NSRDS—NBS 46

National

Bureau

of

Standards
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National Bureau m Standard

Reactivity of the Hydroxyl Radical

in Aqueous Solutions

Leon M. Dorfman

Department of Chemistry

The Ohio State University

Columbus, Ohio 43210

and

Gerald E. Adams

Cancer Research Campaign

Grey Laboratory

Mount Vernon Hospital

Northwood, Middlesex, England

M H \/ U:r\l 1974

U.S. DEPARTMENT OF COMMERCE, Frederick B. Dent, Secretary

NATIONAL BUREAU OF STANDARDS, Richard W. Roberts, Director

Issued June 1973
Library of Congress Catalog Card Number: 72-600321

NSRDS-NBS 46

Nat. Stand. Ref. Data Ser., Nat. Bur. Stand. (U.S.), 46 72 pages (June 1973)

CODEN: NSRDAP

© 1972 by the Secretary of Commerce on Behalf of the United States Government

For sale by the Superintendent of Documents, U.S. Government Printing Office, Washington, D.C. 20402

(Order by SD Catalog No. C13.48:46)

Price: 90 cents, domestic postpaid; 65 cents, GPO Bookstore

Stock Number 0303-01088
Foreword

The National Standard Reference Data System provides access to the quantitative data of physical science, critically evaluated and compiled for convenience and readily accessible through a variety of distribution channels. The System was established in 1963 by action of the President’s

Office of Science and Technology and the Federal Council for Science and Technology, and

responsibility to administer it was assigned to the National Bureau of Standards.

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Research Council of the National Academy of Sciences-National Academy of Engineering. A number of Advisory Panels, each concerned with a single technical area, meet regularly to examine

major portions of the program, assign relative priorities, and identify specific key problems in need of further attention. For selected specific topics, the Advisory Panels sponsor subpanels

which make detailed studies of users’ needs, the present state of knowledge, and existing data re- sources as a basis for recommending one or more data compilation activities. This assembly of advisory services contributes greatly to the guidance of NSRDS activities. The System now includes a complex of data centers and other activities in academic insti- tutions and other laboratories. Components of the NSRDS produce compilations of critically

evaluated data, reviews of the state of quantitative knowledge in specialized areas, and computations of useful functions derived from standard reference data. The centers and projects also establish criteria for evaluation and compilation of data and recommend improvements in ex- perimental techniques. They are normally associated with research in the relevant field. The technical scope of NSRDS is indicated by the categories of projects active or being

planned: nuclear properties, atomic and molecular properties, solid state properties, thermodynamic and transport properties, chemical kinetics, and colloid and surface properties.

Reliable data on the properties of matter and materials is a major foundation of scientific and technical progress. Such important activities as basic scientific research, industrial quality control, development of new materials for building and other technologies, measuring and correcting

environmental pollution depend on quality reference data. In NSRDS, the Bureau’s responsibility

to support American science, industry, and commerce is vitally fulfilled.

Richard W. Roberts, Director

III
'
Contents

Page Chapter I. Introduction 1 Explanation of tables 2 Glossary of abbreviations 2 References 3 Chapter II. Experimental Methods 4 Radiation chemical methods 4 Photochemical methods 6 Chemical methods 7 The Fenton reaction 7 Titanous ion 7 References 8 Chapter III. Kinetic Analysis 9

Direct observation of product formation 9 Direct observation of hydroxyl radical disappearance 10 Competition kinetic measurements 10

Stationary state systems 10 Pulse radiolysis methods 11 Intermittent irradiation 12 References 12 Chapter IV. Physical Properties and Chemical Equilibrium 13 Table 1. Physical properties 14 References 14 Chapter V. Addition Reactions 15 Table 2. Addition reactions 17 References 21 Chapter VI. Hydrogen Abstraction Reactions 22 Table 3. Aliphatic alcohols 23 Table 4. Saturated aliphatic acids 25 Table 5. Polybasic and hydroxy carboxylic acids 27 Table 6. Aliphatic carboxylic esters 28 Table 7. Ethers 29

Table 8. Carbonyl compounds 29 Table 9. Substituted hydrocarbons 30 Table 10. Nitrogen compounds 30 References 31 Chapter VII. Inorganic Electron Transfer Reactions 21 Table 11. Inorganic electron transfer reactions 33 References 37 Chapter VIII. Radical Reactions 38 Table 12. Radical reactions 38 References 39 Chapter IX. Reactions of the Oxide Radical Ion 40 Table 13. Reactions of the oxide radical ion 40

References 42

V
Page Chapter X. Reactions With Biological Molecules 43 Amino acids 43 Enzymes 44

Nucleic acid derivatives 45 DNA 46 Table 14. Sulfur and selenium compounds 46 Table 15. Amino acids 47 Table 16. Simple peptides 49 Table 17. Enzymes 50

Table 18. Nucleic acid derivatives I 50 Table 19. Carbohydrates and related compounds 53 Table 20. Miscellaneous compounds ; 54

References I 56 Chapter XI. Cross-Correlation of Rate Constants 57 Table 21. Comparison of values from different methods 58 References 59

VI
Reactivity of the Hydroxyl Radical

in Aqueous Solutions

Leon M. Dorfman and Gerald E. Adams

The reaction rate data of the hydroxyl radical in aqueous solution are compiled and evaluated in

this critical review. The values are reported in a series of tables covering addition, hydrogen abstraction, inorganic electron transfer and radical reactions. Rate constants for the hydroxyl radical with

biological molecules are included. In addition, the rate constant data for the oxide radical ion are given.

Physical properties are listed and the experimental methods employed in OH radical chemistry are reviewed. An analysis involving rate constant data comparisons is made.

Key words: Abstraction reactions; addition reactions: aqueous solution; biological molecules; electron

transfer reactions; hydroxyl radical; oxide radical ion; radical reactions; rate constants; reference data. Chapter I. Introduction

Water is the most important single solvent in

which the physical and chemical processes that can

occur are of broad importance in so many areas of

the natural sciences. As a result, the rates and

equilibria of chemical reactions in aqueous solution

have been the subject of countless investigations

of almost every conceivable chemical system of

substances soluble in water. Among the important

chemical reactions occurring in water are those of

the free hydroxyl radical, OH, a reactive decomposition product of water which exhibits strong oxidizing properties. Indeed, it is one of the strongest

of chemical oxidizing agents.

Hydroxyl radical reactions are of importance in

both chemical and biological systems. In chemical

systems, such reactions may be fundamental steps

in the hydroxylation and oxidation of aquo-organic

systems, in the oxidation of inorganic ions and ion

complexes, and in the initiation of polymerization.

In aqueous solutions which have been exposed to ionizing radiation, the hydroxyl radical is one of the

two principal reactive species formed in the decomposition of the water. Subsequent oxidation

reactions are the result of its formation. It is, in

fact, the application of fast reaction methods in

radiation chemistry which has provided the major

part of the new information on the reactivity of the

hydroxyl radical, to which this monograph is directed. In radiation biology, since 80 percent of the cell consists of water, reactions of the hydroxyl radical

as a reactive transient are important in biological

damage. The effects are indirect and evidence has

recently been accumulated which shows that a substantial part of this indirect damage to the cells,

in many systems, is due to oxidative effects which

may be attributable to the hydroxyl radical. In- formation from such studies, moreover, has a bearing on our understanding of molecular processes in

biological systems in general.

The chemical reactions of the hydroxyl radical

in water may be classified as four different types.

These are:

(a) Addition

OH+C6 H6 = (OH)C6H6- (1)

in which the hydroxyl radical adds to an unsaturated compound, aliphatic or aromatic, to form a free

radical product. The addition to aromatic compounds, from which various cyclohexadienyl

radicals are formed, is typical;

(b) Hydrogen Abstraction

OH+ CH3 OH= • CH2 0H + H2 0 (2)

to form a free radical and water;

(c) Electron Transfer

OH+ [Fe(CN)6 ]

4 -= [Fe(CN)6 ] 3 - + OH- (3)

to form ions in a higher valence state, or an atom or

free radical if a mononegative ion is oxidized, and

the hydroxide ion;

(d) Radical Interaction

0H+0H=H2 02 (4)

in which the hydroxyl radical reacts with another

hydroxyl radical, or with an unlike radical, to combine or to disproportionate to form a stable product.

1
A subclassification of these four reaction types

exists since the hydroxyl radical has more than one

form. OH behaves like a weak acid, ionizing in very

strongly basic solution to form O

-

, a species whose

reactivity differs substantially from that of OH.

Many separate reactions of each type have been

investigated and the specific reaction rates determined. Precise quantitative information has been

developed principally through the application of the

pulse radiolysis technique. Since the radiolysis of

water, the mechanism of which is fairly completely

understood, produces hydroxyl radicals in known

yields, the use of this fast reaction method has been

most fruitful in determining the chemical reactivity

of OH.

The observation of the spectra of cyclohexadienyl

radicals by Dorfman and his co-workers [1—3]

1

, and

their determination of rate constants for several addition reactions of OH, led to subsequent measurements

of such rate constants for a large number of organic

compounds. The work of Rabani and Matheson

[4, 5] on the reactivity of the hydroxyl radical with

ferrocyanide ion, provided an example of electron

transfer in a reaction which has proved useful as a reference reaction. The generality of such electron

transfer reactions of OH with inorganic anions and

complexes was demonstrated by Adams and Boag

[6, 7] who observed the formation of optically

absorbing transients from anions of Group 4-7 of

the periodic table. A large part of the work which has

provided the information contained in this monograph has evolved from studies of this type.

A critical appraisal and compilation of available

data constitutes the substance of this monograph.

Any quantitative model for a reacting system rests on a knowledge of the individual rate constants for

the elementary reactions of the reactive transient

species. Most satisfactory tests of the validity of

such models require a knowledge of these individual

rate constants. It is our hope that the contents of

this critical compilation will prove useful in different areas of research.

Explanation of Tables

The information contained in these tables represents a critical evaluation as well as a compilation.

Accordingly, the tables contain not only the values

and the cogent parameters of the system, but also

brief notations about the method used in each case and the particular experimental conditions which

may be crucial in establishing the validity of the

results obtained. In the case of many of the rate constants determined on a relative basis, the values

presented in these tables will be found not to coincide with the values suggested by the original

authors. Such values have been corrected to bring

them into accord with the most reliable values available for the rate constants of reference reactions

1 Figures in brackets indicate the literature references at the end of each chapter. References are numbered independently for each chapter.

which have been used in the determination of these

relative values. The following notation has been

used to distinguish the characteristics of the values

wherever possible:

*, absolute values — which do not depend upon

the value of a rate constant for some reference

reaction used in competition studies;

t, most reliable values — values of which the

accuracy (within the stated experimental un- certainty or lacking such a statement, within

± 30%) seems least open to question considering such factors as the scatter, or in some cases

the paucity of the experimental results or, more commonly, the complexities in interpretation of the data.

The selection has the following basis: Values

merit the confidence implied by the designation

“most reliable” when they are obtained by methods:

(a) which are as direct as possible,

(b) which involve only few, well-established

assumptions about mechanism, and

(c) from which the evaluation of the data follows

from a closed analytical solution of the rate equations for such a mechanism, or from a computer analysis of these rate equations.

Accordingly it is not surprising that most of

the values so designated have been determined absolutely by pulse radiolysis, or in competition kinetics by pulse radiolysis

rather than by analysis of stable products.

It may be helpful to point out that, conversely,

the following negative criteria have, in general, been

applied in excluding the t designation for many of

the data whenever:

(a) the system involves a complex mechanism

which may not be clearly established, or more

particularly,

(b) the relationship between the species monitored and the reaction of the hydroxyl radical

is not clearly established, or

(c) the degree of scatter of the results is so great

as to raise a question about the meaningfulness

of the average, or

(d) the variation of appropriate physical

parameters covers too small a range to consti- tute a satisfactory test of validity of the kinetic

analysis, or

(e) insufficient information has been provided to judge whether the criteria for inclusion have

been met. Data from the thiocyanate competition method, for example, have not been designated most reliable because of uncertainties

about the mechanism; nor have values for the

reactions with aromatic compounds obtained

by carbon dioxide analysis because of the uncertainty in relating the yield of this stable

product to the rate of the elementary reaction

in question.

The designation of most reliable values should

be viewed as an attempt to provide some guidance

to the reader who is not familiar with the details

of the methods. It is not bestowed as an accolade.

2
Although we may have erred in selecting too few

values as “most reliable*’, it is clear to us that the

inclusion of such a designation enhances the value

of the compilation.

Glossary of Abbreviations

The following abbreviations have been used in the

tables to conserve space:

abs.

addit.

calc.

comp.

comput.

cone.

corr. eferrocyan.

form.

G

irrad.

IP

neut.

absorbed

addition

calculated or calculation

competition or competitive

computer

concurrent

correction or corrected

molar extinction coefficient

ferrocyanide

formation

radiation chemical yield (molecules

reacting or forming per 100 eV

absorbed)

irradiation

2-propanol

neutral

obs. observed or observation

PNBA p-nitrobenzoate ion

PNDA p-nitrosodimethyl aniline

prod. product

P pulse

R radiolysis reac. reaction

rel. relative

RT room temperature

S steady

scav. scavenger or scavenged

thioevan. thiocyanate

Chapter I. References

[1] Dorfman. L. M., Taub, I. A.. Biihler, R. E., J. Chem. Phvs. 36,3051(1962).

[2] Wander. R.. Xeta, P., and Dorfman, L. M., J. Phys. Chem.

72> 2946, (1968).

[3] Xeta. P., and Dorfman, L. M., Advances in Chemistry Series Xo. 81 , Radiation Chemistrv I. 222 (1968). R. F. Gould,

Ed.

[4] Rabani, J., and Matheson, M. S., J. Amer. Chem. Soc. 86,

3175 (1964).

[5] Rabani. J., and Matheson, M. S., J. Phys. Chem. 70 , 761

(1966).

[6] Adams, G. E., and Boag, J. W., Proc. Chem. Soc., 112 (1964). [7] Adams, G. E., Boag, J. W., and Michael, B. D., Proc. Chem.

Soc., 41 1(1964).

492-021 OL - 73 - 2 3
Chapter II. Experimental Methods

Several different methods of generating hydroxyl

radicals in water, or in aqueous solutions, have been

used. Among these are: (1) radiation chemical

methods in which water is decomposed by ionizing

radiation, as, for example, by electrons, (2) photochemical methods in which hydroxyl radicals are formed from water by the absorption of light in the

vacuum ultraviolet, or from hydrogen peroxide in

solution by the absorption of light in the ultraviolet,

and (3) chemical methods in which the hydroxyl

radical is formed in a chemical reaction as in Fenton’s reaction, or the reaction of titanous ion with

hydrogen peroxide.

The determination of the specific reactivity of

hydroxyl in its elementary reactions further requires

that the method of generation permits the isolation

and observation of individual elementary reactions,

preferably by fast reaction methods. The pulse

radiolysis method [1-5] has proved to be by far the

most effective fast reaction technique for the study

of hydroxyl radical kinetics and it is from such

investigations that the bulk of the data tabulated

here have originated.

The brief description of the experimental methods

which follows, is in no way intended to be a manual

of technique. It is simply a brief summary, intended

to provide the general reader with some understanding of the method of generating hydroxyl

radicals and of observing their reactions and

determining from such observations, the individual

rate constants. It is intended to give the reader some

appreciation of the complexities which are en- countered in arriving at the final results.

Radiation Chemical Methods

The decomposition of water as a result of the absorption of energy from ionizing radiation passing

through it produces some stable products and some

reactive intermediates, the identity of which has

been well established by investigations extending

over three decades [6—8] . The principal reactive

intermediates are the hydroxyl radical, which

exhibits oxidizing properties and the hydrated elec- tron (along with a smaller yield of hydrogen atom)

which exhibits reducing properties. The radiation

chemical decomposition of neutral water may be

represented as: HaO-yy* OH,e-q ,H, H2 , H2 02 , H3 0+

q

, OH-q

.(l)

In addition to the oxidizing species, OH, and the

reducing species, e

“

q and H, there is a smaller yield

of molecular products, hydrogen and hydrogen

peroxide. These are thought to be formed, at least

in part, in local regions of high concentration of the

primary species resulting from the spatially inhomogeneous initial distribution of energy in the system,

in regions called spurs and blobs (depending on

their size). These have spherical radii which are

initially less than 100 A. The hydronium ion is formed in the rapid ion molecule reaction which

forms the hydroxyl radical:

H2 0+ + H2 0= H3 0+ + OH (2) This reaction occurs in a time shorter than the

dielectric relaxation time and thus, before the H2 0+

is solvated. Hydrated electrons are formed in less

than 10_1 1 s as the secondary electrons in the

system approach thermalization. Both OH and

e^are thus formed in times shorter than 10-11

s. The hydroxide ion, indicated in reaction (1), is formed by interaction of a hydrated electron with

hydroxyl radical in the spurs.

The yields of these transient species are a function of radiation quality and of pH. For neutral

solutions and with electrons of energy greater than

about 1 MeV the yields [6, 9—12] (represented by

the symbol G) are: G0H = 2.8, Ge ^ =2.8, GH = 0.55,

G\\2 = 0-45, G-h2 O2 = 0-7’ £h3o+ = 2.9 and G0H_ —

0.1 molecules/100 eV. With regard to the study of

the kinetics of the hydroxyl radical then, irradiated

water may be regarded as essentially a two-radical

system of OH and e~ , with H amounting to only 9

percent of the total initial concentration of reactive

radicals. It is also possible to convert the hydrated

electrons into hydroxyl radicals by the use of nitrous

oxide as an electron scavenger. The occurrence of

the overall reaction

e aqT N2 0+ H2 0= N2 + OHT- OH-

(3) the nature of which will be discussed in a subsequent chapter, thus creates very nearly a oneradical system of OH in water, with the initial con- centration ratio [OH]o/[H]o = 10.

It should be pointed out that it is the nature of the

process of energy loss by the ionizing radiation that

makes the radiolysis of water the most convenient

method for generating hydroxyl radicals in aqueous

solutions. The energy loss process, resulting in ioni- zation and excitation of the absorber system by electron impact, is one of coulombic interaction of

incident electrons with the system electrons and

depends roughly on the electron density of the

4
components of the system. In dilute solutions of any

solute, therefore, it is the water which absorbs

almost all the energy, leaving the solute molecules

relatively unaffected. This is in contrast to the partition of incident energy in the photolysis of dilute

solutions, where the high energy photons necessary for the decomposition of water or hydrogen

peroxide will also be absorbed quite strongly by most

solutes of interest even at low concentration.

It should also be understood that the energy of

the incident electrons is not a physical parameter

fundamental to the investigations since a continuous

electron energy degradation spectrum exists in the

system.

Although the initial spatial distribution of the re- active transients in irradiated water is nonhomogeneous and has itself been the subject of direct

investigation [13] on a time scale of 10 -11

s, this

heterogeneity is short-lived. The spurs have a

lifetime on the order of 10 -9 to 10-8

s, after which

the distribution of the reactive transients becomes

homogeneous by diffusion. Most kinetic investigations of hydroxyl radical reactivity have been carried

out on a time scale greater than 10-8

s. The hydroxyl

radicals generated in this way are in the ground

electronic state and, presumably, in the ground

vibrational state since collisional de-excitation in

condensed phase would be very rapid.

Since the radiolysis of water provides such a convenient means of producing hydroxyl radicals,

the development of the pulse radiolysis technique

furnished an ideal fast reaction method for the study

of their reactivity. In this method, the radiation is delivered to the solution in a single pulse, the duration of which may range from 10_n to 10-6

s. Most

investigations of hydroxyl radical reactivity have

utilized electron pulses ranging from 50 ns to 1 fi s in duration.

The source of the electron pulse in such studies

may be any one of a number of different types of

electron accelerators. Microwave linear accelerators,

with energies ranging from about 2 to 30 MeV, are used most extensively in such applications. Van de

Graff accelerators, modified to store charge and

deliver a single, high current electron pulse, usually

at 2 or 3 MeV, have been used in several laboratories. The Febetron2

, an impulse machine utilizing a capacitor bank with a Marx surge circuit to deliver

a pulse of fixed length at 2 MeV, is a convenient

source for pulse radiolysis. X rays have been used

infrequently because of the conversion loss and have

not been applied in any of the work on hydroxyl

radicals. Detailed characteristics of most of the sources used have been presented in a recent book

[4] on this subject.

The characteristics of the electron pulse, which

are of importance in fast reaction studies are its duration and time-profile and its amplitude. Pulse

- Certain commercial products and instruments are identified in this work in order to specify adequately the experimental procedure. In no case does such identification imply recommendation or endorsement by the National Bureau of Standards.

lengths ranging from 1 ns to 1 /jls or longer are readily available; pulse lengths of less than 20 ps, from the fine structure of a microwave linac pulse,

have been used in a special application [14]. The

pulses with a duration greater than a few nano- seconds are fairly well represented as a square waveform. Thus, the Van de Graaff at Argonne National

Laboratory, with which pulses as short as 1 ns have

been used, has a pulse with rise time and decay

time of less than 0.5 ns. The microwave linac at the

Ohio State University, with which pulses generally

longer than 10 ns have been used, has rise and decay

times of 5 ns. It is an important convenience in the

interpretation of kinetic data that the time-profile

of the pulse approximates to a square wave—

particularly that the falling edge is represented

by a step function.

The pulse current must be high enough so that a

single pulse delivers sufficient power to the system in

order to produce an observable concentration of the

transient species of interest. With the shortest

electron pulses, currents in the range of several

amperes to several tens of amperes have been used;

with the longer pulses, currents are generally of the

order of 0.1 A to almost 1 A. In any event, the

magnitude of the product of current, duration, and

voltage of the pulse is such that, in most applications, a single pulse will deliver in the range of 0.02

to 5 J. With most systems, this is a sufficient power

level to produce an “instantaneous” concentration of

transient species in the range of 10 -7 M to almost

10-4 M. Such a concentration is readily detectable

by a variety of methods.

The solution into which this energy perturbation

is injected is generally contained in a quartz cell of rectangular or cyclindrical design with a volume

generally larger than a few ml, and, perhaps, up to several tens of ml, depending upon the electron

beam energy. The power level is insufficient, therefore, to produce any significant temperature rise in

the system.

The reaction of interest may be observed by monitoring one of the properties of either the reactive

intermediate or of a product formed directly in the

elementary reaction, over an appropriate period of

time following the electron pulse. Properties which

have been used are: optical absorption, optical emission if this occurs, electron spin resonance absorption if the requirement of time resolution permits,

and electrical conductivity if the species is charged.

Optical absorption is the most versatile of these

methods and has proved to be by far the most ex- tensively used. In this case, the optical absorption

of a monitoring light beam passing through the reaction cell is measured by fast spectrophotoelectric

recording at an appropriate wavelength, starting at, or before, the time of the pulse and extending after it for several half-times of the observed reaction. A

typical rate curve for the formation of a product in a hydroxyl radical reaction is shown in figure 1. The

identity of the absorbing species may, in some cases, be known from previous work in which the spectrum

5
FIGURE 1. Rate curve for the reaction of the oxide radical ion,

0~, with ethanol obtained by observing the formation of the

• C 2 H4 O- radical ion at 360 nm following a 300. ns electron pulse

in an aqueous solution at pH 13.92 containing 4.3 X 10 -4 M

ethanol.

The fraction of the light transmitted is shown as a function of time at a sweep speed of 1/is/cm on the oscilloscope. Data taken from reference [7], chapter IX.

has already been assigned; in others, it may be the

subject of some speculation in the event the identity

is based on the chemical behavior of the system.

Rate constants for reactions of the hydroxyl radical,

with the exception of the combination reaction and

the reaction with molecular hydrogen, have not

been determined by observing OH directly. An

optical absorption in the far uv [15, 16a] thought

to be that of the hydroxyl radical in water, is too

weak, and in a spectral region too readily obscured

by most solutes to be used for this purpose. Instead,

the rate curves for product formation have been

used.

In many cases, as will be explained, rate constants

have been determined by competition, in which the

relative concentration, rather than the rate, of a product formed is determined. Such competition

methods may utilize a steady radiation source such

as a Co60 y-ray source rather than a pulsed electron source and will, in that case, involve the relative concentration of a stable product rather than that of an

intermediate.

Another radiation chemical method which has

been used effectively for the determination of rate constants of two important reactions of OH, is the

equivalent of the rotating sector method. A pulsed

Van de Graaff accelerator, in which the pulse duration and period could be varied, was used.

Photochemical Methods

Water vapor absorbs strongly in the vacuum ultra- violet region of the spectrum, the continuum

extending from about 145 to 200 nm. This absorption

was, at first, thought to result in the photodissociation reaction:

H2 0 » [H2 0*] » H2 + O. (4) However, it was argued, on energetic grounds, that

the dissociation is not possible for wavelengths

longer than about 180 nm and that the primary

dissociation step is: H2 0 > [H2 0*] » H + OH. (5)

The absorption spectrum in liquid water is displaced slightly to longer wavelength. The first indication that reaction 5 also occurs in the liquid phase, was the demonstration that ferric ion is formed in the

flash photolysis of aqueous solution of ferrous ion

with light of wavelength below 200 nm [16b|. In an extension of this work [17], the observation that

h2 o2 is a major product in the flash photolysis of

deaerated water, provided fairly reliable confirmation that this reaction takes place. There is no ex- perimental evidence that solutes, certainly at concentrations less than 1 M, interact with the

primary excited state of water. Although liquid water

does not absorb above 200 nm, some mercury photosensitized decomposition has been observed in

aqueous solution, although the quantum yields are rather low [18].

It was reported [19] that hydroxyl radicals are formed by the photodecomposition of hydrogen

peroxide with light of wavelength 253.7 nm:

H2 02 >OH + OH. (6)

Under some conditions the quantum yields for

H2 02 removal can be extremely high, suggesting

the occurrence of chain processes, although, at high light intensities, the chain length decreases.

A study of the kinetics of H2 02 photodecomposition

indicated, initially, a primary quantum yield of

unity [20]. However, a subsequent investigation

[21] of the photo-oxidation of formic acid in solution

containing H2 02 showed that the true quantum

yield is 0.5. The kinetic behavior of the decomposition product is identical to that of the oxidizing

species formed in the y-radiolysis of water [22]

and there is now little doubt that it is the free

hydroxyl radical.

The fast-flow technique has been used in an ESR

study [23] of the structure of radicals formed by

reaction of OH produced by H2 02 photolysis. In

this technique, which, in some respects, is similar to that used in the fast flow ESR studies with the

Ti3+ /H2 02 reagent described below, H2 02 solution is photolyzed just before it is passed through the cavity

of the ESR spectrometer. Under appropriate conditions, radicals formed by reaction of OH with a second solute are sufficiently long-lived to permit

their ESR spectra to be recorded. As with the other

flow experiments, the strength of this technique lies in its suitability for studying the structures of

radicals formed by reaction of OH.

Although attempts have been made to use H2 02 photolysis as a source of OH radicals for kinetic

studies [24, 25], its usefulness is restricted to those

few systems where the absorption of the solute at this wavelength is low relative to that of H2 02 . It has been shown [26] that solutions of ferric ion can initiate vinyl polymerization when irradiated

with light of wavelength 365 nm and oxidize aromatic

compounds, including benzene itself [27], to the

corresponding phenols. The evidence is strong that

the primary photochemical reaction involves an

internal electron transfer reaction in a ferric ion

complex which leads to ejection of the hydroxyl

radical from the solvation shell:

6
Fe 0H2 + —»Fe2+ + OH. (7)

In the case of benzene, the primary quantum

efficiency for light of wavelength 313 nm is 0.13 [27].

There are no reports of this system being used for

determining any OH reactivities.

Chemical Methods

The Fenton Reaction

In 1894, Fenton [28] reported that a variety of

a-hydroxy acids could be oxidized by mixtures of

hydrogen peroxide and ferrous salt. Subsequently,

it was shown that polyhydric alcohols, sugars,

amino compounds, and phenols were similarly

oxidized while, at higher temperatures, some simple

aliphatic alcohols also reacted. The reactions

appeared to be specific to ferrous ion and were dependent upon pH.

In the thirties, the classic work of Haber and

Weiss, continued later by Weiss, led to the postulation of the hydroxyl radical as an intermediate in

the Fenton reaction [29]. This did much to clarify

the mechanisms of the many oxidation processes

initiated by this reagent.

To explain the observation that ferrous ion was a much better catalyst than the ferric ion in the

dehydrogenation of various substrates by hydrogen

peroxide, it was proposed that the initial step is the

reaction:

Fe2+ +H2 02 -^Fe3 + + 0H + 0H-. (8) Adequate confirmation of the OH radical as an intermediate, obtained first from an investigation of the

chain polymerization of olefins [30] was provided by

evidence from a variety of other systems [31-34].

The mechanism of oxidation of organic substrates

by the Fenton reagent can be complex. An initial examination of the kinetics of oxidation of a range of

compounds [33, 341 appeared to indicate that the

reactions could be classified into “chain” and “nonchain” processes. It was proposed that OH radicals

formed in acid solution by reaction (8) react with an

oxidizable substrate RH, by hydrogen abstraction

to form the radical R •

. Since H2 02 is present this

radical may react in a chain propagation step to regenerate OH:

R +H2 02 ^R0H+0H. (9)

In such cases, low concentrations of ferrous ions

could initiate the chain oxidation of an excess of

organic solute. If, however, reaction (9) were slow,

chain termination could occur by radical-radical

reaction by oxidation by Felll. Since ferrous ion can compete with the substrate for OH, it was

proposed that kinetic analysis of the data should

yield the rate constant ratio for these two reactions.

A large number of relative rate constants were

obtained by this method, although it is now known

that the mechanisms proposed are incomplete. No

account was taken of either the reaction of the or- ganic radical with oxygen or the formation of H02 in the chain propagation step:

OH+ H2 02-» H02 + H2 0. (10)

The overall reaction mechanism is sensitive, therefore, to the relative concentrations of the various

solutes. Nevertheless, the kinetic data are interesting from the historical viewpoint in that they

represent the first systematic study of OH reactivity

generally.

Although the Fenton reaction has been used in

more recent years for a study of some OH rate constants [25] the complexities of the oxidation

mechanisms seriously limit its reliability as a general

kinetic method. As is the case with the titanium/

H2 02 system described below, the usefulness of the

Fenton reaction lies more in its application in fast- flow/ESR techniques where the emphasis is on the

structure of the radicals formed by reactions of

OH rather than on the kinetics of the processes.

Titanous Ion

Titanous ion, Ti3+

, reacts with H2 02 in a process

analagous to the Fenton reaction, although it has

been reported that the reaction is faster [35].

Ti3 + + H2 02 -*Ti4+ + 0H + 0H-„ (11)

A convenient and fruitful method of studying re- actions of the OH radical generated by this reaction

has been used [36, 37] in which the reactants,

titanous ion, H2 02 , and an organic substrate, RH,

are mixed in a rapid-flow apparatus. The combined

solution is then passed through the cavity of an

electron spin resonance spectrometer a short time

after mixing and the ESR spectra of the transient

free radicals formed by reaction of OH with the

substrate are recorded.

The mechanism of the titanous ion/H2 02 reaction

is undoubtedly complex. The OH radical produced

in (11) reacts with both Ti3+ and H2 02 forming,

respectively, Ti4+ and H02 radicals. The relative

efficiency with which OH can react with an added

substrate then depends on the relative solute concentrations. However, further complications arise

from the existence of complexes between H2 02 and

the Ti4+ ions formed in the reaction. In the ESR

spectrum observed after mixing Ti3+ and H2 02 , two

singlet lines are observed which have been at- tributed to separate species in view of the different

time constants for the decay [38, 39]. There is little doubt that the singlets are due to complexes of

titanium ions and oxyradicals [40, 41] and recent

evidence [42] indicates that the radical involved in

both complexes is H02 . It has been suggested [40] that the reactive

agents in the oxidation of organic substrates are complexes containing titanium ions and OH radicals

and not free OH and that they are responsible for

7
the singlet spectra. Double mixing experiments have

shown, however, that this is not so T431. When Ti3 ^ and H2 O2 are brought together a

short time before an organic reactant is added, both

singlets are still observed. They are not quenched

and are certainly due to relatively unreactive

species. As would be expected, unless an oxidizable

organic substrate is available for reaction immediately the OH radical is formed, OH will react

with other species in the system. However, there seems little doubt that, despite the complexity of

the reaction when such a compound is available at the time of mixing, the reacting species is the free,

uncomplexed hydroxyl radical. This is confirmed

further by the considerable data concerning the

nature of the radicals formed from various organic

substrates. Where comparisons are possible, agreement with pulse radiolysis data on radical assignments are good. Examples of reactions studied by

the flow method include addition of OH to aromatic

[44] and various unsaturated aliphatic compounds

[45, 46].

The strength of the fast flow/ESR method lies in the facility for determining the structure of the

radicals produced by reaction of OH. However, the

limitations imposed by the time resolution available

with the method and the complexities of the

mechanisms involved, make it less suitable for the

quantitative measurement of the kinetics of OH

reactions.

Chapter II. References

[1] Dorfman, L. M., and Matheson, M. S., Progress in Radiation

Kinetics, Vol. Ill, 237 (1965).

[2] Keene, J. P., J. Sci. Instr. 41, 493 (1964).

[3] Boag, J. W., Actions Chimiques et Biologiques des Radiations, Chap. I, Series VI, M. Haissinsky, ed. (Masson et Cie, Paris, 1963).

[4] Matheson, M. S., and Dorfman, L. M., Pulse Radiolysis

(M.I.T. Press, Cambridge, Mass., 1969).

[5] Dorfman, L. M., Science 141, 493 (1963).

[6] Allen, A. O., The Radiation Chemistry of Water and Aqueous

Solutions (D. Van Nostrand Co. Inc., New York, 1961).

[7] Hart, E. J., and Anbar, M., The Hydrated Electron (John

T. Wiley, New York, N.Y., 1970).

[8] Mozumder, A., and Magee, J. L., Radiation Res. 28.

203 (1966).

[9] Allan, J. T., and Scholes, G., Nature 187, 218 (1960).

[10] Rabani, J., Mulac, W. A., and Matheson, M. S., J. Phys.

Chem. 69, 53 (1965).

[11] Rabani, J., and Stein, G., J. Chem. Phys. 37, 1865 (1962).

[12] Schmidt, K. H., and Ander, S. M., J. Phys. Chem. 73,2846

(1969).

[13] Bronskill, M. J., Wolff, R. K„ and Hunt, J. W„ J. Chem.

Phys. 53, 4201 (1970).

[14] Bronskill, M. J., and Hunt, J. W., J. Phys. Chem. 72, 3762

(1968).

[15] Thomas, J. K., Rabani, J., Matheson, M. S., Hart, E. J., and Gordon, S., J. Phys. Chem. 70, 2409 (1966). [16a] Pagsberg, P., Christensen, H., Rabani, J., Nillson, G., Fenger, J., and Nielsen, S. O., J. Phys. Chem. 73, 1029

(1969). [16b] Baxendale, J. H., and Mansell, A. L., Nature 190, 622

(1961).

[17] Baxendale, J. H., Radiation Res. 17, 312 (1962).

[18] Melville, H. W., Proc. Roy. Soc. A, 157, 621 (1936).

[19] Milas, N. A., Kurz, P. F., and Anslow, W. P., J. Amer. Chem. Soc. 59 , 343 (1937).

[20] Dainton, F. S., and Rowbottom, J., Trans. Faraday Soc. 49, 1160 (1953).

[21] Weeks, J. L., and Matheson, M. S., J. Amer. Chem Soc. 78, 1273 (1956).

[22] Hochanadel, C. J., Radiation Res. 17, 286 (1962).

[23] Livingston, R., and Zeldes, H., J. Chem. Phys. 44, 1245

(1966).

[24] Berces, T., and Trotman-Dickenson, A. F., J. Chem. Soc. 4281 (1961).

[25] Kralic, I., in The Chemistry of Ionization and Excitation,

eds. G. R. A. Johnson and G. Scholes (Taylor and Francis

Ltd., London, p. 303, 1967).

[26] Uri, N., Chem. Rev. 50, 413 (1952).

[27] Baxendale, J. H., and Magee, J., Trans. Faraday Soc. 51,

205 (1955).

[28] Fenton, H. J. H., Chem. Soc. (London) 65 , 899 (1894).

[29] Haber, F., and Weiss, J. J., Proc. Roy. Soc. A147 , 332

(1934).

[30] Baxendale, J. H., Evans, M. G., and Park, G. S., Trans. Faraday Soc. 42, 155 (1946). [31a] Koltoff, I. M., and Medalia, I. A., J. Amer. Chem. Soc. 71,

3777 (1949). [31b] Koltoff, I. M., and Medalia, I. A., J. Amer. Chem. Soc. 71,

3784 (1949).

[32] Barb, W. G., Baxendale, J. H., George, P., and Hargrave,

K. R., Trans. Faraday Soc. 47 , 462 (1951).

[33] Merz, J. H., and Waters, W. A., J. Chem. Soc. S15 (1949).

[34] Merz, J. H., and Waters, W. A., Disc. Faraday Soc. 2, 179

(1947).

[35] Cahill, A. E., and Taube, H. J. Amer. Chem. Soc. 74,

2312 (1952).

[36] Dixon, W. T., and Norman, R. O. C., Nature 196, 891

(1962).

[37] Dixon, W. T., and Norman, R. O. C., J. Chem. Soc. 3119

(1963).

[38] Piette, L. H., Bulow, G., and Loeffler, K., Amer. Chem.

Div. Petrol Chem. 09—019 (1964).

[39] Sicilio, F., Florin, R. E., and Wall, L. A., J. Phys. Chem.

70, 47 (1966).

[40] Chiang, Y. S., Craddock, J., Mickewich, D., and Turkevich,

J., J. Phys. Chem. 70, 3509 (1966).

[41] Fischer, H., Ber. Bunsenges. Phys. Chem. 71, 685 (1967).

[42] Czapski, G., Levanon, H., and Samuni, A., U.S. Atom. Energy Comm. Report No. NYO-3753-9 (1969).

[43] Jefcoate, C. R. E., and Norman, R. O. C., J. Chem. Soc.

(B), 48 (1968).

[44] Lindsay-Smith, J. R., and Norman, R. O. C., J. Chem. Soc. 2897 (1963).

[45] Smith, P., and Wood, P. B., Can. J. Chem. 45, 649 (1967).

[46] Dixon, W. T., Norman, R. O. C., and Buley, A. L., J. Chem.

Soc. 3625 (1964).

8
Chapter III. Kinetic Analysis

The rate constants in this compilation have been

determined by one or another of the following

methods, for which the kinetic analysis will be

indicated and some of the requirements and pit- falls mentioned. Three of these methods lead to absolute values for the rate constant, while a fourth,

very commonly used, leads to relative values in

which only the ratio of the rate constant to that of a competing reference reaction is known.

Direct Observation of Product

Formation

Direct observation of the rate curve for the

formation of a product is feasible if the product, P,

has a sufficiently strong and accessible optical

absorption,

OH + X = P. (1) P may be an adduct free radical, where X represents

a stable molecule or ion, a free radical formed by

abstraction of hydrogen, or a radical or ion-product

of the oxidation of an ion. If no concurrent reaction

of hydroxyl radical such as OH + OH = H,02 (2)

is competitive with reaction (1), a condition that

will obtain if the ratio /ci[X]o/&2 [OH]o is sufficiently

large, say, greater than 50, the rate equation for

product formation is, simply,

^jP=*,[OH][X], (3)

which reduces, for the condition [X]o > [OH] to ^=*;[OH]. (4)

Since the formation of P and the disappearance of

OH are complementary, the integrated form of

this equation, in terms of the optical density,

Dt , of P at any time, is:

log (Dx - Dt ) - log Dx = - 0.4343*;*. (5)

k[ , the first order rate constant, may then be obtained

from the slope of a plot of log {Dx — Dt ) versus time,

which should be linear. The absolute value for k\,

the bimolecular rate constant, is then obtained from

k\ — &]7[X]o (6)

In this application, P must be the only species

absorbing at Ai, the monitoring wavelength, whose

concentration changes sensibly during the observation. It is generally not satisfactory, because of the

insensitivity of log plots, to rely on a linear plot

at only a single concentration of X, but rather more

desirable to demonstrate constancy of k\, with

variation of [X] 0 over a reasonable range. If several

products are formed concurrently from OH and X

in reaction (1), ki will be equal to the sum of the rate constants for the individual reactions.

If the rate of reaction (2) is not negligible relative

to that of (1), some deviation from linearity will be

observed and the occurrence of (2) must be taken

into account in the treatment of the data. This can

be done quite readily by analog or digital computer

calculation since the value for k2 , as will be seen, is known absolutely. In some cases the reaction of e~q and of H will not interfere in the foregoing direct

determination of kx

. In others where one or other

may result in the formation with a rate constant

differing from k[ of a product also absorbing at Aj,

it is necessary to convert e~q into OH by means of

reaction (3) chapter II, and to take into account the

reactivity of H, the concentration of which then

amounts to only 10 percent of that of OH.

An experimental condition which has occasionally

been overlooked in such rate constant determinations and which can result in small, but significant errors, is the nonuniformity of the radiation flux throughout the cell volume. A variation of the initial concentration of reacting species along the path of

the analyzing light has no effect [1, 2] upon rate constants for first order reactions and only a small

effect in the case of second order reactions. Variation across the light path in the direction of the

electron beam, if these are at right angles, has a small effect on first order constants and may

have a significant effect upon second order constants, as has been indicated with numerical

examples [1, 2]. A considerable number of absolute rate constants

for reactions such as addition to aromatic molecules,

electron transfer from anions, and a special case of

hydrogen abstraction, have been determined by

such observation of product formation in pulse

radiolysis. Included are some reactions which may

be used as reference reactions in competition methods. With proper attention to the experimental

conditions and care in analysis of the data, absolute

rate constants have been obtained with an uncertainty as low as ± 10 percent in the accuracy,

which represents very nearly the best attainable.

9
Direct Observation of the Hydroxyl

Radical Disappearance

This method, which is, in principle, the most

desirable one, has proved feasible [3~5] for only

two reactions, the recombination, reaction (2), and

the reaction with molecular hydrogen. As will be

seen in chapter IV, a weak optical absorption at wavelengths below 280 nm, seen in irradiated water

under appropriate conditions, is thought to be that

of the hydroxyl radical. By monitoring this absorption in the presence of a sufficient concentration of

hydrogen, an approximate value for the rate constant of the reaction

0H + H2 = H2 0 + H (7) known more accurately from other methods, was

determined. In this case for the condition [H2]o [OH]o, the integrated form of the differential rate equation, in terms of the optical density of OH, is

log Dt — log Do = — 0.4343/r^. (8)

k 7 may then be obtained absolutely from the slope

of the straight line resulting from a plot of log Dt versus time, if [H 2]o is known.

With N2 0 present to convert e~q to OH, the rate constant for reaction (2) was determined. If (2) were the only reaction occurring, the integrated

form of the differential rate expression for the disappearance of OH, in terms of its optical density,

would be:

l/D,-l/D„=(^\)t, (9)

where e* is the molar extinction coefficient of the

OH

hydroxyl radical at the wavelength, A., of observation. e* may be determined provided the dose is known, since the yields of OH and of e~q are accurately known. The absolute value for /c 2 may

then be determined from the slope of a straight line

plot of l/Dt versus time. Since there is some contribution from the reaction

0H + H=H2 0, (10)

this was taken into account in evaluating A; 2 . The limitation on the application of this method

stems from the fact that the absorption of OH is very weak and is in a region of the spectrum in

which most solutes as well as H02 , an intermediate

formed in the radiolysis, absorb rather strongly.

Competition Kinetic Measurements

Here only a relative rate constant can be

determined.

Stationary State Systems

While it is true that the pulse radiolysis technique

has provided most of the accurate OH-rate data

now available, much of the basic information con- cerning the general mechanisms of reaction of OH

has derived from the considerable effort directed

to the study of the stationary-state radiolysis of

aqueous solutions. In such systems, irradiated at the dose rates available with conventional x- or y-ray sources, the duration of the irradiation is many orders of magnitude greater than the lifetime

of the OH radical formed under these conditions.

Consequently, throughout the course of the radiolysis, this species is present at a constant, but ex- tremely low, concentration. Its reactions cannot be

observed directly and the determination of the

reaction mechanisms depend entirely on the qualitative and quantitative estimation of the final, stable

reaction products.

In aqueous solutions, therefore, the reactivity of

the OH radical must be obtained by analysis of the

yields and distribution of these final products. It can only be expressed relative to some other

reference rate constant which itself has to be

determined absolutely by some other method. The

reliability of rate data so determined is dependent,

therefore, on the accurate knowledge of the overall

reaction mechanism. However, in spite of these

difficulties, useful rate data have been obtained from

stationary state kinetic competition analysis. In principle, two general methods are employed

for the determination of a relative rate constant for

for reaction of OH with a given solute. In the first method, the kinetic analysis is based upon the effect

of an additive on the yield of a stable product

which results from reaction of OH with the solute.

In the second, the competition is measured by the

effect of the additive on the rate of removal or modification of the original solute.

The formic acid system [6] is an example of the

former method. OH radicals react with this solute

to form the carboxyl radical, COOH:

OH + HCOOH -> H2 0 + COOH. (11)

In the presence of ferric ion, at low pH, carboxyl

radicals are oxidized to C02 which can be collected

and measured after the radiolysis is complete:

COOH + Fe3+ -> C02 + Fe2+ + H+. (12)

With a second solute present, which reacts competitively with OH, the yield of C02 will decrease.

If C(C02 ) is the radiation yield of carbon dioxide

resulting entirely from reaction (12), then the effect on this yield of an added solute is given by the

equation:

...

1 _ = 1+ MX]

G(C02 ) A„[HCOOH]’

where [X] is the concentration of the added solute

and kx and k n are the respective rate constants for

reaction of OH with each solute. The relative rate constant ratio kx/kn is obtained from the data ex- pressed graphically according to eq (13).

10
Examples of the second type of stationary state competition system includes those in which the

highly-colored compound, paranitrosodimethylaniline, PNDA [7], and the pyrimidine base, thymine

[8-10], are used as reference solutes.

The compound PNDA has an intense optical

absorption band with a maximum at 440 nm where

the molar extinction coefficient is 3.42 X 10

cm-1. The reaction product of OH with PNDA,

believed to be an adduct radical, has a much weaker

absorption at this wavelength. Consequently, ir- radiation of an aqueous solution of this compound

leads to bleaching of the solution. In the presence

of a second solute which can compete effectively

for the OH radical, the extent of the bleaching is reduced. Simple kinetic analysis of the data yields

the relative rate constant ratio for reaction of OH

with the two solutes.

The thymine method is similar in principle to that employing PNDA as the reference solute. The

presence of the 5:6 double bond in this molecule

gives rise to a strong optical absorption band at 264 nm (e= 7.95 x 103A/-1 cm-1

). It is well es- tablished that OH radicals add to the 5:6 double

bond in thymine and, since this reaction destroys

the chromophore, reaction of OH can be followed by

direct measurement of the radiation-induced bleaching. Provided that the ultimate product of reaction of

OH with a competitive solute does not contribute

to the final absorption (or, if so, the extent of the

contribution must be known) the relative rate constant for the solute can be determined.

Although both these competition methods have

been used extensively, their reliability depends

ultimately on the validity of the overall mechanisms

assumed for the competition. Both solutes have

been used, however, as reference standards for

competition studies in pulse radiolysis where less

emphasis need be placed on knowledge of the

overall reaction mechanism. This is discussed later

in chapter XI.

Pulse Radiolysis Methods

Several competition methods are available where

the pulse radiolysis technique is used for obtaining

relative OH rate data, although, as before, the

validity of absolute rate constants derived from the

data depends upon the accuracy with which the

reactivity of OH with the reference solute can be

determined.

As with the stationary state systems, two general

spectrophotometric methods are available. In the

event that the reference solute has suitable optical

absorption properties, the reaction of OH radicals can be measured by observation of the bleaching of

this absorption. The reaction of a competitive solute

is indicated by a reduction in the extent of the

bleaching, provided the transient product has a weaker absorption in the relevant region of the

spectrum. Alternatively, the reference solute may

react with OH to form a transient product with a much stronger absorption than the stable solute.

In this case, provided the product of OH reaction

with the competitive solute does not absorb strongly

in the same region, the competition is followed by

measurement of the suppression of the transient

absorption.

The competition kinetics of such systems are as

follows: A solution containing a reference solute,

A, of concentration [C] and a concentration [S]

of a test solute B is irradiated by a short electron

pulse. The OH radicals then react with either solute

in proportion to the products of the concentration

and rate constant of the respective reactions:

k OH + A — absorbing product

kb OH + B — nonabsorbing product.

If A0 is the optical density of the absorbing product

when solute B is absent and A is the optical density

when solute B is present, then it follows that

Ao_,. A& [S]

A Aa [C] (14)

A plot of AJA against the ratio of solute concentrations is linear with the slope equal to kb/ka . The rate constant kb can be derived, assuming

that the reference rate constant, ka , can be meas- ured directly.

The carbonate radical, COg- was the first refer- ence system used in pulse radiolysis for the determination of OH rate data [11, 12]. The carbonate ion

reacts with OH to form the transient radical, CO^,

which has an absorption maximum at 600 nm. Reaction of OH with a competitive solute suppresses this

absorption band. An advantage of this system is that

it can be used in aerated solution, although the drawbacks include the rather low extinction coefficient

(e500 = 1.8 X 103M-1 cm-1

) and the obvious restriction to alkaline solutions.

Other systems were introduced in which the

extinction coefficient of the reference absorption

band is much higher. These include the halide and

pseudohalide radicals, I<r [4], and (CNS)^ [12],

which have absorption maxima at 390 nm and

480 nm respectively. The serious disadvantage

with systems of this type, which, in some cases may

invalidate the method, is due to the fact that the

reference absorptions are not formed in a one-step

process. In the thiocyanate system, for example,

the absorbing radical, (CNS)^, is formed ac- cording to OH + CNS- — CNS *, (15)

CNS + CNS- ^CNS2-. (16)

It has been shown [13] that reaction (16) is somewhat

slower than reaction (15) and is thus rate determining. Since the measured rate constant (6.6 X 10 9 M-1 s

-1

) for the formation of the absorption refers

492-021 OL - 73 - 3 11
to reaction (16), rate constants for other solutes

measured by competition using this value are too

low. Nevertheless, as will be discussed later, the

large amount of published rate data for this system

can be successfully normalized by arbitrary fitting

with apparent self-consistency. Added kinetic

complications which would arise if competition

reactions of the uncomplexed CNS radical were

to occur, seem to be unimportant in this system.

In similar competition studies where the Br2~ absorption, formed from OH oxidation of bromide

ions is used as the reference absorption, these

complications are probably present [12]. The competition between carbonate and bromide ions for

OH can be followed either by monitoring the CO;f

absorption or that due to Brf\ In either case, the

rate constant for reaction of OH with Br~ was found

to be 1.8X108 M~x s

-1

. However, the value obtained from direct observation of Br^- formation is considerably larger. Pulse radiolysis systems in

which the competition is measured by the effect of

the second additive on the OH-induced bleaching

of an absorbing reference solute, have been used

fairly successfully. Both the thymine and paranitrosodimethyl aniline methods, used originally under

steady state conditions, have been adapted for use

in pulse radiolysis experiments. As discussed

previously, the validity of the stationary state experiments depends on the validity of the reaction

mechanism assumed. However, in pulse radiolysis,

since the competition is followed by direct observation of the extent of reaction between OH and the

reference solute, subsequent radical reactions

which could complicate the overall mechanism,

become irrelevant.

A pulse radiolysis reference system which has

much to commend it, is that in which ferrocyanide

ion is used as the reference solute [14, 15]. The

ion is oxidized by OH rapidly and quantitatively in

the simple, one-electron transfer reaction:

OH + [Fe(CN)6 ] 4 - OH- + [Fe(CN)6 ] 3 -. (17)

The product, ferricyanide ion, is stable and has a

characteristic absorption band with a maximum at 410 nm (e 410 =1000 M_1 cm-1

). Further, it can be

used over the entire range of pH. Although radicals

produced by reaction of OH with the competitive

solute may, in some cases, react rapidly with the

ferricyanide ion formed in (17) at the radiation

doses which may be used in this technique, the

rate of such reactions are still too slow to prevent

accurate measurement of the initial values of the

absorption.

Intermittent Irradiation

Intermittent irradiation [16] may be employed

with application of the rotating sector method [17].

An absolute value for ki and a value for k-i relative

to k7 have been determined by this method. The

intermittency was provided by a pulsed, 1.5 MeV

Van de Graaff accelerator, with variable pulse

length and variable frequency (although the off/on

ratio was fixed at 7.5) with the period for a cycle

variable from 10“5 to 5 s. The product, of which

the yield was measured, was hydrogen peroxide at steady state, [H2 02] ss , using intermittent irradiation and with a steady beam, [H2 02] b

. Data were

presented of [H2 02] ss/[H2 02] b as a function of r, the pulse duration. With a risetime of 1 /as the pulse

has a square waveform which simplifies the interpretation of rotating sector data.

A modification of the kinetic analysis normally

used in the rotating sector method (in which product

formation rate is proportional to the average con- centration of the rate controlling transient) was

necessary, since the concentration of measurable

products here is at steady state. The analysis in- volves the overall mechanism for the water radiolysis. The experimental quantity, [H2 02 ] ss / [H2 02 ] b, is a complex function of (a) r and the off/on ratio,

(b) the beam intensity, (c) the relative rate and,

hence, rate constants of the individual reactions,

(d) the ratio [02 ]/[H2 02 ] under steady beam

conditions and, (e) the yield of OH and of H2 . The

function is solved by trial and error by fitting to a curve of H2 02ss / [H2 02 ] b versus /c 2 [H2 02 ] b r from which the value for &2 is found. Notwithstanding the complexity of the function used, the

uncertainty in &7 is only ± 10 percent. Since

A: 2 /£f is also obtained in this experiment, a relative

value for the recombination rate constant is determined. The reader is referred to the original

papers for the detailed interpretation of the data.

Chapter III. References

[1] Sauer, M. C., Jr., Argonne National Laboratory Report

No. 7327, (May 1967).

[2] Boag, J. W., Trans. Faraday Soc. 64, 677 (1968).

[3] Thomas, J. K., Rabani, J., Matheson, M. S., Hart, E. J., and

Gordon, S., J. Phys. Chem. 70, 2409 (1966). [4] Thomas, J. K., Trans. Faraday Soc. 61, 702 (1965). [5] Pagsberg, P., Christensen, H., Rabani, J., Nilsson, G., Fenger, J., and Nielsen, S. O., J. Phys. Chem. 73, 1029

(1969). [6] Baxendale, J. H., and Smithies, D., Z. Phys. Chem. 7, 242

(1956). [7] Kraljic, I., and Trumbore, C. N., J. Amer. Chem. Soc. 87,

2547 (1965). [8] Ward, J. F., and Myers, L. S., Jr., Radiation Res. 26, 483

(1965). [9] Scholes, G., and Willson, R. L., Trans. Faraday Soc. 63,

2983 (1967). [10] Scholes, G., Shaw, P., Willson, R. L., and Ebert, M., in Pulse Radiolysis, ed. M. Ebert et al., p. 151 (Academic

Press, London & N.Y., 1965). [11] Adams, G. E., and Boag, J. W., Proc. Chem. Soc. 112

(1964). [12] Adams, G. E., Boag, J. W., and Michael, B. D., Trans. Faraday Soc. 61, 1417 (1965). [13] Baxendale, J. H., Bevan, P. L. T., and Stott, D. A., Trans. Faraday Soc. 64, 2389 (1968). [14] Adams, G. E., Boag, J. W., and Michael, B. D., Trans. Faraday Soc. 61, 492 (1965). [15] Willson, R. L., Greenstock, C. L., Adams, G. E., Wageman,

R., and Dorfman, L. M., Int. J. Radiat. Phys. and Chem.

3,211 (1971). [16] Schwarz, H. A., J. Phys. Chem. 66, 255 (1962). [17] Burnett, M., and Melville, H. W., Techniques of Organic

Chem., Vol. 8, A. Weissberger, Ed. (Interscience Pub.,

New York, N.Y. 1953).

12
Chapter IV. Physical Properties and Chemical Equilibrium

Those physical properties of the hydroxyl

radical in water which pertain most directly to its chemical reactivity are the pK for its ionic dissociation, which has been accurately determined, and

its diffusion coefficient, which has been estimated.

Several other properties, such as the optical

absorption spectrum, electron paramagnetic reso- nance absorption spectrum, and electron affinity

which are also known, are given here.

The hydroxyl radical is a neutral free radical.

The determination that it bears zero charge when

formed in water radiolysis was made in two ways,

by comparison of the reactivity [1] of OH produced

in radiolysis with that produced photochemically

and by the use of tests of the kinetic salt effect

[2]. In the former, the chemical behavior of photolytic and radiolytic OH was shown to be the same;

in the latter experiments the absence of any kinetic

salt effect showed that the oxidizing species in

water radiolysis bore zero charge. This conclusion was an important step in identifying the species as OH. The formation of hydroxyl radicals in irradiated ice and in various irradiated hydrated crystals, has also been shown [3, 4] from its ESR

spectrum at 4.2 K and 77 K. This spectrum consists

of an asymmetric doublet in accord with the interaction of the unpaired electron with the hydrogen

atom. The isotropic hyperfine splitting in ice was — 26.3 Oe compared with —26.7 Oe in the vapor

phase [5] and a calculated value [6] of —24.2 Oe.

This observation of OH in irradiated ice further

supports the conclusion that it is formed in water,

but to the best of our knowledge, the ESR spectrum

has not been observed in the liquid phase, very

likely because of the low steady state concentration

and short lifetime.

The electron affinity of the hydroxyl radical in

the gas phase does not appear to be known accurately, and experimental estimates cover a substantial range. A value [7, 8] of about 2 eV or

slightly higher, seems to be correct. The heat of

hydration of the hydroxide ion is estimated [9] to be about 88 kcal/mol. The sum of this value and the

electron affinity of OH in the gas phase would then

place the electron affinity of OH in water at about

5.8 eV.

The hydroxyl radical behaves like a weak acid,

undergoing ionic dissociation in strongly basic

solution:

0H+0H-^0- + H2 0. (l) The pK for this equilibrium is 11.9 ±0.2. Since

0~, the basic form of the hydroxyl radical, exhibits

substantially different chemical reactivity than the

more familiar form, OH, we may expect the apparent

reactivity of the hydroxyl radical to show a marked

pH-dependence in the region of pH from about

10.5 to 13. This difference in reactivity was, in fact, used

[10] to determine the pK in the following way. The

reactivity of OH with ferrocyanide is much greater

that that of 0~.

OH+ [Fe(CN)6 ] 4 -=OH-+ [Fe(CN)6 p-, (2) H2 0

0-+ [Fe(CN)6 ] 4 - - 2 OH-+ [Fe(CN)6 ]

3 -. (3) This rate of formation of ferricyanide ion was observed by pulse radiolysis as a function of pH in

the appropriate range. With k2 > k3 , the following

equation holds:

log (k2/k — 1) = pH — pK (4) where k is the apparent rate constant for reaction

of both OH and O

-

. The data gave pK= 11.9 ±0.2.

This was also confirmed in later experiments in

which the reactivity of hydroxyl with carbonate

ion was studied [11], which gave pA=11.8±0.2

and with thiocyanate ion [12] which gave

pK— 11.9±0.15.

The reverse of (1), the reaction of O

- to form OH,

involves a water molecule rather than the hydronium

ion which is present in very low concentration at such high pH.

The reactions of OH and of O

- are listed in

separate chapters in this compendium, where the

difference in reactivities may be readily compared.

These differences in reactivity with compounds such

as oxygen and aromatic molecules are so large as to be qualitative.

The hydroxyl radical in water appears to show

a weak optical absorption in the far ultraviolet,

which has been recorded in pulse radiolysis

studies [13-15], In the gas phase, OH exhibits a

series of fairly strong absorption bands [16] at 280 to 306 nm. In water, a weak absorption, which is thought to be that of the hydroxyl radical, is seen,

starting at about 300 nm and increasing at shorter

wavelengths to 200 nm, the limit of measurements

made. This absorption, which seems to be continuous, is shown in figure 2 in which data from two

groups of investigators is included. If any fine

structure existed, it would not likely be seen in such

spectrophotometric measurements with a relatively

large band pass. The identification of this absorption with OH is based upon two observations:

13
FIGURE 2. The absorption spectrum attributed to the hydroxyl

radical in water obtained by pulse' radiolysis of aqueous solutions containing either N2 O or H 2 O2 to scavenge the hydrated electron. The molar extinction coefficient is plotted against wavelength. O Data from reference

[15], chapter IV, in which the solution contains 2 mM N2 O, and the absorption is corrected for H 2 O2 , H and OH # Data from reference [13], chapter IV, in which the

solution contains either ImAf N2O or 0.2 mM H2 O2 , and the absorption is corrected

for H2O2.

(1) with N20 present, the optical density is doubled

in accord with the conversion of e~ to OH and, aQ

(2) the decay kinetics of the absorption in this sys- tem in which [OH]o/[H] 0 = 10, and there are, ostensibly, no other radicals present, gives values

for &oh+oh and &0H+H2 in quite good agreement

with the values obtained by other methods. Molar

extinction coefficients of 370 M~l cm-1 at 260 nm [13]

and 530 A/-1 cm-1 at 230 nm [15], in fairly good

agreement, have been reported from the two

laboratories. The suggestion has been made [15]

that this absorption is a red-shift of the water

continuum resulting from a partial electron transfer

from the first excited singlet state of water to a neighboring OH radical.

It should be understood that experiments in this

spectral region, involving as they do a weakly

absorbing transient, are difficult and complex in

that there are overlapping absorptions from species

such as H, OH-

, and H2O2. However, appropriate

corrections [15] have been made and the agreement

in the observation lends confidence to the validity

of the assignment. The existence of the absorption

maximum reported [15], does, however, depend

upon the optical density measurement at a single

wavelength. A somewhat similar absorption thought

to be that of O

- has been observed [17] in strongly

basic solution, but no definitive evidence for the

assignment has been obtained.

The diffusion coefficient of the hydroxyl radical

in water was estimated [18] from the value determined for the rate constant of the reaction of

hydroxyl radical with catalase, which is 8.3 X

lQi° jyj -

s

-i 25 °C. The diffusion coefficient was

obtained from this value and from the Smoluchowski [19] equation on the assumption that the reaction was diffusion-controlled and that the catalase was stationary, with OH being the only diffusing

partner. With the reasonable approximation that

the sum of the radii of interaction is equal to the

molecular radius of the catalase molecule, a value

of 2.3 X 10~ 5 cm 2 s -1 at 25 °C was estimated for

the diffusion coefficient. The assumption is also

inherent in taking the rate constant to be diffusioncontrolled, that no factor of geometry is involved in the value for the specific rate. If this were not the case, the value determined would then represent a lower limit for the diffusion coefficient of OH.

Table 1 summarizes these physical properties of

OH.

Table 1 . Physical properties of OH in water

Optical absorption at A. sS 300 nm

Molar extinction coefficient J

£260 — 370 M-1 cm"1

l e

230 = 530 M-1 cm'1 pK 11.9±0.2

Diffusion coefficient 2.3 X 10 -5 cm 2 s _l ESR spectrum (ice) Doublet

Electron affinity (gas phase) 2 eV

Electron affinity (aqueous-calc.) 5.8 eV

Heat of ionization 20 0.44 eV

Chapter IV. References

[1] Hochanadel, C. J., Radiation Res. 17,286(1962).

[2] Hummel, A., and Allen, A. O., Radiation Res. 17,302 (1962).

[3] Matheson, M. S., and Smaller, B., J. Chem. Phys. 23 , 521

(1955).

[4] Gunter, T. E., J. Chem. Phys. 46, 3818 (1967).

[5] Radford, H. E., Phys. Rev. 126, 1035 (1962).

[6] Kayama, K., J. Chem. Phys. 39, 1507 (1963).

[7] Page, F. M., Advances in Chemistry Series No. 36,

Free Radical in Inorganic Chemistry, p. 68 (1962).

[8] Branscomb, L. M., and Smith, S. J., Phys. Rev. 99, 1657

(1955).

[9] Benjamin, L., and Gold, V., Trans. Faraday Soc. 50, 797

(1954).

[10] Rabani, J., and Matheson, M. S., J. Phys. Chem. 70, 761

(1966).

[11] Weeks, J. L., and Rabani, J.,J. Phys. Chem. 70,2100(1966).

[12] Adams, G. E., Boag, J. W., Currant, J., and Michael, B. D.,

in Pulse Radiolysis, p. 117, ed. M. Ebert et al. (Academic

Press, London and New York, 1965).

[13] Thomas, J. K., Rabani, J., Matheson, M. S., Hart, E. J., and Gordon, S., J. Phys. Chem., 70, 2409 (1966).

[14] Thomas, J. K., Trans. Faraday Soc. 61, 702 (1965).

[15] Pagsberg, P., Christensen, H., Rabani, J., Nilsson, G., Fenger, J., and Nielsen, S. O., J. Phys. Chem. 73, 1029

(1969).

[16] Oldenberg, O., and Rieke, F. F., J. Chem. Phys. 6, 439

(1938).

[17] Boag, J. W., and Adams, G. E., Cellular Radiation Biology,

p. 64 (The Williams and Wilkins Co., Baltimore, 1965).

[18] Henglein, A., Karmann, W., Roebke, W., and Beck, G., Makromolekulare Chemie 92, 105 (1966).

[19] Smoluchowski, M. V., Z. Physik. Chem. 92, 129 (1918).

[20] Baxendale, J. H., Ward, M. D., and Wardman, P., Trans.

Farad. Soc. 67,2532(1971).

14
Chapter V. Addition Reactions

Hydroxyl radicals add very readily to aromatic

molecules to form the corresponding hydroxycyclohexadienyl radicals and to unsaturated aliphatic

compounds to form hydroxyalkyl radicals. Observation of the rate curve for formation of the cyclohexadienyl radicals in pulse radiolysis, the spectra

of which were first reported [2] for benzene and a

series of substituted benzenes, permits the determination of absolute rate constants for these reactions. The various substituted cyclohexadienyl

radicals which may be formed exhibit a strong

optical absorption in the uv and, therefore, represent a convenient transient product for monitoring

the reaction. Rate constants have thus been determined absolutely for addition to compounds such

as benzene [1, 2], benzoic acid [3], hydroxybenzenes [4], nitrobenzene [5] and many others.

OH + C6 H6 = (OH)C6 H6 • ( 1 ) OH + C6H5 COOH = (OH)C6H5 COOH. (2) The occurrence of such addition reactions, which

compete overwhelmingly with hydrogen abstraction

from these molecules, was first demonstrated by

kinetic and isotopic studies [ 1] and confirmed by

ESR observation [6] of the hydroxycyclohexadienyl

radical in a flow system in which OH was generated

chemically by the reaction of titaneous ion with

hydrogen peroxide. However, just as the hydroxyl

radical adds readily in reactions ( 1 ) and (2), so does

the hydrogen atom add readily to form cyclohexadienyl radicals.

H + CeHe = C6 H7 • , (3)

H + C6 H5 COOH = • C6 H6 COOH. (4)

The hydroxycyclohexadienyl radicals and the cyclohexadienyl radicals of the same compound exhibit

rather similar, overlapping optical absorption

spectra with molar extinction coefficients of comparable magnitude [2-5]. Consequently, any attempt

to observe the reaction by monitoring the optical

absorption in a system containing both hydroxyl

radicals and hydrogen atoms will result in monitoring both OH-addition and H-addition concurrently.

Since the rate constants for OH-addition are, in

general, somewhat different from those for H-atom

addition, unless the conditions of the system are arranged to resolve the reactions of the hydroxyl

radicals or the hydrogen atoms, a composite of the

two rate constants will be obtained. Moreover, the

hydrated electron may also react with the solute

molecule to complicate the system even further by

the formation of a transient with an overlapping

absorption unless these complications are eliminated.

These complications have been eliminated, in

the case of the reactants cited [2-5], as well as for

many other reactants, by the use of selective

scavengers. The hydrated electron has been scav- enged by the use of N2 0 in many systems.

e aq + N2 O = N2 + 0“, (5) O- + H2 0 = OH + OH". (6) As has been pointed out in chapter II, this converts

the system almost completely to a one-radical

system with [OH]o/[H]o = 10, in which rate constants such as k\ and &2 may be determined ac- curately with only a small correction for the effect

of the concurrent reactions (3) and (4) respectively.

This correction may, nevertheless, amount to as much as 5 to 8 percent, depending on the particular

compound and the experimental conditions. Oxygen

has also been used [5] as a scavenger to remove

both eaq and H to form a relatively unreactive

transient.

eaq + 02 — 02 , (7) H + 02 = H02 . (8)

£3 , &4 , and other rate constants for analagous addition reactions of hydrogen atoms, the values of

which are needed to provide accurate, if small,

corrections in determining k\ and &2 , have been

determined from experiments in which OHradicals are eliminated by the use of selective

scavengers such as deuteromethanol [2, 3] or

thallous ion [5]. With such necessary attention to the details of the system, rate constants with an

uncertainty as low as ± 15 percent have been determined for many OH-addition reactions. Some rate constants for OH-addition reactions have also been

determined from competition kinetic studies in

pulse radiolysis as well as in steady state irradiations. The values obtained by all these methods,

along with pertinent information about the conditions are given in table 2, page 17.3 One such condition, the pH of the solution, is particularly worthy of note since it may affect the

value of the rate constant. Many of the compounds

listed in table 2 undergo an ionic dissociation, for

3 All subsequent tables are placed at the end of the chapter in which they are dis- cussed. See contents for page numbers of specific tables.

15
which the equilibrium is pH-dependent. The rate constant for OH-addition may be different, as for

benzoic acid and benzoate ion [3], for the neutral

molecular form and the molecule ion. Consequently,

the pH of the solution must be accurately known,

especially in the range near the pK of the reactant

molecule if an ionic dissociative equilibrium occurs. There is a complication inherent in many of these

addition reactions, the details of which are difficult to determine and which has, consequently, not been

adequately defined for most of the reactants in

table 2. For substituted aromatic compounds, or polycyclic compounds, a number of different

isomeric hydroxycyclohexadienyl radicals may be

formed by the addition of the hydroxyl radical. The

determination of the rate constant for OH-addition

by observation of the transient product in pulse

radiolysis, or by competition studies, reveals only

the value for the overall rate constant, and not the

separate rate constants for formation of the individual isomers in concurrent reactions. The absence

of this information in no way invalidates the rate constants in table 2, which are thus the sum of rate constants for such individual reactions. For those

compounds about which information on the isomeric

distribution has been obtained, the details are given in the table.

Rate constants have been determined for a

sufficient variety of substituted aromatic compounds

to permit some assessment to be made of the effect

of substituent groups upon the reactivity of aromatic

compounds toward hydroxyl radicals. Such an assessment of the rates of OH-addition as an electrophilic substitution reaction has been presented

[7, 8] in terms of the Hammett equation [9],

log

l OH + PhX

' OH + PhH

crp. (9)

where cr is a substituent constant defined by the

log of the ratio of the acidity constant for a substituted benzoic acid to the acidity constant for

benzoic acid, and &oH + Phx and ^oH + PhH are the OH

addition rate constants for the substituted benzene

respectively, p is a constant for the specific reaction.

Figure 3 shows [8] a plot of the cr-values for a number

of mono-substituted benzenes and p-substituted

benzoate ions as a function of log ^OH + PhX using a ^OH + PhH

set of absolute rate constants. The data indicate a

correlation with a value p = 0.5 obtained from the

slope, suggesting that the addition reaction is an

electrophilic substitution. There may be some

question about the significance of this empirical

correlation in view of the fact that the rate constants cover a relatively small range, are all fairly close to the diffusion controlled limit, and show a substantial

degree of scatter of the data presented [7, 8] in such a plot.

Many of the foregoing studies, in which the rates of formation and the optical absorption spectra of a great variety of cyclohexadienyl radicals have been

determined, have been quite successfully directed

toward the exposition of the properties of these

radicals. Thus, information has been obtained about

the dimerization, the detachment of water from

hydroxy- and dihydroxycyclohexadienyl radicals,

the pK of many of the radicals formed, and the slow

decomposition of the dimers of the cyclohexadienyl

radicals. These properties, although important to an understanding of the chemistry of the systems, are not our concern in this monograph.

log jWiiL

* OH + PhH

FIGURE 3. Correlation of rate constants for hydroxyl radical addition reactions with Hammett's equation.

The value for the Hammett cr is plotted against the value of the Hammett function

in which £0H + Phx is the rate constant for the monos ubstituted benzene or the p-substi- tuted benzoic acid and ^om-pim is the rate constant for benzene or benzoic acid.

16
Table 2. Addition reactions

Reactant k{M~' s-») T(°C) pH Ref. Method Comments

acetanilide 5.0 X 109 25 9 [7] S irrad. with PNDA Rel. to A'oh + pnda^ 1-25 X 10 10

acetophenone t*(6.5±0.7) x 109 RT 7 [8] PR prod. form. Absolute. N2 0 added. H-corr.

4.8 x 109 25 9 [7] S irrad. with PNDA Rel. to Aqh+pnda = 1-25 x 10 10

5.2 x 109 RT [17] PR comp, with ferrocyan Rel. tO k oh + ferrocyan. = 9.3 X 109

* 5.4 x 109 RT [17] PR prod. form. Absolute. N2 0 added

acrylamide (3.3 ±0.6) x 109 18-25 10.7 [10] S irrad. 14 C02 form. Rel. to kOH + benzoate 3.6 X 109 from benzoic acid with ratio = 0.59, e~ scav.

aq by 02 (3.3±0.8)X 109 20 ±3 6 [11] PR comp, with OH + CNS" to 11X10,0

4.7 x 109 RT [17] PR comp, with ferrocyan Rel. tO k oh + ferrocyan. = 9.3 X 109 *6.8 x 109 RT [17] PR prod. form. Absolute. N2 0 added

aniline (6.9±0.9) x 109 18-25 10.7 [10] S irrad. 14 C02 form. Rel. tO k OH + benzoate 5.6 X 10 from benzoic acid with ratio = 1.24, e~ scav.

aq by 02 8.9 x 109 25 9 [7] S irrad. with PNDA Rel. to A: oh + pnda = 1-25 X 1

0

1

0

/Vv/V-dimethyl 8.9 x 109 25 9 [7] S irrad. with PNDA Rel. to Aqh+pnda^ 1-25 X 10 10

aniline

anisole t*(1.2±0.3) X 10'° RT 7 [8] PR prod. form. Absolute. N2 0 added. H-corr.

6.0 x 109 25 9 [7] S irrad. with PNDA Rel. to A0h+pnda = 1-25 X 1010

benzamide 4.4 x 109 25 9 [7] S irrad. with PNDA Rel. to Aqh+pnda— 1 -25 x 10'°

benzene t*(7.8± 1.1) X 109 RT 7 [8] PR prod. form. Absolute. N2 O added. H-corr.

(6.7 ±0.7) x 109 18-25 10.5 [10] S irrad. 14 C02 Rd. tO A‘oh+ benzoate 5.6 X 109 form, from benzoic acid with ratio = 1.2. e~ scav. by 02 (4.7 ± 0.8) x 109 18-25 3 [10] S irrad. 14 C02 Rel. tO A‘oH+benzoic = 4.3 X 109 form, from benzoic acid with ratio = 1.1 *(3.3 ± 0.8) x 109 25 3 [12] PR prod. form. Absolute. No H-scav. used.

.’. cone. H-addit.

3.2 x 109 RT 7 [13] PR comp, with hr form. Rel. to A,oH+I" with ratio =

0.31, but doubtful since

1 + 1 - may be rate

limiting. 5.0 x 109 RT 6-7 [14] PR comp, with OH + CNS- Rel- “‘t-'l-lxlO"

with ratio = 0.45

(4.8 ±0.5) x 109 RT 2. 0-2.

2

[15] PR comp, with thymine Rel. to A‘OH+thymine~ 4.7 X 109 with ratio = 1.02 (4.3±0.9) x 109 23 neut. [1] PR prod. form. Absolute. No H-scav. used. but may be trace O2 present. t(7.6± 1.9) x 109 RT neut. [16] PR comp, with PNBA- ion Rel. to Aoh+pnba = 2.6 X 109 e~ removed by scav.

aq J

benzene-^ (4.7 ± 0.9) x 109 23 neut. [1] PR prod. form. Absolute. No H-scav. used, but may be trace O2 present.

benzene-sulfonamide 2.8 x 109 25 9 [7] S irrad. with PNDA Rel. to Aqh + pnda —’ 1-25 X 10 10 or Aoh + c2 h5oh = 1-85 X 10 9

benzene-sulfonate ion 3.0 x 109 25 9 [7] S irrad. with PNDA Rel. to A'qh+pnda~ 1.25 X 1010 t *(4.7 ±0.6) x 109 RT 7 [8] PR prod. form. Absolute. N2 0 added. H-corr.

17
Table 2. Addition reactions — Continued

Reactant kiM*1 s-') r(°C) pH Ref. Method Comments

benzoate ion 4.2 X 109 25 9 [7] S irrad. with PNDA Rel. to Aqh + pnda = 1*25 X 10 10

t *(6.0 ±0.7) x 109 RT 6-9.4 [8] PR prod. form. Absolute. N2 0 added. H-corr.

t *(5.7 ±0.2) x 109 RT [17] PR prod. form. Absolute. N2 0 added. No Hcorr. 5.5 x 109 RT 6-7 [14] PR comp, with OH + CNS- Rd-'o t.-'l-lxlO"

5.4 x 109 RT [17] PR comp, with ferrocyan. Rel. tO AroH+ferrocyan. 9.3 X 109

p-aminobenzoate ion 7.9 x 10s 25 9 [7] S irrad. with PNDA Rel. to &oh+pnda = 1-25 X 1010

p-bromobenzoate ion 3.1 x 109 25 9 [7] S irrad. with PNDA Rel. to A'oh+pnda =1 .25 x 10ul

p-chlorobenzoate ion t*(5.0±0.8)x 109 RT 6-9.4 [8] PR prod. form. Absolute. N2 0 added. H-corr.

3.2 x 109 25 9 [7] S irrad. with PNDA Rel. to A'oh+pnda ~ 1-25 X 1010

p-fluorobenzoate ion 3.5 x 109 25 9 [7] S irrad. with PNDA Rel. to A'oh+pnda = 1.25 X 1010

p-hydroxy-benzoate *(9±2)X 109 RT 7 [8] PR prod. form. Absolute. N2 0 added. H-corr.

ion 5.4 x 109 25 9 [7] S irrad. with PNDA Rel. to A:oh+pnda = 1-25 X 1 O'

0

p-methoxy-benzoate 5.0 x 109 25 9 [7] S irrad. with PNDA Rel. to A'oh+pnda = 1-25 X 1010 ion (anisate ion)

p-methylbenzoate ion 4.4 x 109 25 9 [7] S irrad. with PNDA Rel. to A: 0h+pnda = 1.25 X 1010 (4-toluate ion)

p-nitrobenzoate ion t *(2.6 ±0.4) X 109 RT 6-9.4 [8] PR prod. form. Absolute. N2 0 added. H-corr.

2.0 x 109 25 9 [7] S irrad. with PNDA Rel. to Atqh+pnda = 1.25 X 10'°

benzoic acid t *(4.3 ±0.8) x 109 25 3 [3] PR prod. form. Absolute. N2 0 added. H-corr.

*(2.1 ±0.3) x 109 25 3 [12] PR prod. form. Absloute. No. H-scav. used .\ cone. H-addit.

(5.6 ± 1.1) X 109 18-25 3 [10] S irrad. 14 C02 form, from Rd. to ^oh+ benzoate 5.6 X 10^ benzoic acid. with ratio= 1.0, e~ scav. ’ aq by 02

benzonitrile t*(4.9±0.6) X 109 RT neut. [8] PR prod. form. Absolute. N2 0-added. H-corr.

3.3 X 109 18-25 10.7 [10] S irrad. I4 C02 form, from Rd. to AoH+benzoate 5.6 X 10^ benzoic acid with ratio = 0.59, e~ scav.

aq by 02 3.6 x 109 25 9 [7] S irrad. with PNDA Rel. to Atqh+pnda = 1.25 X 1010 * 8.5 x 109 RT 6.3 [39] PR prod. form. Absolute. N2 0 added. No corr. H-atom

benzyl alcohol *(8.4± 1.2) X 109 RT neut. [8] PR prod. form. Absolute. N2 0-added. H-corr.

carbon monoxide 5.4 x 108 25 7 [18] Phot. H2 02 sol. Rel. to A:oh + Hs02 = 4.5 X 10 7 7.1 X 108 20-25 1 [19] Fenton’s reagent comp. Rel. to A0H+Fe2+ = 3.3 x 108 with ratio = 2.14

carbon dioxide < 106 RT neut. [20] PR prod. form. Absence of transient prod, sets

limit

chlorobenzene (6.2 ±0.7) X 109 18-25 10.7 [10] S irrad. 14 C02 form. Rel. tO AroH+benzoate 5.6 X 10 from benzoic acid with ratio = 1.1, scav. by 02

1 ,3-cyclohexadiene t (9.8 ± 2.5) x 109 RT neut. [16] PR comp, with PNBA- iorn Rel. to Aoh + pnba~ 2.6 X 10 9 eaq removed by scav. H abstraction amounts to 30 percent

18
Table 2. Addition reactions — Continued

Reactant k { M~l s- 1 ) T(°C) pH Ref. Method Comments

1 ,4-cyclohexadiene t (7.7 ± 1.9) x 109 RT neut. [16] PR comp, with PNBA- ion Rel. to &oh+pnba = 2.6 X 109 e

-

q removed by scav. H-abstraction amounts to 45 percent

cyclohexene (8.8±2.2) X 109 RT neut. [16] PR comp, with PNBA- ion Rel. to &OH+PNBA— = 2.6 X 109 e

- removed by scav.

aq J

dimethylsulfoxide 7.1 x 109 RT neut. [20] PR comp, with OH + CNSRel. to =1.1X10'° thiocyan.

dimethylthioether 8.7 x 109 RT neut. [20] PR comp, with OH +

methanol

Rel. to =1.1X10'° thiocyan. with ratio = 0.79, as sec- ondary standard

dioxane 2.4 x 109 RT 7 [13] PR comp, with If form. Rel. to /toh-h- with ratio = 0.23, but doubtful since I + I - may

be rate limiting

1.7 x 109 RT 2. 0-2.

2

[15] PR comp, with thymine Rel. to A: 0H+thymlne = 4.7 X 109 with ratio = 0.37

ethylene 1.8 x 109 RT neut. and

acid

[21]

[22]

PR comp, with OH + CNS- Rel- to 1-1X10"

Calc, although details not given

p-hydroxynitrobenzene (p-nitro- phenol)

t*(3.8±0.6)X 109 RT 1.3 [23] PR prod. form. Absolute. N>0 added. Evidence indicates reac.

is not addition, but with

nitro group

hydroquinone 2.1 x 10’° RT 6-7 [14] PR comp, with OH+CNS- Re'- >° C- =1-1x10'"

nitrobenzene t *(3.2 ± 0.4) X 109 RT neut. [8] PR prod. form. Absolute. N2 0 added. H-corr.

3.4 X 109 25 9 [7] S irrad. with PNDA Rel. to &oh+pnda= 1-25 X 10'° 2.2 X 109 18-25 10.5 [10] S irrad. ' 4 CO> form, from

benzoic acid

Rel. to A'oH+henzoate *^.6 X 10' with ratio = 0.39. e

- scav. by 02 t *(4.7 ± 0.5) X 109 RT 1 [5] PR prod. form. Absolute. 0> to scav. e

-

q nitromethane < 9 X 106 *RT neut. [24] PR comp, with OH+CNS- Rel- >0 = 1.1 X10'»

Upper limit only

nitromethane anion t*(8.5± 1.5) x 109 RT neut. [25] PR prod. form. Absolute. Addit. reac. with

anionic form of molecule

nitrosobenzene 1.9 x 10'° RT neut. [26] PR comp, with OH+CNS- Rel- to = 1.1X10'”

Reac. is not addit. to ring, but nitroso group

p-nitroso-/V,/V-dimethylaniline1.25 x 10'° 22-25 9 [27] S irrad. using PNDA

bleaching

Rel. to &oh+c2 h5oh = 1-85 X 10 ’ with ratio = 6.8 *1.8X10'° RT 9 [28] PR of PNDA bleaching Absolute. N2 0 added. Authors

question use of PNDA as

ref. reac. t* 1.25X10’° RT neut. [29] PR prod. form. Absolute. N2 0 added. No Hcorr. Low pulse intensity

recommended

phenate ion 9.1 x 109 18-25 10.7 [10] S irrad. ,4 C02 form, from

benzoic acid

Rel. to ApH+benzoate 5.6 X 10’ with ratio = 1.62, e

-

q scav. by 02

492-021 OL - 73 - 4 19
Table 2. Addition reactions — Continued

Reactant k{M~' s- 1 ) r(°C) pH Ref. Method Comments

phenol t *(1.4± 0.3) X 10 10 19-23 7. 4-7.

[4] PR prod. form. Absolute. N2 0 added. No corr. for H-addit.

8.6 x 109 25 9 [7] S irrad. with PNDA Rel. to A)oh+pnda = 1.25 X 10'° 6.0 X 109 18-25 [10] S irrad. I4 C02 form, from Rd. to ^OH+benzoate 5.6 X 109 benzoic acid with ratio=1.08, e~

q scav. by 02 1.8X1010 RT 6-7 [14] PR comp, with OH+CNS- Rel.to«U„=l-lX10'»

with ratio = 1.6

/3-phenylacetate (2- t*(7.9± 1.1) x 109 RT 6-8 [8] PR prod. form. Absolute. N2 0 added. H-corr. phenylethanoate) 4.4 X109 25 9 [7] S irrad. with PNDA Rel. to &oh+ pnda= 1-25 X 10'°

phenylacetate 5.0 X109 25 9 [7] S irrad. with PNDA Rel. to A: 0h+pnda~ 1-25 X 10'°

phenylhydroxylamine t*1.5x 10'° RT 3.7-11.5 [30] PR prod. form. Absolute. N2 0 added. No. Hcorr. 2.0 x 1010 RT [30] PR comp, with OH + CNS- Re3 to = 1.1 X101 "

pyridine (3.1 ±0.6) XI

9 RT neut. [31] PR comp, with OH + CNS- Rel. to ^h‘0Cyan. = l-l X H*10 *1.8 x 109 21 7 [33] PR prod. form. Absolute. N2 0 added. No Hcorr.

pyridinium ion (3.4 ± 0.6) xl

7 RT 1 [31] PR comp, with OH + CNS- Rel- to 1.1 X10>»

2,2'-dipyridyl *6.2 X 109 21 9.3 [33] PR prod. form. Absolute. N20 added. No. Hl corr.

4,4'-dipyridyl *5.3 x 109 21 9.3 [33] PR prod. form. Absolute. N2 0 added. No Hcorr.

salicylate ion 5.6 X 109 18-25 10.7 [10] S irrad. 14 C02 form, from Rel. to ^()H+benzoate 5.6 X 10 benzoic acid with ratio = 1.0, e

-

, scav. by 02 t*1.2 x 10'° RT 7 [32] PR prod. form. Absolute. N2 0 added. H-corr.

(2.0 ±0.2) x 1010 RT 7 [32] PR comp, with OH+ CNS- Rel. to &[h'oeyan. = l-l X 1°10

(9.4 ±0.1) x 109 22-25 9 [27] S irrad. with PNDA Rel. to A: 0h+i>nda= 1-25 X 10'°

terephthalate ion 3.2 x 109 25 9 [7] S irrad. with PNDA Rel. to &oh+pnda== 1-25 X 10'°

toluene *(3.0 ±0.7) x 109 25 3 [12] PR prod. form. Absolute. No H-scav. used, .’. cone. H-addit

20
Chapter V. References

[1] Dorfman, L. M., Taub, I. A., and Buhler, R. E., J. Chem.

Phys. 36, 3051 (1962).

[2] Sauer, M. C., and Ward, B., J. Phys. Chem. 71, 3971

(1967).

[3] Wander, R., Neta, P., and Dorfman, L. M., J. Phys. Chem.

72,2946(1968).

[4a] Land, E. J., and Ebert, M., Trans. Faraday Soc. 63,

1181 (1967).

[4b] Adams, G. E., Michael, B. D., and Land, E. J., Nature 211, 293 (1966).

[4c] Adams, G. E., and Michael, B. D., Trans. Faraday Soc. 63, 1171(1967).

[5] Asmus, K. D., Cercek, B., Ebert, M., Henglein, A., and

Wigger, A., Trans. Faraday Soc. 63 , 2435 (1967).

[6] Dixon, W. T., and Norman, R. O. C., Proc. Chem. Soc. 97

(1963).

[7] Anbar, M., Meyerstein, D., and Neta, P., J. Phys. Chem.

70, 2660 (1966).

[8] Neta, P., and Dorfman, L. M., Advances in Chemistry

Series No. 81, Radiation Chemistry I, 222 (1968).

[9] Hammett, L. P., Physical Organic Chemistry, Chap. 7 (McGraw-Hill Book Co. Inc., New York, N.Y., 1940).

[10] Matthews, R. W., and Sangster, D. F., J. Phys. Chem. 69,

1938 (1965).

[11] Chambers, K. W., Collinson, E., Dainton, F. S., Seddon,

W. A., and Wilkinson, F., Trans. Faraday Soc. 63,

1699 (1967).

[12] Dorfman, L. M., Taub, I. A., and Harter, D. A., J. Chem.

Phys. 41, 2954 (1964).

[13] Thomas, J. K., Trans. Faraday Soc. 61, 702 (1965).

[14] Adams, G. E., Boag, J. W., Currant, J., and Michael, B. D., Pulse Radiolysis, P. 131, Ed. M. Ebert et al., Academic

Press, New York, N.Y. (1965).

[15] Scholes, G., Shaw, P., Willson, R. L., and Ebert, M., Pulse

Radiolysis, p. 151, Ed. M. Ebert et al., Academic Press, New York, N.Y. (1965).

[16] Michael, B. D., and Hart, E. J., J. Phys. Chem. 74, 2878

(1970).

[17] Willson, R. L., Greenstock, C. L., Adams, G. E., Wageman,

R., and Dorfman, L. M., Int. J. Rad. Phys. and Chem. 3, 211 (1971).

[18] Buxton, G., and Wilmarth, W. K., J. Phys. Chem. 67,

2835 (1963).

[19] Dainton, F. S., and Hardwick, T. J., Trans. Faraday Soc. 53, 333 (1957).

[20] Meissner, G., Henglein, A., and Beck, G., Zeitschrift fur Naturforschung, B22, 13 (1967).

[21] Cullis, C. F., Francis, J. M., Raef, Y., and Swallow, A. J., Proc. Roy. Soc. A300, 443 (1967).

[22] Swallow, A. J., Advances in Chemistry Series No. 82,

Radiation Chemistry II, Ed. R. F. Gould, 499 (1968).

[23] Cercek, B., and Ebert, M., Advances in Chemistry Series No. 81, Radiation Chemistry I, 210 (1968).

[24] Asmus, K. D., Henglein, A., and Beck, G., Ber. Bunsengesell. fur Phys. Chem., 70, 459 (1966).

[25] Asmus, K. D., and Taub, I. A., J. Phys. Chem. 72, 3382

(1968).

[26] Asmus, K. D., Beck, G., Henglein, A., and Wigger, A., Ber. Bunsengesell. fiir Phys. Chem. 70, 869 (1966).

[27] Kraljic, I., and Trumbore, C. N., J. Amer. Chem. Soc. 87,

2547 (1965).

[28] Dainton, F. S., and Wiseall, B., Trans. Faraday Soc. 64,

694 (1968).

[29] Baxendale, J. H., and Khan, A. A., Int. J. Rad. Phys. and

Chem., 1, 11 (1969).

[30] Wigger, A., Henglein, A., and Asmus, K. D., Ber. Bunsengesell. fiir Phys. Chem. 71, 513 (1967).

[31] Cercek, B., and Ebert, M., Trans. Faraday Soc. 63, 1687

(1967).

[32] Amphlett, C. B., Adams, G. E., and Michael, B. D., Advances in Chemistry Series No. 81, Radiation Chemistry

I, 231, Ed. R. F. Gould (1968).

[33] Simic, M., and Ebert, M., Int. J. Rad. Physics and Chem.

3, 259 (1971).

[34] Chutny, B., and Swallow, A. J., Trans. Faraday Soc. 66,

2847 (1970).

21
Chapter VI. Hydrogen Abstraction Reactions

Fully saturated organic compounds and many

unsaturated molecules such as aldehydes and

ketones react with hydroxyl radicals by simple

hydrogen abstraction to form water and a carbon

radical

RH + 0H=R- + H2 0 (1)

In some cases, however, the possibility exists that

the detailed mechanism of reaction involves, ini- tially, electron abstraction to form a cationic specieswhich then rapidly deprotonates

RH + OH = RH+ + OH- (2)

RH+ = R+H+ (3)

Since OH is a strong electrophile, rate constants for

hydrogen abstraction are, in general, particularly

sensitive to the influence of inductive effects in the

structure. In addition to effects on the overall reaction rate, such effects influence the actual site of

the hydrogen abstraction process. This is well illus- trated by the data for mono- and polyhydric alcohols

given in table 3 of this chapter.

For the primary straight chain alcohols, the rate constants increase progressively from 8 X 108 M~l s

_1

for methanol to about 5-6 X 109 for alcohols containing five or more carbon atoms. This latter value

probably represents a diffusion-controlled limit for

reactions of these compounds. The trend undoubtedly illustrates the effect of the inductive effect of

the aliphatic carbon chain on the electron density at the a-carbon atom, although there is evidence that

OH attack becomes multifocal as the chain length

increases [1, 2]. Information from experiments

where OH is generated either by pulse radiolysis or by photolysis of hydrogen peroxide indicates that

the proportion of a-carbon attack decreases from

100 percent in methanol to 37 percent in normal

butanol. However, as might be expected, chain

branching increases the probability of reaction at this position. In isopropanol, about 95 percent of

OH radicals react at the a-carbon atom compared

with only 66 percent in normal propanol. In the polyhydric alcohols, the effect of carbon chain length on

the overall reactivity is less evident.

In the carboxylic acids (tables 4 and 5), the OH

reactivities are generally lower than those for the

alcohols. Further, there is a pronounced effect of

pH on the rate constants, the values for the anions

being usually greater than those for the unionized

structures. It would be expected that resonance

stabilization in the carboxyl group would tend to hinder OH attack at this group and if, therefore, the

/3, or more distant, carbon atoms were the only

available reaction sites, it is not surprising that the

rate constants are lower than those for the corre- sponding alcohols. Formic acid is an exception since an a-hydrogen is available in this molecule and is, therefore, more reactive than acetic acid. Substitution of an OH group in the a-position has a marked

effect on reactivity due to the effect on the electron

density at this site. Lactic acid is about 20-25 times

more reactive than acetic acid. The effect of halide

substitution at this position is less, although it is

still evident.

Data for carboxylic esters (table 6) are more limited, although where multiple determinations are

available, e.g., methyl acetate, the agreement is good. As might be expected the overall reactivities

of the esters lie between those of the aliphatic alcohols and acids from which they are derived. It can be seen that trends exist in the data. For a given

ester group, the rate constants increase with chain

length in the acid stem, e.g. butyrate > propionate >

acetate. As has been pointed out [3] ethyl formate is an exception to the trend, although, in this instance,

the reactivity is increased considerably by the pres- ence of the cx-hydrogen atom. For the various acetates, the rate constants also increase with chain

length, the value for the n-propyl derivative is an

order of magnitude greater than that for methyl

acetate. Electrophilic attack by OH on the carbonyl group

is most unlikely in these esters. Although it is diffi- cult to draw definite conclusions as to the site of

attack, the apparent effect on the rate constants of

the chain lengths of both the acid and the ester stems suggests that both regions of the molecules

are reaction sites. This aspect has been discussed in more detail elsewhere [3]. In the ethers (table 7), where the results of several determinations are available, the agreement is generally good. As would be expected, the data for

both diethyl ether and dioxan indicate that the OH

reactivities of ethers are generally unaffected by

variation in pH.

Rate constants for reactions of OH with miscella- neous carbonyl compounds are given in table 8. Understandably, acetone is the ketone for which

the most data are available. The agreement between

the values obtained by relative methods using five

different reference solutions is clearly excellent,

the overall spread between the separate mean values

being less than 10 percent. As is the case with the

alcohols and esters, the increasing influence of the

inductive effect of the alkyl group is again evident

in the four simple monoketones. However, the value

for acetone seems abnormally low, since the

increase from acetone to methylethyl ketone is

22
much greater than that observed in the corresponding alcohols. This may be due to the greater influ- ence in acetone of keto-enol tautomerization.

Aldehydes appear to be more reactive, as might

be expected, since, in solutions, these compounds

have some gem-diol character which should increase

the electron availability at the a-carbon atom.

The effect of miscellaneous substitutents is indicated in table 9.

Table 10 contains rate data for miscellaneous

nitrogen compounds. Information on amino com- pounds is scarce. However, there has been a systematic study of aliphatic amides [4], which has shown

that, in general, these compounds are very reactive.

The compound, CH3 CO N(CH3 )CH2 CH2 N(CH3 )CO

CH3 has been isolated from irradiated acetamide

solution [4] showing that the principle site of OH

attack on this molecule occurs at an N-methyl group.

Table 3. Aliphatic alcohols

Reactant k{M 1 s

-1

) pH Ref. Method Comments

methanol 7.35 X 108 2.0 [3] PR comp, with CNS- ReLtoP?!- = 1.1 x 1010 thiocyan. (7.6 ± 0.5) X 108 2.0 [5] S irrad. comp, with thymine Rel. tO ^oH+thymine^^- 7 X l(j9 8.6 X 108 6 [6 ] S irrad. comp, with Br- Rel. to &oh+C2 H50h =1-85 X 109 4.7 X 108 neut. [7] PR comp, with I - Rel. to &oh+i-= 1-02 X 10 10 (uncertain)

t 8.0 x 108 neut. [8 ] PR comp, with benzoate Rel. to A^QH+benzoate 5.6 X 10

18.3 X 108 neut. [8 ] PR comp with phenylacetic acid Rel. to &oh+pa= 7.9 X 109 t8.4 x 108 neut. [8] PR comp, with pN02 benzoate Rel. to ^qh+pnba^ 2.6 X 109 8.02 x 108 7.0 [3] PR comp, with CNS- Rel. to A"1 - = 1 . 1 X 1010 thiocyan. t9 x 108 neut. [9] PR comp, with Cu2+ Rel. to A: 0h+cu2 +~3.5 X 108 t 8.6 x 109 neut. [ 10] PR comp, with ferro cyanide Rel. tO &OH+ferrocyan. = 9.3 X 109 1.08 x 109 9 [ 11 ] S irrad. comp, with PNDA Rel. to£0H+PNDA = 1-25 X 10 10 1.00 X 109 9 [ 12 ] S irrad. comp, with PNDA Rel. to A:oh + pnda~ 1-25 X 10 10 8.6 x 10 8 6 [6] S irrad. comp, with Br- Rel. to &OH+ ethanol = 1-85 X 10 9 9.5 X 108 10.5 [13] S irrad. comp, with benzoate Rel. to &oh+ benzoate 5.6 X 109

methanol-^ 4.2 X 108 6 [ 6 ] S irrad. comp, with BR- Rel. to Atqh+CzHsOh = 1-85 X 109

ethanol 1.64 x 10 9 2 [3] PR comp, with CNS- Rel. to A: rel - = 1.1 X 10 10 thiocyan. (1.75±0.15) x 109 2 [5] S irrad. comp, with thymine Rel. tO ^OH + thymine 4.7 X 10‘ (1.7

0. 2 ) x 109 2 [14] S irrad. comp, with thymine Rel. tO A: 0h + thymine 4.7 X 109 7.2 X 108 neut. [7] PR comp, with I - Rel. to &oh + i-= 1-02 X KB0 1.59 x 109 3 [13] S irrad. comp, with benzoate Rel* to koh

+

benzoate 5.6 X 109 (1.55 ±0.2) X 10 9 5 [14] S. irrad. comp, with thymine Rel. tO &OH + thymine 4.7 X 10' 1.15 X109 6.6-10.5 [15] S irrad. comp, with ferrocyan. Rel. to A:qh + ferrocyan. = 9.3 X 109 1.8 X109 7 [3] PR comp, with CNS- Rel. to&rel =1.1X1010 thiocyan. 1.8 X109 7 [16] PR comp, with H2 O2 Rel. to &0H+H2O2 = 4.5 x 10 ‘ tl.73 x 10 9 neut. [ 8] PR comp, with benzoate Rel. to A^oh+ benzoate 5.6 X 10

tl.83 X 109 neut. [8 ] PR comp, with phenylacetic acid Rel. tO ^OH + phenyl. A = 7.9 X 109 tl .81 x 10 9 neut. [ 8] PB comp, with pN02 benzoate Rel. to koh + pnba = 2.6 x 10 9 tl.85 x 10 9 neut. [ 10] PR comp, with ferrocyan Rel. to ^OH+ ferrocyan. = 9.3 X 109 tl .8 x 109 neut. [9] PR comp, with Cu2+ Rel. to Aoh + Cu2 + = 3.5 X 109 (1.82±0.1) X109 9 [ 11 ] S irrad. comp, with PNDA Rel. tO&OH + PNDA = 1-25 X 10 10 1.54 x 109 10.5 [13] S irrad. comp, with benzoate Rel. tO Aoh + benzoate 5.6 X 109

ethanol d$ 1.14 x 109 6 [ 6] S irrad. comp, with Br- Rel. to ^oh + C2H5OH = 1-85 X 10 9

/3-bromoethanol 7.7 x 108 9 [17] S irrad. comp, with PNDA Rel. to ^oh + C2H5OH = 1.85 X 109

/3-chloroethanol 9.2 x 108 9 [17] S irrad. comp, with PNDA Rel. to &0H+C2 H5OH = 1-85 X 109

2 -ethoxyethanol 1.68 x 109 9 [ 6] S irrad. comp, with PNDA Rel. to ^oh+C2H5oh = 1 -85 X 109

n-propanol (2.8 ±0.3) X 109 2 [14] S irrad. comp, with thymine Rel. to ^oH + thymine = 4.7 X 109 (2.6 ±0.3) x 109 5 [14] S irrad. comp, with thymine Rel. to kOH + thymine = 4.7 X 109 2.45 X 109 7 [3] PR comp, with CNS - ReL to k’^- 1 . 1 X 10 “

t2.7 x 109 neut. [ 10] PR comp, with ferrocyan. Rel. to koh + ferrocyan. = 9.3 X 109 2.72 x 109 9 [6] S irrad. comp, with PNDA Rel. to &oh+c2 h.oh~ 1-85 X 109

23
Table 3. Aliphatic alcohols — Continued

Reactant k(M~' s

-1

) pH Ref. Method Comments

isopropanol (1.82 ± 0.15) x 109 2 [5] S irrad. comp, with thymine Rel. to ^OH + thymine 4.7 X 109 2.2 x 109 6 [6] S. irrad. comp, with Br- Rel. to ^oh+c2 h5 oh = 1-33 X 109

(2.0 ±0.2) x 109 2 [14] S irrad. comp, with thymine Rel. tO ^OH + thymine 4.7 X 109 (2.0 ± 0.2) x 109 5 [14] S irrad. comp, with thymine Rel. tO Aqh + thymine' 4.7 X 109 1.74 x 109 neut. [7] PR comp, with I - Rel. to Ar 0H + i-~ 1-02 X 10 10 t2.01 x 109 neut. [10] PR comp, with ferrocyan Rel. tO ^’oH + ferrocyan. 9.3 X 109 (2.13 ±0.1) x 109 9 [11] S irrad. comp, with PNDA Rel. to k oh

PNDA ~ 1.25 X 10 10 2.0 x 109 9 [6] S irrad. comp, with PNDA Rel. to £oh+c2 h5 oh = 1-35 X 109

1.84 x 109 ? [18] PR comp, with CNS- Rel. to A: rel = 1.1 X 1010

thiocyan. aD isopropanol 1.44 x 109 6 [6] S irrad. comp, with Br- Rel. to £oh+c2 h5 oh = 1-35 X 109

n-butanol (4.0 ±0.4) x 109 2 [14] S irrad. comp, with thymine Rel. tO &OH + thymine 4.7 X 109 (4.0 ±0.4) x 109 5 [14] S irrad. comp, with thymine Rel. tO (ioH + thymine 4.7 X 109 t4.0 x 109 neut. [10] PR comp, with ferrocyan. Rel. tO ((oH + ferrocyan. 9-3 X 109 3.7 x 109 7 [3] PR comp, with CNS- Rel. to£rel = 1.1 X 10 10 thiocyan. 3.7 x 109 9 [6] S irrad. comp, with PNDA Rel. to &oh+c2 h 5 oh = 1-85 X 109

isobutanol (3.3 ±0.3) x 109 2 [14] S irrad. comp, with thymine Rel. tO ((oH + thymine 4.7 X 109 3.3 x 109 7 [3] PR comp, with CNS- Rel. to k

r

?\- =1.1X1010 thiocyan. 3.5 x 109 9 [6] S irrad. comp, with PNDA Rel. to £oh+c2 h5 oh = 1-85 X 109

secondary (2.3 ±0.25) x 109 2 [14] S irrad. comp, with thymine Rel. tO ^OH + thymine 4.7 X 109 butanol 3.1 X 109 7 [3] PR comp, with CNS - Rel. to k

rel 1.1 x 10 10 thiocyan. 2.4 X 109 9 [6] S irrad. comp, with PNDA Rel. to &oh+c2 h 5 oh~ 1-85 X 109

t-butanol (6.3 ±0.7) x 108 2 [14] S irrad. comp, with thymine Rel. tO ^OH + thymine 4.7 X 109 t5.2 x 108 neut. [10] PR comp, with ferrocyan. Rel. tO k oh + ferrocyan. 9-3 X 109 4.2 x 108 7 [3] PR comp, with CNSRel. to a =1.1 x io 10 thiocyan. 4.7 x 108 9 [6] S irrad. comp, with PNDA Rel. to A: 0h+C2 H5oh~ 1.85 X 109

n-amyl-alcohol 4.7 X 109 9 [6] S irrad. comp, with PNDA Rel. to A: 0h+C2H5oh~ 1-85 X 109

f-amyl-alcohol 1.8 x 109 9 [6] S irrad. comp, with PNDA Rel. to &oh+C2 H5oh = 1-85 X 109

n-pentanol (4.5±0.5) x 109 2 [14] S irrad. comp, with thymine Rel. tO A qh + thymine 4.7 X 109 (4.8±0.5) x 109 5 [14] S irrad. comp, with thymine Rel. tO ^OH + thymine 4.7 X 109

n-hexanol 5.2 x 109 2 [14] S irrad. comp, with thymine Rel. tO A oh + thymine 4.7 X 109

n-heptanol (5.4±0.5) X 109 2 [14] S irrad. comp, with thymine Rel. tO &OH + thymine 4.7 X 109

n-octanol (5.6 ±0.7) x 109 2 [14] S irrad. comp, with thymine Rel. tO ^OH+thymine 4.7 X 109

allyl-alcohol 1.97 x 109 7 [3] PR comp, with CNS- Rel. to £

rel - = 1.1 x 10 10 thiocyan.

ethylene glycol (1.5 ±0.15) x 109 2 [14] S irrad. comp, with thymine Rel. tO ^OH + thymine 4.7 X 109 1.14 x 109 6 [6] S irrad. comp, with Br- Rel. tO &oh + C2H5 OH~ 1-85 X 109 1.44 x 109 7 [3] PR comp, with CNS- Rel. •°*SiU„ = 11x10,0

1 1 .47 x 109 neut. [10] PR comp, with ferrocyan. Rel. tO k oh + ferrocyan. 9-3 X 109 1.51 x 109 9 [6] S irrad. comp, with PNDA Rel. tO £oh + C2H5OH~ 1-85 X 109

1 ,2-propanediol ( 1.6 ± 0. 15) x 109 2 [14] S irrad. comp, with thymine Rel. tO ^OH + thymine 4.7 X 10

1.58 x 109 6 [6] S irrad. comp, with Br- Rel. to &0H + C2H5OH — 1-85 X 109 1.68 x 109

. 7 [3] PR comp, with CNS -

Rel. to k

r

l\- = 1.1 X 10 10 thiocyan. 1.66 x 109 9 [6] S irrad. comp, with PNDA Rel. to &oh + C2 H5oh~ 1.85 X 109

24
Table 3. Aliphatic alcohols — Continued

Reactant k(M~' s1 ) pH Ref. Method Comments

i ,.5-propanediol 2.9 X 109 6 [6 ] S irrad. comp, with Br- Rel. to A: 0h+C2H5 oh = 1-35 X 109 2.4 X 109 9 [6 ] S irrad. comp, with PNDA Rel. to &0H+C2 H5 OH = 1-35 X 109

diethylene 2.0 x 109 9 [6 ] S irrad. comp, with PNDA Rel. to &oh+C2H5 oh~ 1.85 x 109 glycol

glycerol (1.64 0. 16) x 109 2 [14] S irrad. comp, with thymine Rel. tO ^OH + thymine = 4.7 X 109 1.59 x 109 7 [3] PR comp, with CNS - Rel. toArel- =1.1X10*°

thiocyan. tl.9 x 109 neut. [10] PR comp, with ferrocyan Rel. to Ar ( ) H+ fl

. IT,)t

. van. = 9.3 X 10 9 2.04 x 109 9 [11] S irrad. comp, with PNDA Rel. to A* oh + pnda = 1-25 x 10 10 1.85 x 109 9 [ 6] S irrad. comp, with PNDA Rel. to A,)H+C2H . ()H = 1.85 x 109

1,3-butane-diol 2.2 x 109 7 [3] PR comp, with CNS- Rel. to A

rel - = 1 . 1 X 10 *° thiocyan. 2.3 x 10 9 9 [ 6] S irrad. comp, with PNDA Rel. to Aoh + CjHoOh = 1-35 X 109

2,3-butane-diol 1.35 x 109 7 [3] PR comp, with CNS - Rel. toA rel - =1.1X10*°

thiocyan.

1,4-butane-diol 3.2 x 109 7 [3] PR comp, with CNS - Rel. to A

"' 1 =1.1X10*°

thiocyan. 3.0 x 10 9 9 [6] S irrad. comp, with PNDA Rel. to A()h+c 2 h 5 oh = 1.85 X 109

erythritol 2.0 x 10 9 9 [6] S irrad. comp, with PNDA Rel. to A()H+ c2 h5 oh = 1-35 x 10 9

pentaerythritol 3.2 x 109 9 [6] S irrad. comp, with PNDA Rel. to A0H + C2H.-.011 = l- 8o x 10 9

ribose 2.1 x 109 9 [19] S irrad. comp, with PNDA Rel. t() AOH+ PNDA = 1-25 x 10*°

D-glucose (1.9 ± 0.15) x 109 2 [14] S irrad. comp, with thymine Rel. to A< ) H + thymine = 4.7 X 10 9

ascorbic acid 1.2 x 10 10 1 [3] PR comp, with CNS- Rel. to A" 1 - = 1 . 1 X 10 *° thiocyan.

sucrose 2.4 x 109 2 [14] S irrad. comp, with thymine Rel. to Aon + thymine ~ 4. t X 10 9

Table 4. Saturated aliphatic acids — ^Monobasic)

Reactant k(M * s *) pH Ref. Method Comments

Formic acid 1.36 X109 1.0 [7] PR comp, with I - Rel. to Aow + / -= 1-02 x 10*° 1.25X108 1 [3] PR comp, with CNS- Rel. toA rel =1.1X10*°

thiocyan. 1.53 X 10* acid [14] S irrad. comp, with thymine Rel. tO A0 H-thymine = 4.7 X 109 (interpolated from pH curve)

1.2 X 10 s 0.05 M [20] S irrad. comp, with isopropanol Rel. to A0h+ip = 2.0 x 109 h,so4 2.45 X109 neut. [7] PR comp, with I - Rel. to Aow+/ _= 1.02 x 10*° 4.2 x 10 7 [21] S irrad. comp, with H2 O2 Rel. to Aoh *

H

2O2 = 4. 5 x 10' 3.1 x 109 neut. [14] S irrad. comp, with thymine Rel. to AoH + thymine = 4.7 X 10 9 (interpolated fron pH curve)

2.7 x 109 7 [20] S irrad. comp, with isopropanol Rel. to A0h+ip = 2.0 X 109 +2.8 x 10 9 neut. [10] PR comp, with ferro Rel. to Aoh + ferrocyan. = 9.3 X 109 4.0 x 109 9 [11] S irrad. comp, with PNDA Rel. to Aoh+ pnda = 1-25 x 10'° 3.5 x 109 10.7 [13] S irrad. comp, with benzoate Rel. tO Aqh + benzoate 5.6 X 10' 1.2 x 109 13 [20] S irrad. comp, with isopropanol Rel. to A oh * ip = 2.0 x 10 9

25
Table 4. Saturated aliphatic acids— (Monobasic) — Continued

Reactant k(M-' s-') pH Ref. Method Comments

acetic acid 2.3 x 10 7 1.0 [3] PR comp, with CNS~ Rel. to &

rel =1.1X10'"

thiocyan. *(9±0.4) x 106 1.0 [7] PR disappearance of OH spectrum Absolute

1.4 x 107 1.0 [7] PR comp, with I~ Rel. to k0H + l ~ — 1.02 X 10 10

2.0 x 107 2 [14] S irrad. comp, with thymine Rel. tO ^oh + thymine = 4.7 X 10 9 t7x 10 7 neut. [10] PR comp, with ferrocyan. Rel. to ^ ()H + ferrocyan. = 9.3 x 109 t*8.5 x 10 7 neut. [10] PR prod. form. Absolute measurement NjO

added No corr. for H

reaction (8.8±0.5) x 10 7 9 [11] S irrad. comp, with PNDA Rel. to koH + pnda = 1.25 x 10'° 7.2 X 10 7 9 [6] S irrad. comp, with PNDA Rel. to ^oh + C2H5OH = 1 .85 X 10 ' 6.2 x 10 7 10.7 [13] S irrad. comp, with benzoate Rd. tO A oh + benzoate 5.6 X 10*

bromoacetic 4.4 X 10 7 9 [6] S irrad. comp, with PNDA Rel. to A:qh + C2H5OH = 1 -85 X 10" acid

chloracetic acid

4.3 X 10 7

(7. 0 + 0.7) X 10 7

12

[3] [14]

PR comp, with CNS~

S irrad. comp, with thymine

Rel. to ^

=1.1X10'"

thiocyan. Rel. to /r 0H + thymine = 4.7 X 10" 5.5 x 10 7 9 [6] S irrad. comp, with PNDA Rel. to /r 0H + C2H50H = 1.85 X 10"

fluor-acetic 3.0 x 10 7 9 [6] S irrad. comp, with PNDA Rel. to &oh + c2 h5 oh= 1-85 X 10" acid

methoxy-acetic 6.0 x 108 9 [6] S irrad. comp, with PNDA Rel. to &oh + < 2 H 5oh= 1 - 85 X 109 acid

cyano-acetic 1.54 X 10 7 9 [6] S irrad. comp, with PNDA Rel. to A: 0h + ( 2 h5 oh = 1-85 X 109 acid

ethylene- 3.5 x 109 9 [19] S irrad. comp, with PNDA Rel. to &oH +pNdA = 1-25 X 10'° diamine

tetracetic acid

trimethyl 1.4 x 109 9 [6] S irrad. comp, with PNDA Rel. to /foH + t 2 h5 oh = 1-85 X 10 1 0 acetic acid

propionic (4.6±0.5) X 10 8 2 [14] S irrad. comp, with thymine Rel. to /f OH + thymine = 4. 7 X 10 9 acid

7.9 x 108 9 [6] S irrad. comp, with PNDA Rel. to /r 0H + < 2 h soh = 1-85 X 10 9

a-bromo- 2.2 X 108 8.5 [17] S irrad. comp, with PNDA Rel. to A:oh + c2 h5oh= 1-85 X 10

s propionic

acid

pyruvic acid 3.2 x 107 9 [19] S irrad. comp, with PNDA Rel. to A'qh + pnda = 1-25 X 1

1

/3-bromo 2.3 x 108 8.5 [17] S irrad. comp, with PNDA Rel. to ^oh+C 2 Hsoh = 1 -85 X 109 propionic

acid

a-chloro- 2.4 x 108 8.5 [17] S irrad. comp, with PNDA Rel. to k0H+C2HsOH = 1-85 x 109 propionic

acid

/3-chloro- 3.2 x 108 8.5 [17] S irrad. comp, with PNDA Rel. to &0H+C2H5OH = 1-85 X 109 propionic

acid 26
TABLE 4. Saturated aliphatic acids — (Monobasic) — Continued

Reactant k(M~' s1 ) pH Ref. Method Comments

ra-butyric acid (1.64 ±0.2) x 109 2 [14] S irrad. comp, with thymine Rel. to kOH + thymine = 4.7 X 109 1.85 x 109 9 [6] S irrad. comp, with PNDA Rel. to A'oh+c 2 h 5 oh = 1-85 X 109

iso butyric acid 1.26 x 109 9 [6] S irrad. comp, with PNDA Rel. to Aoh+c2 h 5 oh = 1-85 x 109

cyclobutane

carboxylic

acid

3.02 x 109 9 [6] S irrad. comp, with PNDA Rel. to Aqh+c2 h oh = 1.85 X 109

a-amino-nbutyric acid

(3.3 ±0.3) x 108 2 [14] S irrad. comp, with thymine Rel. to Aoh + thymine = 4.7 X 109

/3-amino-n- butyric acid

(6.8 ±0.7) x 107 2 [14] S irrad. comp, with thymine Rel. to A0H + thymine~ 4.7 X 109

y-amino-nbutyric acid

(1.9 ±0.2) x 108 2 [14] S irrad. comp, with thymine Rel. tO A0H + thymine~ 4.7 X 109

2-methyl

butyric acid

2.4 x K)9 9 [6] S irrad. comp, with PNDA Rel. to Aoh+c2 h5oh = 1-85 X 10 9

3-methyl

butyric acid

2.2 x 109 9 [6] S irrad. comp, with PNDA Rel. to Aoh+c 2 h 5 oh = 1-85 X 109

3,3-dimethyl

butyric acid

1.66 x 109 9 [6] S irrad. comp, with PNDA Rel. to A'oh+c 2 hsOh = 1-85 X 109

n-valeric acid 2.9 x 109 9 [6] S irrad. comp, with PNDA Rel. to A0h + c 2 h 5 oh = 1-85 X 109

n-caproic acid 3.9 x 109 9 [6] S irrad. comp, with PNDA Rel. to A‘oh+c 2 HsOh = 1-85 X 109

cyclo-pentane

carboxylic

acid

4.0 x 109 9 [6] S irrad. comp, with PNDA Rel. to A'oh+c 2 HsOh = 1-85 X 109

cyclo-hexane

carboxylic

acid

5.4 x 109 9 [6] S irrad. comp, with PNDA Rel. to A'oh+c 2 HjOh = 1-85 X 109

Table 5. Polybasic and hydroxy-carboxylic acids

Reactant k(M 1 s

1 ) pH Ref. Method Comments

oxalic acid (8 ± 3 ) x 10« 2 [14] S irrad. comp, with thymine Rel. to Aoh + thymine = 4.7 X 109 1.03 x 107 7 [22] S irrad. comp, with methanol and Rel. to A0h+c2 h5oh = 1-85 X 109 ethanol

(8.4 ± 1.5) x 106 9 [11] S irrad. comp, with PNDA Rel. to Aqh + pnda = 1-25 X 10"’

malonic acid (1.7 ±0.15) x 107 2 [14] S irrad. comp, with thymine Rel. to AoH + thymine = 4.7 X 109 3.0 x 108 6-7 [3] PR comp, with CNS- Rel. toA rel =1.1X10'°

thiocyan. 5.5 x 107 9 [6] S irrad. comp, with PNDA Rel. to Aqh + pnda = 1-25 X 1

0

1

0

succinic acid (1.2 ±0.1) x 108 2 [14] S irrad. comp, with thymine Rel. to AoH + thymine =4.7 X 109

glutaric acid (6.1 ±0.5) x 108 2 [14] S. irrad. comp, with thymine Rel. to Aqh + thymine = 4.7 X 109

adipic acid (1.5 ± 0.15) x 109 2 [14] S irrad. comp, with thymine Rel. to Aoh + thymine ~ 4.7 X 109

27

492-021 OL - 73 - 5
Table 5. Polybasic and hydroxy-carboxylic acids — Continued

Reactant k(M~' s- 1 ) pH Ref. Method Comments

pimelic acid (2.6±0.3) x 109 2 [14] S irrad. comp, with thymine Rel. to ^OH+thymine 4.7 X 109

suberic acid (3.5±0.4) X 109 2 [14] S irrad. comp, with thymine Rel. to ^OH+thymine 4.7 X 109

azelaic acid (4.0±0.5) X 109 2 [14] S irrad. comp, with thymine Rel. tO AcQH+thymine 4.7 X 109

sebacic acid (4.7 ±0.5) X 109 2 |14j S irrad. comp, with thymine Rel. tO &0H+thymine 4.7 X 109

citric acid 5 X 107 1 [3] PR comp, with CNS- Rel. to A:";

1 =1.1X10 10 (4.7 ±0.3) X 108 2 [14] S irrad, comp, with thymine Rel. tO &oh+thymine 4.7 X 10'

glycolic acid (4±0.3) x 108 9 [6] S irrad. comp, with PNDA Rel. to &oh + c2 h5 oh = 1-85 X 109 7.2 x 108 Rel. to A: 0h+C2H5oh = 1-35 X 109 9 [6] S irrad. comp, with PNDA

lactic acid 4.3 x 108 1 ‘[3] PR comp, with CNS- Ret. to *[h

e ‘ ocyan = 1.1 X 10'° (5.6±0.5) X 108 2 [14] S irrad. comp, with thymine Rel. tO A: 0h + thymine = 4.7 X 109

malic acid (4.7 ±0.5) X 108 2 [14] S irrad. comp, with thymine Rel. to A^oH+thymine = 4. 7 X 109

tartaric acid (5.2±0.5) X 108 2 [14] S irrad. comp, with thymine Rel. to /ioH+thymine = 4.7 X 109 6.8 x 108 9

[19] S irrad. comp, with PNDA Rel. to &oh+pnda = 1.25 X 1010

tetrahydroxy- 1.3 X109 9 [6] S irrad. comp, with PNDA Rel. to A: 0h + C2H5OH = 1-85 X 109 succinic acid

thioglycolic acid 6.0 X 109 1 [3] PR comp, with CNS- Rel. to 1.1X10'°

TABLE 6. Aliphatic carboxylic esters

Reactant A:(M -1

s -

') pH Ref. Method Comments

ethyl formate 3.8 x 108 6-7 [3] PR comp, with CNS - Rei- to = 1 . 1 x 10"

methyl acetate 1.3 x 108 2 [3] PR comp, with CNS- Rei. to *sU,„ = l.ixio"

1.2 x 108 6-7 [3] PR comp, with CNS- Rel. 10 1 . 1 X 10'°

1.1 X 108 9 [6] S irrad. comp, with PNDA Rel. to &oh+C2 HsOh = 1.85 X 109

ethyl acetate (2.4±0.2) X 108 2 [14] S irrad. comp, with thymine Rel. tO AfoH+thymine 4.7 X 10' 4.0 x 108 6-7 [3] PR comp, with CNS- Re'- totSU.-l.lX10"

isopropyl acetate 4.3 X 108 2 [3] PR comp, with CNS- Rel- to *£«„,= 1.1X10“

4.5 x 108 6-7 [3] PR comp, with CNS- Re'- to 1.1X10'°

n-propyl acetate 1.4 x 109 6-7 [3] PR comp, with CNS - Re'- to tSU.,- 1.1X10'°

methyl propionate 4.5 x 108 6-7 [3] PR comp, with CNS- Rel- to *£«,,„ = 1-1X10"

ethyl propionate 8.7 X 108 6-7 [3] PR comp, with CNS- Rel- to 1.1X10'°

methyl butyrate 1.7 x 109 6-7 [3] PR comp, with CNS- Rel- to tSUan = 1.1X10'°

ethyl butyrate 1.6 x 10H 6-7 [3] PR comp, with CNS- Rel- to «!„„„ = 1.1X10"

diethyl malonate 6.5 x 108 6-7 [3] PR comp, with CNS- Rel. to*"?1: =1.1X10'°

thiocyan.

diethyl succinate 7.8 X 108 6-7 [3] PR comp, with CNS- Rel- <0 tSU„= 1.1X10'°

28
Table 7. Ethers

Reactant A(Af-i s'1 ) pH Ref. Method Comments

diethyl ether 3.9 x 10s neut. [7] P comp, with I - Rel. to Aoh-i-~ 1-02 x 1010 1.36 x 109 9 [6] S irrad. comp, with PNDA ReL to Aoh + C2H5OH = 1.85 x 10 9 (2.4±0.2) x 109 2 [14] S irrad. comp, with thymine ReL to A'0H-rlhymine = 4. t X 109 2.3 x 10s 0.05 M

H2 S04

[20] S irrad. comp, with isopropanol ReL to A 0h^ip~ 2.0 x 109

2.4 x 109 7 [20] S irrad. comp, with isopropanol ReL to A oh-ip=2.0X109 1.74 X109 ~ 13 [20] S irrad. comp, with isopropanol ReL to A Oh-!-ip = 2.0 x 109 dioxane 2.35 x 10s neut. [7] PR comp, with I - ReL to A- OH+r=l-02 Xl(r0

1.74 x 109 2 [14] S irrad. comp, with thymine ReL to Arm-thvminp ~ 4. . x 109

1.85 x 109 9 [6] S irrad. comp, with PNDA ReL to A'oh -r C2 H5 oh = 1-85 x 109

13,5-trioxane 4.9 x 108 9 [6] S irrad. comp, with PNDA ReL to A'oh^CpHsOh" 1-85 X-10 9

tetrahydrofuran 2.7 x 109 9 [6] S irrad. comp, with PNDA ReL to A0H-PC2 H5 OH = 1.85 x 10®

23,6-trimethyl 1.00 x 109 9 [6] S irrad. comp, with PNDA ReL to A* oh ^ C2 H5 oh = 1.85 x 10®

- 133-trioxane (paraldehyde)

diethyleneglvcol

diethylether

3.2 x 109 9 [6] S irrad. comp, with PNDA ReL to Aoh + C2H5OH == 1-85 X 10®

diethoxyethane 2.2 x 109 9 [6] S irrad. comp, with PNDA ReL to n oh

+

C2 h3 oh

1 • 85 x 10®

ethyleneglycol 1.3 x 109 9 [6] S irrad. comp, with PNDA ReL to A'oh + c 2 h5 oh = 1-85 x 10 ® monomethylether

dimethoxy- ethane

1.56 x 109 9 [6] S irrad. comp, with PNDA ReL to A'oh + c2 h5 oh = 1-85 x 10 ®

Table 8. Carbonyl compounds

Reactant A(A#-1 s-

') pH Ref. Method Comments

formaldehyde -2X 109 [23] PR comp, with H2 O2 ReL to Aoh-h2Oi = 4-5 x 10'

acetone 6.6 X 107 2 [14] S irrad. comp, with thymine ReL tO A'oH+thymine = 4. X 10' 7.7 x 107 neut. [7] PR comp, with iodide ReL to Aoh+i- = 1.02 x 1010 9.7 x 107 6-7 [3] PR comp, with CNS- Rel. to C . = 1.1 x 1010 7.3 x 107 9 [6] S irrad. comp, with PNDA Rel. to A'oH-rCjHaOH — 1.85 X 109 12 x 107 [10] PR comp, with ferrocyan Rel. tO A'oH^ferrocyan. = 9.3 X 109 ~ 7 x 107 9 [19] S irrad. comp, with PNDA Rel. to Aqh+pnda = 1-25 x 1010 (9±4) x 10 7 10.7 [13] S irrad. comp, with benzoate Rel. tO A*oh-p benzoate 0.6 X 10^

acetoin 8.5 x 108 2 [3] PR comp, with CNS- Rel. to A

7!

1 : =l.lxl010 thjocyan. 1.5 x 109 neut. [24] PR comp, with CNS- Rel. to A

r

!': =1.1X1010 thiocyan.

diacetyl 1.7 x 10s neut. [24] PR comp, with CNS- ReLtoAf:1 - = 1.1 x 1010 thiocyan.

butyraldehyde 3.5 x 109 2 [3] PR comp, with CNSRel. to A

1*1 =1.1 X 1010 thiocyan.

methylethyl ketone 9x 108 6-7 [3] PR comp, with CNS- Rel. to Afh

el - = 1.1 X 1010 thiocyan.

diethylketone 1.35 x 108 6-7 [3] PR comp, with CNS- Rel. toAfh

el =1.1X10'°

thiocyan.

methyl n-propyl ketone 1.9 x 108 6-7 [3] PR comp, with CNS- Rel. to A^> =1.1X10'°

thiocyan.

29
Table 9. Substituted hydrocarbons

Reactant k(M~ l s-1

) pH Ref. Method Comments

methane 2.4 X 108 9 [6] S irrad. comp, with PNDA Rel. to £oh + c2 h5 oh = 1-85 X 10s '

ethane 1.2 X 109 1.2 [25] S irrad. comp, with formic acid Rel. to ^OH+HCOOH = 1.2 X 108

chloroform 1.4 X 107 9 [6] S irrad. comp, with PNDA Rel. to A:oh + c2 h5 oh~ 1-85 X 109

cyanomethane 3.5 X 106 9 [6] S irrad. comp, with PNDA Rel. to &oh + c 2 H50h~ 1-85 X 109

diethoxyethane 1.55 x 109 9 [6] S irrad. comp, w th PNDA Rel. to ^oh+cjHsOh = 1 -85 X 109

dimethoxyethane 5.7 X 108 9 [6] S irrad. comp, with PNDA Rel. to A: 0h + c2 H50h~ 1-85 X 109

nitromethane 3.1 x 108 9 [6] S irrad. comp, with PNDA Rel. to £oh + c2 h5 oh = 1.85 X 109 t*(8.5± 1.5) x 109 10.5 [26] PR prod. form. Absolute N2 0 added, no correct,

for H addit.

TABLE 10. Nitrogen compounds

Reactant k(M~' s -1

) pH Ref. Method Comments

methylamine 1.9 X 107 5 [69] S irrad. comp, with PNDA Rel. to &oh+c2 h5 oh~ 1-35 X 109 1.8 X 108 neut. [27] PR comp, with CNS- Re'- 1-1 XW*

2.4 X 109 12 [6] S irrad. comp, with PNDA Rel. to A: 0h+C2H5oh — 1.85 X 109 1.15X1010 12 [27] PR comp, with CNS- Rel. to Aff!’ „ = 1.1 X 1010 thiocyan.

ethylenediamine 1.0 X 108 5

[28] S irrad. comp, with isopropanol Rel. tO &OH + isopropanol 2.0 X 109

formamide < 5 x 108 5.5 [29] PR comp, with CNS- R'1 'O«Lv.n= 11X10">

acetamide 1.9 X 108 5.5 [29] PR comp, with CNS- Re!->° =11 *10'”

1.3 X 107 9 [6] S irrad. comp, with PNDA Rel. to &oh + c2 h5 oh~ 1-85 X 109

acrylamide (3.3±0.8) X 109 (3.3±0.2) X 109 neut.

10.7

[30]

[13] PR comp, with CNSS irrad. comp, with benzoate

Rd to =1.1X10'"

Rel. to &oh + benzoate 5.6 X 10^

acetonitrile 3.5 x 106 9 [6] S irrad. comp, with PNDA Rel. to ^oh+CzHsOh = 1-35 X 109

A-methylformamide 1.2 X 109 5.5 [29] PR comp, with CNS- Rel- to ^^= 1.1X10'“

A-dimethylacetamide 1.7 X 109 5.5 [29] PR comp, with CNS- Rel. to Af*

1 - =1.1X10'°

thiocyan.

A-methylacetamide 1.6 x 109 5.5 [29] PR comp, with CNS- Rel. to A:r°!' =1.1X10'°

thiocyan.

A-dimethylacetamide 3.5 X 109 5.5 [29] PR comp, with CNS- Rel. to C! = 11 x 10'°

thiocyan.

dimethylacetamide 1.6 x 109 5-6 [4] PR comp, with CNS- Rel. to A"1 - =1.1X10'°

thiocyan.

trimethylacetamide 1.4 x 109 5-6 [4] PR comp, with CNS- Rel. to A"*- =1.1X10'°

thiocyan.

A-methyldimethyl

acetamide

1.9 x 109 5-6 [4] PR comp, with CNS- Rel- to 1.1X10'"

A-methyltrimethyl

acetamide

2.4 X 109 5-6 [4] PR comp, with CNS- Relto^Un-llx 10'”

30
Table 10. Nitrogen compounds — Continued

Reactant k(M *s ) pH Ref. Method Comments

A-dimethyl trimethyl

acetamide

4.0 X 109 5-6 [4] PR comp, with CNS- Rd- '<> = 1-1 X 10“

A-tert. butylacetamide 1.1 X109 5-6 [4] PR comp, with CNS- *£Lm, =11 XU10

propionamide 7.0 x 108 5-6 [4] PR comp, with CNS- Rel. to Fel- = 1.1 X 1010 thiocyan.

A-methyl propionamide 1.4 x 1(P 5-6 [4] PR comp, with CNS- Rel. to AJJ-.

1 ' = 1.1 x 1010 thiocyan.

Chapter VI. References

[1] Adams, G. E., and Willson, R. L., Trans. Faraday Soc. 65,

2981 (1969).

[2] Burchill, C. E., and Ginns, I. S., Can. J. Chem. 48, 2628

(1970).

[3] Adams, G. E., Boag, J. W., Current, J., and Michael, B. D., Pulse Radiolysis, Ed. M. Ebert, et al., p. 131 (Academic

Press, London and New York, 1965).

[4] Hayon, E., Ibata, T., Lichtin, N. N., and Simic, M., J. Amer. Chem. Soc. 93 , 5388 (1971).

[5] Scholes, G., Shaw, P., Willson, R. L., and Ebert, M., Pulse

Radiolysis, Ed. M. Ebert et al., p. 151 (Academic Press, London and New York, 1965).

[6] Anbar, M., Meyerstein, D., and Neta, P., J. Chem. Soc. (B), 742 (1966).

[7] Thomas, J. K., Trans. Faraday Soc. 61, 702 (1965).

[8] Neta, P., and Dorfman, L. M., Advances in Chemistry Series No. 81, Radiation Chemistry I, 222, Ed. R. F. Gould (1968).

[9] Baxendale, J. H., Fielden, E. M., and Keene, J. P., Pulse

Radiolysis, Ed. M. Ebert et al., p. 217 (Academic Press, London and New York, 1965).

[10] Willson, R. L., Greenstock, C. L., Adams, G. E., Wageman,

R., and Dorfman, L. M., Int. J. Rad. Phys. and Chem. 3, 211 (1971).

[11] Kraljic, I., and Trumbore, C. N., J. Amer. Chem. Soc. 87,

2547 (1965).

[12] Shah, S., Trumbore, C. N., Giessner, B., and Park, W.,

Advances in Chemistry Series No. 81, Radiation Chemistry I, 321, Ed. R. F. Gould (1968).

[13] Matthews, R. W., and Sangster, D. F., J. Phys. Chem. 69,

1938 (1965).

[14] Scholes, G., and Willson, R. L., Trans. Faraday Soc. 63,

2983(1967).

[15] Rabani, J. and Stein, G., Trans. Faraday Soc. 58, 2150

(1962).

[16] Hummel, A., and Allen, A. O., Radiat. Res., 17, 302 (1962).

[17] Anbar, M. , and Neta, P. , J. Chem. Soc. (A), 834 (1967).

[18] Greenstock, C. L., Ng, M., and Hunt, J. W., Advances in Chemistry Series No. 81, Radiation Chemistry I, 379, Ed. R. F. Gould (1968).

[19] Kraljic, I., Chemistry of Ionization and Excitation Ed.,

G. R. A. Johnson and G. Scholes, p. 303 (Taylor and

Francis Ltd., London, 1967).

[20] Hughes, G., and Makada, H. A., Trans. Faraday Soc. 64,

3276 (1968).

[21] Hart, E. J., J. Amer. Chem. Soc. 74, 4174 (1952).

[22] Draganic, I., Proc. Tihany Symp. Akademiai Kiado (Hungary)

p. 129 (1966).

[23] Hart, E. J., Thomas, J. K., and Gordon, S., Radiation Res.

Suppl. 4., 74 (1964).

[24] Lilie, J., Beck, G., and Henglein, A., Ber. Bunsenges. Phys.

Chem. 72, 529 (1968).

[25] Ahmad, M., and Clay, P. G., J. Chem. Soc. 845 (1966).

[26] Asmus, K. D., and Taub, I. A., J. Phys. Chem. 72, 3382

(1968).

[27] Wigger, A., Grunbein, W., Henglein, A., and Land, E. J., Zeit. fur Naturforsch. 24b, 1262 (1969).

[28] Anbar, M., Munoz, R. A., and Rona, P., J. Phys. Chem. 67,

2708 (1963).

[29] Hayon, E., Ibata, T., Lichtin, N. N., and Simic, M., J. Amer.

Soc. 92, 3898 (1970).

[30] Chambers, K. W., Collinson, E., Dainton, F. S., Seddon,

W. A., and Wilkinson, F., Trans. Faraday Soc. 63, 1699

(1967).

31
Chapter VII. Inorganic Electron Transfer Reactions

This chapter lists rate constants for reactions

of OH with neutral and charged inorganic compounds. With few exceptions, reactions of this type

involve simple electron transfer of OH to form the

hydroxide ion with a corresponding change in the

valency state of the donor.

M"+ +OH = M(,l+1)+ + OH- (l)

or M«- + OH= M(n_1)_ + OH- (2)

The data in table 11 are listed according to the periodic classification of the elements.

Although Ho is listed here, the isotope effect

evident in the significant difference between the

OH reactivities of hydrogen and deuterium indicates

that the reactions probably involve direct hydrogen

transfer

H2 + 0H=H2 0+H- (3)

rather than the formation of an intermediate H^ ion:

H2 + OH = H2+ + OH- (4)

However, for convenience, the data for H2 and D2 are given in this chapter and not in Chapter VI. For the group I metal cations, Ag+ [1] and Au+ [2], and, presumably, Cu+ react with OH to form

higher valency states. Surprisingly, the oxidation

of cupric ion to a form of Cu III also proceeds at a

rate which is only an order of magnitude lower than

that for a diffusion-controlled reaction [3]. Interest in the reaction of OH with the powerful

reducing agent, borohydride ion, centered on its possible value as a means of converting OH into

the highly reducing hydrogen atom [4].

BH.r +OH=BH4 +OH- (5) BH4 =BH3 + H- (6)

Preliminary studies indicated, however, that the

BH4 intermediate is sufficiently stable to dissociation to permit it to react with other solutes.

There are some data for reaction of OH with

cations and complex anions of metals of the transition series. With the exception of ferrous ion and

thallous ion, data are sparse for the transition complex cations. For the three complexes listed, there is no change in the coordination number following OH

oxidation, although this is not the case for a complex

of Pt II, [Pt Cl4] =. The one-electron oxidation state of this anion, a complex of Pt III, is unstable and

the reaction proceeds to the ultimate formation of a 6-coordination complex of Pt IV. The initial reaction

is not electron transfer and involves addition of OH

to form an intermediate platinum complex of co- ordination number five [5].

There are adequate data for the reaction of OH

with carbonate ion. Agreement is good, although

there is a pH effect on the rate constant due to the

carbonate-bicarbonate ionic equilibrium. The

CO^ radical ion formed in the reaction gives rise to a strong transient absorption spectrum with a maximum at 600 nm. For this and other reasons, the

reaction has been used as a reference system in

pulse radiolysis competition experiments for the

measurement of a large number of OH rate constants. We recommend that a value of 4.1 X 108 be

used for the reference rate constant in such

determinations.

As would be expected, the reactivity of the reducing oxy-anions of the three-valent Group 5 and the

four-valent Group 6 elements approach diffusionlimited values. Where multiple determinations are

available, the agreement is generally quite good.

There is a considerable amount of data available on the reactivity of the halide ions of Group 7; reactivities for the pseudo-halide, thiocyanate ion,

CNS“, are also included in this section. For both

Cl- and Br_ there is a strong effect of acid pH on

the reactivity and for chloride, the effect is particularly marked due to the complex nature of the overall

oxidation process [6]. There are numerous determinations of the reactivity of bromide ion with,

apparently, good agreement at both acid and neutral

pH.

Oxidation of a halide ion, X

-

, proceeds in two

stages:

OH + X - = X- + OH-, (7) X- + X- “X2-. (8) In competition experiments designed to measure the

OH-reactivity of a second solute, the possibility

exists, therefore, of competitive reaction of the

intermediate species, X-, with both solutes. If this

occurs, relative rate data determined by such

methods could be misleading. Rate constants

32
measured in pulse radiolysis experiments by direct

observation of the rate of formation of the complex

radical depend for their validity on the assumption that, in the overall process, reaction (7) is rate-determining. Experiments have been carried

out [7] which show that this assumption is not

valid. Kinetic analysis of the reaction sequence in

the thiocyanate system indicated that the rate constant of reaction (7) is greater than that of re- action (8). In view of the frequency with which

the CNS- system has been used as a reference sys- tem in kinetic competition studies, the validity of

the rate data so determined is discussed in detail

in the final chapter.

Table 11. Inorganic electron transfer reactions

Reactant kiM-'s-1 ) pH Ref. Method Comments

h2 *(6.0±2) X 107 neut. [8] PR removal of OH

spectrum

Absolute

4.2 xlO7 7 [9] S irrad. comp, with H2 02

Rel. to A:oh+h202 = 4.5 X 107

4.5 x 107 7 [10, 27] Intermittent irradiation Rel. to ^oh+h202 = 4.5 X 107

d2 1.5 XlO7 0.1 N

HC104

[ 11] Comp, with Fe2+ (Fenton)

Rel to i

OH + Fe2+ = 3 x 10 8

Cu2+ *3.5 x 108 7 [3] PR product form. Absolute, N2 0 added

Au+ *(4.7 ±0.8) X 109 7 [2] PR product form. Absolute BH- * 1.2 x 1010 11 [4] PR product form. Absolute

* <4X 108 12.8 [4] PR product form. Absolute (O - rate constant)

Vanadium IV VO+ + (5.0± 1.5) x 108 0.1 [ 12] S irrad. comp, with H2 Rel. to &0H+H2 ” 4.5 x 107

Cr3* (3.2 ±1) x 108 [13] S irrad. comp, with H2 Rel. to k0h+h2 = 4-5 X 107 Mn2+ * 1.4 x 108 neut. [14] PR product form. Absolute. N2 0 added

Fe2+ * > 108 0.1 [15] PR product form. Absolute

3.0 x 108 1 [16] S irrad. Rol. to A’oh+H2 = 4.5 X 107 3.2 x 108 1.6 [17] S irrad. comp, with H2 Rel. to A OH+ H2 = 4.5 x 107 5.0 x 108 2 [18] Fenton reaction, OH

comp, with PNDA

Rel. to Aoh+pnda = 1-25 X 1010

2.6 x 108 2.1 [17] S irrad. comp, with H2 Rel. to A 0h+h2 = 4.5 X 107

[Fe(CN)5 NO]2 -

1.2 x 107 neut. [19] PR comp, with bicar- bonate

Rel. to A

oh + hco _ = 5.5 x107

[Fe(CN)6] 4 -

1.7 x 1010 neut. [20] PR comp, with I - Rel. to Aoh+i-= 1.02 X 1010 t*

(

1.1 ± 0. 2 ) x 10>° neut. [21] PR product form. Absolute. N2 0 added

8.7 x 109 7 [22] PR comp, with ethanol Rel. to A 0H+C2 H5OH = 1-85 X 109 t*9.3 x 109 neut. [23] PR product form Absolute N2 0 added

(T.25±0.2) x 1010 9 [24] S irrad. comp, with PNDA

Rel. to A‘oh+pnda = 1-25 X 1010

[Ru(NH3 )5 N2] 2 + *4.8 x 109 neut. [25] Product form. Absolute. N2 0 added

Ce3+ 7.2 x 107 0.1 [26, 27] S irrad. comp, with formic acid

Rel. to A'oh+H202 = 4.5 X 107

T1+ 2.7 x 109 0.1 [27, 28] S irrad. comp, with Ce3+ ion

Rel. to A0h+h202 = 4.5 X 107

(0.9±0.3) x 10>° 0.1 [ 12] S irrad. comp, with H2 Rel. to A0h+h2= 4-5 x 107 t*(7.6 ± 1 ) x 109 6.5 [29] PR product form. Absolute. N2 0 added

33
Table 11. Inorganic electron transfer reactions — Continued

Reactant pH Ref. Method Comments

[PtCU]2 - *8 X 109 7 [30] PR product form. Absolute. N2 0 added

*8 X 109 > 13 [30] PR product form. Absolute. N2 0 added

[Pt(CN)4]2_ 1.0 X 1010 2 [30] PR comp, with methanol Rel. to ^oh+ch3oh = 8.5 X 108 co3= t*4.2 x 108 10.8 [31] PR product form. Absolute. N2 0 added

4.5 x 108 10.5 [32] S irrad. comp, with Rcl. to &oh+benzoate 5.6 X 10** benzoate

3.8 x 108 10.7 [33] PR comp, with methanol Rel. to A:oh+cjh5oh = 1-85 X 109 and ethanol t*3.65 x 108 11 [34] PR product form. Absolute. N2 0 added

hco3- *1.0 X 107 6.5 [35] PR prod. form. Absolute. C02 added

*1.5 x 107 8.4 [31] PR prod. form. Absolute. N2 0 added

CO 4.5 x 108 neut. [36] PR comp, with H2 02 Rel. to £oh+hj02 ~ 4.5 X 107

CN- (4.5 ±0.3) X 109 9 [24] S irrad. comp, with Rel. to &oh+pnda = 1-25 X 1010 PNDA

C(N02

)3- *3 x 109 neut. [37] PR bleaching of solute Absolute. N2 0 added

Sn++ (2.5±0.3) X 109 0.1 [38] S irrad. comp, with Fe2+ Rel. to £

0H + Fe 2+ = 3 - 5xl°

8 N02“ 5x 10 9 neut. [39] S irrad. comp, with H2 Rel. to kou j. = 4.5 X 107

1.3 X109 [40] PR of NO3 solutions Computer analysis of reactions

7.1 X109 [41] S irrad. comp, with Rel. to &OH+PNDA = 1*25 X 1019 PNDA

(8.1 ±0.4) X109 9 [24] PR comp, with PNDA Rel. to &oh+i>nda = 1-25 x 1010 7.4 X109 10.7 [33] PR comp, with CO3 Rel. to Ar 0H+co;r

=::: 4.1 X 108 8.1 X109 11 [42] PR comp, with CO3 Rel. to /coH+cor = 4.1 X 108 NO (8.8± 1.1) x 109 7 [43] S irrad. comp, with Rel. to &0H+C2H5 OH = 1-85 X 109 alcohols

azide (1.08± 0.05) x 1010 9 [24] S irrad. comp, with Rel. to &oh+pnda = 1-25 X 1010 PNDA

hypophosphite 1.9X109 10.7 [33] PR comp, with CO3 Rel. to &oh+co:t = 4.1 X 108

phosphite 3.9 x 109 10.7 [33] PR comp, with CO= Rel. to A: 0h+co3= = 4.1 X 108 HP02 ~ < 107 neut. [20] PR comp, with iodide Rel. to *OH + I _=1.02X1010

< 5 x 106 [44] PR comp, with CO3 Rel. to /coH+cof = 4.1 X 108

PO5 - < 10 7 [44] PR comp, with CO3 Rel. to &oH+cor = 4.1 X 10 8

p2 o^- < 4 x 106 [44] PR comp, with CO3 Rel. to A: 0H+cof = 4.1 X 108

arsenite (AsOf) (7.6± 0.3) x 109 9.0 [24] S irrad. comp, with Rel. to Aoh+pnda~ 1-25 X 1010 PNDA

9.5 X109 10.7 [33] PR comp, with CO3 Rel. to A: 0h+coT ::= 4.1 X 108

34
Table 11. Inorganic electron transfer reactions — Continued

Reactant kiM-'s-1 ) pH Ref. Method Comments

H2 02 1.2 X107 3 [45] Prod, analysis. High dose

rate radiolysis

Comp, with radical-radical reactions in H2 O/O2 /H2 O2 4.5 X107 neut. [27] Prod, analysis. High dose

rate radiolysis

Comp, with radical-radical reactions in H2 O/O2 /H2 O2 2.25 X107 neut. [20] PR comp, with 1“ Rel. to A:oh + i _=1.02 X,10 10

H2 S 1.83 X1010 6 [46] PR comp, with CNS- Rel. to k

re] =1.1 X1010

thioe.van. H2 SO 3.5 x 105 0.1 [27,28] S irrad. comp, with Ce3+ Rel. to A'oh + h2 o2 = 4-5 X 10' (4.5+0.5) x 105 ' < 0.4 [12] S irrad. comp, with H2 Rel. to A: ()H+ h 2 = 4.5 X 107 (1.2 ± 0.3) X106 0.3-2.

0

[47] S irrad. comp, with Safranine T

Rel. to A: ( )H+t'6H6 = 7.9 X 10 9

KHSO4 1.5 xlO6 neut. [48] PR comp, with methanol Rel. to koii+cHrion = 8.5 X 108

HSO.r 3.9 x 109 [49] PR comp, with CO= Rel. to &oii + co:f = 4. 1 X 10s

s2 o8= < 106 [49] PR comp, with CO3 Rel. to &()| 1+ co= = 4.1 X 108

selenite SeOg 4.6 XlO9 7 [33] PR comp, with methanol Rel. to koH+CiiHsOii = 1-85 X 10 9

tellurite TeOg 3.9 XlO9 10.7 [33] PR comp, with CO3 Rel. to A;()H+ iw =4.1 XlO8

ci- see original paper acidand

alkal.

[6] PR product form, and

comp, with methanol

and ferrocyanide

Rate constant depend, on pH. Cl- concentration and overall ioni strength

*(1.5 ± 0.3) X 10 10 0. 8-3.4 [50] PR prod, form Absolute. Obs. CLf 104 9 [24] S irrad. comp, with PNDA

Rel. to At(,h + i» N|) A = 1.25 X 10 10

C102 1.1 X 109 5.8-6.

0

[51] S irrad. comp, with Br- Rel. toA:()II+Br -=l.l XlO9

Br- 1.02 X 10 10 1.3 [52] S irrad. comp, with ethanol

Rel. to A'oh + cjH-oh = 1-85 X 10 9

*5.0 XlO9 2 [53] PR prod, form Absolute. Obs. Br-,- (1.0+0.15) XlO10 2 [47] S irrad. comp, with Safranine T

Rel. to A- () || +(- 6 ii b = 7.8 X 109

3.9 XlO9 0.05 N

H2 SO4

[54] S irrad. comp, with isopropanol

Rel. to*OH+ ,i> = 2.0xi09

7.2 XlO9 2.7 [52] S irrad. comp, with ethanol

Rel. to A:oh+(-2 ii 5oh = 1-85 X 109

(5.0 + 0.7) x 109 3 [47] S irrad. comp, with Safranine T

Rel. tok( )|| + (- 6 H6 = 7.8 X 109

(2. 2 + 0.3) xlO9 5.5 [47] S irrad. comp, with Safranine TRel. to Atoh-m 6 hs = 7.8 X 109

1.17 x 109 6 [55] S irrad. comp, with ethanol

Rel. to Ar ( )ii + c2 HsoH = 1-85 X 109

1.1 xlO9 6 [52] S irrad. comp, with

ethanol

Rel. to A:()H+e2Hson — 1-85 X 10 9

*1.0 x 109 7 [53] PR prod. form. Absolute. Obs. Br2- 1.14 x 109 7-10.5 [32] S irrad. comp, with benzoate

R. 0 I. to k()\\ + benzoate 5.6 X 10

*(1.2 + 0.15) x 109 7 [56] PR prod. form. Absolute. Obs. Brj

1.08 x 109 8 [55] S irrad. comp, witli PNDA

Rel. to Ar))|| + (-2 || 3 oii = 1 .85 X 109

(1.1 + 0.06) x 109 9 [24] S irrad. comp, witli PNDA

Rel. to*0H + ,.NI)A = 1.25 X10'°

35
Table 11. Inorganic electron transfer reactions — Continued

Reactant A(A/_ 1 s _1

) pH Ref. Method Comments

hypobromite 4.4 X 109 11 [57] PR comp, with carbonate Rel. to Aoh+co? = 4.1 X 108 BrO1.9 x 109 13 [57] PR comp, with carbonate Rel. to AoH+cor = 4.1 X 108

I- 1.38 x 10 10 0.05 N [54] S irrad. comp, with Rel. to A0h+ip = 2.0 X 109

(1.13 ± 0.16) X1010 H2 SO4 2 [47]

isopropanol S irrad. comp, with Rel. to A0h + c6 H6 ~ 7.8 X 109

*3.4 x 10 10 2-7 [7]

Safranine T

PR prod. form. Absolute, Analysis of I + I _ ^I~

(7.5± 1.1) x 109 3-5.5 [47] S irrad. comp, with

equilibrium

Rel. to Aqh + C6 H6 ~ 7.8 X 109

1.6 x 1010 7 [54]

Safranine T

S irrad. comp, with Rel. to Aoh +ip = 2.0 X 109

*(1.02±0.13) x 1010 neut. [ 20]

isopropanol

PR product form. Absolute N2 0 added

1.2 X 1010 9 [42] S irrad. comp, with Rel. to A0h+ pnda —' 1-25 X 1010

(1.09±0.06) x 1010 9 [24]

PNDA

S irrad. comp, with Rel. to A0h+pnda = 1-25 X 1010

1.37 x 1010 10.5 [32]

PNDA

S irrad. comp, with Rel. to -P benzoate 5.6 X 10

7.3 x 109 11 [42]

benzoate

PR comp, with CO3 Rel. to Aoh+ coir = 4. 1 X 108 3.4 x 1010 1.0 M [51] S irrad. comp, with Rel. to Aoh+ ip=2.0 x 109

thiocyanate 8.6 x 109

NaOH

2 [58]

isopropanol

S irrad. comp, with Rel. to A0H + thymine = 4.7 X 109

1.13 x 1010 2 [33]

thymine

PR comp, with C2 H5 OH Rel. to Aoh+c 2 H 50h = 1-85 X 109 *(7.5 ±0.5) X 109 2-10 [59] PR prod. form. Absolute N2 O added

*2.8 x 1010 2-9 [7] PR prod. form. Absolute. Analysis of

9.2 X 109 5 [60] S irrad. comp, with

CNS' + CNS-^CNS2- equilibrium.

Rel. to AoH + thymine = 4. 7 X 109

1.13 x 1010 7 [33]

thymine

PR comp, with C2 H5 OH Rel. to Aoh+c2 h5oh~ 1-85 X 109 *6.6 x 109 7 [35] PR form, of (CNS)jr Absolute N2 O added

1.19 x 1010 9 [18] S irrad. comp, with Rel. to Aqh+pnda = 1-25 X 1010

*(1.08±0.1) X 10'° neut. [61] PNDAPR prod. form. Absolute. N2 O added.

36
Chapter VII. References

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Radiolysis, Ed. M. Ebert et al., p. 207, (Academic Press, London and New York, 1965).

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Radiolysis Ed. M. Ebert et al. p. 217, (Academic Press London and New York, 1966).

[4] Baxendale, J. H., Breccia, A., and Ward, M. D., Int. J. Radiat. Phys. & Chem. 2, 167 (1970).

[5] Adams, G. E., Broszkiewicz, R. K., and Michael, B. D., Trans. Faraday Soc. 64, 1256 (1968).

[6] Anbar, M., and Thomas, J. K., J. Phys. Chem. 68 , 3829

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[7] Baxendale, J. H., Bevan, P. L. T., and Stott, D. A., Trans. Faraday Soc. 64, 2389 (1968).

[8] Thomas, J. K., J. Phys. Chem. 70, 2409 (1966).

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[11] Bunn, D., Dainton, F. S., Salmon, G. A., and Hardwick,

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in Pulse Radiolysis, Ed. M. Ebert et al., p. 221 (Academic

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[15] Keene, J. P., Radiat. Res. 22, 14 (1964).

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[19] Buxton, G. V., Dainton, F. S., and Kalecinski, J., Int. J. Rad. Phys. & Chem. 1, 87 (1969).

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3276 (1968).

[55] Anbar, M., Meyerstein, D., and Neta, P., J. Chem. Soc.

(B), 742 (1966).

[56] Matheson, M. S., Mulac, W. A., Weeks, J. L., and Rabani, J.,

J. Phys. Chem. 70, 2092 (1966).

[57] Buxton, G. V., and Dainton, F. S., Proc. Roy. Soc. A. 304,

427 (1968).

[58] Scholes, G., Shaw, P., Willson, R. L., and Ebert, M., Pulse

Radiolysis, Ed. M. Ebert et al., p. 151, (Academic Press, London and New York, 1965).

[59] Greenstock, C. L., Ng, M., and Hunt, J. W., Advances in Chem. Series No. 81, Radiation Chemistry I, 397, Ed.

R. F. Gould, (1968).

[60] Scholes, G., and Willson, R. L., Trans. Faraday Soc. 63,

2983 (1967).

[61] Ellison, D. H., Salmon, G. A., and Wilkinson, F., Proc. Roy.

Soc. A32 8, 23 (1972).

37
Chapter VIII. Radical Reactions

Rate constants for the reaction of the hydroxyl

radical with another hydroxyl, or with other free

radicals (notably those formed in the radiolysis of

water) have proved to be among the most difficult of the hydroxyl rate constants to determine ac- curately. Nevertheless, values have been obtained,

in one case by several methods, for the following

radical-radical reactions of OH as well as for three

analogous reactions of the deuteroxy radical, OD,

the latter in deuterium oxide, rather than in water.

oh+oh=h2 o2 (1)

oh+o-=ho2- (2)

oh+h=h2 o (3)

OH+eaq =OH- (4)

oh+ho2 =h2 o+o2 (5)

oh+o2-=oh-+o2 (6)

oh+h2 o2+ = h3 o+ +o2 (7)

od+od=d2 o2 (8)

0D+D=D2 0 (9)

OD + eo20 =OD- (10)

k i has been determined, as pointed out in chapter

III, by a method analogous to the rotating sector

technique, ki and k% have been determined by pulse

radiolysis observation of the hydroxyl radical

directly. The molar extinction coefficient must be

known for this calculation, and it should be noted

that there is a weak overlapping absorption of Hatoms. Several of these rate constants have been

determined by computer fitting of over-all product

yields in pulsed and steady irradiations, relative to the rate constant for hydrogen atom recombination.

These values, dependent as they are upon a fit to many parameters, must be regarded as approximate.

An accurate value for kz has been determined by

pulse radiolysis, in competition with the ferrocyanide ion oxidation by OH. For the reactions of

OD, there is some uncertainty, since the pK for OD

is not precisely known, concerning the extent of

involvement ofO -

in the reactions. The values, as well as a brief indication of the method used along

with other pertinent factors, are given in table 12.

Table 12. Radical reactions

Reaction k{M-' s1 ) T(° C) pH Ref. Method Comments

oh+oh=h2 o2 t(4.0± 1.0) x 109 RT neut. [1] P rot. sector Relative to /c oh+ h 2 = 4.5 X 10 7 Mechanism dependent

t(5.5±0.8) X 109 23 acid and

neut.

[2] PR Relative to A: O H + ferrocyan = 0-93 X 10 10

*(5.3 ± 0.5) x 109 RT neut. [3] PR direct obs. of OH Dependent one0H- Absolute. Corr. made for H + OH.

*(5.3±0.5) X 109 RT 3 [4] PR direct obs. of OH Dependent on e<) M

. Absolute. 6X 109 RT acid [6] Prod, yields in P and S

irrad. Comput. fit to over-all mechanism

withA; H+ H = 6 X 109 7 x 109 RT acid [8] Prod, yields in P and S

irrad. Comput. fit to over-all mechanismwith&H +H = 6 X 109 oh+o-=ho2-

=£ 2. OX 10 10 23 basic [21 PR Relative to ^OH + ferrocyan 0.93 X 10 oh+h=h2 o 7 x 109 RT 3 [4] PR direct obs. of OH No corr. for overlapping abs. of H

1.2 X1010 RT acid [6] Prod, yields in P and S

irrad. Comput. fit to over all mechanism

with kH+ H = 6 X 109 3 x 10 10 RT acid [8] Prod, yields in P and S

irrad. Comput. fit to over-all mechanism

with A: h +h = 6 X 109 OH + CaqT- OH- *(3.0± 1.0 X1010 23 basic [5] PR obs. of e~q Mechanism dependent. Absolute.

38
Table 12. Radical reactions— Continued

Reaction kiM-1 s1 ) T{° C) pH Ref. Method Comments

0H+H02=H2 0+02 6 x 109 RT acid [6] Prod, yields in P and S

irrad. Comput. fit to over-all mechanism

withA:H +H = 6 X 109 7.1 X109 25 acid [7] Prod, yields in P irrad. Comput. fit of QH2 O2 ) with ^OH + OH = 6 X 109 1.4 x 1010 RT acid [8] Prod, yields in P and S

irrad. Comput. fit to over-all mechanismwith &h+h = 6 X 109 oh+o2-=oh-+o2 1.01 x 1010 25 >2.74 [7] Prod, yield in P irrad. Comput. fit of QH2 O2 ) with ^OH + OH = 6 X 109 0H+H2 02+ =H30+ + 02 1.27 x 1010 25 < 1.51 [7] Prod, yield in P irrad. Comput. fit of G(H2 02) with ^OH + OH” 6 X 10 9

OD + OD — D2 02 4.00 x 109 23 basic [9] PR obs. of e~aq Concurrent with OD + e~q Uncertainty about 0~

OD+ D= D2 0 2.00 x 10 10 23 basic [9] PR obs. of e~q Concurrent with OD-+- e~q Uncertainty about 0“

OD + eD2o = OD- 2.80 X1010 23 basic [9] PR obs. of e qq Uncertainty about amount of 0~

Chapter VIII. References

[1] Schwarz, H. A., J. Phys. Chem. 66, 255 (1962).

[2] Rabani, J., and Matheson, M. S., J. Phys. Chem. 70, 761

(1966).

[3] Pagsberg, P., Christensen, H., Rabani, J., Nilsson, G., Fenger, J., and Nielsen, S. O., J. Phys. Chem. 73, 1029

(1969).

[4] Thomas, J. K., Trans. Farad. Soc. 61 , 702 (1965).

[5] Matheson, M. S., and Rabani, J., J. Phys. Chem. 69, 1324

(1965).

[6] Thomas, J. K., J. Phys. Chem. 67, 2593 (1963).

[7] Sehested, K., Rasmussen, O. L., and Fricke, H., J. Phys.

Chem. 72,626 (1968).

[8] Fricke, H., and Thomas, J. K., Radiation Res., Supplement

No. 4, 35 (1964).

[9] Hart, E. J., and Fielden, E. M., J. Phys. Chem. 72,577 (1968).

39
Chapter IX. Reactions of the Oxide Radical Ion

The chemical reactivity of the basic form of the

hydroxyl radical, 0~, differs markedly from that of

OH in many reactions. Since the pK is 11.9 for the

equilibrium

0H+0H-^0- + H2 0, (1)

all chemical reactions of the oxide radical ion have

been studied in strongly basic solution. The differ- ence in reactivity of 0~, compared with OH, may

be seen in the following reactions. Whereas the

hydroxyl radical is unreactive toward oxygen, the

oxide radical ion adds readily to form the ozonide

ion:

0-+02 = 0,j\ (2)

While the hydroxyl radical adds readily to aromatic

molecules, the reactivity of O

- toward aromatic

compounds is lower by at least three orders of

magnitude in the specific rate constant. The OH

radical is much more effective than is the oxide

radical ion in oxidizing a number of inorganic

anions. Coulombic repulsion of O

-

, which un- doubtedly accounts for part of this difference, is not great enough to account for all of it, and the

larger part of the effect may be the result of a lower

electron affinity of O

- compared with that of OH.

Hydrogen abstraction reactions of 0

_

, on the other

hand, exhibit a specific reactivity only slightly lower

than that of OH:

O- + C2 H5 OH = OH- + C2 H4 OH (3)

Rate constants for reactions of 0~ have been determined in a few cases by direct observation of a transient product in pulse radiolysis. Thus the

ozonide ion, formed in reaction (2), has a strong

optical absorption with a maximum [1, 6] at 430

nm, which has been used [3] to determine kz absolutely. Similarly, has been determined absolutely

[7] by observing the formation of the a-ethanol

radical, of which the ionic form [8, 9] present at high pH has a conveniently observable optical

absorption in the uv. In other cases, rate constants have been determined in flash photolysis and in pulse radiolysis

by competition kinetic studies [3, 6, 10, 14] using

reaction (3), or some other process as the reference

reaction. Where the desired result is obtained by

monitoring Og, the interpretation of the data is usually quite complex, involving as it does not only

the equilibrium constant for the pH-dependent

equilibrium (1), and the forward and reverse rate constants for the thermal equilibrium (2), as well

as the rate constant for the reaction of interest, but

also the rate constant for the analogous competing

reaction of OH. Notwithstanding the complexity,

competition kinetics with ozonide formation is a feasible method, since the formation rate constant,

kz, is reasonably accurately known and the decomposition constant, &2r , has been determined [10] as 3.3 X 103 s -1

at 25 °C.

Table 13 lists the rate constants for reactions of

0~, among which are to be found examples of addition, hydrogen abstraction, and reaction with other

radicals and radical ions, notably those formed in

the radiolysis of water.

Table 13. Reactions of the oxide radical ion

Reaction k{ M-1 s1 ) T (°C) pH Ref. Method Comments

0~ + O2 — Oj- t*2.5 x 109 RT 13 [3] PR prod. form. Absolute. Observ. O3" Single concen. of O2

*2.5 X 109 RT 13-13.7 [6] Flash, phot. prod, form.

Absolute. Observ. O3" in H2O2 phot. No data given

t*(3. 6+0.4) x 109 20-25 11 & 13 [11] PR prod. form. Absolute. Observ. of Oj form. 4 X 109 23 11.9 [17] PR prod. form. Absolute. Approximate

o- + c6 h5 coo- < 6x 106 25 13 [4] PR comp, with 0;f form.Rel. to A: 0

- + o2 = 2.5 X 109

. Complex kinetics. May be

very much lower.

40
Table 13. Reactions of the oxide radical ion — Continued

Reaction ATM- 1 s-1 ) 7’(°C) pH Ref. Method Comments

0-+H2 0=0H+0H- 5.5 X102

(3.0 xlO4 s1 ) RT 13-13.7 [ 6] Flash phot. Rel. to ^o-+o2 = 2.5 X 109

1.7 X106 (9.2xi07 s-x

)

20-25 11 [12] PR comp, with o-+co2 -

3

Rel. to Ao-+ c2 h5oh = 1 x 109

2 x 10 7 25 13 [10] Calculated Based on equilibrium consts. and Aoh+oh = 4 X 10 9 o-+h2 =oh-+h (8±4) x 10 7 23 13 [13] PR observ. of e~aq Comput. fit with other rate consts. and depend, upon

ee -

q o-+ho2-=oh-+o2- (7±3) x 108 25 13 [4] PR observ. of Of Rel. to l'o-+oj-2.5X 109 8.1 x 108 20-25 11 [ 11 ] PR comp, with OH+CO2 -

Rel. to Aoh+co2 _= 4.1 X 10 8

(5.0± 1.7) x 10 8 RT 13-13.7 [6 ] PR observ. of Of Rel. to A0

-

+ o2 ~ 2.5 X 109 2.74 x 108 24 12-13.5 [21 ] PR comp, with O

- + 02

Rel. to A0 -

+02 = 2.5x 109 Corr. to zero ionic strength

O- + CH3 OH

= ch2 oh+oht(5.2 ± 1.0) x 108 25 13 [10] PR observ. of Of

decay

Rel. to A0

- +0j = 2.5 x 109

t*(5.8±0.8) x 108 25 > 13.3 [7] PR prod. form. Absolute. Observ. of alcohol

radical form. o-+c2 h5 oh t8.9 x 108 RT > 13 [14] PR observ. of Of Rel. to A0 -+o2 ~2.5 X 10 9 = c2 h4 oh+oht (8.4 + 1.6) x 108 25 13 [10] PR observ. of 03- decay

Rel. to A0

- +0l = 2 . 5 x 109

t*(1.13±0.17) x 10 9 25 > 13.3 [7] PR prod. form. Absolute. Observ. of alcohol

radical form. 0-+C2 0l- 1.6 x 107 24 12-13.5 [15] S irrad. C02 form. Rel. to A0 -+oj = 2.5x 109 — C02 + 20H- + COf

O

- + [F e(CH)e]4_

HjO = [Fe(CN)6]3 - + 20H- < 7 x 10 7 23 13.5 [17] PR comp, with OH + ferro- cyanide

Approx, upper limit

o-+co2 -

H= CO3-+ 2 OH- < 10 7 23 10.6-13.5 [16] PR prod. form. Absolute. Approx, limit. o-+hco2-

HjO = HC02 + 20H1.4 x 109 20-25 11 [11] PR comp, with OH+CO2 - Rel. to Aoh + co3 _ ~4.1 X 10 8

0- + N02-

H= N02 +20H- 2.8 x 108 20-25 11 [11] PR comp, with OH + CO2 - Rel. to A0h + co?- = 4.1 X 10 8

HiO O- + I- = I + 20H- 9.6 x 108 20-25 11 [11] PR comp, with OH + CO2 " Rel. to A0h + co|-~4.1 X 10 8

O- + CNSH= CNS+20H1.0 x 109 > 12 [19] PR comp, with O

- + 02

Rel. to A0

- +02 = 2.5 X 109

41
Table 13. Reactions of the oxide radical ion — Continued

Reaction Ar(M"' s-*) T(°C) pH Ref. Method Comments

0+0=Of- 5.8 X 109 RT 13 [3] PR comp, with O3 Rel. to /r0 - + oj — 2.5 X 109

. N2 0

form. added

2 X 109 23 12-13 [18] PR comp, with Rel. to OH + [Fe(CN)6 ] 4 - OH + ferro- cyanide < 1.8 x 109 23 12-13 [17] PR comp, with May be much less OH + ferro- cyanide < 8x 108 23 13 [20] PR comp, with Recalculated from ref. [17] OH + ferro- cyanide

o-+oh=ho2- < 2.6 x 1010 23 12-13 [17] PR comp, with May be 4-fold lower OH + ferro- cyanide

HjO 0- + e~ = OH- (2.2 ± 0.6) x 10>° 23 > 13 [13] PR observ. of e~

aq Comp, fit with other rate constants

? 0

- + O3 —

5X 108 25 13 [10] PR observ. of O3

" decay

Rel. to k0

- +02 = 2.5 X 109

Chapter IX. References

[1] Czapski, G., and Dorfman. L. M., J. Phys. Chem. 68 , 1169(1964).

[2] Adams, G. E., and Boa", J. W., Proc. Chem. Sue., 112

(1964).

[3] Adams, G. E.. Boag, J. W., and Michael, B. D., Proc. Roy

Soc. 289A, 321 (1966).

[4] Felix, W. D., Gall, B. L., and Dorfman, L. M., J. Phys.

Chem. 71,384(1967).

[5] Heidt, L. J., and Landi, V. R., J. Chem. Phys. 41 , 176 (1964).

[6] Behar, D., and Czapski, G., Israel J. Chem. 6, 43 (1968).

[7] Wander, R., Gall, B. L., and Dorfman, L. M., J. Phys.

Chem. 74,1819(1970).

[ 8] Asmus, K. D., Henglein, A., Wigger, A., and Beck, G., Ber. Bunsengesell Physik. Chem. 70, 756 (1966).

[9] Simic, M., Neta, P., and Hayon, E., J. Phys. Chem. 73, 3794

(1969).

[10]

Gall, B. L., and Dorfman, L. M., J. Am. Chem. Soc. 91,

2199(1969).

[11] Buxton, G. V., Trans. Farad. Soc. 65, 2150 (1969).

[12] Buxton, G. V., Trans. Farad. Soc. 66, 1656 (1970).

[13] Matheson, M. S., and Rabani, J., J. Phys. Chem. 69, 1324

(1965).

[14] Adams, G. E., Boag, J. W., and Michael, B. D., Trans. Farad. Soc. 61, 492 ( 1 965).

[15] Draganic, Z. D., Draganic, I. G., and Kosanic, M. M., J. Phys. Chem. 70, 1418(1966).

[16] Weeks, J. L., and Rabani, J., J. Phys. Chem. 70, 2100

(1966).

[17] Rabani. J., and Matheson, M. S., J. Phys. Chem. 70, 761

(1966).

[18] Rabani, J., and Matheson, M. S., J. Am. Chem. Soc. 86 , 3175(1964).

[19] Adams, G. E., Boag, J. W., Currant, J., and Michael, B. D., Pulse Radiolysis, Ed., M. Ebert, et al., p. 117 (Academic

Press, N.Y., 1965).

[20] Rabani, J., Radiation Chemistry of Aqueous Systems,

Ed., G. Stein, p. 229 (Weizmann Press 1968).

[ 21 ] Rabani, J., Advances in Chemistry Series, No. 81, Radiation

Chemistry I, 131 Ed. R. F. Gould (1968).

42
Chapter X. Reactions With Biological Molecules

There is an extensive literature on the reactions

of the OH radical with compounds relevant to, or involved in, many biochemical and biological

processes. Reactions studied include oxidative

deamination of aliphatic amino acids; addition to the free bases, nucleotides, and nucleosides derived

from the nucleic acids; hydrogen abstraction in- volving the thiol group in cysteine and other sulfurcontaining compounds and carbohydrates; and also,

various investigations of OH-oxidation of biologically-active macromolecules including enzymes and

nucleic acid itself. In this chapter the data for the

compounds are presented in separate tables grouped

according to general biochemical type.

Amino Acids

It is now well established that oxidative and re- ductive deamination are major processes in the

radiolysis of dilute aqueous solutions of simple ali- phatic amino acids such as glycine and alanine

[1]. It is believed that OH attack on amino acids

of this type, represented as NH^CH(R)COO~,

occurs mainly at the carbon atom alpha to the

carboxyl or amino group.

OH+ NH+CH(R)COO- = H2 0+ NH+C(R)COO- (1) Dependent upon experimental conditions, the a- amino radical may react by dismutation to form an

intermediate imino-cation which hydrolyzes to yield

ammonia and a keto acid:

2NH3+ C(R)COO- = NH3+ CH(R)COO-

+ NH2 = C(R)COO- (2) NHf = C(R)COO- + H2 0 = NH4 + RCOCOO-

(3)

Although there is an approximate 1:1 stoichiometry

between OH disappearance and ammonia production, this is not so for aliphatic amino acids containing longer side chains. For example, in the radiolysis

of such amino acids in dilute aqueous solution, the

efficiency of ammonia production falls with increasing length of the aliphatic side chain [2]. This is due

to the reduction in the yield of the a-amino radical

relative to that of radicals formed by OH attack elsewhere in the molecule.

Oxidative deamination is not a major process

following the reaction of OH with the ring-containing

amino acids, e.g. tyrosine, phenylalanine, tryptophan, and histidine. As might be expected by

analogy with other aromatic and heterocyclic compounds, OH radicals react predominantly by addition to the ring. In the case of tyrosine, the hydroxycyclohexadienyl radical formed in this reaction

undergoes a unimolecular elimination reaction in

which a molecule of water is lost from the structure

The reaction is acid-base catalyzed.

The rate constants for OH addition to the ringamino acids are an order of magnitude or more

larger than those for the non-ring amino acids, with

the exception of the sulfur-containing compounds.

For this reason therefore, the ring structures in

enzymes are particularly vulnerable to OH radical

attack, reactions which may lead, in some cases, to loss of biological activity.

In the heterocyclic amino acid, tryptophan, there

is evidence of multifocal attack by OH. This is based on the analysis of the stable products fol- lowing radiolysis of aqueous solutions [4] and also, on a study of the reactions of OH generated in the

Fenton reaction [5]. A detailed pulse radiolysis

study of tryptophan and related substituted indoles

not only confirmed that multifocal attack occurs but

has also provided information on which sites in the

ring are involved [6]. The transient absorption maximum due to the OH-adduct has three subsidiary

maxima at 310, 325, and 345 nm. The spectrum was compared with those obtained by OH addition to indoles in which substituent groups were present at various positions in the structures. It was found that

the relative intensities of the three subsidiary max- ima are dependent upon the position of the substit- uent. It was argued that substitution of a methyl

group on various positions of the indole ring should

restrict electrophilic attack by OH at that position.

Complications arising from changes in the electron

density distribution in the molecule due to the inductive effect of the methyl group appear to be negligible since the molecular spectrum of indole is not

very different from those of the substituted indoles.

It was concluded that the effect of substitution on

the relative intensities of the subsidiary maxima in

43
the transient spectra reflect the lowering of the probability of OH attack at the substituted position.

Kinetic data indicated that the OH reactivities at the

three sites in tryptophan were of the same order of

magnitude, from which it was concluded that the

three OH-adducts are formed in approximately

equal yield.

It is evident from table 14, that the sulfur-containing amino acids react extremely rapidly with OH.

Cysteine (RSH) is oxidized to cystine through the

formation of the intermediate radical RS which

results from abstraction of a hydrogen atom from

the thiol group

RSH 4- OH = RS • + H2 0 (6)

It has been shown by pulse radiolysis studies on

cysteine-related compounds including H2 S and simple mercaptans [7, 8], that under nonacidic conditions, the intermediate RS radical complexes with a

solute molecule to form the ion-radical RSSR- according to the equilibria

RSH^±RS- + H+ (7)

RS + RS- ^ RSSR-

. (8)

Cystine is probably formed by dimerization of the

uncomplexed RS radicals

RS + RS = RSSR. (9)

In the presence of oxygen, the high efficiency for

the oxidation of RSH indicates the occurrence of a chain reaction [9]. It has been suggested that the

RS radicals formed in (6) react with oxygen to form

the RS02 radical which propagates the chain by the

H-atom transfer reaction

RS02 + RSH = RS02 H + RS. (10)

absorption band is observed, indicate that the

absorption band is due to a radical anion-dimer

analogous to the species RSSR- formed by OH oxidation of thiols. SH

NH2—C=NH + NH2 —C=NH

s s NH2—C=NH NH2—C=NH + H (13)

Some trends are apparent in the rate constant

data listed in the tables. Clearly the absolute reactivities of OH with amino acids (table 15) and simple

peptides (table 16) vary considerably with the struc- ture of the compound and also with pH, an effect

which is due to the influence of the ionic equilibria

in these solutes. With increasing pH, the net charge on amino acids changes from + 1 in acid solution

through zero to — 1 in alkaline solutions which

favors reaction with the electrophile OH.

The simple aliphatic amino acids, glycine and ala- nine, appear to be the least reactive although when

OH groups or branched chains are present in the

molecules, inductive effects tend to increase the

reactivity. As discussed earlier, the ring-structured

amino acids are among the most reactive due to the ease with which OH addition to the 7r-electron sys- tem can occur. Similar trends are apparent in the

data for the simple peptides although the peptide

linkage evidently increases the reactivity relative to that of the free amino acids. As might be expected,

in peptides where the component amino acids have

different reactivities in the free state, e.g., glutathi- one and glycyltyrosine, the rate constants are comparable in each case to that of the more reactive

amino acid component.

The chain termination step is dimerization of RS

radicals to from cystine. Thiourea is rapidly oxidized by OH radicals [10]. It has been suggested

that the reaction proceeds through a transient intermediate imino radical formed by hydrogen abstraction from the imino form of the solute.

S SH

NH2—C—NH2 = NH2—(i=NH (11)

SH S- NH2—C=NH + OH = NH2—d=NH+ H2 0 ( 12

)

A strong transient absorption band with a maximum

near 400 nm, is formed during the pulse radiolysis

of thiourea solutions [10]. This was attributed originally to the imino-RS radical formed in reaction

(11). However, more recent pulse radiolysis studies

with the analogue selenourea [11], where a similar

Enzymes

Table 17 lists the available data for some relatively

simple enzymes. In most cases, the rate constants

are of the order of 10nA/-1

s

-1

, values which are cal- culated on the basis of the total molecular weight of

the enzymes. Since these compounds contain in excess of 100 amino acids in their structure, the rate constants calculated on the basis of the total molarity of the individual component amino acids are of course considerably smaller. Although attempts

have been made to calculate overall rate constants

from a knowledge of the values for the single units,

such calculations are of doubtful significance in view

of the complex tertiary structure of enzymes and the

varying degrees of accessibility to free radical attack

of different regions in the molecules.

Overall rate constants have been measured by

pulse radiolysis and stationary-state methods. Data

obtained by competition techniques are less reliable

than those determined by absolute methods due to

44
the problems of kinetic inhomogeneity encountered

in systems containing very large macromolecules.

Fortunately, rate constants can be measured quite

conveniently by the absolute pulse radiolysis methods since OH attack on proteins gives rise to strong

transient absorption bands in the ultraviolet region

of the spectrum. These absorption bands are due to addition of OH to the ring-amino acids present in the

structure.Nucleic Acid Derivatives

The relationship between intracellular damage to DNA and the response of the cell (lethal or otherwise) to ionizing radiation is one of the most fundamental problems of radiobiology. It is not surprising

therefore that much effort has been devoted to the

study of the radiation chemistry of DNA and its constituent nucleotides, nucleosides, and free bases.

It is from this field of research that almost all of

the available information on OH reactivities with

these compounds derives. It is relevant therefore

to discuss first some of the early work on the stationary-state radiation chemistry of these compounds.

Dilute aqueous solutions of simple pyrimidine

bases, e.g., thymine, cytosine, and uracil are bleached by ionizing radiation [12, 13]. The chromophore in these solutions is the 5:6 double bond in

the heterocyclic ring which gives rise to an absorption maximum at 260 nm and it was concluded

therefore that the radiation-induced bleaching was

due to saturation of this double bond. This was

subsequently confirmed by measurement of residual

double bonds by bromination with free bromine.

In oxygenated thymine solutions, hydroperoxides

are formed, and it was concluded therefore that OH

radicals formed during the radiolysis add to the

5:6 double bond [14, 15]. The full reaction sequence

is as follows:

O

HN

+ OH

pCH3 ° N oh

H

02

O

A CHs

HN V02

' O^N OH

H

0

A ch3 HNXV-02 h

H (14)

This has since been confirmed in numerous

experiments with thymine and other pyrimidines.

By comparison with the cis and trans isomers of

5-hydroxy-6-hydroperoxythymine and 6-hydroxy-5-

hydroperxoythymine, it was shown chromatographically [16, 17] that the radiation-induced peroxy

compound is a mixture of the cis and trans isomers

of the 6-hydroxy derivative. However, for uracil

and dimethyluracil where hydroxyhydroperoxides

are also produced, there are indications that both

the 6-hydroxy and 5-hydroxy radicals are formed by

OH attack in the bases [18].

An interesting application of the aerated thymine

system, and of relevance to this monograph, is its use as a reference standard for the measurement of

relative rate constants for reactions of OH with

many different solutes [19a, 19b]. The method is based on kinetic analysis of the competition,

OH -F thymine —» loss of chromophore (15)

OH 4- solute—» products, (16)

in which the extent of bleaching of the thymine

chromophor is used to measure the relative rate constant ratio kie/ki^. The application of this kinetic

system is discussed more fully in the final chapter

together with the normalization procedure used for

obtaining absolute values of the rate constants

determined by this method.

In the thymine, the site of OH attack changes

gradually with increasing pH [20]. In solution of

pH > 9, 5-hydroxymethyluracil is one of the overall

products of the oxidation, indicating that under

these conditions some attack occurs at the 5-methyl

group

V

CHc

C

+ OH

pH >9

\H

CH2 OH

H (17)

In the presence of cupric ion, the OH adduct of

uracil is mainly oxidized directly to the vicinal

glycol derivative [21]

H

OH

+ Cu 11 H2 0

VOH

A\OH

H

+ Cu 1 + H+

(18)

although isobarbituric acid is a minor product of

the reaction

VH

< OH

/ n

V/OH

+ Cun > /Si

+ Cu 1 + H+

(19)

Compared with the pyrimidines, the mechanism

of OH-induced oxidation of the purines has received

considerably less attention. Early radiation chemical

45
studies which showed that deamination and ring

opening were important in the overall oxidation

process were explained on the basis of OH-addition

to the 4-5 double bond as the main initiating

reaction [22, 23]

OH

NH2

ns /N

N N

H

OH (21 ) However there is some evidence [24] that OH

attack can occur elsewhere in adenine since small

yields of 8-hydroxyadenine and 4-5-6-triaminopyrimidine are formed in this system.

Further confirmation that the 4-5 double bond is a major locus of OH attack in purines was obtained

from an analysis of the radiolysis products from

several purine bases [25, 26]. It was argued from

conventional purine chemistry that oxidation at the

4-5 position should give, on hydrolysis, 4-5 diketo

products. The overall stoichiometry in the case of

xanthine is given by the equation:

O

II

N N

H H

-t- 4H2 0 + 02

o

HN O

+ 2NH3

//\

O N O

H

+ HCOOH + H2 02

(22) Analysis of irradiated oxygenated xanthine solution

showed that alloxan and ammonia were formed in

relative yields required by this stoichiometry. For

other purines, the degradation products vary al- though OH addition at the 4-5 double bond is in- dicated as the main initiating reaction in each case. Rate constants for reactions of OH with free

bases, nucleosides, and nucleotides are listed in

table 18. In general, the rate constants are near

diffusion-limited values and unlike the reactivities

with the strongly reducing entity, the hydrated

electron, there appears to be little difference between free bases and the sugar phosphate

derivatives. Where the data are available, it can

be seen that the rate constants measured absolutely

by direct observation of the rate of formation of the

OH-adduct, agree well with those determined by

measurement of the rate of bleaching of the

chromophor. This again confirms that the 5-6

double bond in the pyrimidines is the locus of OH

attack. Although there are occasionally very small

changes in reactivity with change of pH, it is clear

that, also unlike reaction with e~

q

, there appears

to be little general effect of tautomerism on the

rate constants for reaction with OH.

DNA

Nucleoprotein in aqueous solution is itself fairly

resistant to OH attack [27] which contrasts markedly

with the sensitivity of free DNA even when present

in solution with equimolar proportions of free protein. This apparent protection of DNA in nucleoprotein is consistent with the proposed structure

for this molecule in which the double helix of DNA

is enveloped by the protein chains. In aqueous

solution therefore, OH radicals formed in the solvent

have restricted access to the DNA core and must

react preferentially with the protein sheath.

In solution, free DNA is inactivated by reaction

with OH, a process which occurs mainly by attack

at the base moieties. However, elimination of free

bases also occurs due to OH attack on the sugar

backbone [22]. The relative yield of free base

measured after hydrolysis of the irradiated DNA,

is about 20 percent of the total base damage. This

is as might be expected since, in general, rate constants for reaction of OH with alcohols and polyhydric compounds (table 19), e.g., ribose, are about an order of magnitude lower than those for addition

to pyrimidine and purine bases.

Table 14. Sulfur and, selenium compounds

Reactant /c(M *s x ) pH Ref. Method Comments

cysteamine (1.47 ±0.1) X 10 10 1.4-9 [8] PR comp, with CNS - Rel. to Hel - =1.1X10 10 thiocyan.

cysteine 1.3 X 10 10 1 [28] PR comp, with CNS - Rel. to kTM\- =1.1X10 10 thiocyan. (5.1 ±0.5) X 109 2 [19] S irrad. comp, with thymine Rel. to koH + thymine 4.7 X 10

cystine (4.8±0.5) x 109 2 [29] S irrad. comp, with thymine Rel. to &OH + thymine = 4.7 X 109

dimethylsulfide *5.2 X 109 neut. [30] PR product, form. Absolute. N2 0 added. No corr. for H-atoms

dimethylsulfoxide 7.0 x 109 neut. [30] PR comp, with CNS- Rel. to Ar]?! = 1.1 X 10 10 thiocyan.

46
Table 14. Sulfur and selenium, compounds — Continued

Reactant £(M- 1 s -1

) pH Ref. Method Comments

glutathione 1.47 X1010 1.0 [28] PR comp, with CNS - Rel. to kTM1: = 1 . 1 X 10 10 thiocyan.

glycylmethionine (4.0 + 0.3) x 108 2 [29] S irrad. comp, with thymine Rel. tO ^OH + thymine 4.7 X 109

(1.1 ±0.25) X 108 2 [29] PR comp, with CNS - Rel. to A:TM1 = 1.1 X 10 10 thiocyan. (2.2 ±0.3) x 108 5 [29] PR comp, with CNS- Rel. to Afel = 1.1 X 10 10 thiocyan.

hydrogen sulfide 1.84 X 10 10 6 [7] PR comp, with CNS - Rel. to kTM1 - = 1 . 1 X 10 10 thiocyan. 9.0 x 109 alk. [7] PR comp, with CNS- Rel. to k

1*': = 1.1 X 1010 thiocyan.

hydrogen selenide (1.0±0.3) x 10 10 1.0 [31] PR comp, with CNS - Mean value calculated three det.

I- and Br- Rel. to ^oh + cns—

== 6.6 X 10^

5.5 X 10 9 8.5-11.5 [31] PR comp, with CNS - ^oh+i- = 7.0 X 109

I - and Br-

^OH+Br- = 5.0 X 109

f-butylmercaptan (1.8±0.3) X 10 10 7 [32] PR comp, with CNS - Rel. to yp

eL = 1.1 X 10 10 thiocyan.

methylmercaptan (3.3±0.5) x 10 10 7 [32] PR comp, with CNS - Rel. to kTM1 = 1.1 x 10 10 thiocyan. (1.0±0.15) x 10 10 11 [32] PR comp, with CNS - Rel. to kit'- =1.1X1010 thiocyan.

2-mercaptoethanol 8.5 x 109 6-7 [28] PR comp, with CNS - Rel. to *"!• =1.1X1010 thiocyan. (2.7 ±0.4) x 10 10 7 [32] PR comp, with CNS - Rel. to kTM1 = 1 . 1 X 10 10 thiocyan. t (6.1 ±0.7) X 109 6.5 [33] PR comp, with ferrocyanide Rel. tO AoH + ferrocyan. = 9.3 X 10 9 t(4.9±0.5) X 109 6.5 [33] PR comp, with nitrobenzene Rel. to A; 0 H + <t>N02 = 4.7 X 109 (6.7±0.1) X109 11 [32] PR comp, with CNS - Reito4;;;My„ =i.ixio“>

methionine (8.2 ±0.7) X 109 5.5-7 [29] PR comp, with CNS - Rel- to iSiTM. = 1 - 1 X 10“

8.5 X109 6-7 [28] PR comp, with CNS - Rel. to kTM1: = 1.1 X 10 10 thiocyan. (6.0±0.5) X 109 2-2.8 [29] S irrad. comp, with thymine Rel. tO &oh + thymine 4.7 X 109

selenourea *6.9 X109 neut. [ 11 ] PR bleaching of solute Absolute

*5.5 X 109 neut. [ 11 ] PR build-up Absolute

1.20 X 10 10 neut. [ 11 ] PR comp, with CNSRel. to kTM1: = 1 . 1 X 10 10 1.14 x 10 10 neut. [ 11 ] PR comp, with ethanol Rel. to A0 H + C 2 H 5 OH — 1-85 X 109 1.17 x 10 10 neut. [ 11 ] PR comp, with methanol Rel. to &0 H + CH3 OH = 0-85 x 109

Table 15. Amino acids

Reactant &(A/ -1 s

-1

) pH Ref. Method Comments

alanine (4.4±0.2) X107 2 [19] S irrad. comp, with thymine Rel. tO &oh + thymine 4.7 X 10

4.65 X 107 6 [35] S irrad. comp, with

ferrocyanide

Rel. to A^oh + ferrocyan 9.3 X 10

(7.7±0.6) x 107 5.5-6 [29] PR comp, with CNS - Rel. to &*} = 1.1 x 10 10 thiocyan. 6.6 x 108 9.75 [35] S irrad. comp, with

ferrocyanide

Rel. to Auqh

i- ferrocyan. 9.3 X 10

arginine ( 6. 6 ± 0. 6 ) x 108 2 [29] PR comp, with CNS - Rel. to *"!• = 1.1 x 10 10 thiocyan. (7.2±0.5) X 108 2 [29] S irrad. comp, with thymine Rel. tO ^OH + thymine = 4. 7 X 109 (3.5±0.3) x 109 6 . 5-7.

5

[29] PR comp, with CNS - Rel- '»*«. = 11 x 10,0

asparagine 3.1 X 10 7 2 [19] S irrad. comp, with thymine Rel. to &oh + thymine 4.7 X 109

aspartic acid ( 2.1 ± 0. 2 ) x 107 6.8 [29] S irrad. comp, with thymine Rel. tO ^OH + thymine 4.7 X 109 (7.5± 1) x 10 7 2 [29] PR comp, with CNS - Rel. to kTM1: = 1.1 x 10 10 thiocyan. 47
Table 15. Amino acids — Continued

Reactant k(M~' s- 1 ) pH Ref. Method Comments

glutamic acid (1.3±0.1) x 108 2 [29] S irrad. comp, with thymine Rel. to &oH + thymine 4.7 X 109

glutamine 1.5X108 2 [19] S irrad. comp, with thymine Rel. to &OH + thymine 4.7 X 109

glycine 1.7 x 10 7 1 [28] PR comp, with CNS - Rel. to kTM1 =1.1X1010 thiocyan.

( 7.2 ± 0.3 ) x 106 2.8 [29] S irrad. comp, with thymine Rel. tO &oh + thymine 4.7 X 109

( 1 .0 ± 0.07 ) x 107 2.8 [29] PR comp, with CNS - Rel. to kTM1: = 1.1X10‘°

thiocyan. 4.5 x 106 5 [36] S irrad. comp, with PNDA Rel. to A:oh + c 2 h 5 oh = 1-35 X 109 (1.7 ±0.17) x 10 7 5.8 [29] PR comp, with CNS - Rel. to k

r

t f

l - _ = 1.1 X 10 10 thiocyan. 5.9 x 106 7 [35] S irrad. comp, with Rel. tO /CoH + ferrocyan 9.3 X 10 9 ferrocyanide

2.7 X 109 9.5 [35] S irrad. comp, with Rel. tO ^OH + ferrocyan 9.3 X 109 ferrocyanide

( 1 .8 ± 0. 15 ) x 109 9.5 [29] PR comp, with CNS - Rel. to kTM'. =1.1X1(P0 thiocyan. 2.6 x 109 10.5 [37] S irrad. comp, with benzoate Rel. tO ^OH + benzoate 5.6 X 10^ 3.8 x 109 10.5 [35] S irrad. comp, with Rel. tO A)qh -1- ferrocyan. 9.3 X 10^ ferrocyanide

2.0 x 109 12 [36] S irrad. comp, with PNDA Rel. to &0H + C2H5OH = 1-35 X 109

histidine (1.9±0.15) x 109 2 [29] PR comp, with CNS - Rel. to k

Tel - = 1.1 X 10 10 thiocyan. (5.0±0.3) X 109 6-7 [29] PR comp, with CNS - = 11 x i°'0

hydroxyproline (3.3 =i= 0.15) x 108 2 [29] S irrad. comp, with thymine Rel. tO k0H + thymine — 4.7 X 109

isoleucine (1.7 ±0.1) x 109 2 [29] S irrad. comp, with thymine Rel. to ^oH + thymine = 4.7 X 109

leucine (

.8 ± 0. 1 ) x 109 2 [29] S irrad. comp, with thymine Rel. tO (cqh + thymine 4.7 X 109 (1.6 ±0.1) x 109 2 [29] PR comp, with CNS - Rel. to kTM1 =1.1X101°

(1.6±0.1) x 109 5.5 [29] PR comp, with CNS - Rel. to kTM1 =1.1X101°

(3.8±0.5) X 109 9.8 [29] PR comp, with CNS - Rel. to kTM]- = 1.1 X 10i°

thiocyan.

lysine (6.0±0.5) X 108 2 [29] S irrad. comp, with thymine Rel. tO ^OH + thymine 4.7 X 109

norleucine 2.8 x 109 2 [19] S irrad. comp, with thymine Rel. tO &OH + thymine 4.7 X 109

norvaline 1.4 x 109 2 [19] S irrad. comp, with thymine Rel. tO &OH + thymine = 4.7 X 109

ornithine 1.6 x 108 2 [19] S irrad. comp, with thymine Rel. tO A: 0 h + thymine 4.7 X 109

phenylalanine (7.0 ± 0.3) x 109 2 [29] S irrad. comp, with thymine Rel. to k0H + thymine — 4.7 X 109 (5.7± 0.5) x 109 2 [29] PR comp, with CNS - Rel. to kTM}- =1.1X101° thiocyan. (5.8± 0.5) x 109 5.5-6 [29] PR comp, with CNS - Rel. to kTM1.- =1.1X101° thiocyan. t6.0 x 109 neut. [38] PR comp, with Rel* to A~qh + ferrocyan. 9.3 X 10

ferrocyanide

t*6.6 x 10 9 neut. [38] PR product form. Absolute. N2 O added. No corr. foi H-addit.

(9.5 ± 1.5) X 10 9 10.6 [39] PR comp, with carbonate Given rel. to ^oh+co2- = 4.1 X 108

proline (2.9± 0.15) X108 2 [29] S irrad. comp, with thymine Rel* to A)qh -p thymine 4.7 X 10

serine (2.5 ± 0.15) X108 2 [29] PR comp, with CNS - Rel. to k TM1 =1.1X101°

thiocyan. (3.2 ± 0.3) X108 5.5-6 [29] PR comp, with CNS - Rel. to *«!• =1.1X101° thiocyan.

threonine (3.6± 0.15) X 108 2 [29] S irrad. comp, with thymine Rel. to -1- thymine 4.7 X 10

48
Table 15. Amino acids — Continued

Reactant A(.V/ -'s

-1

) pH Ref. Method Comments

tryptophan +*( 1.25 ±0.2) X 10 10 1 [6] PR product form. Absolute. N2 O added. No corr. for H-addit.

(1.1 ± 0.25) x 1010 2 [29] PR comp, with CNS - ReL to A j?! = 1.1 X 10 10

thioeyan. (7.0± 0.6) x 109 2 [29] S irrad. comp, with thymine Rel. tO A()h + thymine = 4.7 X 109 (1.4± 0.25) x 1010 6.1 [29] PR comp, with CNS - ReL to k*} = 1.1 X 10 10 tmocyan. t*(1.2±0.3) X1010 8.8 [6] PR product form. Absolute. NNO added. No corr. for H-addit.

tyrosine (2.2 + 0.5) XIO10 10.6 [39] PR comp, with CO9- Given rel. to AoH-hCo*- = 4.1 X 10®

valine (6.6± 0.5) x 108 2 [29] S irrad. comp, with thymine ReL to A0h + thymine = 4.7 x 10 9

Table 16. Simple peptides

Reactant Ar( l/ ^s 1 ) pH Ref. Method Comments

acetylalanine 4.6 x 108 9.2 [41] PR comp, with CNS - Rel. to A

reL

thi»cyan. = 1.1 XlO10

acetylglycine 4.2 x 108 8.7 [41] PR comp, with CNS - ReL to A"'1 ’ lhit>cyan. = 1.1 X 1010

acetylglycylglycine 7.8 x 108 8.6 [41] PR comp, with CNS -

Rel. to Ar rel thioeyan. = 1.1 x 10 10

alanylglycine (1.4 =t 0.8) x 107 2 [29] S irrad. comp, with thymine Rel. to AOH-f thymine 4.7 X 10 H

glycylalanine (1.7 it 0.15) x 10 8 2 [29] S irrad. comp, with thymine Rel. tO A() H _ lhvmim.= 4.7 x 10 9 (3.5 ±0.3) x 108 5.5-6 [29] PR comp, with CNS - Rel. to A

reL

Ihiocyan. = 1.1 X 10 10

glycylglycine (1.2 ±0.1) x 108 2 [29] S irrad. comp, with thymine ReL to AoH+thvmim. — 4.7 x 109 (1.6 ± 0.1) x 108 2.2 [19] PR comp, with CNS - ReL to A"*! = 1.1 X 10 10

4.4 x 108 4.2 [41] PR comp, with CNS - Rel. to A

rel = 1.1 x 10 10

(2.2 ±0.2) x 108 5.5-6 [29] PR comp, with CNS - ReL to A

rel thi<»cyan. = 1.1 X 10 10 2.7 xlO8 6-7 [28] PR comp, with CNS - Rel. to A

reL = 1.1X10'°

thioeyan. 5.2 x 109 10.5 [41] PR comp, with CNS - Rel. to A

rel- = 1.1 x 10 10 thioeyan.

glycylglycvlglycine (1.4±0.07) XlO8 2 [29] S irrad. comp, with thymine Rel. tO Aqh + thymine — 4.7 X 10 9 (2.4 ±0.25) x 108 2.8 [29] PR comp, with CNS - Rel. to A

rel thioeyan. = 1.1 x 10'° 7.3 x 108 5.4 [41] PR comp, with CNS - Rel. to A

reK

thioeyan. = 1.1 x 10 10

(3.3 ±0.7) x 108 5. 5-6.0 [29] PR comp, with CNS - Rel. to A

rel- thioeyan. = 1.1 x 10>° (1.75±Oe 15) x 10 9 8.5 [29] PR comp, with CNS - Rel. t0 ^thioeyan/= 1.1 X 1010 5.0 x 109 10.6 [41] PR comp, with CNS - Rel. to kKh

thioeyan. = 1.1 X1010

glycylglycylglycyl- (2.2±0.6) X 108 2 [29] S irrad. comp, with thymine Rel. tO Aoh + thymine - 4.7 X 109 glycine

(3.5±0.3) x 108 2.4 [29] PR comp, with CNS - Rel. to A

rel- thiocyan. = 1.1 x 10 10 (4.5±0.5) xlO8 5. 5-6.0 [29] PR comp, with CNS - Rel. to A

1*1 - ihiocyan. = 1.1 x 10 10

( 1.2 ±0.

1

) x 109 7.7 [29] PR comp, with CNS - Rel. to A

rel - thiocyan. = 1.1 x 10 10

(3.0 ± 0.

1

) x 109 9.5 [29] PR comp, with CNS - Rel. to A

7

:

1 thioeyan. = 1.1 x 10'°

glycylisoleucine (2.7 ±0.15) x 109 2 [29] S irrad. comp, with thymine Rel. to Aqh + thymine = 4.7 X 10 9

glycylleucine (2.3 ±0.15) x 109 2 [29] S irrad. comp, with thymine Rel. tO Aqh + thymine — 4. 7 X 109

glycylpenylalanine (8.2±0.6) XlO8 2 [29] S irrad. comp, with thymine Rel. tO Aqh + thymine = 4.7 X 109

49
Table 16. Simple peptides —Continued

Reactant k(M~' s -

') pH Ref. Method Comments

glycylproline (1.3 ±0.1 ) x 109 2 [29] S irrad. comp, with thymine Rel. tO ^oH + thymine 4.7 ^ HP

glycylserine (5.3±0.3) x 108 2 [29] S irrad. comp, with thymine Rel. tO kyH + thymine ~ 4.7 X 109

glycyltyrosine (9.0 + 0.6) x 109 2 [29] S irrad. comp, with thymine Rel. to ^OH + thymine~ 4.7 X 109

glycylvaline (1.1 ±0.1 ) x 109 2 [29] S irrad. comp, with thymine Rel. tO ^OH + thymine 4.7 X 109

histidylhistidine (9.0 ± 0.3) x 109 5. 5-6.

[29] PR comp, with CNS - Rel. to k

Tel =1.1X10'°

thiocyan.

Table 17. Enzymes

Reactant k(M-' s -1

) pH Ref. Method Comments

alcoholdehydro- 1.6 X 10" 9 [42] S irrad. comp, with PNDA Rel. to &<)H+i>.M)A = 1-25 X 10'° genase (yeast) 2.2 X 10" neut. [43] PR comp, with nitrobenzene Rel. to A)()h+<i> 4\()2 = 4.7 X 10 9

catalase 1.4 x 10" basic [44] PR comp, with CNS - Rel. to k

n' u =1.1X10'°

thiocyan.

lysozyme t4.9 x 10'° neut. [45] PR product form. Absolute. N2 O added. No corr. for H-addit.

5.1 x 10'° neut. [46] PR comp, with CNS - Rel. to A: ,el =1.1X10'°

thiocyan. 1.86 x 10'° 9 [42] S irrad. comp, with PNDA Rel. to &<>h+i>m)a = 1-25 x 10'°

ribonuclease 4.3 X 10'° 6.8 [47] PR comp, with CNS - Rel. toA: rel =1.1X10'°

thiocyan.

a8.7 x 10'° 6.8 [47] PR comp, with CNS - Rel. to A: rt“' =1.1X10'°

thiocyan.

trypsin t(8.2 ± 0.7) x 10'° 7.1 [48] PR prod. form. Absolute. N2 0 added. No corr. for H-addit.

(1.6 ±0.1) x 10" 7.4 [48] PR comp, with glucose Rel. to & ()H + t4 iucose~ 1.9 x 109

trypsinogen (1.6 ±0.1) x 10" 7 [48] PR comp, with glucose Rel. tO ^oh + ulucose = 1-9 X 10°

a T= 60 °C

Table 18. Nucleic acid derivatives

Reactant k{M~ l s-») pH Ref. Method

adenine (8.8±0.8) X 108 2 [29] PR comp, with CNS - (3.8±0.15) X 109 5-5.5 [29] PR comp, with CNS - (2.8±0.5) X 109 6.5 [49] PR comp, with CNS - (5.0±0.5) x 109 7.3 [29] PR comp, with CNS -

adenosine (1.75±0.15) x 109 2 [29] PR comp, with CNS - (3.8±0.3) x 109 5.2 [29] PR comp, with CNS -

(4.2 ±0.4) x 109 7.6 [29] PR comp, with CNS -

adenosine-5-phosphate (1.25±0.12) x 109 2 [29] PR comp, with CNS - (3.0±0.3) x 109 5.2 [29] PR comp, with CNS -

4.0 x 109 9 [42] S irrad. comp, with PNDA

deoxyadenylic acid (1.37 ±0.12) x 109 2 [29] PR comp, with CNS - (3.5±0.3) x 109 6.4 [29] PR comp, with CNS -

Comments

Rel- t°Cocya,= l-lXl010

Rel.

Rel.

Rel.

toto

to

k

Te '- thiocyan. k

Tel

thiocyan. k

Tel - thiocyan.

1.1 x 10'°

1.1 x 10 10

1.1 x 10 10

Rel.

Rel.

Rel.

toto

to

k

rel

thiocyan. k

Te'- thiocyan. k

rel thiocyan.

1.1 X 10 10

1.1 x 10 10

1.1 x 10 10

Rel. toA: rel =1.1X10'°

thiocyan. Rel. to k

rel =1.1X10'°

thiocyan. Rel. to A: 0h+pnda = 1-25 X 10'°

Rel.

Rel.

to

to

k

Te ' thiocyan.

A: rel - thiocyan.

= 1.1 x 10'° = 1.1 x 10'°

50
Table 18. Nucleic acid derivatives — Continued

Reactant k{M-' s -1

) pH Ref. Method Comments

cytosine (3.1 ±0.15) x 109 2 [29] PR comp, with CNS - Rel. to k

reL = 1.1 X 10 10

thioc.van. (4.5 ±0.3) x 109 5-6 [29] PR comp, with CNS - Rel. to k

TeL = 1.1 X 10 10

thioc.van. (4.9 ±0.3) X 109 7.4 [29] PR comp, with CNS - Rel. to k

TeU = 1.1 X 10 10

thiocyan.

cytidine (3.3 ±0.3) x 109 2 [29] PR comp, with CNS - Rel. toA: rel - = 1.1 X 10 10 thiocyan. (4.9 ±0.5) x 109 5.2 [29] PR comp, with CNS - Rel. toA: rel = 1.1 X 1010

thioc.van. (4.6 ±0.5) x 109 7.2 [29] PR comp, with CNS - Rel. toA: re1 ' = 1.1X10'°

thioc.van.

cystosine-5- (2.5 ±0.25) x 109 2 [29] PR comp, with CNS - Rel. toA: rel - = 1.1 X 10 10

thiocyan. phosphate

(4.4 ±0.5) x 10 9 7.4 [29] PR comp, with CNS - Rel. toA: rel = 1.1 X 10 10

thioc.van.

deoxycytidylic acid (3.0 ± 0.15) x 10 9 2 [29] PR comp, with CNS - Rel. toAr reL = 1.1 X 10 10

thiocyan. (3.8 ± 0.3) x 109 4.3 [29] PR comp, with CNS - Rel. to Ar rel - = 1.1 X 10 10 thiocyan. (5.0 ± 0.3) x 109 6.7 [29] PR comp, with CNS - Rel. to k

reL = 1.1 x 10 10

thioc.van.

guanosine 7.6 x 109 9 [42] S irrad. comp, with PNDA Rel. to &()H+ i>Nr>A = 1-25 X 10 10

deoxyguanylic acid (4.7 ± 0.5) X 109 2 [29] PR comp, with CNS - Rel. toA: rel - — 1.1 X 10 10

thiocyan. (6.8 ±0.5) x 109 6. 5-7.0 [29] PR comp, with CNS - Rel. toA: rel - = 1.1 X 10 10 thiocyan.

orotic acid 5.1 x 109 neut. [54] PR comp, with ferrocyanide Rel. tO &OH + fcrrocyan. ~ 9.3 X 109

isoorotic acid 3.9 x 109 neut. [54] PR comp, with ferrocyanide Rel. to £,) H + ferr»c.van. = 9.3 X 109

dihydroorotic acid 2.9 x 109 neut. [54] PR comp, with ferrocyanide Rel. tO &()H + ferrnc.van.= 9.3 X 109

orotidine 3.9 x 109 neut. [54] PR comp, with ferrocyanide Rel. tO A:()|| + f,.rroc.van. = 9.3 X 109

thymine (3.7 ± 0.2) x 109 0. 7-7.0 [50] S irrad. loss of chromophor Rel. to ethanol taking ^OH + CalUOH = 1-85 X 10 ' 5.2 x 109 1 [28] PR comp, with CNS - Rel. toA: rel - =1.1 X 1010

thioc.van. (5.2 ±0.2) x 109 2 [29] PR comp, with CNS - Rel. to A: reL -1.1 X1010

(7.8 ± 1.0) x 109 2 [19] S irrad. loss of chromophor Rel. to A:oh+(’bh« — 7-8 X 10 9 (5.0 ±0.2) x 109 5-5.5 [29] PR comp, witli CNS - Rel. to k

rel = 1.1 X 10 10

thioc.van. (5.3 ±0.2) x 109 7.2 [29] PR comp, with CNS - Rel. to k

n' u =1.1X10'°

thiocyan. *7.4 x 109 7 [51] PR product form. Absolute. N2 O added. No corr. for H-addit. 4.9 x 109 7 [52] S irrad. loss of chromophor Comp, with methanol and ethanol.

Rel. to £oh+c2 h5oh = 1-85 X 109

+ 4.7 X 109 neut. [38] PR comp, with ferrocyanide Rel. to Ar0H+feirocyan. = 9.3 X 109 t*5.1 x 109 neut. [38] PR bleaching chromophor Absolute. N2 0 and 02 added

(7.6 ±1.1) X 109 neut. [53] PR comp, with CNS- Rel. to AT*!- X 1.1 X 10'°

thiocyan. *(7-4±0.5) x 109 neut. [53] PR bleaching of chromophor Absolute. No corr. for H-addit. *(4.6±0.3) x 109 neut. [53] PR product form. Absolute. No corr. for H-addit.

6.2 x 109 9 [42] S irrad. comp, with PNDA Rel. to A'oh+pnda" 1-25 X 10'° +*3.9 x 109 11 [51] PR product form. Absolute. N2 0 added. No corr. for H-addit. +*1.1 X109 12.4 [51] PR product form. Absolute. N2 0 added. No corr. for H-addit.

51
Table 18. Nucleic acid derivatives — Continued

Reactant k{M~l s' 1 ) pH Ref. Method Comments

dihydrothymine (4.7±0.5) x 109 neut. [53] PR comp, with CNS" K®1- «® *&»«,= 1.1 X 10”

t*2.2 x 109 neut. [53] PRproduct from. Absolute. No corr. for H-addit. t*2.2 X 109 neut. [51] PR product form. Absolute. N2 0 added. No corr. for H-addit. *4 x 108 12.4 [51] PR product form. Absolute. N2 0 added. No corr. for H-addit.

thymidine (4.6 ±0.5) X 109 2 [29] PR comp, with CNS" Rel. tok]?1 - =1.1X1010 (5.0±0.5) X109 5 [29] PR comp, with CNS" Re|-'°*SU.n = 1 - lxl0'° (4.6 ±0.5) x 109 7.4 [29] PR comp, with CNS"

t*4.7 x 109 7 [51] PR product form. Absolute. N2 0 added. No corr. for H-addit. t*2.1 x 109 12.4 [51] PR product form. Absolute. N2 0 added. No corr. for H-addit.

thymiuylic acid (4.3±0.5) x 109 2 [29] PR comp, with CNS" Rel. to A"!- = 1.1 X 1010 (amm. salt) (5.3 ±0.5) x 109 6.5-7.

[29] PR comp, with CNS” Rel. to = 1.1 X 1010 thiocyan.

uracil (4.8±0.5) X 109 2 [29] PR comp, with CNS" Rel. to n =1.1 X 1010 (5.2 ±0.7) x 109 5 [29] PR comp, with CNS"

(7.3 ±0.9) x 109 6.5 [49] PR comp, with CNS" Rel 'o = 1.1 Xl0'»

t*(7.4± 1.0) x 109 7 [49] PR loss of chromophor Absolute. No corr. for H-addit.

(7.3 ±0.9) x 109 neut. [53] PR comp, with CNS"

t*(6.0±0.3) X 109 neut. [53] PR loss of chromophor Absolute. No corr. for H-addit. t*(6.5±0.7) X 109 neut. [53] PR product form. Absolute. No corr. for H-addit.

(5.3 ±0,5) x 109 7.3 [29] PR comp, with CNS" Re'- >° = 1-1 x l 0

' 0 6.8 x 109 9 [42] S irrad. comp, with PNDA Rel. to &oh+pnda = 1-25 X 1010

5-bromouracil 4.3 x 109 9 [42] S irrad. comp, with PNDA Rel. to A'qh+pnda ~ 1-25 X 1010 t*3.6 x 109 7 [40] PR prod. form. Absolute. N2 0 added. No corr. for H-addit.

dihydrouracil (1.2 ±0.15) X 109 neut. [53] PR comp, with CNS" Rel. to ^Jhj^yan. = 1-1 x 10‘°

uridine (4.2 ±0.6) x 109 neut. [53] PR comp, with CNS" Rel- >o *&.„.= “I X 10'° t*(6.0±0.3) x 109 neut. [53] PR loss of chromophor Absolute. No corr. for H-addit. t*(6.5±0.7) x 109 neut. [53] PR product form. Absolute. No corr. for H-addit. (4.3±0.6) x 109 6.5 [49] PR comp, with CNS" Rel- to = 1-1 X 10'°

uridine mono- (5.1 ±0.8) x 109 6.5 [49] PR comp, with CNS" Rel- to *;j!ocy>1, = 1.1 X10'»

phosphate (mixed)

(5.1 ±0.7) X 109 neut. [53] PR comp, with CNS" Rel- to «!„„, = 1.1 X10'»

t*(4.6±0.3) x 109 neut. [53] PR loss of chromophor Absolute. No corr. for H-addit. t*(4.0±0.4) X 109 neut. [53] PR product form. Absolute. No corr. for H-addit.

uridylyl 3' —> 5' (5.3±0.6) X 109 neut. [53] PR comp, with CNS" Rel. to Affilbcyan. = 1.1X10'°

uridine (UpU)

t*(3.8±0.2) x 109 neut. [53] PR loss of chromophor Absolute. No corr. for H-addit.

(calc, per unit base)

oligouridylic acid (4.0±0.6) x 109 neut. [53] PR comp, with CNS" Re'- to 1.1X10'”

t*(4.3 ±0.2) x 109 neut. [53] PR loss of chromophor Absolute. No corr. for H-addit.

(calc, per unit base)

polyuridylic acid ( 1.5 ± 0.15) x 109 neut. [53] PR comp, with CNS" Rel. to Af®.

1 ' =1.1X10'°

thiocyan.

52
Table 18. Nucleic acid derivatives — Continued

Reactant k(M~l s-1 ) pH Ref. Method Comments

DNA *6 x 108 7 [51] PR product form. Absolute. N2 0 added. No corr. for H-addit. Based on mean nucleotide

mol. wt. of 350

*6 x 108 12.4 [51] PR product form. Absolute. N2 0 added. No corr. for H-addit. Based on mean nucleotide

mol. wt. of 350

1013 9 [42] S irrad. comp, with PNDA Rel. to Ar0H+PNDA = 1.25 X 1010 (mol. wt. = 5 X 106

) *1.3 x 10 13 7.5 [34] PR product form. Absolute. N2 0 added. No corr. for H-addit. (mol. wt. = 5 X 106

)

Table 19. Carbohydrates and related compounds

Reactant kiM-h-1 ) pH Ref. Method Comments

ascorbic acid 1.20 x 1010 1 [28] PR comp. with CNS- Rel. to ^thiocyan. = 1.1 X1010

2 -acetamido-2 -deoxyD-glucose

3.07X10® [55] PR comp. with CNS- Rel. to ^thiocyan. = 1 . 1 X 10*°

2 -acetamido-2 -deoxyZ)-galactose

1.62X10® [55] PR comp. with CNS- Rel. to ^th'ocyan. = 1.1 X 10*°

/V-acetylglucoseamine 3.07 x 10® [56] PR comp. with CNS- Rel. to ^thiocyan. = 1.1 X 10*°

2 -amino-2 -deoxy-Z)-

galactose (HC1)

1.77 x 10® [55] PR comp. with CNS- Rel. to

thiocyan. = 1.1 X 10*°

chondroitin-4-

sulfate I

8.0 x 108 [55] PR comp. with CNS- Rel. to /c[h!; icyan. = 1-1 X 1010 Cone. = mol. equiv./liter

( 1 . equiv. = average mol. wt. of hexose

unit)

chondroitin-6- sulfate I

6.8 X108 [55] PR comp. with CNS“ ReL to = 1.1 X 1010 Cone. = mol. equiv./liter

( 1 . equiv. = average mol. wt. of hexose unit)

/V-desulfated heparin 8.0 x 108 [55] PR comp. with CNS- Rel. to kTM!' = 1 . 1 X 10*° thiocyan. Cone. = mol. equiv./liter

(1 equiv. = average mol. wt. of hexose unit)

2 -deoxy-2-sulfo- amino-D-glucose

2.12 x 10® [55] PR comp. with CNS- Rel. to Af*

1 - thiocyan. = 1.1 X 10*°

erythrytol 2.0 x 10® 9 [36] S irrad. comp, with PNDA Rel. to &0H+C2 H5OH — 1-85 X 10®

glucose 1 x 1010 1.9 x 10® 2 [57]

[19] PR comp, with I~ S irrad. comp, with thymine

Rel. to Aoh+p~ 1.0 x 10*° Rel. to AOH+thymine = 4.7 X 109

glucuronic acid 3.09 x 10® [56] PR comp. with CNS ReL t0 ^thiocyan. = 1 . 1 X 10*°

D-glucuronate (Na) 3.09 x 10® [55] PR comp. with CNS- ReL t0 thiocyan. = 1.1 X 10'°

heparin 3.7 x 108 [56] PR comp. with CNSCone. = mol. equiv./liter

(1 equiv. = average mol. wt. of hexose unit.) 53
Table 19. Carbohydrates and related compounds—Continued

Reactant k(M~l s” 1 ) pH Ref. Method Comments

hyaluronic acid 6.7 x 108

(1.34±0.07) x 109

[56]

[58]

PR comp, with CNSPR comp, with CNS-

=i> x io'° Cone. = mol. equiv./liter

(1 equiv. = average mol. wt. of hexose unit.) Rel. to^*|ocyan. = l.lx l0'» Cone. = mol. equiv./liter

(1 equiv. = average mol. wt. of hexose unit.)

hyaluronic acid

(sulfated)

6.0 X 108 [55] PR comp, with CNS- Rel. to*Sicyan =l.lX10‘«

Cone. = mo. equiv./liter

(1 equiv. = average mol. wt. of hexose unit.)

keratan sulfate 7.8 x 109 [59] PR comp, with CNS- Rel. to A:!?)- =1.1X10'°

thiocyan. Cone. = mol. equiv./liter

(1 equiv. = average mol. wt. of hexose unit.)

pentaerythrytol 3.2 x 109 9 [36] S irrad. comp, with PNDA Rel. to A:oh+c2h5oh = 1-35 X 109

ribose 2.1 X 109 9 [42] S irrad. comp, with PNDA Rel. to A:0h+pnda = 1-25 X 10'°

sucrose (2.5 ±0.3) X 109 2 [19] S irrad. comp, with thymine Rel. tO A^oH+thymine 4.7 X 109

Table 20. Miscellaneous compounds

Reactant k{M-'s -

') pH Ref. Method Comments

benzene sulfonate 2.9 x 109 [56] PR comp, with CNS- Rel. to kTM'- =1.1X10'°

thiocyan.

ethane sulfonate 1.0 x 107 [56] PR comp, with CNS - Rel. to cyan . = 1-1 x 10'°

glutamine (1.4± 0.15) X108 2 [19] S irrad. comp, with thymine Rel. to A)pH+thyminc = 4.7 X 109

poly(ethylenesulfonate)1.2 X107 [56] PR comp, with CNS - Rel. to ^thiocyan. = 1- 1 X 10'°

poly(styrenesulfonate)

3.3 x 108 [56] PR comp, with CNS - Rel. to ^thiocyan. = 1-1 X 10'°

toluene sulfonate 3.0 X109 [56] PR comp, with CNS- Rel. to ^thiocyan. = 1.1X10'°

nor-pseudo-pellertierine-N-oxyl

(NPPN)

(7.4± 1) X 109 alk. [60] PR comp, with COf Rel. to A:oh+co^‘~4.1 X 108

sodium sulfacetamide 8.1 x 109 [61] S irrad. comp, with PNDA Rel. to A: ( , h+ [>nda = 1-25 X 10'° (Values interpolated from data)

sulfanilic acid 6.3 x 109 [61] S irrad. comp, with PNDA Rel. to Ac 0h+pnda = 1-25 X 10'° (Values interpolated from data)

sulfanilamide 6.0 X 109 [61] S irrad. comp, with PNDA Rel. to At0h+pnda = 1-25 X 10'° (Values interpolated from data)

triacetoneamine-/Voxyl

(4.3 ±0.5) x 109 alk. [60] PR comp, with COj Rel. to Aoh+co? = 4. 1 X 108

54
Table 20. Miscellaneous compounds—Continued

Reactant kiM-1 s1

) pH Ref. Method Comments

eosin 2 x 1010 8.5 [62] PR comp, with CO= Rel. to ^oh+co? = 4. 1 X 10®

eosin-Y 1.4 x 109 10.5 [63] PR comp, with CO= Rel. to A:0h+co= = 4.1 X 108

eosin dianion *1.7 x 109

(i) *6 x 108

(ii)

9 [64] PR product form. Absolute. Computer analysis.

(i) electron transfer reaction

(ii) OH-addition

fluoroscein *(1.6±0.3) X109

(i) *(1.4±0.2) X109

(ii)

9.2 [65] PR product form. Absolute computer analysis.

(i) electron transfer reaction

(ii) OH-addition

indole ( 1 .37 rt 0.5 ) X 1010 9 [66] S irrad. comp, with PNDA Rel. to tryptophan

^OH+tryptophan 1-25 X 1010

indoline (2.02±0.14) x 1019 9 [66] S irrad. comp, with PNDA Rel. to tryptophan

^OH+ tryptophan 1.25 X 1019

1-methylindole (1.44±0.01) x 1010 9 [66] S irrad. comp, with PNDA Rel. to tryptophan

^OH+tryptophan 1.25 X 1010

2-methylindole 1.44 x 1010 9 [66] S irrad. comp, with PNDA Rel. to tryptophan

^OH+tryptophan 1.25 X 1010

3-methylindole 1.05 X1010 9 [66] S irrad. comp, with PNDA Rel. to tryptophan

^OH+tryptophan 1-25 X 1010

1 ,2-dimethylindole (1.25±0.02) x 1010 9 [66] S irrad. comp, wdth PNDA Rel. to tryptophan

^OH+tryptophan 1.25 X 1010

1 ,3-dimethylindole (1.01 ±0.08) X 1010 9 [66] S irrad. comp, with PNDA Rel. to tryptophan

^OH+tryptophan 1-25 X 1010

2,3-dimethylindole (1.26 ±0.01) X 1010 9 [66] S irrad. comp, with PNDA Rel. to tryptophan

^•OH-t- tryptophan 1.25 X 1010

5-methylindole (1.66±0.06) X 1010 9 [66] S irrad. comp, with PNDA Rel. to tryptophan

^OH+tryptophan 1.2o X 1010

5-chloroindole (1.91 ±0.06) X 1010 9 [66] S irrad. comp, with PNDA Rel. to tryptophan

^OH+tryptophan 1-25 X 1019

5-bromoindole (1.57±0.18) X 10 10 9 [66] S irrad. comp, with PNDA Rel. to tryptophan

^OH+tryptophan 1.25 X 1019

5-hydroxyindole (1.67±0.10) X 1010 9 [66] rrad. comp, with PNDA Rel. to tryptophan

^OH+tryptophan 1.25 X 1019

5-methoxyindole (1.39±0.04) X 1010 9 [66] S irrad. comp, with PNDA Rel. to tryptophan

^OH+tryptophan 1.25 X 1019

5-amido indole (3.17 ±0.31) X 1010 9 [66] S irrad. comp, with PNDA Rel. to tryptophan

^OH+tryptophan 1.25 X 1019

5-nitroindole (1.25 ± 0.24) X1010 9 [66] S irrad. comp, with PNDA Rel. to tryptophan

^OH+tryptophan = 1-25 X 1019

5-cyanoindole (1.06 ±0.24) X 1010 9 [66] S irrad. comp, with PNDA Rel. to tryptophan

^OH+tryptophan = 1-25 X 1019

indole-5-acetic acid (0.79 ±0.07) X 1010 9 [66] S irrad. comp, with PNDA Rel. to tryptophan

^OH+tryptophan 1.25 X 1019

55
Chapter X. References

[1] Garrison, W. M., Current Topics in Radiation Research,

p. 45, Eds. M. Ebert and A. Howard (North Holland

Publishing Co., Amsterdam, 1968).

[2] Holian, J., and Garrison, W. M., unpublished results. (May

be obtained from L. M. Dorfman, Ohio State University,

Columbus, Ohio 43210.)

[3] Land, E. J., and Ebert, M., Trans. Faraday Soc. 63, 1181

(1967).

[4] Jayson, G. G., Scholes, G., and Weiss, J., Biochem. J. 57,

386 (1954).

[5] Eich, E., and Rochelmeyer, H., Pharm. Acta. Helv. 41,

109(1966).

[6] Armstrong, R. C., and Swallow, A. J., Radiation Res. 440,

563 (1969).

[7] Karmann, W., Meissner, G., and Henglein, A., Zeit. fiir Naturforsch. B22,273 (1967).

[8] Adams, G. E., McNaughton, G. S., and Michael, B. D., The Chemistry of Ionisation and Excitation, p. 281, Eds.

G. R. A. Johnson and G. Scholes, (Taylor and Francis

Ltd., London, 1967).

[9] Swallow, A. J., J. Chem. Soc., 1334 (1952).

[10] Charlesby, A., Fydelor, P. J., Kopp, P. M., Keene, J. P., and

Swallow, A. J., Pulse Radiolysis, p. 193, Ed. M. Ebert

et al., (Academic Press, London and New York, 1965).

[11] Badiello, R., and Fielden, E. M., Int. J. Rad. Biol. 17, 1 (1970).

[12] Barron, E., Johnson, P., and Cobure, A., Radiation Res. 1,

410 (1954).

[13] Scholes, G., Ward, J. F., and Weiss, J. J., J. Mol. Biol. 2,

379 (1960).

[14] Scholes, G., Weiss, J. J., and Wheeler, C. M., Nature 178,

157 (1956).

[15] Daniels, M., Scholes, G., Weiss, J. J., and Wheeler, C. M.,

J. Chem. Soc., 226 (1957).

[16] B. Ekcrt and R. Monier, Nature 184, BA58 (1959).

[17] Latarjet, R., Ekert, B., Apelgot, S., and Reybeyrotte, N.,

J. Chim. Phys. 58, 1046 (1961).

[18] Scholes, G., and Weiss, J. J., Nature 185,305 (1960). [19a] Ward, J. F., and Myers, L. S., Jr., Radiation Res. 26, 483

(1965). [19b] Scholes, G., and Willson, R. L., Trans. Faraday Soc. 63,

2983 (1967).

[20] Myers, L. S., Jr., Ward, J. F., Tsukamoto, W. T., Holmes,

D. E., and Julia, J. P., Science 148, 1234 (1965).

[21] Holian, J., and Garrison, W. M., Nature 212,394 (1966).

[22] Scholes, G., and Weiss, J. J., Exp. Cell. Res. Suppl. 2, 219

(1952).

[23] Scholes, G., and Weiss, J. J., Biochem. J. 53, 567 (1963).

[24] Conlay, J. J., Nature 197, 555 (1963).

[25] Holian, J., and Garrison, W. M., J. Phys. Chem. 71, 462

(1964).

[26] Holian, J., and Garrison, W. M., Chem. Comm. 14, 676

(1967).

[27] Emmerson, P. T., Scholes, G., Thompson, D., Ward, J. F., and Weiss, J. J., Nature 187, 319 (1960).

[28] Adams, G. E., Boag, J. W., Currant, J., and Michael, B. D., Pulse Radiolysis, p. 131, Ed. M. Ebert et al. (Acad. Press, London and New York, 1965).

[29] Scholes, G., Shaw, P., Willson, R. L., and Ebert, M., Pulse

Radiolysis, p. 151, Ed. M. Ebert et al. (Acad. Press, London

and New York, 1965).

[30] Meissner, G., Henglein, A., and Beck, G., Zeit. fiir Naturforsch. 22, 13 (1967).

[31] Schoeneshoefer, M., Karmann, W., and Henglein, A., Int.

J. Rad. Phys. and Chem. 1,407 (1969).

[32] Karmann, W., Granzow, A., Meissner, G., and Henglein, A.,

Int. J. Rad. Phys. and Chem. 1, 395 (1969).

[33] Jayson, G. G., Stirling, D. A., and Swallow, A. J., Int. J. Rad. Biol. 19,143(1971).

[34] Scholes, G., Willson, R. L., and Ebert, M., Chem. Comm.,

17 (1969).

[35] Rabani, J., and Stein, G., Trans. Faraday Soc. 58, 2150

(1962).

[36] Anbar, M., Meyerstein, D., and Neta, P., J. Chem. Soc., 742 (1966).

[37] Matthews, R. W., and Sangster, D. F., J. Phys. Chem. 69,

1938 (1965).

[38] Willson, R. L., Greenstock, C. L., Adams, G. E., Wageman,

R., and Dorfman, L. M., Int. J. Rad. Phys. and Chem. 3,

211(1971).

[39] Chrysochoos, J., Radiation Res., 33,465 (1968).

[40] Zimbrick, J. D., Ward, J. F., and Myers, L. S., Jr., Int. J. Rad. Biol. 16,505 (1969).

[41] Simic, M., Neta, P., and Hayon, E., J. Amer. Chem. Soc. 92,

4763 (1970).

[42] Kraljic, I., Chemistry of Ionisation and Excitation, p. 303, Eds. G. R. A. Johnson and G. Scholes (Taylor and Francis

Ltd., London, 1967).

[43] Davies, J. V., Ebert, M., and Quintilliani, M., in Radiation

Protection and Sensitisation, p. 87, Eds. H. L. Moroson

and M. Quintilliani (Taylor and Francis Ltd., London,

1970).

[44] Henglein, A., Karmann, W., Roebke, W., and Beck, G., Makromol. Chem. 92, 105 (1966).

[45] Adams, G. E., Aldrich, J. E., Cundall, R. B., and Willson,

R. L., Int. J. Rad. Biol. 16, 333 (1969).

[46] Davies, J. V., Ebert, M., and Shalek, R. J., Int. J. Rad. Biol. 14, 19 (1968).

[47] Braams, R., and Ebert, M., Advances in Chemistry Series No. 81, Radiation Chemistry I, 464, Ed. R. F. Gould

(1968).

[48] Masuda, T., Ovadia, J., and Grossweiner, L. I., Int. J. Rad.

Biol. 20,447 (1971).

[49] Greenstock, C. L., Ng, M., and Hunt, J. W., Advances in Chemistry Series No. 81, Radiation Chemistry, 397, Ed. R. F. Gould (1968).

[50] Ward, J. F., and Myers, L. S., Jr., Radiation Res. 26, 483

(1965).

[51] Myers, L. S., Jr., Hollis, M. L., and Theard L. M., Advances

in Chemistry Series No. 81, Radiation Chemistry I, 345, Ed. R. F. Gould (1968).

[52] Loman, H., and Blok, J., Radiation Res. 36, 1 (1968).

[53] Greenstock, C. L., Hunt, J. W., and Ng, N., Trans. Faraday

Soc. 65,3279 (1969).

[54] Greenstock, C. L., Trans. Faraday Soc. 66, 2541 (1970).

[55] Moore, J. S., Phillips, G. O., Davies, J. V., and Dodgson, K. S., Carbohyd. Res. 12,253 (1970).

[56] Balazs, E. A., Davies, J. V., Phillips, G. O., and Scheufele,

D. S., J. Chem. Soc., 1420(1968).

[57] Davies, J. V., Griffiths, W., and Phillips, G. O., Pulse

Radiolysis, p. 181, Ed. M. Ebert et al. (Academic Press, London and New York, 1965).

[58] Balazs, E. A., Davies, J. V., Phillips, G. O., and Young, M. D., Radiation Res. 31,243 (1967).

[59] Jooyandeh, F., Moore, J. S., Morgan, R. E., and Phillips, G. O., Radiation Research 45, 455 (1971).

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355 (1971).

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Res. 46, 236 (1971).

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Chem. 71,1629(1967).

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2018 (1968).

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56
Chapter XI. Cross-Correlation of Rate Constants

It will be seen from the considerable number

of OH rate constants listed in the tables that the

large majority of those determined indirectly were

obtained using one of three independent competition methods. The respective reference solutes in

question are thiocyanate ion, thymine, and paranitrosodimethyl aniline (PNDA) [1—3], While the

thiocyanate system is restricted to the pulse

radiolysis method, the other two solutes have been

used to determine OH rate data by both stationarystate and pulse-radiolysis methods. The thiocyanate

method utilizes the intense transient spectrum of

the thiocyanate radical-ion (CNS)^ as the reference

absorption. This radical is formed in the electron

transfer reaction:

CNS- + OH = CNS • +OH- (1) CNS • + CNS-^ (CNS)2-. (2) The experimental rate constant for its formation

is 6.6 X 109Af -1s -1

, which does not, in fact, corre- spond to k i. This value had been adopted in numerous publications for calculation of rate data for a wide variety of inorganic, organic, and

biological molecules. Because of its extensive usage

the matter of the validity of the method merits

careful scrutiny. It has been shown [4-6] that

reaction (1) is not the rate-determining process for

formation of (CNS)^ and, therefore, absolute rate constants calculated on the basis of k i = 6.6

X 109 A/ -1

s -1 are all too low.

One detailed kinetic study of this system [4] has indicated an approximate value for k\ of

2.8X 10 10 M-1

s -1

. Inspection shows that when

several independent estimates of the OH reactivity

of a given solute are available, the value determined

by the thiocyanate method and calculated relative

to this value for k\ is, in all cases, too large by a

factor of about three.

The data for ethanol serve to illustrate this point.

Excluding values obtained by the thiocyanate

method, there are 14 independent estimates of

this rate constant listed in table 3, all of which, with

two exceptions, fall within the narrow range

1.6— 1.9 X 10 9 M-1

s -1

. When the rate constant

obtained by use of the thiocyanate system is cal- culated on the basis of k\ = 2.8 X 10 10 Af -1

s -1

, the

value obtained, 4.6 X 109M-1 s

-1

, is clearly

anomalous. In order to examine whether, in cases where replicate measurements by other methods

are available, the thiocyanate data can be successfully normalized, we selected ethanol as a secondary

standard, adopting, from competition data, the

absolute value 1.85 X 109 M-1

s -1 for the rate constant. All rate constants listed in this compilation

which have been determined by the thiocyanate

method, have been expressed relative to the above

values for ethanol. From this value it follows that

the corresponding calculated value for the rate constant of reaction (1) is 1.1 X 10 10 A/ -1

s -1. Toward the completion of this compilation a new

determination [5] of k\ gave the value Ai = (1.08

±0.10) X 10 10 M ’s l

, in excellent agreement with

the value based on ethanol as a secondary standard.

When the value 1.1 X 10 10 is used for calculating

other rate constants determined using the

thiocyanate method, agreement is generally good

when the existence of independent estimates of

these rate constants permits comparison. However,

in spite of this apparent agreement, the discrepancy

in the experimental determinations of the reference

rate constant k i may raise questions as to the validity

of the thiocyanate method particularly since the

reference radical, (CNS)2-

, is not formed in a onestage process.

There are over 140 solutes listed in this compilation for which rate constants have been determined

by this technique. In order to investigate whether

this large volume of data is at all reliable, we have

examined methods of normalizing the values in

terms of other standards. There are several solutes

which can be used with confidence for this purpose.

Multiple independent determinations are available

for several of the alcohols. In most cases, the agreement is very good, e.g. , methanol and ethanol. The

scatter in the ethanol rate constant is small and

there are eight independent estimations for which

the scatter is within about ±10 percent of the mean

value of 1.85 X 109M-1 s

-1

. Similar agreement is apparent for methanol. All rate constants listed in

this compilation which have been determined by the

thiocyanate method have been expressed relative

to the above mean value for ethanol. Although the

calculated value of 1.1 X 1010 is in excellent agreement with the more recent determination of k\ =

1.08 X 1010

, all data obtained by the thiocyanate

method carry the qualification in the comment

column kTM!- — 1.1 X 10104/-1 s

-1

. thiocyanate

Inspection of the separate tables shows that

normalization based on this value is successful in

almost all cases where independent determinations

permit a direct comparison. For comparison, a

selection of these data, along with others, is listed

in table 21. For such a large number of solutes, it seems most unlikely that the satisfactory agreement

with independent data could arise by a chance

cancellation of errors. Despite the existing disagree57
Table 21. Comparison of values from different methods

(M-'s- 1 X 10- s

)

methanol

ethanol n-propanol

2-propanol

n-butanol

f-butanol ethylene glycol

glycerol

acrylamide

benzoate

acetophenone..

phenylalanine..

thymine

uracil acetone

ferrocyanide...

PNDA

thiocyanate

Solute Absolute measurement

Ferro- cyanideThio- cyanate Thymine PNDA Benzoate

6.8

5.9 6.0

6.6

4.7

6.6

0.86

1.85 2.7 2.0

4.0

5.2 1.47

1.9 4.7

5.4

5.2 6.0

4.7

0.12

0.77

1.72 2.5

1.8 3.7

4.2 1.44 1.59

3.3

5.5

5.8

5.2 6.0 0.097

0.76

1.7 2.7

1.9 4.0

6.3

1.5 1.64

1.0 1.82

0.80

1.73

2.1

5.0

4.8

0.066

6.8 0.08 0.09

9.3 12.5

b 11.0 9.2

a Values shown in this table are, in many cases, an average of several determinations. Thus, the value for the absolute determination

for benzoate, 5.9, is the average of the two values 5.7 and 6.0 from table 2.

b This value of 11.0 for thiocyanate is the value obtained in competition studies with ethanol, taking ioH+r2 H 5 oH = 1.85 X 10 9

, and

has been used as the reference value in the competition studies with CNS -

ment in the absolute value of the rate constant for

reaction (1), it seems clear that the large amount of

published data for OH reactivity obtained by the

thiocyanate method need not be discarded.

The thymine and PNDA methods used most

frequently for stationary-state determination of OH

rate constants, are based upon radiation-induced

bleaching of the respective solutes:

solute + OH —» loss of chromophore. (3)

In the presence of a second solute, the competition

is measured by the decrease in the amount of OHinduced loss of the reference chromophore absorption. Both methods have limitations, as is stressed

by the respective authors of the original papers on

the subject. The methods can be used for the determination of the reactivity of a given solute only

under conditions where other reactions which can produce absorption at the reference wavelength

are either absent or can be quantitatively assessed.

Despite this limitation and, also, certain pH restrictions, data obtained by both these methods show

generally good agreement with data obtained by

other methods.

The chromophore in thymine is the 5—6 double

bond which exhibits an absorption maximum at 260 nm. Reaction of OH occurs by addition to the

double bond resulting in a loss of this absorption.

The subsequent fate of this OH-adduct depends

upon the nature of the competing solute. However,

in all cases studied, the overall result of a sta- tionary-state irradiation is a decrease in absorption

at 260 nm.

In principle, the absolute rate constant for the

thymine reaction, (3), can be determined either by

direct observation of the indirect bleaching due to OH addition, or by measurement of the rate of

formation of the transient product which has an

absorption at longer wavelengths. In neutral solution, values determined by these methods (table 18)

range from 4.6-7. 4 X 109A/_1 s

_1

. The rate constants

determined by the reference to the ethanol standard

and by competition with ferrocyanide ion all fall within a narrower range and we have adopted as the

best value for this rate constant, the value

4.7 X 109A/_1

s _1

. Due to mechanistic requirements, rate constants

determined by the PNDA method have been

measured, in most instances, at pH 9. There are two independent estimates of the absolute value of

the reference rate constants, namely, 1.25 X 10 10 M~ x s -1

[7] and 1.8 X 10 10 A/ -1

s _1

[8J. Unfortunately,

they do not agree. However, since calculation of the

rate constant on the basis of the ethanol standard

agrees precisely with the lower absolute measurement, we have adopted this value for calculation of

the rate data, from competition studies, listed in the

various tables. It is clear that, in general, the consistency is good when independent measurements

permit direct comparison.

Although the ferrocyanide method (pulse radiolysis) has been used to determine relatively few rate 58
constants, it is evident from table 21 that agreement

with values determined by absolute methods is generally very good. There is little doubt attached to the

authenticity of the absolute value of the reference

rate constant and, in addition, the reference reaction is a one-electron transfer reaction in which both

the reactant (ferrocyanide ion) and the product

(ferricyanide ion) are stable chemical species. We

consider, therefore, that the evidence to date justifies the acceptance of this method as a reliable

competition method for the determination of OH

rate constants. Chapter XI. References

[1] Adams, G. E., Boag, J. W., Currant, J., and Michael, B. D., Pulse Radiolysis, p. 131, Ed. M. Ebert et al. (Acad. Press, London and New York, 1965).

[2] Scholes, G., Shaw, P., Willson, R. L., and Ebert, M., Pulse

Radiolysis, p. 151, Ed. M. Ebert et al. (Acad. Press, London

and New York, 1965).

[3] Kraljic, I., and Trumbore, C. N., J. Amer. Chem. Soc. 87, _ 2547 (1965).

[4] Baxendale, J. H., Bevan, P. L. T., and Stott, D. A., Trans. Faraday Soc. 64,2389(1968).

[5] Edison, D. H., Salmon, G. A., and Wilkinson, F., Proc. Roy.

Soc. A32 8,23 (1972).

[6] Behar, D., Bevan, P. L. T., and Scholes, G., J. Phys. Chem.

76,1537(1972).

[7] Baxendale, J. H., and Khan. A. A., Int. J. Rad. Phys. and

Chem. 1,11,(1969).

[8] Dainton, F. S., and Wiseall, B., Trans. Faraday Soc., 64, 694

(1968)

Most of the writing and compilation of tables in

this monograph was done while one of us (LMD) was

a Fellow of the John Simon Guggenheim Foundation,

at the Royal Institution of Great Britain, in London.

I am grateful to the Guggenheim Foundation and to my host at the Royal Institution, Professor Sir

George Porter. We are indebted to Dr. Alberta B. Ross of the

Radiation Chemistry Data Center, University of

Notre Dame, for providing us with many helpful

lists of references. We acknowledge, with thanks,

the help of Mrs. Mary Cook and Mrs. D. Woodman in

typing the manuscript with its extensive tables.

59
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1. PUBLICATION OR REPORT NO.

NSRDS - l'IBS-46

2. Gov’t Accession No.3. Recipient’s Accession No.

4. TITLE AND SUBTITLE

Reactivity of the Hydroxyl Radical in Aqueous Solutions

5. Publication Date June 1975

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7. AUTHOR(S)

Leon M. Dorfman and Gerald E. Adams

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15. SUPPLEMENTARY NOTES

16. ABSTRACT (A 200-word or less factual summary of most significant information. If document includes a significant

bibliography or literature survey, mention it here.)

The reaction rate data of the hydroxyl radical in aqueous solution are compiled and

evaluated in this critical review. The values are reported in a series of tables

covering additon, hydrogen abstraction, inorganic electron transfer and radical

reactions. Rate constants for the hydroxyl radical with biological molecules are included. In addition, the rate constant data for the oxide radical ion are given.

Physical properties are listed and the experimental methods employed in OH radical

chemistry are reviewed. An analysis involving rate constant data comparisons is

made.

17. KEY WORDS (Alphabetical order, separated by semicolons) Abstraction reactions; addition reactions;

aqueous solution; biological molecules; electron transfer reactions; hydroxyl radical;

oxide radical ion; radical reactions; rate constants; reference data.

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