^^->}^ and ^2= — }^<-^— ^0 are orientation dependent. For a polycrystalline assembly it is found then that

the first transition will be very intense, since all its intensity is con- centrated at one field value. On the other hand the other two transitions will have their intensity smeared out with weak maxima atdzZ^.

Hyperfine coupling with the nucleus of spin 1 leads to the expectation

that the spectrum will consist of three intense lines and a weak triplet on either side of them. In fact Cole, Harding, Pellam, and Yost

[22] had reported such weak satellites in the electron spin resonance spectrum of nitrogen atoms trapped in a nitrogen matrix, and since

then other workers [21, 9] have made the same observation. Actually

only four of the six predicted weak lines have been observed, the other

two being hidden by the outer pair of the central intense triplet with

which they coincide. Cole and McConnell [9] have actually succeeded

in demonstrating that the intensity of the outer pair of the central

triplet is enhanced relative to the central line due to this coincidence.

The zero field splitting parameter D was determined to be 31 ±6 Mc.

Wall, Brown, and Florin [44] have observed similar satellites with

N^^. In an argon matrix, however, they did not observe them and

conclude from this that the nitrogen atoms are situated in glassy

regions in the matrix. Jen et al. [21] report that the appearance of

the satellites depends on the rate of deposition of the matrix, fast deposition leading to their appearance while slow deposition does not. This observation confirms the theory of Cole and McConnell since

fast deposition has been observed to lead to higher degree of order in the matrix whereas almost amorphous matrices have been observed on slow deposition at very low temperatures.

The failure to detect some free radicals by electron spin resonance

techniques has been suggested to be due to possible crystal field perturbations. Jen et al. [21] propose that it may not have been

possible to detect trapped oxygen atoms due to crystal field splitting

causing a nondegenerate ground state. They point out, however,

that this is only possible in even-numbered electron systems due to Kramer's theorem. This explanation, they continue, cannot be in- voked to explain their failure to detect chlorine atoms. It may be

84
that ^-electrons are yery seriously perturbed b}" a matrix and the resonance spectrum is so broadened as to be undetectable.

3.4. Neighboring Nuclear Spin Interaction

Zeldes and Livingston first reported weak satellites in the spin resonance spectrum of hydrogen atoms in solid acids. They found

that the separation of the weak lines from the main features was

35.1 Mc which compares with 36.12 Mc for the proton magnetic

resonance as reported in the literature. Zeldes and Livingston [45]

therefore proposed that the satellites were due to simultaneous spin

flips of neighboring protons, caused by weak magnetic dipole-dipole

coupling between the electronic spin of the electron and the nuclear

spin of the proton. The intensity ratio between the satellites and the

main lines was reported to be about 1 :35 and this value could be fitted to a rough theory to give a distance to the neighboring proton. This

theory has been refhied by Trammell, Zeldes, and Livingston [41] and

the distance calculated from the intensity ratio was 1.8 A, a very

reasonable value. These authors also give generally applicable for- mulas for similar events involving other systems. Livingston,

Zeldes, and Taylor [22] point out that the hydrogen atom must be at

rest in order for the phenomenon to be observed. This is therefore

the only really direct evidence for impeded diffusion of a free radical

in a matrix. Jen et al. [21] report two pairs of weak satellites in the

electron resonance spectrum of atomic deuterium in deuterium molecular crystal matrix and one pair as discussed above for atomic

hydrogen in solid molecular hydrogen crystal. The second pair has

a separation from the main line of exactly twice that of the first pair

and has been mterpreted as representing a simultaneous deuteron

nuclear spin transition of AMi=2 induced by quadrupole coupling

arising from the quadrupole moment of the deuteron. These authors

also discuss the possibility of a spin flip of one proton in view of the

fact that this would result in the conversion of an ortho-hydrogen to a para-hydrogen or vice versa. They conclude that this is not likely

and believe that the electron spin transition could cause both protons

of an ortho-hydrogen to undergo spin flips. Their scheme is not

clear, however, and the problem they raise cannot be considered to have been solved satisfactorily.

3.5. Line Width

The line widths of electron spin resonance spectra of free radicals

in matrices has not been treated in any detail. It is possible that

this parameter could give information concerning the interactions of the trapped species with the environment. Smaller, Matheson, and

Yasaitis [40] have interpreted their observed line widths for hydrogen

and deuterium in terms of interaction with neighboring nuclei. Jen

et al. [21] have discussed line width in terms of a number of parameters

and conclude that it is mainly due to anisotropic broadening in the

polycrystalline assembly of which they believe their matrices to consist.

4. Nuclear Spin Species Conversion

It seems worth while to add a word about nuclear spin conversion

at low temperatures, since this process may depend on interaction

85
with paramagnetic species in the environment. It has been shown

that conversion does take place allowing thermal equilibrium to be

established at low temperatures with remarkable efficiency in NH2

trapped in a matrix [35]. On the other hand, this does not seem to be

so for the methyl radical. The intensities of the hyperfine components

in the electron spin resonance have been observed to be in the ratio

1:3:3:1 [21,39, 43] and they would be predicted to be of equal intensity

if only one nuclear spin species were present. McConnell [25] has,

however, suggested that serious hindering of the rotation could be

responsible for this result. The problem of interconversion between

different nuclear spin species of molecules is a very basic one and

deserves serious attention. It is also possible that the efficient interconversion observed for NH2 is caused by intramolecular effects such

as the presence of the nuclear spin of the nitrogen nucleus.

The author acknowledges many stimulating and illuminating dis- cussions with Dr. K. Dressier in the course of the preparation of this

review and also is indebted to Drs. J. L. Franklin and H. P. Broida

for allowing the use of their, as yet unpublished, review paper.

5. References

[1] F. J. Adrian, Bulletin of the A.P.S., Series II, 4, No. 3 (March 1959). 1959 March Meeting at Cambridge, Mass., Paper KA 1. [2] E. D. Becker, G. C. Pimentel, and M. Van Thiel, J. Chem. Phys. 26, 145

(1957).

[3] H. P. Broida and M. Peyron, J. phys. rad. 19, 480 (1958).

[4] II. P. Broida and M. Pevron, to be pubUshed.

[5] H. P. Broida and M. PeVron, J. Chem. Phvs. 30, 139 (1959).

[6] E. Catalano and D. E. Milligan, J. Chem. Phys. 30, 45 (1959).

[7] T. Cole and J. T. Harding, J. Chem. Phys. 38, 993 (1958).

[8] T. Cole, J. T. Harding, J. R. Pellam, and D. M. Yost, J. Chem. Phvs. 37, 593 (1957).

[9] T. Cole and H. M. McConnell, J. Chem. Phys. 39, 451 (1958).

[10] K. Dressier, to be published.

[11] K. Dressier, to be pubhshed.

[12] K. Dressier and D. A. Ramsay, J. Chem. Phys. 37, 971 (1957).

[13] K. Dressier and O. Schnepp, to be published.

[14] S. N. Foner, C. K. Yen, E. L. Cochran, and V. A. Bowers, J. Chem. Phvs.

38, 351 (1958).

[15] S. N. Foner, E. L. Cochran, V. A. Bowers, and C. K. Jen, Phys. Rev. Letters

1, 91 (1958).

[16] H. Goldberg and C. M. Herzfeld, Bui. A.P.S. Series II, 3, No. 3, 180 (May

1, 1958).

[17] G. Herzberg, Spectra of diatomic molecules 559-560 (D. Van Nostrand Co.,

Inc., New York, N.Y., 1959).

[18] C. M. Herzfeld and H. P. Broida, Phys. Rev. 101, 606 (1956).

[19] C. M. Herzfeld, Phys. Rev. 107, 1239 (1957).

[20] C. K. Jen, S. N. Foner, E. L. Cochran, and V. A. Bowers, Phys. Rev. 104,

846 (1956).

[21] C. K. Jen, S. N. Foner, E. L. Cochran, and V. A. Bowers, Phys. Rev. 113, 1169 (1958).

[22] R. Livingston, H. Zeldes, and E. H. Taylor, Phys. Rev. 94, 725 (1954^.

[23] R. Livingston, H. Zeldes, and E. H. Taylor, Disc. Faraday Soc. 19, 166

(1955).

[24] M. McCarty, Jr. and G. W. Robinson, Mol. Phys. 3, 415 (1959).

[25] H. M. McConnell, J. Chem. Phys. 39, 1422 (1958).

[26] M. S. Matheson and B. Smaller, J. Chem. Phys. 33, 521 (1955).

86
[27] R. M. Hexter and D. Milligan, Proceedings of the Fourth International SYmposiiim on Free Radical Stabilization, National Bureau of Standards,

Washington, D.C. (August 1959).

[28] I. Xormanand G. Porter, Proc. Roy. Soc. (London) A230, 399 (1955).

[29] H. S. Peiser, in Formation and trapping of free radicals, Chpt. 9, A, M. Bass and H. P. Broida, eds. (Academic Press, New York, N.Y., 1960).

[30] G. J. Brealev and M. Kasha, J. Am. Soc. 77, 4462 (1955).

[31] D. A. Ramsay, J. Chem. Phvs. 25, 188 (1956).

[32] D. A. Ramsay, Mem. Soc. Rov. Sci. Liege 18, 471 (1957).

[33] G. W. Robinson and M. McCartv, Jr., J. Chem. Phvs. 28, 349 (1958).

[34] G. W. Robinson and ^I. McCarty, Jr., J. Chem. Phys. 28, 350 (1958).

[35] G. W. Robinson and yi. ]\IcCarty, Jr., Faraday Soc. Informal Discussion on

Free Radical Stabilisation, Sheffield University (Sept. 1958). Summaries

of Papers, p. 58.

[36] G. W. Robinson and M. McCartv, Jr., J. Chem. Phys. 30, 999 (1959),

J. Am. Chem. Soc. 81, 4472 (1959).

[37] O. Schnepp, to be published.

138] L. J. Schoen and H. P. Broida, to be pubhshed.

[39] B. Smaller and M. S. Matheson, J. Chem. Phys. 28, 1169 (1958).

[40] B. Smaller, M. S. :Matheson, and E. L. Yasaitis. Phvs. Rev. 94, 202 (1954).

r41] G. T. Trammeh, H. Zeldes. and R. Livingston, Phvs. Rev. 110, 630 (1958).

[42] M. Van Thiel, E. D. Becker, and G. C. Pimentel, J. Chem. Phvs. 27, 95

(1957^.

[43] L. A. Wall, D. W. Brown, and R. E. Florin, J. Phvs. Chem. 63, 1762 (1959).

[44] L. A. Wall, D. W. Brown, and R. E. Florin, J. Chem. Phvs. 30, 602 (1959).

[45] H. Zeldes and R. Livingston, Phys. Rev. 96, 1702 (1954). Washington 25, D.C, November 18, 1959.

87

9. Summary of the Fourth International Symposium

on Free Radical Stabilization

Maurice W. Windsor^

1. Introduction

The Fourth International Symposium on Free Radical Stabilization, sponsored by the National Bureau of Standards, was held in Washington, D.C., from August 31 to September 2, 1959 at Dunbarton

College. The first of the Symposia on Free Radicals took place in 1956 and was organized by Professor P. A. Giguere of Laval University,

Quebec. It was apparent that there was considerable interest in the

general topic of free radicals and a second symposium was held at the

National Bureau of Standards in Washington, D.C., in September 1957.

By that time, a 3-year program on the stabilization of free radicals,

sponsored by the Department of Defense,had been instituted under the

direction of Dr. H. P. Broida. Following in the now established

tradition, Professor G. Porter arranged for a Faraday Society informal

discussion on free radical stabilization early in September 1958 at

Sheffield University in England. The decision to hold the fourth

SATuposium in Washington, D.C., was largely based on the fact that

the Free Radical Program at the National Bureau of Standards was

scheduled to end in September 1959 after running its term of 3 years. Holding the meeting in Washington would then give overseas visitors

and other visitors the opportunity to inspect the research taking place

in the program prior to its termination. The decision was made to hold the fifth symposium at Uppsala, Sweden in July 1961.

The title of the Fourth Symposium was ''Trapped Radicals at Low Temperatures". The number of papers (29) in no way re- flected the very considerable growth of interest in the subject over

the past 3 years. To prevent the program from becoming unwieldy

and to maximize the participation of workers outside the National

Bureau of Standards, no papers had been accepted from members of the National Bureau of Standards program. Instead, their work was

described in several survey papers. Two such papers were devoted to surveying respectively the experimental and the theoretical aspects of the National Bureau of Standards program. The other survey papers, while placing emphasis on a review of work done at the National

Bureau of Standards, had a somewhat broader scope and attempted to define the state of accomplishment in a number of areas which also served as the theme for each session. These were as follows:

(1) Low-Temperature Chemistry;

(2) Methods of Production of Trapped Radicals, and Physical

Properties of Radical-trapping Solids;

(3) Identity and Concentrations of Trapped Radicals ; and

(4) Interactions of Free Radicals with Solids.

In addition to the survey papers, 22 contributed papers, grouped

under the general headings given above, were presented plus 6 five- minute papers describing recent work.

1 Guest scientist from Space Technology Laboratories. 89
Except for the surveys, which were allotted 30 min, each author was given 10 minutes to highlight the important points made in his

paper. The success of a symposium may be measured largely by the

amount of discussion that takes place. This depends quite crucially on the amount of time available for discussion. The various session

chairmen deserve the utmost praise for keeping the meeting on

schedule.

For those interested in statistics, the distribution of papers by

(a) author's country, and (b) the techniques used are given in tables

1 and 2. In all there was a total of 318 registrants from 12 countries.

In addition to the technical sessions, a round table discussion was

held on 'Tuture Trends in Trapped Radicals Research". Giguere

(Laval University) presided over a panel comprised of: FernandezMoran (Massachusetts General Hospital) ; Pimentel (University of

California, Berkeley) ; Rice (Catholic University) ; Broida (National

Bureau of Standards) ; and Porter (Sheffield University) . There was

also a program of sightseeing for the ladies and a symposium banquet.

The guest speaker at the banquet was Mr. Lewis Strauss who, in an

address entitled '^A Lilliputian in Laputa", reminisced on his past

association with such scientific giants as Albert Einstein, Enrico Fermi,

Ernest Lawrence, Leo Szilard, and John von Neumann.

Table 1. Disirihution of papers hy country {including 5-min papers).

Canada 2 France 2

Israel 1 Sweden 1 UK 4 USA - 22 USSR... 2

Table 2. Distribution of papers among techniques used

Liquid nitrogen temperatures 15 Liquid helium temperatures 9 Electron spin resonance 10 UV spectro-scopy 5 IR spectroscopy 3 Thermometric and calorimetric 3 Microwave discharge 3 UV photolysis 8 X-rays 1 7-rayS-, 6 Atomic beams 1 Strong magnetic fields 2

An exhibition of glass and metal Dewars developed during the

National Bureau of Standards program was arranged by Hofman

Laboratories and by H. S. Martin Company, and a display of publications was put on by the National Bureau of Standards. Also at- tracting attention were the galley proofs of a much anticipated book.

The Formation and Trapping of Free Radicals, to be published shortly

by Academic Press, edited by Bass and Broida, with chapters contributed by several members of the Free Radicals Program, and ranging

in scope from fundamentals to the importance of trapped radicals in astrophysics, biology, and propulsion.

90
2. Flavor of the Symposium

Seen in overview, this year's symposium had a decidedly more

fundamental flavor than those held in previous years. This was

reflected by the decreased niunber of participants from industry compared to universities and government laboratories. In particular,

there was only passing mention of trapped radicals in the fields of astronomy, biology and rocket fuels, all three of which had papers

devoted to them at earlier meetings. It has become apparent since

then that, discounting the possibility of radically new discoveries,

trapped free radicals are not the answer to the missile-man's prayer.

One cannot blame the rocketeers for being disenchanted by the pros- pects of harnessing the energy (all 1 kcal/mole of it) of a fraction of

a percent of nitrogen or hydrogen atoms trapped in a solid which has

to be kept at liquid helium temperatures. In the fields of astronomv

and biology probably all has been said that can safely be said and

perhaps a trifle more at the present state of our knowledge.

The dramatic blossoming of the free radicals field has now faded, but

the seeds of interest have been widely disseminated. Those who

came to admire only the flower have gone home again but those whose

interest is more deeply rooted are waiting to see what the seeds will produce. This is all to the good. The study of free radicals and

unstable chemical species is a fascinating subject and has been so for a devoted few for a number of decades.

3. The Technical Sessions

3.1. Session A: The National Bureau of Standards Free Radicals

Research Program

Chairman: R. D. Huntoon (National Bureau of Standards)

(1) Organization and Rationale of the Free Radicals Program—J. W. Mover (General Electric Companv, Santa Barbara,

Calif.).

(2) The Experimental Aspects of the National Bureau of Standards

Free Radicals Research Program—M. D. Scheer (National

Bureau of Standards).

(3) A Survey of the Theoretical Work on Trapped Radicals at the

National Bureau of Standards—^C. M. Herzfeld (National

Bureau of Standards).

This session was devoted to reviewing the National Bureau of Standards program. Mover (General Electric Company) covered the

organization and rationale of the program; Scheer (National Bureau of Standards) covered the experimental research program; and Herzfeld

(National Bureau of Standards) covered the theoretical aspects of the

program. Moyer described how the National Bureau of Standards

Free Radicals Program was conceived as an experiment in research

management in which about 30 senior scientists of proven ability would

be gathered together and allowed to determine their own research program. Red tape was cut to a minimum and a horizontal-type organization of staff itself reduced formality and encouraged collaboration

and interchange of ideas. By such means it was hoped to produce

a creative and stimulating atmosphere with few constraints—^an environment which should keep a varied group of scientists happy and

91
imbued with a high morale. Participants were drawn from industrial

organizations and from universities, both here and abroad. The

National Bureau of Standards was chosen as the site for the program

because of the unique facilities it had to offer in the way of physical

plant, services, and supply of liquid helium, and the '^neutral" at- mosphere favorable to industrial participation. The program was

directed by Dr. Broida as section chief and Dr. Bass as assistant chief. Those responsible are to be congratulated on foreseeing that the

major purpose of a program such as this was not to produce exotic

propellants, nor yet even to make studies of their feasibility, but rather

to pioneer the opening up of an exciting and provocative frontier of science. The major product of the National Bureau of Standards Free

Radicals Research Program, underlined by the more than 100 research

papers produced, has been kiiowledge—knowledge of how to make free radicals, knowledge of the conditions under which they may be stored,

of their identity, their concentrations, their properties and their re- actions, and finally and perhaps most important of all, knowledge of

how to do chemistry at temperatures close to the absolute zero of temperature. And in addition to knowledge has come also a host of new

questions, new problems, and new ideas. And that is as it should be. In his review of the experimental program, Scheer (National Bureau

of Standards) said that 0.1 percent appears to be a typical figure for

the maximum concentration of radicals that can be stabilized at low

temperatures. Larger amounts are very unlikely.

Of the various processes involved in the production and stabilization

of free radicals, probably the most complex and least understood is the

process of deposition. Except for the recent studies of Baurer, Foner

and Mauer, and Windsor, little work has been done in this area. From

this work one concludes that large differences in temperature can exist

between the cold substrate and the growing surface of the deposit, es- pecially when the solid is disordered by the presence of atoms, radicals or foreign molecules and has in consequence a thermal conductivity

several thousand times less than that of the perfect crystal. The true temperature of deposition thus becomes, by reason of the drastically

lowered conductivity, a very sensitive function of the flow rate. The experiments of Schoen, Schnepp, and Dressier were cited on

the perturbations of atomic and molecular spectra caused by various

matrices and the useful information about the local environment to which this can lead.

According to Scheer, the following areas deserve increased attention

in the future:

(1) Deposition process,

(2) Low temperature chemistry,

(3) Interaction of trapped species with the matrix,

(4) Kinetics of crystallographic changes at ver}^ low temperatures.

Herzfeld (National Bureau of Standards) praised the close interaction between experimentalists and theorists within the Free Radical

Program. He highlighted the following topics: deposition, emission

and absorption of energy, and disappearance of free radicals. The

common weakness of all the deposition theories is that they ignore

temperature gradients and diffusion. The statistical theories lead to predicted trapped concentrations between 5 and 17 percent depending on the coordination number. These are all upper limits. The comparison with experiments is not good—maximum concentrations ex- perimentally observed being of the order of a few hundredths of 1

92
percent. Jackson's dynamic theory comes closer to experimental real- ity in predicting a maximum stable concentration of the order of 0.7 percent. In the field of spectroscopy, considerable success has been obtained

in accounting for the nitrogen afterglow. The mechanism by which

the excited atoms are produced is still poorly understood. Herzfeld

concluded that the two most important new fields are:

(1) The theory of chemical condensation,

(2) The photochemistr}^ of solids. 3.2. Session B: Low-Temperature Chemistry

Chairman: G. Porter (Sheffield University)

(1) Hydrogen Atom Reactions with Toluene—R. B. Ingalls (Atomics International).

(2) Low-Temperature Irradiation of Mixtures of HBr and C2H4

D. A. Armstrong and J. W. T. Spinks (University of Saskatchewan).

(3) Radical Reactions at Low Temperatures as Studied by ESR—

M. Fujimoto, D. J. E. Ingram, and M. C. Saxena (University

of Southampton, England).

(4) Kinetic Studies of the Disappearance of Hydroxyl Radicals in Ice at Low Temperatures—J. M. Flournoy, L. H. Baum,

S. Siegel, and S. Skolnik (Aerojet-General Corporation).

(5) Free Radical Conversions in Solids—V. V. Voevodsky (Academy of Sciences, Moscow).

(6) Review: Low-Temperature Chemistry—R. Klein (Olin-Mathieson Chemical Corporation and National Bureau of Standards).

Low-temperature chemistry is the study of chemical reactions

below an arbitrarily specified temperature of 150° K. Below this temperature, only reactions with activation energies of a few kilo- calories per mole or less proceed at a measurable rate. This simplifies

the system by greatly restricting the class of possible reactions. Also,

small differences in activation energy will cause measurable changes

in observed rates. The disadvantage is that the processes of escape

from the solvent cage and diffusion through the solvent may well

become rate controlling steps. Klein (Olin-Mathieson) and Scheer

(National Bureau of Standards) deserve praise for opening up this

field by their studies of H-atom addition reactions to solid olefins in the region of 77° K. Ingalls (Atomics International) described

similar work in which he studied the addition and abstraction reactions between deuterium atoms and liquid toluene between —88°

and -79° C.

Another way of initiating chemical reactions at low temperatures

is by gamma-irradiation of a mixture. This was done by Armstrong

and Spinks (University of Saskatchewan) who studied the addition

of HBr to C2H4 both in the liquid just above its melting point at -165° C and in the solid at -196° C.

Fujimoto, Ingram, and Saxena (Southampton University) photolyzed H2O2 in hydrocarbon glasses at 77° K and attributed the

observed changes in ESR spectra to reactions of OH radicals with

the substrate to produce secondary radicals. In the case of OH

radicals produced in ice by gam.ma-irradiation, Flournoy, Baum,

Siegel, and Skolnik (Aerojet-General Corporation) found that the

93
rate determining process for disappearance of the ESR spectrum was

migration of the radicals through the ice. In the ensuing discussion there was controversy over the assignm.ent

of the ESR spectrum to the OH radical. Both Livingston (Oak

Ridge) and Foner (APL) stated that in polycrystalline specimens,

crystal anisotropy would smear out the spectrum, and that an unambiguous assignment could be made only by careful work using single

crystals.

The ''cage" effect came in for considerable discussion. Pimentel

(University of California, Berkeley) believed that for methyl radicals

at 20° K in a CD4 matrix, energies of 3 or 4 times the activation energy

for D atom abstraction were necessary to overcome the cage effect. However, McNesby (National Bureau of Standards) pointed out

that this could be accounted for by the ambiguities in the m.agnitude

of the activation energy and by a reasonably large steric factor. In

addition, Willard (University of Wisconsin) and Cochran (APL)

presented evidence for decreased quantum yields in glassy solids at low temperatures which they attributed to the cage effect. To sum up, it appears that, while hydrogen atoms can readily

diffuse through a solid film at 77° K, for larger radicals, such as iodine

atoms or methyl and ethyl radicals, the process of escape from the

solvent cage and diffusion through the solvent environment may well

become rate controlling.

In the field of techniques, Thomas (Shell Research Ltd. and National Bureau of Standards) has developed a rotating Dewar for use

at liquid helium temperatures. By depositing two reactants on the

rotating drum from, jets on opposite sides of the cylinder, a kind of

helical sandwich roll can be built up. It is hoped that in this way

secondary reactions between the reactants can be largely prevented

The Low-Tem.perature Chemistry Session ended with a survey paper

by Klein (Olin-Mathieson and National Bureau of Standards) which

summ.arized National Bureau of Standards work in this field. 3.3. Session C: Methods of Production of Trapped Radicals

and Physical Properties of Radical Trapping Solids

Chairmen: R. C. Herman (General Motors Corp.)

S. Claesson (Uppsala University, Sweden)

(1) Studies of Some New Matrices for Radical Stabilization—

H. T. J. Chilton and G. Porter (University of Sheffield,

England)

(2) A Calorimetric Study of Trapped Radicals Produced by Gamma

Radiation—R. L. Arnett, E. D. Guth and J. R. Berreth

(Phillips Petroleum Company, Bartlesville, Oklahoma).

(3) Photolysis of Chromate, Permanganate, and Related Compounds in Rigid Media at Low Temperatures—U. Klaning

and M. C. R. Symons (University of Southampton, England).

(4) High Resolution Infrared Spectroscopy of Solids: Intermolecular Interactions in the Solid State; Extent of Freedom of Rotational Motion of Small Molecules in Inert Gas Matrices

R.'M. Hexter and D. Milligan (Mellon Institute, Pittsburgh),

(5) Atomic-Beam Studies on the Formation of Free-Radical

Sohds—R. T. Brackmann and W. L. Fite (General Atomics,

La Jolla, California).

(6) Review: Methods of Production of Trapped Radicals and

94
Properties of Radical-Trapping Solids—A. Thomas (Shell

Research, Chester, England, and National Bureau of Standards)

In the past few j^ears it has become apparent that the physical

properties of the matrices used for trapping radicals are extremely important. This includes not only such obvious factors as rigidity and

chemical inertness but less obvious ones such as specific crystalline

structure, thermal conductivity, molecular weight, and the velocity of sound in the matrix materials.

Porter and Chilton (Sheffield University) described a novel attempt

to stabilize aromatic radicals in a matrix of KCl. The work was done

at about —110° C and only fairly stable radicals such as diphenyl

methyl could be trapped. It appears that trapping occurs actually in microcrystals of the solute rather than in the matrix itself. Arnett (Phillips Petroleum) described calorimetric work on gammairradiated samples of ammonia, argon, and nitrogen at 4.2° K. Measurements showed that only a few percent of the energy put in by

gamma-irradiation was stored in the solid. He confirmed the curious

effect noted earlier by other workers in the field that stored energy was

found to be released at temperatures below that at which it was stored.

Klaning and Symons (Southampton University) described the photolysis of anions, such as permanganate, in alcohol solvents at 77° K.

The observed reaction leads to tervalent manganese and oxygen and

they believe that the primary step involves four electrons and the

extrusion of a molecule of O2. By observing the changes in high resolution infrared spectra of small

molecules, such as H2O, NH3, and CH4, in rare gas matrices at 4° K,

Hexter and Milligan (Alellon Institute) have obtained information on

the freedom of rotational motion in rare gas matrices at 4.2° K.

Brackmann and Fite (General Atomic) described experiments on

trapping hj^drogen atoms at liquid helium temperatures using an atomic beam technique. With a beam of intensity 10^"^ to 10^^ particles per second, no evidence was obtained of stabilization of atoms even when a spin-alined beam was used. However, at flow rates of about

10^* particles per second, periodic energy releases during the condensation of hydrogen occurred. Later experiments showed that considerable reflection of a mixture of H atoms and H2 molecules occurred from

a surface even at temperatures as low as 2.2° K.

During the discussion of the effects of the matrix on the trapped

molecule, Dressier (National Bureau of Standards) described a spectroscopic observation of the cage effect, in which changes in the Schumann-Runge absorption bands of oxygen are observed in an argon

matrix. The importance of the thermal conductivity of the matrix was emphasized by both Windsor and Herzfeld who agreed that at

liquid helium temperatures the disorder introduced in a lattice by the

presence of trapped atoms and radicals could well reduce the thermal

conductivity by several orders of magnitude due to the scattering of phonons that would take place. Windsor described experiments similar to those of Brackmann and Fite in which energy pulses and pressure

pulses had been observed during the deposition of discharged streams

of hydrogen, deuterium, and nitrogen. He emphasized that the temperature of the growing surface of a deposit may be several degrees

above that of the substrate and attributed the temperature pulses

to the periodic recombination of atoms which took place when the

growing surface of the disordered deposit reached a critical tempera95
ture. In the case of molecular hydrogen the observed temperature

pulses are attributable to a small amount of water vapor impurity.

It was generally agreed that we need to know much more about the

physical properties of the matrices used for trapping atoms and free radicals. Fortunately, studies of the effects of the surrounding molecules on the spectra of trapped atoms and radicals provide us with a powerful tool for learning about the structure of the matrix.

3.4. Session D : Identity and Concentrations of Trapped Radicals

Chairman: G. Pimentel (University of California)

(1) ESR Spectra of Trapped Radicals Produced by Photolysis of Group IV and V Hydrides—E. L. Cochran (Applied Physics

Laboratory, Silver Spring, Maryland).

(2) Isolation and Identification of New Oxides of Nitrogen—H. A.

Bent (University of Minnesota).

(3) Free Radicals in Organic Solids—R. L. McCarthy, R. G.

Bennett, E. J. Burrell (E. 1. DuPont de Nemours, Inc., Wilmington, Delaware)

(4) Electron Spin Resonance Spectroscopy of Radicals Formed by

X-irradiation of Amino Acids and Related Compounds-

C. A. McDowell and A. J. Horsfield (University of British

Columbia)

(.5) Experiences with Sulfur Radicals Trapped at 77° K—S. Sunner

(University of Lund, Sweden)

(6) Survey of the Status of Research on Identity and Concentrations of Trapped Radicals—R. Zwanzig (National Bureau

of Standards)

Spin resonance played a large part in the work described at this

session. Cochran (APL) described experiments in which hydrides of carbon, silicon, nitrogen, and phosphorus were photolyzed at high di- lutions in an argon matrix. There does not seem to be much doubt

about the assignment of the spectra of the simpler radicals such as CHg,

CD3, and SiHg. The spectrum obtained for SiHg is identical with an

earlier unknown spectrum obtained by many workers with an electrical discharge in hydrogen, thus confirming the idea that silane was

being formed in small amounts by the reaction of hydrogen atoms

with the glass or quartz walls of the discharge tube. Burrell (DuPont)

described the observation of free radicals produced by gamma-irradiation of organic solids. The importance of using single crystals in ESR

work was emphasized by McDowell and Horsfield (University of

British Columbia) who described experiments on the X-irradiation of amino acids and related compounds. The complex spectra obtained were strongly dependent on the crystal orientation. They concur with the interpretation of Ghosh and Whiffen that the observed spectra

are probably due to NH+g, CH2CO-2.

In addition to radicals, thermodynamically unstable new forms of molecules may often be trapped in films frozen at 4.2° K. Bent (University of Minnesota) has evidence for several new oxides of nitrogen

from infrared studies. The spectra of the frozen oxides were very

sensitive tQ choice of matrix and dilution ratio, window temperature

and the rate of deposition. Self matrices and matrices of argon,

nitrogen, oxygen, carbon dioxide, nitrous oxide, and hydrogen were

used. In the case of NO2 at high dilution the spectrum of the monomer

is obtained. However, during warmup dimerization occurs and

several forms of the dimer have been isolated.

96
Simner (University of Lund, Sweden) described attempts to measure

recombination energies of organic radicals trapped in an organic matrix

at 77° K. Aliphatic disulphides were chosen and the radicals were produced by photolysis. A new absorption band attributable to the

thiyl radical was observed and at the same time the disulphide spectrum

decreased in intensity. However, the primary reaction products appeared to react with the matrix material (mixtures of 3-methyl pentane

or method cyclohexane in six parts of isopentane) even at 77° K.

During the discussion, several people emphasized that the very

sensitivity of the spin resonance technique brings with it a considerable lack of specificity and ambiguity in assignment. In addition to

single crystal studies, therefore, gross chemical analysis by standard

techniques of samples studied by spin resonance is to be strongly

recommended.

In his review of this field, Zwanzig (National Bureau of Standards)

discussed the various techniques which have been used for the indentification of trapped radicals and for estimating their concentration.

Warmup and heat release experiments agree with ESR in giving concentrations of a few tenths of a percent. Fontana obtained a similar

figure for nitrogen atoms by a magnetic susceptibility method.

3.5. Session E: Interactions of Free Radicals with Solids

Chairmen: R. Livingston (Oak Ridge National Laboratory)

N. Kurti (Oxford L^niversity)

(1) Multiple Trapping Sites for Hydrogen Atoms in Various Matrices—S. N. Foner (Applied Physics Laboratory, Silver

Spring, Maryland)

(2) Interconversions of Ions and Radicals in Radiolysis—V. L. TaProse and E. L. Frankevitch (Academy of Sciences,

Moscow)

(3) Intermolecular Interactions of Trapped Species with the Solid

at 4.2° K—M. McCarty, Jr., and G. W. Robinson (Johns

Hopkins University).

(4) Isothermal Decay of the a (5229 A)—Radiation from Trapped

Nitrogen Atoms—R. A. Hemstreet and J. R. Hamilton

(Linde Company, Tonawanda, New York).

(5) Magnetic Interactions of Free Radicals at Very Low Temperatures and in Strong Magnetic Fields—^W. E. Henry (U.S.

Naval Research Laboratory, Washington, D.C.).

(6) Review: The Interactions between Trapped Species and the

Matrix—O. Schnepp (Israel Institute of Technology and

National Bureau of Standards).

(7) Summar}^ of the Symposium—M. W. Windsor (Space Technology Laboratories, and National Bureau of Standards)

By studAang the interactions of free radicals with their molecular

environment, one can get information about the structure of the matrix. Foner (APL) described ESR experiments on hydrogen atoms

trapped in various rare gas matrices at liquid helium temperatures.

Perturbations produced b}^ the matrix cause deviations of the hyperfine coupling constant and the electronic ^'-factor from the free atom

value. H-atoms could either be deposited from a discharge or produced photolytically in situ. The spectra for deposited H-atoms could

be explained in terms of a single site which is probably a substitional

site. These same sites plus additional ones also are found in the

539369—60 8 97
pliotolytic experiments. These may be attributable to interstitial

sites, but owing to lattice distortion the effect on the hyperfine spectrum is difficult to calculate. In the case of xenon an additional splitting occurs caused by the magnetic moment of the xenon nucleus.

Pertubations by the matrix also affect the electronic spectra of

trapped species. McCarty and Robinson (John Hopkins University)

observed shifts in the electronic absorption spectra of Hg, NH, and

C2 in rare gas crystals at 4° K. These are explained quantitatively on the basis of a Lennard-Jones potential between the trapped species and the rare gas atoms. Good agreement is obtained except in cases where strong angular dependent forces differing appreciably in the two electronic states are present. Later, Schnepp (Israel Institute

of Technology and National Bureau of Standards) reported similar

experiments with manganese and magnesium atoms in matrices of

argon, krypton, and xenon. Although the spectra obtained lie close to those of the gas-phase in energy, the splitting of orbital degeneracies indicates a high degree of asymmetry in the atomic environment.

Hemstreet and Hamilton (Linde Company) have measured the

decay rates of the alpha lines of the nitrogen atom afterglow in the

range 4° to 30° K. If the temperature be raised in a step-wise

fashion, each increase in temperature causes an increase in glow in- tensity followed by a decay at constant temperature. The glow may

be continually restimulated by raising the temperature and even at 30° K active species still remain trapped, as indicated by the intense

green glow observed when the temperature was raised at the end of

each run. The initial decay rate increases with temperature. After

about 10 min., all the curves tail off to an approximately equal rate. Much discussion on the nitrogen afterglow followed this paper and it was apparent that although the observed spectra correlate well with

known emissions of atomic nitrogen, the mechanism by which the excited atoms are produced in the solid is still very poorly understood.

The two main mechanisms are

(1) Excited nitrogen atoms are trapped during the deposition process. These have a very long lifetime, perhaps longer than 20 hr. During warmup they diffuse to radiative sites. It transpired that the

experiment which might settle this point, namely the storage of a sample for a period much longer than 20 hr, had never been done.

(2) Excited atoms are produced during the recombination process, the energy of recombination of two ground state atoms serving to

raise a third atom to an excited state. To test this mechanism it seems important to know not only the spectrum of the afterglow but

also the spectrum of the flashes which occur during deposition and

during warmup and also the lifetime of the light emission from the

flashes. It is apparent from Hemstreet's work that the nitrogen atoms

trapped in the solid cover a range of energies. Windsor (Space Technology Laboratories and National Bureau of Standards) and Hemstreet (Linde Company) both reported observations of quasi-periodic

temperature pulses which are coincident with the light flashes during

the deposition of discharged nitrogen.

Henry (U.S. Naval Research Laboratory) described an investigation of the magnetization of the free radical diphenyl-picryl hydrazyl

(DPPH) in magnetic fields up to 60,000 gauss and at temperatures in

the liquid helium range. Pressures of 10,000 Ib/in.^ appeared to in- crease the magnetic interaction and may possibly cause recombination

of free radicals.

98
Leach (Laboratory of Chemical Physics, Paris) discussed the effect of rigid media on photochemical processes in the benzene molecule.

Owing to the high symmetry of benzene there may be qualitative

changes in the rigid media due to the effects of external perturbations on the effective symmetry of the molecule.

In his survey paper on the interactions between trapped species

and the matrix, Schnepp discussed the effect of matrix perturbations on the various molecular degrees of freedom.

The symposium ended with a summary by Windsor (Space Technology Laboratories).

4. Summing Up

The subject of free radical stabilization at low temperatures has

outgrown the excesses of adolescence and has now reached the fruitful stage of maturity. Stimulated by the 3-year program at the National

Bureau of Standards, research activity in the field of free radicals will almost certainly continue to increase in the future. In addition to the

research that has been accomplished, much else of importance has also been gained, especially in the fields of arts and techniques, due in large

part to the fine efforts of Stober (National Bureau of Standards). A

large array of experimental Dewars, both metal and glass, has been developed at the National Bureau of Standards and many of these are now available commercially.

Up to the present, and this is quite apparent from a cursory examination of the program of the symposium, most of the work at liquid

helium temperatures has been carried out in the United States. This

is because of the abundant supply of helium from natural sources in

this country and the paucity of such supply abroad, most of what is available being jealously guarded by the low temperature physicists.

However, this situation looks as if it will be remedied in the near

future and we can then expect a worldwide increase in chemical ex- periments in the liquid helium temperature range.

5. The Future

The following is one man's opinion of those areas most deserving of our attention in the future in the field of free radicals research at low

temperatures.

5.1. Mechanism of Deposition and Condensation

The processes occurring when a gas at 300° K condenses on a surface

held at 4° K are very poorly understood theoretically, and there is virtually no experimental information available. It is apparent that

the thermal diffusivity of the deposited solids will play a crucial role,

since it will largely determine the temperature gradient existing between the liquid helium-cooled substrate and the growing surface of the deposit which is in thermal contact with the incoming gas stream.

If the temperature gradient is large, the true temperature of the growing surface may be many degrees above that of the substrate. The

structure and degree of disorder of the condensed solid will therefore

be a function of many variables, including the gas flow-rate, the gas

temperature, the heat of condensation, the thermal diffusivity of the

99
portion of deposit which has already condensed and the thickness of that deposit. And in turn the thermal properties of the deposit will depend crucially upon the degree of disorder. We thus have to con- tend with a nonlinear and highly complex system.

5.2. The Structure of Solids

Before we can properly understand the properties of solids relevant

to the trapping of free radicals and their subsequent recombination

during warmup, we must increase our understanding of the matrix

itself. Strangely enough the study of trapped radicals themselves

promises to give the answers to many of these questions. By using a trapped radical as a probe and observing the effects of the molecular

environment on the energy levels of the radical, as observed by all branches of spectroscopy from the infrared to the ultraviolet and by

spin resonance, much information can be gained about the structure

of the matrix.

5.3. Low-Temperature Chemistry

This is a field so far largely unexploited. In the gas phase at room

temperatures few reactions are simple, most proceeding by a complicated series of steps. At low temperatures and in rigid matrices many

of the secondary reactions are prevented and it becomes possible to study the kinetics of primary processes and measure their energies of

activation. It is also apparent that besides free radicals, thermodynamically unstable new forms of otherwise stable molecules may be

trapped at low temperatures. Besides the intrinsic interest of such

species from the point of view of molecular architecture and bond

energies, there is real hope that the use of low temperatures may en- able the industrial chemist to gain a very nice control of chemical

reactions. Low-temperature chemistry may well mark the beginning

of a new era of precision in chemical synthesis.

Washington 25, D. C, December 16, 1959.

100
10. Bibliography of the Free Radicals Research Program

Published: NBS Papers on Trapped Radicals Prior to 1957

Reference No. Author Title

1 Broida, H. P. Phosphoresence of atoms and

Pellam, J. R. molecules of solid nitrogen at

4.2° K.

2 Broida, H. P. A note on the preparation of Pellam, J. R. solid ozone and atomic oxygen.

3 Bass, A. M. Spectra emitted from solid ni- Broida, H. P. trogen condensed at 4.2° K

from a gas discharge. 4 Broida, H. P. Abundance of free atoms in solid Lutes, O. S. nitrogen condensed at 4.2° from a gas discharge.

5 Herzfeld, C. M. Interpretation of spectra of Broida, H. P. atoms and molecules in solid nitrogen condensed at 4.2° K.

Journal

Phys. Rev. 95,

845-46 (1954).

J. Chem. Phys.

33, 409 (1955). Phys. Rev. 101,

1740-47 (1956),

J. Chem. Phys.

34, 484-85

(1956). Phys. Rev.

101, 606-11

(1956).

6 Broida, H. P.

7 Broida, H. P. Peyron, M.

Dibeler, V. H.

Reese, R. M.

Franklin, J. L. Herzfeld, CM.

10 Herzfeld, C. M.

Bass, A. M.

11 Minkoff, G. J. Scherber, F. I. Stober, A. K.

1957

Stabilization of free radicals at low temperatures.

Luminescence de Fazote solide

(4.2° K) contenant desatomes

ou radicaux libres. Effet de

la dilution par I'argon. Ionization and dissociation of oxygen difluoride by electron impact.

Theory of the forbidden transi- tions of nitrogen atom.s trapped in solids. Frozen free radicals. Suppression of bubbling in boiling refrigerants. 1958

Ann. N.Y. Acad.

Sci. 67, 530-45.

J. phys. radium

18, 593-95.

J. Chem. Phys.

27, 1296-97. Phys. Rev.

107, 1239-45.

Sci. American

March, 90-102. Nature (Lond.)

180, 1413-14.

12 Bass, A. M.

Broida, H. P. 13

14

16

Black, I. A.

Bolz, L. H.

Brooks, H. P. Mauer, F. A.

Reiser, H. S. Broida, H. P. 15 Broida, H. P. Peyron, M.

Broida, H. P. Peyron, M.

17 Fontana, B. J.

Absorption spectra of solids condensed at low temperatures from electric discharges.

A liquid-heHum cold cell for use with an X-ray diffractometer.

Trapped radicals. Luminescence de I'azote solide

(4.2° K) contenant des atomes

ou radicaux libres. Effet de

traces d'oxygene, d'hydrogene

et de vapeur d'eau. Evaporation of active species

trapped in a solid condensed

from, "discharged" nitrogen.

Thermom.etric study of the fro- zen products from the nitro- gen microwave discharge.

J. Mol. Spect.

3, 42-53. NBS, J. Research

61, 367-71.

Endeavour

17, 208-15.

J. phys. radium

19," 480-84.

J. Chem. Phys.

28, 725-26.

J. Appl. Phvs.

39, 1668-73.

101
No. Author

18 Franklin, J. L. Dibeler, V. H.

Morris, P. P.

19 Franklin, J. L. Dibeler, V. H.

Reese, R. M.

Krauss, M.

20 Franklin, J. L. Herron, J. T. Bradt, P. Dibeler, V. H.

21 Golden, S. 22 Harvev, K. B. Bass, A. M.

23 Herron, J. T.

Franklin, J. L. Bradt, P. Dibeler, V. H.

24 Horl, E. M.

Marton, L. 25 Jackson, J. L. 26 Jackson, J. L. Montroll, E. W,

27 Klein, R.

Scheer, M. D.

28 Klein, R.

Scheer, M. D.

29 Klein, R.

Schoen, L. J. 30 Klein, R. Simpson, J. A. 31 Leder, L. B. Marton, L. 32 Leder, L. B.

Sim.pson, J. A. 33 Minkoff, G. J. Scherber, F. I. 34 Minkoff, G. J. Scherber, F. I. 35 Moyer, J. W.

36 Reese, R. M.

Dibeler, V. H.

Franklin, J. L, 37 Schoen, L. J. Kuentzel, L. Broida, H. P.

38 Bass, A. M.

Kessler, K. G. 39 Bass, 'A. M.

Mover, J. W.

40 Bolz, L. H.

Boyd, M. E. Mauer, F. A.

Reiser, H. S.

Title Journal Mass spectrometric study of the NBS, J. Research

rate of thermal decoro.position 61, 41-46.

of hydrazoic acid. Ionization and dissociation of J. Am. Chem. Soc. hydrazoic acid and methyl 80, 298-302.

azide by electron impact.

Mass spectrometric study of the J. Am.. Chem. Soc. decomposition of hydrazoic 80, 6188-90.

acid by the electric discharge. Free-radical stabilization in J. Chem. Phys.

condensed phases. 39, 61-71. Infrared absorption of oxygen J. Mol. Spect.

discharge products and ozone 3, 405-10.

at 4° K.

Kinetics of nitrogen atom re- J. Chem. Phvs. combination. 29, 230-3f (L).

Cryostat for electron bom.bard- ment and electron diffraction work.

Stabilized free radicals. Free radical statistics. The addition of hydrogen atom.s

to solid olefins at —195°.

The reaction of hydrogen atom.s with solid olefins at —195°.

Energy of dissociation of the C—H bond in the form.yl

radical. Production of a square temperature wave in filaments operating at low temperatures.

Temperature dependence of the

characteristic energy loss of electrons in alum.inum_.

Improved electrical differentia- tion of retarding potential measurements.

Suppression of bubbling in boil- ing refrigerants as an aid in optical measurements. Energy release from, discharged

m.onatom.ic gases trapped at 4° K.

Free radicals research s^anposium. Electron impact studies of sulfur dioxide and sulfuryl fluoride.. Glass Dewars for optical studies

at low temperatures.

1959

Rev. Sci. Instr. 29, 859-61.

J. Wash. Acad. Sci. 48, 181-85.

J. Chem. Phys.

28, 1101-9.

J. Am. Chem. Soc.

80, 1007 (L).

J. Phys. Chem.

62, 1011-14.

J. Chem. Phys.

29, 953-54 (L). Rev. Sci. Instr. 29, 770-73. Phys. Rev.

112, 341-43. Rev. Sci. Instr. 29, 571-74.

J. Opt. Soc. Am..

48, 358.

J. Chem. Phys.

28, 992-93 (L). Phvsics Today

il, 14-16 (Feb.).

J, Chem. Phys.

29, 880-83. Rev. Sci. Instr. 29, 633-38.

Large-aperture grating spectrograph utilizing commercial camera components.

Free radical chemistry.

A re-examination of the crystal structures of « and /3 nitrogen.

J. Opt. Soc. Am.

49, 1223-25. Chem. Eng. News

37, 50-6 (Aug. 24). Acta Cryst.

12, 247-48 (L).

102
Xo. Author

41 Bolz, L. H.

Mailer, F. A.

Peiser, H. S. 42 Bolz, L. H.

Mailer, F, A.

Peiser, H. S.

43 Broida, H. P.

44 Dibeler, V. H.

Franklin, J. L. Reese, R. M.

45 Dibeler, V. H.

Franklin, J. L. Reese, R. M.

46 Dressier, K.

47 Foner, S. X.

Mailer, F. A.

Bolz, L. H.

48 Fontana, B. J,

49 Franklin, J. L. Broida, H. P. 50 Gager, W. B.

Rice, F. O.

51 Goldberg, H.

52 Griffing, V. Jackson, J. L. Ransil, B. J. 53 Harvey, K. B. Brown, H. \V.

54 Harvey, K. B.

Giguere, P. 55 Herron, J. T.

Dibeler, V. H.

56 Herron, J. T.

Franklin, J. L. Bradt, P. 57 Herron, J. T.

Franklin, J. L. Bradt, P. Dibeler, V. H.

58 Horl, E. M.

59 Horl, E. M.

60 Horl, E. M.

61 Jackson, J. L.

Title Journal Low-temperature X-ray studies J. Chem. Phys.

of Rice's blue material. 30, 349-50 (L) Exploratory study, by low-temperature X-ray diffraction techniques, of diborane and

the products of a microwave

discharge in diborane. La recherche sur les radicaux

libres au National Bureau of Standards. Electron impact studies of hydrazine and the methyl-substituted hydrazines. Electron impact studies of hydrazine and the methyl-substituted hvdrazines.

Absorption spectrum of vibrationally excited X2 in active

nitrogen.

Argon deposition on a 4.2° K

surface. Magnetic study of the frozen products from the nitrogen

microwave discharge.

Trapped energetic radicals.

Paramagnetic resonance spectra

of active species. Blue material from hydrozoic acid. Crystal field theory for systems

of light atoms. Magnetic interaction of H3. Etude aux infrarouges de cer- tains solides condenses a partir de de charges en phase

gazeuse. An infrared study of hydrogen

bonding in solid H2O2 and

H2O-H2O2 mixtures.

Studies of the evaporation of condensates containing nitro- gen atoms. Mass spectrometric study of the

reactions of some hydrocarbons with active nitrogen. Kinetics of nitrogen atom re- combination.

J. Chem. Phys.

31, 1005-7.

J. chim. phys.

56, 813-16.

J. Am. Chem. Soc.

81, 68-73. Advances in Mass

Spectrometrv, J. D. Waldron,"Ed.,

(Pergainon Press, London, 1959). 443-57.

J. Chem. Phys.

30, 1621-22 (L).

J. Chem. Phvs.

31, 546-48 (L).

J. Chem. Phvs.

31, 148-53": Ann. Rev. Phvs. Chem. 10, 145-68.

J. Chem. Phvs.

31, 564 (L). Ph. D. Thesis Univ. of Md.

June 1959.

J. Chem. Phys.

30, 1066-74.

J. chim. phvs.

56, 745-48.

J. Mol. Spect.

3, 36-45.

J. Chem. Phvs.

31, 1662-65. Can. J. Chem.

37, 579-82.

J. Chem. Phvs.

30, 879-85".

Light emission from solid nitro- gen during and after electron bombardment.

The absorption spectrum of solid Qi-oxygen. Electron bombardment of solid acetone. Reactions en chaine de radicaux

geles.

J. Mol. Spect.

3, 425-49.

J. Mol. Spect.

3, 548-53.

J. Chem. Phys.

31, 564-65 (L).

J. chim. phvs.

56, 771-78.

103
No.

62

63

64

65

66

67

68

70

71

72

73

74

75

76

77

78

79

80

Author

Jackson, J, L.

Jackson, J. L.

Klein, R.

Klein, R.

Scheer, M. D.

Klein, R. McNesbv, J. R.

Scheer, M. D.

Schoen, L. J. Kruger, J. Ambs, W. J. McNesby, J. R.

69 Minkoff, G. J.

Minkoff, G. J. Scherber, F. I. Gallagher, J. S. Peyron, M.

Broida, H. P.

Pevron, M.

I-I(3rl, E. M.

Brown, H. W.

Broida, H. P. Peyron, M.

Horl, E. M.

Brown, H. W.

Broida, H. P.

Radford, H. E. Ruehrwein, R. A. Hashman, J. S. Scheer, M. D.

Klein, R,

Schoen, L. J. Rebbert, R.

Wall, L. A. Brown, D. W.

Florin, R. E.

Wall, L. A. Brown, D. W.

Florin, R. E. Windsor, M. W.

Title The dynamic stability of frozen

radicals. Part I: Description

and application of the model. The dynamic stability of frozen

radicals. Part II: The for- mal theory of the model.

Adsorption, diffusion, and evaporation of carbon monoxide

on tungsten.

Reaction of hydrogen atoms

with solid oxygen at 20° K.

Kinetics of D atom reactions with H2, CH4, and D2CO.

Optical measurements on thin

films of condensed gases at low temperatures.

Photolysis of oxygen in solution

in liquid nitrogen and in liq- uid carbon monoxide at 1849

A. Frozen free radicals.

Energetic species trapped at

4.2° K from gaseous dis- charges.

Spectra emitted from solid nitro- gen condensed at very low

temperatures from a gas dis- charge.

Spectroscopic evidence for tri- atomic nitrogen in solids at very low temperature.

Etude spectroscopique des produits de la decharge electrique

dans I'azote condenses a Tetat

solide a tres basse temperature, preuves en faveur de

I'existence d'azote triatomi- que dans le solide. Paramagnetic resonance in the

free hydroxyl radical. Formation of ozone from atomic oxygen at low temperatures.

The double bond isomerization

of olefins by hydrogen atoms

at -195°.

Electrical discharge induced lu- minescence of solids at low

temperatures.

Atoms and free radicals by 7-

irradiation at 4.2° K.

Electron spin resonance spectra

from gamma-irradiated solid nitrogen.

Volcanoes on the moon.

1 Bass, A. M.

Papers in Process, January 1960 Frozen free radicals?

Journal

J. Chem. Phys.

31, 154-57.

J. Chem. Phys.

31, 722-29.

J. Chem. Phvs.

31, 1306-13.

J. Chem. Phys.

31, 278-79 (L),

J. Chem. Phvs.

30, 58-60."^

J. Opt. Soc. Am.

49, 1195-98.

J. Chem. Phvs.

31, 283-84\L)

Progress in Cryogenics, 1 Aca- demic Press, New York.

J. Chem. Phys.

30, 753-58.

J. Chem. Phys.

30, 139-50.

J. Chem. Phvs.

30, 1304-10.

J. chim. phys.

56, 736-44.

II Nuovo Cimento

14, 245-47.

J. Chem. Phys.

30, 823-26.

J. Phys. Chem.

63, 1517-18

(Note).

J. Mol. Spect.

3, 417-24.

J. Phys. Chem.

63, 1762-69.

J. Chem. Phys.

30, 602-3 (L). Nature (Lond.)

184, 1556-57 (L).

N.Y. Herald Trib. Eng. News Supp.

(Feb.) 104
No.

82

83

84

85

86

89

90

9192

Avthor

Bass, A. M.

Broida, H. P. (Editors) Bass, A. M.

Broida, H. P. (Editors) Baurer, T.

Bolz, L. H.

Mauer, F. A.

Peiser, H. S. Bolz, L. H.

Mauer, F. A. McMurdie, F. H.

Peiser, H. S. Broida, H. P. Nicholls, R. W.

Broida, H. P. Peyron, M.

Brown, D. W.

Wall, L. A. Brown, N. L.

Barfield, R. N.

Clarke, J. T. Gorden, R. Comeford, J. J. Gould, J. H.

Title Final report of the NBS free radical research program

Formation

radicals & trapping of free

93 Donn, B.

94 Dressier, K.

95 Dressier, K.

90 Dressier, K.

97 Dressier, K.

Schnepp, O. 98 Dressier, K.

Schnepp, O. 99 Edwards, J. W.

100 Edwards, J. W.

Hashman, J. S. 101 Goldberg, H.

Herzfeld, C. M.

102 Goldberg, H.

Herzfeld, C. M.

103 Herron, J. T. 104 Herron, J. T.

Dibeler, V. H.

105 Herron, J. T.

Dibeler, V. H.

100 Herzfeld, C. M.

107 Herzfeld, C. M.

Scheer, M. D.

Some experiments on the deposition of gases at 4.2° K. Some observations on the crys- tallographic properties of sol- id argon.

Solid phases of NHg.

Phosphorescence of nitrogen and

nitrogen-argon deposited

films at 4.2°K. Emission spectra of N2, O2, and

NO molecules trapped in solid matrices. Gamma-irradiation of liquid and

solid oxygen.

Shielded coaxial leads for low- temperature electrical measurements. The thermal conductivity of condensed films: methane.

Infrared spectra of solid hydrocarbons at very low

temperatures.

Trapped radicals in astrophysics

(chapt. 11). The vacuum ultraviolet absorption spectra of solid carbon monoxide and nitrogen.

Absorption spectra of atomic

mercury and xenon in solid matrices. Electronic absorption spectrum

of NH3 at 4.2° K. The absorption spectra of solid methane, ammonia and ice in the vacuum ultraviolet. The Schumann-Runge bands of oxygen in solid phases.

Chemical and physical studies of trapped radicals (chapt. 8). Reaction of oxygen and atomic

hydrogen.

On the nature of the crystal field approximation.

Theory of spectra of trapped oxygen atoms. Reaction of nitrogen atoms with cyanogen bromide.

Electron impact study of the cyanogen halides. The ionization potentials of

fluorine. A survey of theoretical work on

trapped radicals at the National Bureau of Standards.

Qualitative comments on the

physical and chemical processes in trapped radical systems.

Journal NBS Monograph

No. 12. (Academic Press, New York, New

York, 1960). NBS, Tech. Report.

J. Chem. Phys,

J. Chem. Phys.

J. Chem. Phvs.

32, 023-24 (L),

J. Chem. Phys.

In press.

J. Phys. Chem.

Rev. Sci. Instr. In press.

J. Chem. Phys.

33, 705-10.

J. Mol. Spect.

Trapping of Free

Radicals. See Reference No. 83.

J. Chem. Phvs.

J. Chem. Phys.

J. Chem. Phys.

J. Chem. Phys.

J. Chem. Phys.

Trapping of Free

Radicals. See Reference No. 83.

J. Am. Chem. Soc.

J. Chem. Phys.

Phys. Rev.

J. Am. Chem. Soc.

J. Am. Chem. Soc. In press.

J. Chem. Phys. (L). In press. NBS Monograph

No. 12. See Ref- erence No. 82. NBS Monograph

No. 12. See Reference No. 82.105
No, Author

108 Horl, E. M.

Marton, L. 109 Jackson, J. L.

110 Klein, R.

111 Klein, R.

Horl, E. M.

112 Klein, R.

Scheer, M. D.

Waller, J. G. 113 Kruger, J. Ambs, W. J.

114 Leifer, A.

Barfield, V. S. Mador, I. L. 115 Mador, I. L.

Leifer, A.

Barfield, V. S. 116 Mauer, F. A.

117 Mauer, F. A.

Bolz, L. H.

118 McNesby, J. R.

Scheer, M. D.

Klein, R.

119 Minkoff, G. J. 120 Moyer, J. W.

121 Reiser, H. S.

122 Peyron, M.

123 Radford, H. E.

Rice, F. O. 124 Reed, S. G.

Herzfield, C. M.

125 Ruehrwein, R. A. Hashman, J. S. Edwards, J. W.

126 Scheer, M. D.

127 Schnepp, O.

128 Schnepp, O.

129 Schnepp, O.

Dressier, K.

Title Electron diffraction studies on

solid a-nitrogen. Free radical trapping—theoretical aspects (chapt. 10). Low-temperature chemistry.

Journal Acta Cryst. In press. Trapping of Free

Radicals. See Reference No. 83. NBS Monograph

No. 12. See Ref- ence No. 82.

J. Chem. Phys.

1, 307-8 (L). Isotope exchange processes in solid nitrogen under electron bombardment.

The reaction of hydrogen atoms

with solid propene at low

temperatures.

The reaction between copper and species condensed at very low temperatures from a gas

discharge.

Sublimation of hydroxylamine J. Phys. Chem

hydrochloride.

J. Phys. Chem.

Electrochem.

Soc.

Photolysis of hydroxylamine

hydrochloride.

Low-temperature

(chapt. 5). Techniques

Lattice constants and thermal

expansion coefficients for neon, argon, xenon, and krypton.

The isotope effect in the hydro- gen atom-formaldehyde re- action. Frozen free radicals. Organization and rationale of the NBS free radicals program.

Structure of solids containing

trapped radicals (chapt. 9). Quelques spectres d'emission de

molecules piegees dans I'azote ou r argon solide a la temperature de I'helium liquide.

Green and purple sulfur electron: spin resonance studies. Theory of flame propagation in

solid nitrogen at low temperatures.

Chemicals reactions of free radicals at low temperatures.

The experimental aspects of the National Bureau of Standards free radical program. The interactions between trapped

species and the matrix. The absorption spectra of dis- persions of magnesium and of manganese in rare gas matrices. The absorption spectra of solid xenon, krypton, and argon in the vacuum ultraviolet.

J. Phys. Chem.

Trapping of Free

Radicals. See Reference No. 83.

J. Chem. Phys.

In press.

J. Chem. Phys.

(Interscience Pub.,

Lond.) In press. NBS Monograph

No. 12. See Ref- erence No. 82. Trapping of Free

Radicals, See Reference No. 83. Proc. 4th Mtg.

Mol. Spect., Bologna (Pergamon Press, Lond.).

J. Chem. Phys.

J. Chem. Phys.

33, 1-2.

J. Phys. Chem.

NBS Monograph

No. 12. See Ref- erence No. 82. NBS Monograph

No. 12. See Ref- erence No. 82. Phvs. and Chem.

Sohds.

J. Chem. Phys.

In press.

106
No. Author

130 Schnepp, O.

Dressier, K.

131 Schoen, L. J. 132 Schoen, L. J. Broida, H. P. 133 Schoen, L. J. Broida, H. P. 134 Stober, A. K.

135

136

137

138

139

140

141

142 143

144

Thomas, A. Thomas. A.

Wall, L. A.

Wall, L. A. Brown, D. W.

Windsor, M. W

Windsor, M. W

Windsor, W

Zahn, C. A. Zwanzig, R. W.

Zwanzig, R. W.

145 Zwanzig, R. W

Title The photolysis of ammonia in a

solid matrix at low temperatures. Triboluminescence in a mercury

bubbler. Spectra emitted from rare gas- oxygen solids during electron bombardment.

Effect of structure on the spectra

emitted by solid nitrogen

during electron bombardment.

A method for efficiently pro- viding low-temperature liq- uids on a large scale to an

accelerated experimental pro- gram. Low-temperature chemistry: A

rotating cryostat for mixing

reactants at 4.2°K. Methods of production of trapped radicals and properties of radical-trapping solids. Electron spin resonance studies

of free radicals in irradiated

materials. Storage of atoms and free radicals at 4° and 20° K. Trapped radicals in propulsion

(chapt. 13). Temperature effect on molybdenum oxide as a detector

for hydrogen atoms. Summary of the Fourth Inter- national Symposium on Free

Radical Stabilization. A Dewar system for low-temperature experiments.

Collision of a gas atom with a

cold surface. Identity and concentrations of trapped radicals. Intermolecular forces from optical spectra of impurities in molecular crystals.

Journal

J. Chem. Phys.

In press. Can. J. Phys.

J. Chem. Phys.

In press.

J. Mol. Spect.

(Note). Proc. 10th Intern. Cong, on Refrig. In press.

Trans. Faraday

Soc. NBS Monograph

No. 12. See Ref- erence No. 82. Proc. A.S.T.M.

J. Chem. Phys.

Trapping of Free

Radicals. See Reference No. 83.

J. Chem. Phys. (L). NBS Monograph

No. 12. See Ref- erence No. 82. Rev. Sci. Instr. In press.

J. Chem. Phys.

NBS Monograph

No. 12. See Ref- erence No. 82. Mol. Phys.

In press.

Patents

No.

146

147 148

149

Inventor Title Broida, H. P. Free-radical stabilization. Pellam,

Golden,

J. R.

S. Ruehrwein, R. A.

Scheer, M,

Klein, R.

Method of free radical production and stabilization. Method for making ozone. D. Process for low-temperature

hydrogenation of olefins.

U.S. Patent No.

2,892,766 June

30, 1959. 2,924,562 February 9, 1960. Patent application

pending. Patent application

pending.

107
Motion Picture Film

No. Title 150 Bass, A. M. Trapping of free radicals at low For sale or loan Richardson, temperatures. from: Office of W. P. Technical Infor- mation, National Bureau of Standards, Washington 25, D.C.

108
Author Index

Name and Reference No. Ambs, W. J. 67, 113

Bass, A. M., 3, 10, 12, 22, 38, 39, 81, 82, 83, 150

Barfield, R. N., 90

Barfield, V. S., 114, 115 Baurer, T., 84

Black, I. A., 13

Bolz, L. H., 13, 40, 41, 42, 47, 85, 86, 117 Bovd, M. E., 40

Bradt, P., 20, 23, 56, 57

Broida, H. P., 1, 2, 3, 4, 5, 6, 7, 12, 14,

15, 16, 37, 43, 49, 71, 72, 73, 82, 83,

87, 88, 131, 132, 146 Brooks, H. P., 13 Brown, D. W., 78, 79, 89, 138 Brown, H. W., 51, 70, 71 Brown, N. L., 90

Clarke, J. T., 91

Coineford, J. J., 92

Dibeler, V. H., 8, 18, 19, 20, 23, 36, 44,

55, 57, 104, 105 Donn, B., 93

Dressier, K., 46, 94, 95, 96, 97, 98, 128, 129 Edwards, J. W., 99, 100, 125

Florin, R. E., 78, 79

Foner, S. N., 47 Fontana, B. J., 17, 48

Franklin, J. L., 8, 18, 19, 20, 23, 36,

44, 49, 56, 57

Gager, W. B., 50

Gallagher, J. S., 70

Giguere, P., 54

Goldberg, H., 51, 101, 102 Golden, S., 21, 147 Gorden, R., 91

Griffing, V., 52 Harvev, K. B., 22, 53, 54 Hashman, J. S., 75, 100, 125 Herron, J. T., 20, 23, 55, 56, 57, 103,

104, 105

Herzfeld, C. M., 5, 9, 10, 101, 102, 106,

107, 124

Horl, E. M., 24, 58, 59, 60, 72, 73, 108, 111 Jackson, J. L., 25, 26, 52, 61, 62, 63, 109

Name and Reference No.

Kessler, K. G., 38

Klein, R., 27, 28, 29, 30, 64, 65, 66,

76, 110, 111, 112, 118, 149 Krauss, M., 19 Kruger, J., 67, 113 Kuentzel, L., 37

Leder, L. B., 31, 32

Leifer, A., 114, 115

Lutes, O. S., 4 Mador, I. L., 114, 115 Marton, L., 24, 31, 108 Mauer, F. A., 13, 40, 41, 42, 47, 85,

86, 116, 117 McMurdie, F. H., 86 McNesby, J. R., 66, 68, 118 Minkoff, G. J., 11, 33, 34, 69, 70, 119

Montroll, E. M., 26

Morris, P. P., 18 Mover, J. W., 35, 39, 120

Nicholls, R. W., 87

Pellam, J. R., 1, 2, 146

Reiser, H. S., 13, 40, 41, 42, 85, 86, 121 Pevron, M., 7, 15, 16, 71, 72, 73, 88, i22 Radford, H. E., 74, 123

Ransil, B. J., 52 Rebbert, R., 77 Reed, S. G., 124

Reese, R. M., 8, 19, 36, 44

Rice, F. O., 50, 123 Richardson, W. P., 150 Ruehrwein, R. A., 75, 125, 148

Scheer, M. D., 27, 28, 65, 66, 76, 107,

112, 118, 126, 149

Scherber, F. I., 11, 33, 34, 70 Schnepp, O., 97, 98, 127, 128, 129, 130 Schoen, L. J., 29, 37, 66, 77, 131, 132, 133 Simpson, J. A., 30, 32

Stober, A. K., 11, 134 Thomas, A., 135, 136

Wall, L. A., 78, 79, 89, 137, 138

Waller, J. G., 112 Windsor, M. W., 80, 139, 140, 141 Zahn, C. A., 142 Zwanzig, R. W., 136, 143, 144, 145

109
Articles in NBS Technical News Bulletins on the Free Radicals

Research Program

Low-temperature storage of free radicals 40, 112-14

(1956) Free-radicals research program 41, 1-2

(1957) Free radicals symposium 41, 200-1

(1957) 1959 Low-tem.perature production of ozone 43, 29

Glass Dewars for trapped radicals studies 43, 40-42

Fourth International Symposium on Free Radical Stabilization 43, 62 Plans for the Fourth International Free Radicals Symposium 43, 139 Providing liquid refrigerants to research workers 43, 146-47

Spectroscopic studies of trapped radicals 43, 164-65

Patent for free-radical stabiKzation 43, 175

Highlights of the Fourth International Symposium on Free Radical

Stabilization 43, 201-2

Studies of reactions with low^-activation energies 43, 206-7

110

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