NATIONAL BUREAU OF STANDARDS REPORT

4649

MICROWAVE SPECTROSCOPY OF LABILE MOLECULES

I* Transition Types, Frequencies, and Intensities

R* J$ Kurland and D.* E. Mann

Thermodynamics Section

Heat and Power Division

March 1956

<NB^>

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NATIONAL BUREAU OF STANDARDS REPORT

NBS PROJECT NBS REPORT

0302-10-2606 4649

MICROWAVE SPECTROSCOPY OF LABILE MOLECULES

I. Transition Types, Frequencies, and Intensities

R* J. Kurland and D* E. Mann

Thermodynamics Section

Heat and Power Division

Sponsored in part by:

United States Air Force through the

Air Force Office of Scientific Research of the

Air Research and Development Command

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NATIONAL BUREAU OF STANDARDS

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BIBLIOGRAPHICAL CONTROL SHEET

AFOSR TN- -

ASTIA Document No. AD

1. Originating Agency and Monitoring Agency:

O.A. National Bureau of Standards, Washington, C. C.

M.A. Physics Division, Air Force Office of Scientific Research

2. Originating Agency and Monitoring Agency Report Numbers :

0. A. N«,S.B. Report No. 4&49

M. A. AFOSR TN- -

AD

3* Title and Classification ; Microwave Spectroscopy of Labile Molecules. I. Transition Types

Frequencies, and Intensities. (Unclassified)

4. Personal Authors:

Kurland f R? J.

Mann, D. E«?

5. Date of Report : March, 195&

6. Pages :

7. Illustrative Material : None

8.

Prepared for Contract : C.S.O. 680-56-19
9.

Prepared for Project ; AFOSR Physics 1-15

10. Security Classification

Unclassified

11. Distribution limitations ; \

None

12. Abstract : The available data for a number of labile diatomic and some

triatomic molecules have been reviewed and summarized in order

to classify their expected microwave spectra by type, frequency,

and intensity. Estimates are given for lamda~, and rho-» type

multiplet transitions as well as for pure rotational spectra

expected to fall in a readily accessible region of the microwave

spectrum. Some discussion of methods of preparation of certain

unstable species suitable for microwave investigations is also

given
FOREWORD

This report is the first of a projected series which will describe

various stages in an attempt to observe microwave spectra of free radicals

and other labile molecular species.

Comments on the research herein described and the exchange of reports

are invited. Communications may be addressed to Dr, D, E, Mann, Room 204,

Building 8, National Bureau of Standards, Washington 25, D, C,

C, W, Beckett,

Chief, Thermodynamics Section

F, G, Brickwedde

Chief, Heat and Power Division
MICROWAVE SPECTROSCOPY OF LABILE MOLECULES

I. Transition Types, Frequencies and Intensities

Rj- J* Kurland*

*” asd EW E, Manfi

National Bureau of Standards, Washington 25, D* C,

ABSTRACT

The available data for a number of labile diatomic and some

triatomic molecules have been reviewed and summarized in order to

classify their expected microwave spectra by type, frequency, and

intensity* Estimates are given for lamda-, and rho^type nrultiplet

transitions as well as for pure rotational spectra expected to fall

in a readily accessible region of the microwave spectrum. Some

discussion of methods of preparation of certain unstable species

suitable for microwave investigations is also given*

^ This work has been supported in part by the United States Air Force

through the Air Force Office of Scientific Research of the Air Research

and Development Command under C*S*0* 6BO-56-19* ^ National Research Council - National Bureau of Standards Postdoctoral

Research Associate for 195&*
THE MICROWAVE SPECTROSCOPY OF FREE RADICALS, LABILE

MOLECULES, AND HIGH TEMPERATURE SPECIES

I. Types, Frequencies, and Intensities of Expected Transitions

INTRODUCTION

This report is intended to serve as a preliminary guide in the

formulation of a program for the microwave spectroscopy of labile or

high-temperature species. Three questions must be answered about any

such species to be investigated: do sufficiently intense transitions

fall in the accessible microwave frequency range; is there a convenient

source for the species; are there means by which the concentration of

the species in the absorption cell can be monitored or estimated.

In many cases spectral and kinetic data for a species are not

reliable or available. Further study by means of optical spectroscopy

might be used to obtain the needed data in some of these cases.

PREDICTION OF THE SPECTRUM

The types of transitions the frequencies of which might fall

within the microwave region may be conveniently classified as follows:

1. Electric Dipole Transitions,

a. Rotational Transitions.

This classification includes those transitions which are

essentially between rotational states. Molecules in states are

treated in texts on conventional microwave spectroscopy^-. For non-

/ 2 n linear polyatomic free radicals (e.g® CIO2 or NO2 ), the orbital

1
angular momentum of the odd electron is quenched by the lower than

symmetry and the effect of the unpaired electron spin is manifested

as fine structure in the lines corresponding to rotational transitions.

The microwave transitions of those diatomic radicals for which the p or

A.-type multiplet splitting is small may also be conveniently put in

this classification.

(i) . Diatomic Molecules

The transition frequencies are given by

v = 2B (J + l) + fine structure terms.

The fine structure terms may be estimated from the considerations given

in Island 2«). The rotational constant B = h/&n^I0 may be obtained

from previous spectroscopic work or estimated by use of a likely value

for the intemuclear distance. The internuclear distance itself may

be estimated from distances in similar diatomics or from values of the

appropriate covalent bond radii. The rotational constants of diatomic

species observed spectroscopically is given in Table I.

»

2
Table I

Rotational Transitions of Diatomics

Species r

* (A) Be (lO%c) Transition** + x)

(icAmo)

Fine Structure***

AgBr(Xx Z) 2.5 o*i8 4 «- 3 1*4 -

AgClCX1!.) 2,2 0.39 2«-l 1.6 -

Agl (X1^) 2.7 0.12 5 4 1.2 -

AlBrCX1

!.) w 0.477+ 2 1 1.81 -

AlClCX1

!.) - 0. 720+

2 <— 1 2.88 -

A1F(X1 £) 1,8 1,4 1 0

1

—

1

2.8 - AU(X1 r) 2.4 0,38 2 <r- 1 1.6 -

AgO(X2 r) 2*0 0.85 1^-0 1.7 p-type, ~300Mc

A10(X2£ ) - 1.92

+ ' 1 ^— 0 3*84 p-type, />*600Mc

AsO(X2tti ) lf 7 1»3 ii <-4 3*9 4-type, r*1700Mc

AsNCX1 ^.) V 1,6 l^-o 3.2 -

BaBr(X2

X.) 3.0 o»n 6 «-5 1.3 p-type, ^lOOMc

BaCl(X2r ) 2,9 0,21 3 ^-^ 1*3 p-type, ^600Mc

3aF(X2£ ) | 2.6 j 0,45 2 <- 1 1# 8 p-type, ^800Mc

Bal (X2£ ) 3,2 0,075 8 <;— 7 1.2 p-tjrpe, ^60Mc

BaO(XX

2:)

• 0,936* l<r- 0 1.87 -

BaSCX1?: ) 2*25 0,38 . 2 4- 1 1.5 -

BBr(XXZ

)

- 1*47* 1 — 0 2.94 -

BClCX1! ) - 2 , 07*1<V 0 4,14 -

BeCl(X2r ) - 1 (2»4)

+ i<— 0 4,8 p-type, ^lOOMc

i

3
Table 1 (cont f d)

Rotational Transitions of Diatomics

Species r* (A) Bg(lcAMc) Transition** 2B(J

+ 1) Fine Structure***

(icAmc)

BiBr(X3Z ?) 2.6 0.13 5 <6-4 1.3 P-type(?)

?

BiCl(X3 £ ?) 2.5 0.27 3 4—

2

1.6 p-type(?), ?

BiF(X3L ?) 2.2 0.60 1 <£—0 1.2 p-type (?), ?

BiKX^X- ?) 2.8 0.080 8 4-7 1.3 p-type(?), ?

BiO(X2

ni?)

2

2.2 0.69 ii . i J-2 <r 2 2.1 4-type (?), ?

BrO(X2tTl?) 1.8 1.2 14— o 2.4 4-type (? ) , ?

CaBr(X2S:) 2.0 0.48 2^—1 1.9 p-type, ^p80Mc

CaCl(X2£ ) - (0.84)

+ 14r- 0 1.68 p-type , r^lOOMc

CaF(X2

£.

)

- (0.966) + 14— 0 1.93 p-type, ^140Mc

Cal (X2X

)

2.2 0.32 2<r—

1

1.3 p-type, ?

CaOCX1! ) 2.0 1.1 14—0 2.2

CaSCX1^ ) 2.3 0.52 2 4 1 2.0 -

CBr(X2m)

2

1.9 1.3 -L2 2 3.9 4-type, ?

CCl(X2m)

2

1.8 1.8 -1 1 1-L2

^

2 4.8 4-type, r-*20QMc

CSe(XXZ ) 1.7 1.8 14-0 3.6 -

CP(X2

S.) - 2. 40

+ 14—0 4.8 p-type , -170Mc

CdBr(X%I

)

2.6 0.16 44-3 1.3 p-type?, ?

CdCl(X2£ ) 2.4 0.32 2*4— 1 1.3 p-type, -lOOMc

CdF(X2 £) 2.1 0.75 14- 0 1.5 p-type, ?

CdI(X2

X.

)

2.8 0.11 64-5 1.3 p-type, ^--30Mc

4
Table 1 (cont t d)

Rotational Transitions of Diatomics

Species r* (A) B

e (lOilMc) Transition^ 2B(

J

+ 1)

N

(IcAmc)

Fine Structure'*’**’

C10(X2

rri?) 1.7 1.6 ii^i 4.8 \-type, ?

CoBr(?) ? 9* - - -

CoCl(?) ?

9• - - ->

CoO (?) 9 ? - -

CrO (?) 9 9 - - -

CuBr(X1

Z.) 2.2 0f30 2^—1 1,2 -

CuCl(X12: ) 2«0 0.54 2 — 1 2^2 -

GuF(X1X-) — 1.14

+ 1*^-0 2.2£ -

Cul(X1r ) 2.4 0.21 3 <e-2 1.3 -

cuo(x2

j;

)

1.8 1.3 1<- 0 2.6 p-type, ?

FeCl(X6X ?) 2*2 | 0.51 2 4— 1 2.0 p-type, ?

FeO(?) 1.9 . 1.2 1^—0 2.4 ?

GaO(X22_) ?

9 -

GaBr(X1

Z-

)

? ? - “

GaCl(X1S.) 9• ; 9• - -

GaF(X1r )

9 9 -

Gal(X1ZL )

9 ? - - -

GeBr(x2TTi)

2

2.4 0.24 2i*-ii 1.2 \-type, ^- 350Mc

GeCl(X2m)

2

2.2 0.44 li^i 1.3 X,-type, ^VOOMc

5
Table 1 (cont’d)

Rotational Transitions of Diatomics

Species r* (A) B

6 (1cAmc) Transition** 2B(fJ + 1)

(I0%c)

Fine Structure***

GeF(X^ni) 1.9 0.90 2.7 k-type, ^2000Mc

GeOCX1

!.

)

- l.U+ 1 <- 0 2.82 -

GeSCx1^.) 2.0 0.56 2 4-1 2.2 -

GeSeCX1^) 2.4 0.23 3^2 1.4 -

GeTe (X^S ) 2.5 0.18 4 ^— 3 1.4 -

HgBr(X2 £ ) 2.5 0.14 5 4- 4 1.4 P“type, ^140Mc

HgCl(X2r ) 2.2 0.34 2 4—1 1.4 p-type, ^l300Mc

HgF(X2r ) 2.0 0.69 1 4— 0 1.4 p-type^ ^2500Mc

Hgl(X2Z) 2.7 0.089 7<-6 1.2 P“type, ?

TnBr(X^S-) 2.5T 0.161 4 ^-3 1.29 -

InClCX3^

) - 0.351+ 2-4-1 1.41 -

Ini (XXE-

)

a,86+ 0.105 6 -4— 5 1.26 -

InO(X2£ ) 2.2 0.77 14—0 1.5 p-type , ^700Mc

I0(X2m?)

2

2.1 0.85 -! 1 >1 -*-2 2 2.6 X-type, ?

LaO(X2£ ) 7 — — p-type, ?

MgBr(X2£ ) 2.2 0.51 24—1 2.3 p-type, ~100Mc

MgCl(X2£L) 2.1 0.85 14- 0 1.7 p-type, ~100Mc

MgF (X2

2- ) - 1.55+ 1 4— 0 3.10 p-type,

^

80Mc

Mgl(X2Z ) 2.4 0.43 24—

1

1.7 p-type, ?

MgO(xl2L

)

- 1.72+ 14—0 3*44 -

6
Table 1 (cont t d)

Rotational Transitions of Diatomic

s

Species ** (A) B

6 (1cAmo) Transition** 2B(h + 1)

(IcAmc)

Fine Structure'5***

MgsU^T) 2.1 0.90 1<— 0 1.8 -

KnBr(x7si ) 2.5 0.24 3 « 2 1.4 p-type, ?

MnCl(xt) 2.4 0.39 2 ^ 1 1.6 p-type, ?

tfaF(x7z;

)

2.1 0.82 1 0 1.6 p-type, ?

Mnl(x7r_) 2.7 0.18 4 < 3 1.4 p-type, ?

IfaO(X ?) 2.1 0.93 1« 0 1.9 9

NiBr(X^ni?)

2

2.2 0.31 2|«— 1| 1.6 \-type, ?

NiCl(X2m?)

2

2.1 0.54 i| h 1.6 \-type, r-' 300Mc

NiOCX1^-) 1.8 1.2 1 <— o 2.4 -

NBr(X?) 1.6 1.7 1<— 0 3*4 9

PbBr(X2m)

2

2.7 0.12 5i ^— 4i 1.3 \-type, /^2000Mc(?)

PbCl(X2

ni) 2.5 0.26 2j <— l| 1.3 \-t3Tpe j ^ 4000Mc(?)

PbFCx^i)

2

2.2 0.58 Xj4—

|

1.7 X-type,

^

4000Mc

PbI(X2

tri)

2

2.8 0.079 7i ^6i 1.2 X-type f ?

PbOCX^) 2.1 0.31 1<— 0 1.6 -

PbSCX1^.

)

- 0.318+ 2^— 1 1.27 -

PbSeCX1

^:

)

2.5 0.14 5 <— 4 1.4 -

FbTeCx1

!. ) 2.7 0.085 7 <:— 6 1.2 -

pnCx1^. ) - 2.35* 1 <T~ 0 4.70 -

SbCl(X*) 2.4 0.32 2 < 1 1.3 9

3bF(X?) 2.1 0.70 1<—- 0 1.4 9

7
Table I (cont f d)

Rotational Transitions of Diatomic

s

Species i# (A) —co

<D s o' >>

<r Transition^ 2B(^ + 1) (icAmc)

Fine Structure ****

SbN(XX

i:) 2.0 0,95 1 ^— 0 1.9 -

SbO(X2m)

2 2,1 .0,81 ii<-| 2.4 4-type, ^3000Mc

Sc0(x2

l.) 9 “ , - - p-type ?

(x1

!! ) 1*7 1.3 1 0 2.6 - SiBr(X^m)

2

2.1 0.58 Is <-h 1.7 4-type, r^200Mc

SiCl(X2m)2

1.9 0.90 li 2.7 4-type, ^200Mc

SiF(X2m)

2 - 1. 26+

. li s 3.84 4-type , ~ 300Mc

SiN(X2£-

)

- 2,19+ 1-^0 4.38 p-type, ?

SiOCX1!.) - 2.18^ i<— o 4.36 -

SiS(XX

£.

)

- 0.906+

1 <^- 0 1.81 -

SiSe (X1!: ) 2*0 0.58 2 <— 1 2.3 -

SiTe(X1

r:

)

2.2 0.45 2 ^— 1 1.8 -

Sn3r(X2rri) 2.5 0.16 4i <- 3i 1.4 4-type,

^

500Mc

SiiC1(X2

tti)

2

2.4 0,33 2j <-ii 1.6 4-type, ^lOOOMc

SnFCx^ni)

2

2.1 0.70 I2 2 2.1 4-type, /^3l00Mc

SnOCx1

^.

)

- 1.06+ l<r— 0 2.12 - SnSCX-t ) - 0.471 2 <— 1 1.84 -

SnSetX1^ ) 2.2 O .23 3 <— 2 1.4 - SnTeCX1^

)

2.3 0.16 4 ^— 3 1.3 -

S0(X3Z ) - 2.n

+

4.22 p-type, —i,00Mc

8
Table I (cont f d)

Rotational Transitions of Diatomic

s

Species r* (A) B

s (1cAmc) Transition** 2B(jJ + 1)

(icAmc)

Fine Structure***

SrBr(X^£ ) 3.0 0.13 5 <— 4 1.3 p-type, ^lOOMc

SrCl(X2 I-) 2.9 6.24 3<—

2

1.4 p-type , r-' 200Mc

SrF(X2Z ) 2.6 0.4S 2^—1 1*9 p-type , ^

400Mc

SrI(Ji 2Z.

)

3*2 0.093 7^-6 1*3 p-type, (small)

SrOCX1

!. ) - 1.01+ 1<- 0 2.02 -

SrS(X:LX-

)

2.2 0.44 2<-l l.S -

TeCKX1^:

)

1*9 0.99 1<—0 1.9 -

TlBrCX1

!;

)

2.68+ 0.12 5 4 1.2 -

TlClCX1!.) 2.55

+ 0.26 3<- 2 1.6 - tif^sl) 2.2 0.58 2^-1 2.3 -

Til (X^X. ) 2.87+ 0.078 8<— 7 1.2 -

WO (X?) - - - - ?

YbCl(X2£ ) - - - - ?

YbO (X? ) - - - - 9

YO(X2J ) - - - - ?

ZnBr(X2 £

)

2.7 0.19 4<— 3 1.5 p-type, r^70Mc

ZnCl(X2r ) 2.6 0.33 2<— 1 1*3 p-type , ^140Mc

ZnF(X2r ) 2.3 0.65 1<—0 1.3 p-type, r^240Mc

Znl(X2I ) 2.9 0.34 5 <— 4 1.4 p-type, ^

50^c

9
Table I (cont’d)

Rotational Transitions of Diatomic

s

Species r* (X) Bg(lcAMc) Transition"**' 2B(jJ + 1)

(icAmc)

Fine Structure***'

Zn0(xl£) 1.3 1.2 1^-0 2.4 -

ZnSCX1!.) 2.1 0.54 2/f— 1 2.2 -

ZnSeCX3! ) 2.2 0.37 2<—

1

1.5 -

ZnTeCX1

!! ) 2.4 0.19 4<— 3 1.5 -

* Estimated from considerations given in section l.a. ** J f ^- J for or ^tti (Hund f s case (a))

Nf ^N for or ^rr, (Hund f s case (b)) *** Estimated from considerations given in Section l.c and 2 ; see

also tables 2 and 3 ^ Quoted by Herzberg^*

10
(ii) Bent Triatomic Molecules * For most triatomic molecules the transition

frequencies will depend strongly on the valence angle 9. Since the

structure of most triatomic radicals and labile molecules is not well

known or predictable, the spectrum cannot be forecast accurately.,

The situation may be illustrated by considering MX2 molecules of

C2v symmetry (e.g. CF2

). The rotational constants may be written

A. = <*- M „ 1+cos© m

B - ^ , 1+cos 9

C = AB , A+B

/ where h ; here m is the mass of atom,M,fti is the mass of atom X,

Srr^m’d^

M is the total mass of MX2 , d is the M-X bond distance and 9 is the X—M-X

angle. Usually the bond distance can be predicted much more accurately than

the angle, which is very sensitive to the type of bonding present.

Moreover, for 9 greater than 90° (which is nearly always the case)

f bf -type selection rules will hold for MX2 type molecules: = ±1 ,... ?

AKq_ = ±1, ±3 , in the notation of King, Hainer and Cross. This requires

that the relatively large rotational constant A - B+C enters into the

2

transition frequencies:

w = (§i£) j (j+i) + (a - !±£) k_-, 2

+ 2 2

x

or v ^ j (B+C) (J+l) + (A-BfC) ACK^2

)!,

2

(where,, for purposes of illustration, only the symmetric rotor approximation

to the energy has been explictly written Consequently large J will be

involved in those transitions occurring in an accessible frequency range*

11
4

.

.

- : -v-'

‘ / . C

.. V •

.

vi J - - » ‘

.

. v *

.

• c l

•
v~

CF2 Transitions: (J - ^

Jo,J° For dc-F " 1.30A.

Figure 1
This situation makes the prediction of the spectrum difficult; the

rotational constants must be accurately known if the relatively small

difference of two large quantities is to yield useful information*

Several examples will illustrate the situation:

CFg_

The frequencies of the transition series (J~l)^ ^0,J

are plotted in figure 1 against the angle F-OF = 0* It is evident

that for values of Q up to 130®, transitions will occur within the

K-band, although for the larger Q, the transitions will be of higher

J and consequently more difficult to identify. Also, centrifugal

distortion effects will be greater for larger J. There will be transition

series for higher values of also, but since the frequencies of these

have even a stronger dependence on A - B*C and, therefore, on ©, even

2

higher J will be involved, A plot of the frequencies of these latter transitions will not be very useful*

NH2

As is shown in Table II, the rotational constants must be known

very accurately in order to predict the spectrum successfully.

Table II* (Assumed N-H distance - 1,0 A)

e (°) A(105mc) B(105mc) C(105mc)

90 5.69 5.05-f 2.69

no 8.68 3.77 2.63

130 15.88 3.07 2.54

12
It is possible that

CHg

The situation is similar to that for

for both of these light species no transition occurs within an accessible

frequency region, or that, as in the case of water, only one occurs.

NSg

If a structure similar to NC^ is assumed for NS28 - angle S-N-Sb?» 134®

and N-S bond distance = 1«19 + O ft 33A (the N-0 bond distance plus the

difference between the 0 and S covalent bond radii^), then

A = 1,251 x 105 Me , B = 4^045 x 1C)3 Me , C = 3^907 x 103 Mc^

and the transitions occurring in the microwave region would be

15. , r 16 2 12,200 Me,

1>15 0*16

17]^ 17 : 28,400 Me©

“0,12

^

“1,11 : 25 »°°° Mc *

HCO

Ramsay^ gives for the rotational constants of HCO, B s 4<?498 x 10^ Me,

C = 4*209 x 1CA Mc 9 The corresponding moments of inertia are consistent

with the following structure: rQ_^ = 1^08 - 0*02 Af = 1*19 ± 0 o 01 A,

and angle H-C-0 = 120 ±4% It is therefore likely that the major comr- ponent of the dipole moment is along the a-*axis e A series of Q-branch

transitions between asymmetry doublet components should fall in the K-band

region (AJ = 0, - 1# AK = 0, AK^_ = 1)^ The frequencies of these

13
transitions are given approximately by

V = ( B - °) J (J + 1) = 1.445 J (J + 1) X 103 Me.,

2

or

v = 17,340 Me., ^13 ^ 3^2^

v

82 28 , 900 Me . ^ 424""^ ^

13 9

v = 43,350 Me. , 515

-^ 5x4-

The lack of information about the structure of HO2 precludes a

reliable prediction of its spectrum.

b. Lambda-doublet Transitions.

These transitions occur for molecules irf tt states. The

transitions are between electronic states whose degeneracy is removed by

the interaction of the electronic angular momentum with the molecular

rotation. The frequencies of the transitions depend on the rotational

states involved. The following summarizes material given in reference 1,

p chapter 7 and reference 3 , chapters 2 and 3 * Only *rr states are of

interest here.

(i) For Hundf s coupling case a* the frequency of a transition between the lambda doublet components is given by

V = a(J + 1/2), 1/2,

v = b(J2 - 1/4) (J + 3/2), ru= 3/2

(case a corresponds to the electronic orbital angular momentum and spin

angular momentum coupled strongly to the figure axis 5 -n- =SL+Z 9 the

sum of the projection of the orbital and spin angular momentum on the

figure axis. J is the resultant of and 0, the angular momentum

14
corresponding to the molecular rotation* -A- = l/2 is the ground state

when the spin coupling constant A is positive; -r^= 3/2 is the ground

state for A negative.)

(ii) Hund f s case bs

The lambda-doublet separation is given by

v = q N(N + 1),

where N is the quantum number for the resultant of the nuclear rotation,

0 and-/U • (In Hundf s case b the electronic spin angular momentum is coupled

more strongly to the overall rotation than to the figure axis.)

(iii) Intermediate Couplings

As the rotation energy becomes comparable to the

spin doubling energy a transition between Hundf s case a and b takes place.

Thus, for the lighter diatomic molecules (e.g. hydrides) we may have intermediate coupling for fairly low values of J (or N).

z For the general case of intermediate coupling Van Vleck gives

v = !_ (J+1/2) [(2+a, /b*)(1* +4 ]

,

2 X x

where

X = -[A/B(a/B%) + 4(J+1/2) 2 ]

1'/2

2 2 here, A positive corresponds to the tri state, A negative to the tt^/2

For A positive and the ^3/2 state the positive square root is taken

equal to X, as for A negative and the ^ra. Af /Bf is approximately equal

to A/B but is actually proportional to the quotient of the matrix elements

of Aly and Bly, taken between the ^tt state and excited X states.

15
(iv) Estimation of the constants q and a. The constants q and a may be estimated by means of

formulas given by Mulliken and Christy?, These formulas are based on the

hypothesis of f pure precession* -which considers that -A*., the projection on

the figure axis of the electronic orbital angular momentum, is the projection

of the orbital angular momentum of the odd electron. This orbital angular

momentum has quantum number Jt and precesses about the figure axis. The

molecular rotation interacts, both magnetically and mechanically, -with the

angular momentum of the odd electron thus giving the tt stab© a small amount

of r state character. This hypothesis holds best for molecules for which the

1 united atomt representation of the electronic wave-function is not a bad

approximation. It still may be useful, however, in setting the order of

magnitude of q or a for other diatomics. From the pure precession hypothesis

Mulliken and Christy obtain for the constants a and q:

a r 2ABJUJL +1) = L A3

hv(n,Z ) hv(rr,z )

q

& m L B

2

hv(n, r. ) hv(rr,x.)

where hv (tt, z) is the energy difference between the ground tt state and the

excited X. state mixed in by the interaction of the molecular rotation and

the electronic orbital angular momentum,

(v) Intensities

.

The absorption coefficient e< may be evaluated by

means of the expressions given in reference 1, chapter 1, for the rotational

transition absorption coefficient of diatomic molecules, if the dipole

16
matrix element for a pure rotational transition is replaced by that for

a transition between lambda-doublet components. The resulting formulas

are, at T * 300°K,

case a:

o<= 5oxlO-18 '/ exp [-HBJ(J+1)

j

(J + l) kT

case b:

= 5.5xl0“18 BX~ u' x>2

exp [-hBK(H+l)l

(J + l) kT

where v and B are in lie, and \x 9 the dipole moment, is in Debyes. The

dipole moment may be estimated from bond moments, when reported, or from

the electronegativities of the two atoms. The line width parameter for

pressure broadening has been assumed to be 10 Mc/mm. Hg.

Table III gives calculated, and when available, observed values for

the lambda-doubling constants and the frequencies and intensities of

any transitions occurring in the microwave region. The data has been

taken, for the most part, from reference 3 and Table 1, The principal

uncertainty is in B, where this has to be determined from an estimated

intemuclear separation.

2. Magnetic Dipole Transitions

The material in this section has been summarized from reference

1, chapter 7*

2 3 Radicals in T and states exhibit p-type multiplet splitting.

While electric dipole transitions between the components of these raultiplets

are forbidden, magnetic dipole transitions are not. Although the magnetic

17
dipole of a radical or biradical is several orders of magnitude less than

that of the usual electric dipole (several Bohr magnetons compared to

several Debyes), the tmasition may still be quite strong®

a® states

The components of the p-type doublet have energies given by

W” = BN(N+1) + N, J - N + i;

2

W» - BN(N+l) - Y (N+J), J - N - J.

The frequency of the p-type doublet transition is then

v = irl (n+i)

Using the wpure precession” hypothesis, Mulliken and Christy obtain

v = 2AB JL Cl+l) , hv(n, 1

)

where A is the spin coupling constant of the *rr state "mixed” in by the

interaction of the electronic spin with the molecular rotation®

In Table IV are given the coupling constants calculated from the

3 Christy and Mulliken formula or obtained from Herzberg* s compilation®

The intensities of these transitions are given by

9 X 10-21 B (N+l) 2y‘

2

exp C-BN(N+l)] , (2H+X) kT

where B, v are in Me and the temperature is taken to be 300°K® A pressure

broadening parameter of 10 Mc/mm. Hg® has been assumed.
Table III

Species a (Me) q(Mc) Transitio: 1 v(Mc) ^(cirT1

)

AsO(X2

tti) 1700 J=9i 17,000 5xl0“6

BN(x3rr) Insuffie:Lent data - - -

BrO(X2

rri) Insufficient data - -

• f- CCl(X2m)

2

200 - - -

CF(x2ni) 308 - - -

CH(X2

tia) 800 N=4 16,000 5xlO-6

C 10 (X2m)

2

Insufficient data - “ -

GeBr(X2m)

2

350 - - - GeClCX^i)

2

630 - - —

GeF (X2TM.)

S

2000 J=9| 20,000 2.1x10"5

NiCl(X2m)

2

300 - - -

N0(X2ni) 600 ( calc

2

355 (obs.r

NS(x2ra) 700 _ _

2 (^330 obs)'

ohCx2^) 1300 ( calc .

)

4#4x10“5

2 1090 (obs • ^intermediate1

1190 (case b)

HS(X2rro) 320 J=9i 19,20a 1.3xl0-?

PbBr(xtn) 2000 (if A = 1CA cm

-1

) J=9i 20,000 2.57xlO“6

FbClCX^i) 4000 (if A = lcA cm

-1

) J=4i 20,000 2xl0-6

PbF(X^nj) 4000 ( A == 8.2x10^ cm*”*4 J=4s 20,000 1x10-5

PbH(X2

v ni)

2

3.2x105 - -

19
Table III (cont T d)

Species a (Me) q(Mc) Transition v(Mc) *(cn

T

1

)

P0 (X2m)

2

500 - - -

SbOCX^ni) r^3000 24,000 3xlO“6

SiBr(X2mj

2

250 - - “ SidCX2^)

2

200 - -

SiF(X2m)

2

320 - - SiHCX^i) 4 . 0x103* —200--- J=9i 19,500 Ixl0_?

2

(if v (tt, £ ) = 3x104

SnBr(X2m)

2

4S0 - «*

SnCl(X2m)

2

1000 J=9i 10,000 5xlO“7

SnF(X2

Tvi) 3100 J“4h 16,500 lxlO~5

SnH(X2rri) 5400 J=3| 21,600 8x10”

5

20
b. 3£ States

In addition to the sort of interaction that occurs in

2

2 states, the two unpaired spins of a bi-radical may interact with each

other. If this interaction is small compared to the rotational energy, a

first order expression given by Kramers for the multiplet splitting can be

used.

W = BN(N+1) A + 7r(N+l), JwN+1

.

1+ 1

’ 2(N+1)

W = BN(N+1), J=N

W = BN(N+1) - - YN, J-N-l.

2W

If the spin-spin interaction is large compared to the rotational energy

or of comparable magnitude, then a second order expression given by Miller

and Townes should be used:

W = BN(N+1) -X- JL + U2 - 2X(B- _|L)+ (2N-l) 2 (B- 2^)

2

]

2

—(2N—l)B, J = N-l , W = BH(H+l) y •

. J = «

'' j. W = BN(N+1) -X - - it? - 2X(B + (2N+3) 2

(B )

2

]®

+(2K+3)B, J = N+l.

The intensities are given as = 2«4x10“20 B N(2N+?) e , J« N <r?J=N+l

N+l

= 4,£xl0ff20 B (2N-1) (N+l) e , J=N*» J-N-l

2N

21
at 300°K and for an assumed pressure broadening parameter of 10 Mc/ram.Hg,

If the species is non-polar, then the line width will be much smaller, and

one will gain in intensity by a factor of 5-10,

In Table V are listed the expected frequencies and intensities for a

number of transitions of molecules. The fine structure constants are

taken from reference 3> except where otherwise noted.

22
Table IV

Species

(X2

£.) ^

(Me) Transition

(p-type doublet)

v 6ic)

j AgO 340

AlO 633*

BaJBr 120

BaCl 65O

BaF 820

Bal 60

BaH (10,000)

5 , 580* N=4 25,100

-A

2 x10 0

BeCl 100

! BeF -90

BeH 125

BO | r** -1000

GaBr 80

CaCl c+ 100

CaF ~iko

GaH (1250)

1350* N=9 13,000 5x10^'

Cal ?

CdCl 100

CdH 32,000 N-l 48,000 lxlO

"6

CN -1270 N=19 25,000 3 xlO“8

23
Table IV (cont t d)

- Species

jf

(Me) Transition v(Mc) c< (cm"1

) (p-type doublet) -

CP

(-150)

-170*

- —

CuO 9• GaO 9

HgBr 140

HgCl 1260 N-19 24,600 7X10-8

HgF 2500 N=9 23,600 3x10-8

HgH (90,000)

64,500*-

Hgl 9

InO r^'JOO

LaO 9

MgBr 100

MgCl 100

MgF so

~MgH ( 105)

750*

;

Mgl ?

ScO 350

SiN 9

SrBr 100 . SrCl 200 j

SrF 400

24
Table IV (cont f d)

Species

(x*£) y

(Me) Transtion

(p-type doublet)

v (me) oc ( cirT J

SrH ( 9000) 366O* N=5 19,600 2xl0-7

YbGl 103

ZnBr 70

ZnCl 140

ZnF 240

ZnH ( 11, 300) 7,620* N=3 26,600 5x10“®

Znl 50

* Quoted by Herzberg

25
Table V

p-Type Triplet Transitions of Molecules

(A) Insufficient data

(B) N4

(A3£ u) X = 39,000 Me '

B = 43,200 Me

Y

= -90 Me

'

N+l<r N

N c o' (cnT-4

2 29500 4x10"5

31100*

4 36000* 1x10“^

(C) NH A = I35OO Me

B = 4495OO Me

Y = 1200 Me

H 4-N+l

N v*(Mc) cX/enT1

) 1 13200 5xl0“6

2 15200 6xlO-6

3 16800 7xl0“6

4

•

18300

•

8xl0“6

to •

to

8

to

to 23450

to

to5xlO-7

26
N <r N-l

N v*(Mc) ^(cuT1

) 1 258OO 1.6xl0"5

2 15600 3x10“°

3 12600 2xl0-6

PH. X = 21990 Me

B * 253300 Me

y - -2340 Me

N ^ N+l

N v*(Mc) ^(cnf1

)

1 22280 7xl0-6

2 25870 IxlO-5

3 28910 2xlO_5

4 31700 2xlO“5

•

8

•

•

#

•

7 39420 lxlO~5

N<- N-l

N v*(Mc) (cm"1

)

1 4165O 2xl0“5

2 24620 7x10“5

3 19280 4x10-6

4 15740 3x10-6

5 12800 1x10-6

27
(E) 32 X

^

105 - 106 Mo

11

B = 7920 Me 11

jV-103 Me

11

N <r N-l

-

N < ? ; cJlcuT1

)

3 4.7520 -X* 3x10-7

SO 4^30000 -50000 Me

12

B = 21099 Me

X= 90 Me12

N «r N+l

N v*(Mc) o< (cm”1

) 1

• ^

40000 Me

*

2 . 0xl0~6

•

•#

N

•

•

+A*-(N+l) X

•

•

N N—

1

N v*(Mc)

2

#

^40000 Me

•

4xlo

"6

V •

N

•

•+ x + Nr

•#

* Calculated by use of Kramer’s first-order formula*

^ Intensity calculated for T - 700°K; the temperature dependence

of the line width for non-polar S2 has been assumed to be of the

_ i_ -1 c /2 form Av 2

; the temperature dependence of ai is then (^)

„

^ X si 2M(M-1)(B- V2) 2 4x103 Me

X + V/2 -B

28
PRODUCTION OF RADICALS , METASTABLES AND HOT MOLECULES

Since the field of free radical reactions is so wide, only the particular

cases relevant to the present microwave program will be discussed here in any

detail. A recent survey of free radical and atomic reactions, with particular

attention to organic systems, is given by Steaeie^. Earlier work dealing

with the role of atomic and free radical species in electrical discharge

processes is reviewed by Glockler and Lind^*.

The survey herein given is neither complete nor detailed with regard

to the number of systems or to the explicit experimental techniques that

can be used. It was thought more desirable to cull from the general

references cited and from other sources those systems of most interest from

a spectroscopic point of view. When a choice of particular species is

made as being most suitable for further investigation, the less general

considerations can be brought forth.

Radicals or metastables may be produced by thermal, photolytic, or

discharge techniques. The radicals may result either from direct dissociation of the parent species or from chemical reactions with other species.

Thermal Methods

In the thermal production of radicals two cases may be distinguished?

(l) the radicals or metastables are in thermal equilibrium with the parent

species; (2) the radicals are short-lived (from 10 ^ to 0.1 seconds)

intermediates in pyrolytic reactions.

For the first case, fairly high temperatures are required to get even

a moderate concentration of radicals. For example, in the thermal dissociation of water vapor, 2H20 -*H2+20H, a temperature of 1700° K is required to

29
get an equilibrium partial pressure of about 0.05 mrru Hg. OH from 1 mm.

Hg. HgO'.'HL Temperatures of the order of 103°C are required to get

appreciable partial pressures of most inorganic free radicals - e.g.

l6

Hgl, HgCl* If the bond to be broken in the pyrolysis is weak, then

the required temperature will be lower. For example, the dissociation

17

of I2 is 99 percent complete at about 800°K ' - the I-I bond energy is

about 35 kcal/mol. as compared with an 0-H bond energy in H2O of about

100 kcal/mol. Several particular cases are of interest where an adequate

vapor pressure of the radical or molecule is achieved for moderate

temperatures (^700°C, say).

IS S2S At 400° C and a total pressure of 0.1 mm* Hg.^ sulfur in the

18

vapor phase is almost completely S2 ? At 350°C and a total pressure of

0.1 mm. Hg* the partial pressure of S£ is 0.07 mm9 Hg.j the other species

in the sulfur vapor are Sg and Sg. At 300°G and a total pressure of

sulfur vapor of 0.1 mm* Hg. , the partial pressure of is' 0.013 mm. Hg.

In Table V are listed some molecules whose vapor pressure should be

sufficient for microwave purposes in the given temperature range. The

19 data are taken from Stullt s compilation .

Table 71

0-300°C 300-500°C 5OO-7OO0 G p^lmmsHg.

T1F GeO GeSe

T1C1 GeS GeTe

Til InBr

InCl

39
For the situation 'where the radicals are not in thermal equilibrium

with the parent species, as is the case for most organic free radical

reactions, the variety of species which usually occur in the reactions of

the pyrolytic intermediates complicate the picture. It also should be

noted that the radicals themselves may be thermally unstable at sufficiently

high temperatures. The strength of the bohd to be broken may, even for

the non-equilibrium situation, give a measure of the temperature required

for sufficient yield. For example, benzyl iodide is quantitatively

pyrolyzed to benzyl radicals at a temperature of 670°C, while at a

temperature of 1150°C only about 10 percent benzyl radicals are detected in

the pyrolysis of toluene .

It may be noted in passing that the picolyl radical should have kinetic

22 23 properties similar to the benzyl as was shown by Swarzc * J in studies

on the pyrolysis of toluene and picoline. Thus it should be possible to

should be more suitable for microwave study than the benzyl, having a larger

dipole moment.

In general one might expect the pyrolysis of Iodides, RI R + I, to be

effected at lower temperatures than those of other halides. The reverse

reaction is fast, however, so that silver or some other I fixing agency

is required. Also, reactions among the intermediates, as mentioned above,

may complicate the picture. Although one might in principle be able to

obtain such radicals as CF2

CI, CH^CFCl, etc. by the pyrolysis of the

corresponding iodides, the kinetics of such reactions are not yet well

prepare The picolyl radical
enough known to be sure that these radicals would be the principal steadystate constituents. The pyrolysis of other organic systems is more likely

to be complicated by many intermediate species® The pyrolysis of organometallic, e.g. HgCCH^^, compounds proceeds at fairly low temperatures (200-

700° C) but the resulting radicals, having a low dipole moment, are not

likely to be particularly suitable for microwave studies*

SO has been prepared f,in considerable yield” by the reaction

180° - 350°C

2Ag + SOCI2 > 2AgCl + SO,

the yield increasing at higher temperatures.*^

A whole class of similar reactions, the sodium diffusion flame

reactions, has been reviewed by Steacie*^* The reactions are of the

type

200 - 300°C

RX + Na(g) ^ NaX + R,

where X is any halogen* For Iodides, corresponding to the weaker G-I

bond, reaction occurs at almost every collision, while for F, less than

£

one collision in 10 is effective* In principle one might obtain such

radicals as CF2CI from such reactions as

CF2C1I + Na(g) —> GF2C1 + Nal(s ),

since, as in photolysis, the reaction is selective to the C-I bond®

The partial pressure of Na in these reactions is about 10"^ mm Hg* and

that of RX about 0.1 mm. Hg*, so that the yield is small; however, the

Na partial pressure could be increased. Because of the lack of information about the lifetime of the radicals and about reactions subsequent

to RX + Na —^ NaX + R, one cannot say that R* would be present in high

concentration in the steady state.

32
Photoloysi3

The production of radicals by photolysis may be effected in two -ways:

by steady irradiation or by short bursts of very intense radiation (flash

photolysis). The yield of radicals by use of the former technique -will

be much less, in general, than by use of the latter, as is illustrated

in Ramsay1 s experiments with the continuous2^ and flash2"'7 photolysis of

NH^ to give

A number of radicals suitable for microwave study have been produced

by flash photolysis: CH,CN,0H28

, HCO29, NH3°, HE,27

, CIO31’ 32

. All

but the last of these have lifetimes of the order of milliseconds. CIO

has a lifetime (i,e. the time required for the pressure of CIO to go

from 0.1 mm. Hgt to 0.01 mm. Hg.) between 0.05 seconds and three seconds,

as indicated by the kinetic studies of Porter and Wright^2

. The continuous photolysis of a number of organic compounds has been

studied extensively. Although the initial step, the breaking of some

weak bond - e.g. a C-I bond-is simple and well-established, the ensuing

reactions may be, as in pyrolyses, quite complicated. CH^ produced by

the photolysis of ketene has a lifetime of approximately 0.2 seconds33 # Sensitizers may be used to absorb the radiation and transfer it to

the species to be dissociated. That is,

M + hv —» M*

M*+AB —* M + A + 5.

CF2 is probably produced by the Xenon sensitized photolysis of CF^. Hg,

Cd and Zn may also be used as sensitizers:

33
M + hv M*

m + RH —MH + R

m —^ m + h

HgH is very unstable; its lifetime is probably 10“5 that of CdH, for

-which latter the resonance absorption has been observed. It is probable

that even though CdH molecules in the ground state are produced in the

photolysis of Cd vapor in H2, the yield is small-^.

Electrical Discharges

Electrical discharges may be used to produce radicals and metastables

in two ways: (l) by direct dissociation or chemical reaction in the discharge,

or (2) by production of an active species in the discharge and reaction of it

outside the discharge to give another labile species. Many diatomic and a

few triatomic labile molecules have been produced by reactions in a discharge. The discharge processes are in general more violent and less

selective than those occuring in continuous photolysis or pyrolysis, so

that polyatomic radicals do not result. However a number of polyatomic

radicals have been postulated as intermediates when relatively longlived species-H, 0, OH, active nitrogen, for example - are flowed from the

13 discharge and attack stable molecules. Steacie cites a number of such

reactions, which are supported by more or less indirect evidence.

Among those radicals produced directly within the discharge for which

kinetic data is available are the following:

35 NH has been produced by discharges through HN^ ; its lifetime should

be about the same as NH produced by the pyrolysis of HNo: 10“^ seconds

34
at a pressure of HN^ of 0,07 mm* Hg,-^, NH and NH2 have also been produced

. 37 by discharges through ammonia and hydrazine^ * HO2 is produced from discharges through H2O2 in yields of about 0,1

to 1 percent at moderate flow rates

^2 i s l°wer state in the transition corresponding to the

first positive bands present in the active nitrogen afterglow* This

afterglow can be made to persist, by suitable wall conditioning, for

several hours after a discharge through M2 is cut off. Since the transi1 tion to the %. ground state is forbidden, the radiative lifetime of the

state is relatively long. However, mass spectrometric studies of the

afterglow at N2 pressures of about 1 mm, Hg, indicate that N atoms rather

39 than excited N2 molecules are the principal constituent of the afterglow , CIO should result when a discharge is passed through mixtures of CI2 and C>2 ? The kinetic properties of CIO produced from a discharge should be similar

to those of CIO produced by photolysis, although the initial yield from

the discharge might be smaller*

SO has been prepared by passing SO2 through a discharge; the yield

of 10$ is increased to 40$ if sulfur vapor is mixed with the SO^ , The spectra of a number of other diatomic radicals have been observed

in emission and absorption in discharges. However, detailed kinetic data

are not available for these®

Several relatively long-lived metastable molecules have been produced

by dissociation in a discharge,

CH2 has been detected by the formation of (TeCH2)n

polymers when

the products of a discharge through ketene or methane are passed over

tellurium mirrors^. Its lifetime should be about the same as when

35
produced by photolysis - about 0*2 seconds* CH2 has not yet been

observed spectroscopically, however*

CF2 has been studied by means of its ultraviolet absorption spectrum

and its lifetime estimated to be of the order of several seconds^- * It

has been prepared by the dissociation of compounds containing CF2 groups,

although it was not observed in discharges through CF^% Wall conditions,

it should be noted may be important; CF2 was not observed when the discharges were carried out in glass tubing whose diameter was less than

about 40 mm. CSe might be expected to have a lifetime of several seconds f if the relative lifetimes of CO (stable) and CS (about 30 minutes when produced by a discharge through CS^~) are any indication, CSe has been seen

in emission in discharges through CH2CI2 and Se^« it should be possible

to prepare it by passing CSSe through a discharge.

Among those species produced by reactions of active species flowed

from the discharge, and for which there is spectroscopic evidence are

those occurring in the following reactions:

H + 0, > OH* + 02

^

(where OH# is vibrationally excited (v ^ 9) ) I active nitrogen + Br2 —> NBr;

^

9 active nitrogen + 82(g) NS;

1 /

(the lifetime of NS has been reported to be of the order of seconds) 4

; 0 + HCHO —> OH + CHO;

^

0 + CH3OH—

>

0H + CH„0j *7

(the OH and HCHO were observed spectroscopically; further data would be

required before one could say that a sufficiently high steady-state

36
concentration of CH^O could be maintained: also the OH and HGO were

observed in emission - the reaction is luminescent - rather than

absorption so that no estimate of the yield could be made).

Among those reactions for which the intermediates are inferred

are those of the types^

H + RH —> H

2

+ R-

,

0 + RH —> OH + R*,

(here one interestihg possibility is 0 + CH^COH—^OH + CH^CO),

Active nitrogen + RCH = CH2 RCH - C^-^-RCH: + C^N.^

N

In all these cases the variety of intermediates is likely to produce

situations as complicated as those occurring in photolysis and pyrolysis.

37
MONITORING THE CONCENTRATION OF RADICALS

Inasmuch as the experimental conditions required for an

optimum concentration of radicals in the absorption cell -will not

be known a priori, there should be available some device which can

detect the radicals in concentrations less than that required for the

observation of their microwave spectra© The experimental conditions can

then be varied to maximize the radical concentration to a point where

their spectra should be observable© This monitoring device is particularlyrequired if there is a lack of information for positively predicting the

frequencies and intensities of the microwave lines. Even in such a

relatively simple system as that of H^O passed through a discharge to

give OH, a history of the microwave investigations shows that a means,

independent of the microwave line itself, for estimating the concentration of OH in the absorption cell would be desirable and almost necessary.

Obviously the monitoring device need not be used in conjunction with the

spectrograph, but may be used to estimate concentrations on a set-up

similar to that which will finally be used.

Three sorts of monitoring systems might be used.

Chemical Tests

A number of specific reactions, such as the color change of certain

oxides on reaction with atomic hydrogen, have been used for particular

radicals. The most general method, however has been to analyze directly the

products of reactions involving free radicals or the products of the

reaction intermediates with various test substances. For example, the

formation of a dark blue paramagnetic solid, with composition (NH) , X

38
±n a cold trap through which is passed the products of a discharge

through HN^, has been used to infer the presence of the NH radical^*

However, in that the analysis of products is not a direct indication

of the concentration of radicals at a particular site and instant, it

may be misleading. Such was shown to be the case for the microwave

work on the OH radical^ . In even such a relatively simple system as

H2O passed through a discharge, the amount of H

2O

2 trapped out did not

correlate with the actual concentration of OH in the microwave absorption cell.

Absorption Spectra

The minimum partial pressure which may be detected in absorption

by a spectrograph with good resolving power and dispersion is of the

order of 10~3 mm. Hg. However only diatomic and a few triatomic species

can be observed at these pressures. Porter^ gives the requirements that

an optical spectrograph for the study of labile molecules in absorption

must meet.

Paramagnetic Resonance Absorption

51

A paramagnetic resonance spectrograph such as that used by Dehmelt

in his study of atomic phosphorus would be capable of detecting a minimum

partial pressure of about 10”^ mm. Hg. of free radicals and would therefore be ideal for the detection of polyatomic free radicals whose optical

spectra could not be observed. The searching problem present in microwave

spectroscopy would be minimized* Furthermore, valuable information about

the free radicals could be obtained from their paramagnetic resonance

spectra* However the construction of such a spectrograph would certainly

be almost as major an undertaking as the construction of a microwave

spectrograph.

39
Mass Spectrometer

A mass spectrometer would be the ideal tool for monitoring not only

radical concentrations but also those of stable species in free radical

reactions^ However the same objection which might be applied to a paramagnetic resonance spectrograph as a possible monitoring device can be

applied to the mass spectrograph - its construction would be as big and

complex a task as that for the microwave spectrograph itself*

40
REFERENCES

^See, for example, Townes and Schawlow, Microwave Spectroscopy, (McGraw-Hill Book Co., New York 1954, Chap. 1-4.

2The case of NO2 has been treated by C. C. Lin, Ph.D. Thesis,

Harvard University, 1955

3g. Herzberg, Spectra of Diatomic Molecules 2nd Ed. (D. Van Nostrand

Co., Inc. New York 1950) Table 39, pp. 501 et« seq.

^W. Gordy, Microwave Spectroscopy. Wiley & Sons, Inc. New York, 1953

Table 84, PP 309

3d. A. Ramsay, J. Chem. Phys . 21, 960 (1951)

6j. H. Van Vleck, Phys. Rev. 33, 4^7 (1929)

?R. S. Mulliken and A. Christy, Phys. Rev. 38 , 87 (1931)

^L. Pauling, The Nature of the Chemical Bond . 2nd Ed. Cornell University

Press, Ithaca, N. Y. , 1947 pp67

9r. F. Barrow, Proc. Phys. Soc . 67A, 365 (1954)

10*4. H. Hebb, Phys. Rev. 49, 610 (1936)

X

S. M. Naude, Ann. Physik 3 , 201 (1948) Naude evaluates X and using

Kramer* s first order formula. This is incorrect since X»B for

the ground electronic state.

l^E. V. Martin, Phys. Rev. 37, 226 (1931) ^he same comment applies here

as was given in ^l. In this case X^B, and the spectral data are

less consistent, so that the determination of X is more uncertain

for ^0 than for

41
13e. W. R. Steacie, Atomic and Free Radical Reactions . 2nd Ed.,

Reinhold Publishing Corp., New York, 1954*

^G. Glockler and S* C. Lind, The Electrochemistry of Gases and Other

Dielectrics . John Wiley & Sons, New York, 1939

-^Boenhoffer and Richards, Z. Physik Chem. AI39, 75 (1928)

l^See for example, Donnees SpectroscopiquesConcernani les Molecules

Diatomiques . Hermann & Go., Paris, 1951

^From Table 1, p. 18, Reference 13

^Kelley, U. S. Bur. Mines. Bull. No. 406, pp.7-8

R. Stull, Ind. & Eng. Chem. 39, 517 (1947)

2%. P. Lossing, K. U. Ingold and I. H. S. Henderson, J. Chem. Phys .

(in Press)

21k. U. Ingold and F» P, Lossing, J a Chem. Phys. 21, 368 (1953)

22m. Swarzc , J. Chem. Phys . 16, 128 (194&)

23

j

# s. Roberts and M# Swarzc, J. Chem. Phys a 16, 9^1 (194S)

24h. Cordes and P. W. Schenk, Z. Electrochem. 39, 594 (1933)

25Ref. 13, p. 20 et. seq, Yol. I, Chap XIII, Vol. II

2%. A* Ramsay, J

? Chem. Phys. 21, 165 (1953)

2?D. A. Ramsay and G. L. Herzberg, Disc. Faraday Soc . 14, 11 (1953)

2%. G. W. Norrish, Disc. Faraday Soc . 14 . 16 (1953)

29g. Herzberg and D. A. Ramsay, Proc. Roy. Soc.

A

. 223, 34 (1955)

3°D. A. Ramsay, J. Phys e Chemt £2, 415 (1953)

31g. Porter, Disc. Faraday Soc . 9, 60 (1950)

32g. Porter and F. J. Wright, Disc. Faraday Soc . 14, 23 (1953)

42
3%. G. W. Norrish and G. Porter, Disc 0 Faraday Soc* 2, 97 (1947)

34c. R. Masson and E. W, R. Steacie, J,, Chem» Phys a 18, 210 (1950)

35F t 0* Rice and M. Freamy, J» Amer* Chem* Soc . 75 $ 54S (1953)

0. Rice and M. Freamy, J B Amerft Chenu Soc Q 73 , 5529 (1951)

3?P. J. Dyne, Disc„ Faraday Soc . 14, 112 (1953)

3&S. n. Foner, R9 L*. Hudson and R. M* Fristrom, John Hopkins Appl* Phys . Lab , private communication,

39j# Berkowitz, Ph.D a Thesis, Harvard University, (1955)

40pw Goldfinger, P. Le Goff and M? Le Tprt, Third Syrno^ on Combustion .

1949, p* 446

^R. K« Laird, E9 B. Andrews, R. F. Barrow, Trans. Faraday Soc . 46 , 8O3

42p # j # Dyne, Can^ J» Phys . 31, 453 (1953)

43r* F. Barrow, Proc. Phys^ Soc., A 51 , 9&9 (1939)

^J. D* McKinley, Jr. David Garvin and M. J. Boudart, J» (jhenuPhys .

23 , 7S4 (1955)

^k. Elliot, Proc„ Royf , Soc, AI69 , 469 (1939)

46r# f« Barrow, Disc a Faraday Soc . 9, SI (1950)

47k. H. Geib and W. M. Vaidya, Proc B Roy. Soc. A17S, 351 (1941)

4&C. A* Winkler and H* I. Schiff, Disc* Faraday Soc. 14, 63 (1953)

49Reference 1, Chap. 13

5°T. M. Sanders, Jr., A. L. Schawlow, G. L. Dousmanis and G. H. Townes,

J~ Chem Phys . 22, 245 (1954)

5lDehmelt, Phys ? Rev. 99, 5^7 (1955)

43
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