Relaxation and Reactivity of Singlet Oxygen - Advances in Chemistry

70 Relaxation and Reactivity of Singlet

Downloaded by UNIV MASSACHUSETTS AMHERST on September 7, 2012 | http://pubs.acs.org Publication Date: January 1, 1968 | doi: 10.1021/ba-1968-0077.ch070

Oxygen S. J .

ARNOLD,

M.

KUBO,

a n d E. A. OGRYZLO

University of British C o l u m b i a , Vancouver 8, B. C., Canada

Measurements of some energy transfer, physical quenching, and chemical reaction processes of singlet oxygen are presented. The results of these measurements and those obtained previously are analyzed in an attempt to assess the fate of O ( Δg) and O ( Σg ) in oxidation systems. 1

1

2

'Two

+

2

e l e c t r o n i c a l l y excited singlet states of o x y g e n a r e l o c a t e d 22.5 a n d

37.5 k c a l . a b o v e t h e t r i p l e t g r o u n d state.

T h e o n e at 22.5 k c a l . is

c o m m o n l y d e s i g n a t e d a A a n d w i l l b e a b b r e v i a t e d A. T h e h i g h e r o n e 1

has t h e t e r m s y m b o l b X 1

X 2/ 3

1

g

a n d w i l l b e r e f e r r e d to as

+

g

T h e g r o u n d state

w i l l b e a b b r e v i a t e d S . B e c a u s e their r e l a t i v e i m p o r t a n c e i n c h e m i 3

c a l reactions has n o t y e t b e e n d e t e r m i n e d , these t w o e x c i t e d states are r e f e r r e d to as singlet oxygen. T h i s p a p e r considers t h e various p h y s i c a l a n d c h e m i c a l processes w h i c h t h e t w o species c a n u n d e r g o , a n d a n att e m p t is m a d e to assess their r e l a t i v e i m p o r t a n c e i n o x i d a t i o n processes.

Experimental A t y p i c a l flow system u s e d to p r e p a r e A molecules f o r k i n e t i c studies is s h o w n i n F i g u r e 1. O x y g e n at a pressure b e t w e e n 1 a n d 10 torr is passed t h r o u g h a m i c r o w a v e discharge. T h e atoms are r e m o v e d w i t h a m e r c u r i c oxide r i n g i m m e d i a t e l y after t h e discharge. T h e c o n c e n t r a t i o n is m e a s u r e d at o n e p o i n t i n t h e t u b e b y t h e heat l i b e r a t e d w h e n t h e m o l e c u l e s a r e d e a c t i v a t e d o n a c o b a l t w i r e . R e l a t i v e concentrations o f e x c i t e d molecules a l o n g t h e o b s e r v a t i o n t u b e c a n b e m e a s u r e d w i t h a m o v a b l e interference filter a n d p h o t o m u l t i p l i e r . T h e details o f these m e t h o d s h a v e b e e n d e s c r i b e d (1,3,4). T a n k o x y g e n is n o r m a l l y selected for l o w n i t r o g e n content a n d u s e d w i t h o u t f u r t h e r p u r i f i c a t i o n . Q u e n c h i n g gases w e r e treated o n l y t o r e m o v e h i g h e r b o i l i n g i m p u r i t i e s , e s p e c i a l l y water. 1

133 In Oxidation of Organic Compounds; Mayo, F.; Advances in Chemistry; American Chemical Society: Washington, DC, 1968.

134

OXIDATION

OF

ORGANIC

COMPOUNDS

III

Radiative Relaxation I n the absence of a n y external p e r t u r b a t i o n b o t h A a n d *2 o x y g e n a

d o not emit a n y m e a s u r a b l e electric d i p o l e r a d i a t i o n . H o w e v e r , w i t h a l i f e t i m e of 7 s e c , *2 c a n g i v e rise to m a g n e t i c d i p o l e r a d i a t i o n at 7619 A . , a n d * A c a n g i v e rise to m a g n e t i c d i p o l e r a d i a t i o n at 12,683 A . w i t h a l i f e t i m e of 45 m i n u t e s .

I n a c o l l i s i o n w i t h another m o l e c u l e

electric

d i p o l e transitions b e t w e e n these states are m a d e m o r e p r o b a b l e , a n d the Downloaded by UNIV MASSACHUSETTS AMHERST on September 7, 2012 | http://pubs.acs.org Publication Date: January 1, 1968 | doi: 10.1021/ba-1968-0077.ch070

r a d i a t i v e l i f e t i m e c a n be shortened. T h e exact r a d i a t i v e l i f e t i m e of singlet o x y g e n i n a c o l l i s i o n c o m p l e x is d i f f i c u l t to estimate because the d u r a t i o n of a c o l l i s i o n is u n c e r t a i n . H o w e v e r , w i t h a reasonable estimate of a b o u t 10

1 3

sec. for this c o l l i s i o n t i m e , the f o l l o w i n g r a d i a t i v e lifetimes for a

n u m b e r of c o l l i s i o n complexes can be c a l c u l a t e d f r o m the i n t e g r a t e d ab s o r p t i o n coefficients.

r = 4 sec.

(1)

12,700 A . and 15,800 A* W A

t—

1.5 sec.

(2)

6,340 A . and 7,030 A . r = 15 sec. 7,620AT T

3

(3)

2 S 3

= 0.3 sec.

(4)

3,808 A . and 3,612 A . T

= 1.7 sec.

(5)

4,773 A

W h e n the c o l l i s i o n c o m p l e x is m a d e u p of t w o excited molecules, a n o v e l energy p o o l i n g process occurs i n w h i c h the energy of t w o molecules appears i n a single p h o t o n . T h e above list shows that the p r o b a b i l i t y of s u c h a c o o p e r a t i v e event c a n b e c o m p a r a b l e w i t h that for a one-electron transition. H o w e v e r , because the f r a c t i o n of molecules i n a state of c o l l i s i o n is s m a l l , r a d i a t i v e r e l a x a t i o n is not responsible for the d e c a y of a significant n u m b e r of e x c i t e d molecules u n d e r the u s u a l e x p e r i m e n t a l c o n d i t i o n s . F o r example, the strongest i n d u c e d r a d i a t i o n for A occurs at 6340 a n d 7030 A . If this w e r e the o n l y m o d e of decay, the o b s e r v e d l i f e t i m e at 1 a t m . w o u l d be 10 s e c , whereas the o b s e r v e d l i f e t i m e is m u c h less t h a n 1 sec. T h e 6340-A. b a n d is, h o w e v e r , a c o n v e n i e n t a n d sensitive emission for m o n i t o r i n g the singlet d e l t a c o n c e n t r a t i o n since the e m i s s i o n intensity is p r o p o r t i o n a l to the square of its c o n c e n t r a t i o n . X

3

In Oxidation of Organic Compounds; Mayo, F.; Advances in Chemistry; American Chemical Society: Washington, DC, 1968.


70.

ARNOLD E T A L .

Figure 1.

Relaxation

and

Reactivity

135

Core of flow system used for quenching studies

Energy Transfer F o r efficient transfer of electronic excitation to another m o l e c u l e , t h e acceptor must possess a n e x c i t e d electronic state at or n o t too f a r b e l o w that of t h e d o n o r . N o t m a n y m o l e c u l e s c a n meet this r e q u i r e m e n t w h e n the d o n o r is singlet oxygen. W e h a v e o b s e r v e d energy transfer to t h e f o l l o w i n g species, ( a ) another A m o l e c u l e , ( b ) v i o l a n t h r o n e ( d i b e n z a n A

t h r o n e ) , ( c ) n i t r o g e n d i o x i d e , a n d ( d ) i o d i n e atoms.

T h e mechanism

of energy transfer to ( b ) is n o t w e l l u n d e r s t o o d a n d w i l l b e d e s c r i b e d elsewhere ( 7 ) . S i n c e ( c ) a n d ( d ) are n o t d i r e c t l y r e l a t e d to the subject of h y d r o c a r b o n o x i d a t i o n , they w i l l n o t b e discussed i n a n y d e t a i l . T h e transfer to i o d i n e atoms is u n d o u b t e d l y the most efficient process w h i c h w e h a v e o b s e r v e d ( 2 ) , a n d this c a n b e a t t r i b u t e d to the fact that t h e 2

? i / 2 state of i o d i n e lies 22 k c a l . a b o v e the -P3/2 g r o u n d s t a t e — i n almost

perfect resonance w i t h A o x y g e n . T h e transfer to n i t r o g e n d i o x i d e is 1

m u c h less efficient a n d appears to i n v o l v e energy transfer f r o m X a n d A l

]

to raise the acceptor to a r a d i a t i n g state w h i c h lies about 60 k c a l . above the g r o u n d state ( 2 ) . 5

A—^A Transfer.

Since the ^

state lies 15 k c a l . above the * A , t h e

latter c a n act as a n acceptor as w e l l as a d o n o r i n an energy d i s p r o p o r t i o n a t e process: 1

Two

A + iA-> £ + 1

3

2

(6)

laboratories h a v e r e p o r t e d rate constants f o r this r e a c t i o n . A

v a l u e of 1.8 X 1 0 liters/mole/sec. w a s r e p o r t e d b y Y o u n g a n d B l a c k ( 8 ) , 7


136

OXIDATION

OF

ORGANIC COMPOUNDS

III

a n d a v a l u e of 1.3 X 1 0 liters/mole/sec. w a s m o r e r e c e n t l y r e p o r t e d b y 3

Arnold and Ogryzlo (3).

B e c a u s e of the large d i s c r e p a n c y b e t w e e n these

t w o values w e h a v e a t t e m p t e d a t h i r d d e t e r m i n a t i o n b y m e a s u r i n g the rate of * A r e m o v a l d i r e c t l y . T h e results of these measurements are s h o w n i n F i g u r e 2. T h e A c o n c e n t r a t i o n w a s v a r i e d b y c h a n g i n g the p o w e r f e d X


i n t o the discharge.

A s s u m i n g that i n a d d i t i o n to R e a c t i o n 6, w h i c h is

['AgJ Figure 2.

x lO Moles/1 6

Rate of singlet delta decay (R = d [ A ] /dt) divided by singlet delta concentration as a function of the singlet delta concentration J

second o r d e r i n A , w e c a n h a v e w a l l a n d gas-phase q u e n c h i n g that is X

first order i n A , the rate e q u a t i o n b e c o m e s : X

R

=

^

A / [

^

=

1

* Q [

A ] = *

Q

1

+

A ]

+

* D [

* D [

1

1

A ] »

A J

T h e slopes of the lines i n F i g u r e 2 are t h e n e q u a l to fci>. T h e average v a l u e w e o b t a i n is 3 X 1.3 X (3)

10

3

10

4

liters/mole/sec.

This value can be compared w i t h

liters/mole/sec., p r e v i o u s l y r e p o r t e d b y A r n o l d a n d O g r y z l o

f o r R e a c t i o n 6.

T h e t e c h n i q u e u s e d i n the earlier measurement

is

q u i t e difficult, a n d p o s s i b l y the v a l u e is s o m e w h a t l o w . H o w e v e r , it is also c o n c e i v a b l e that the process w e h a v e m e a s u r e d is not the slower,


70.

ARNOLD

Relaxation

ETA L .

and

137

Reactivity

s p i n - f o r b i d d e n R e a c t i o n 6, b u t t h e m o r e r a p i d s p i n - a l l o w e d process, R e a c t i o n 7. -» 2 + 2

*A + *A

3

(7)

3

A d e c i s i o n b e t w e e n these possibilities m u s t a w a i t f u r t h e r measurements. W e c a n o n l y c o n c l u d e that t h e v a l u e of k

lies b e t w e e n 1.3 X 1 0 a n d 3

Ct

3 X 1 0 liters/mole/sec. 4

A n i m p o r t a n t consequence of the o c c u r r e n c e of R e a c t i o n 6 is that *S is c o n s t a n t l y b e i n g f o r m e d i n a n y system w h i c h contains A. Downloaded by UNIV MASSACHUSETTS AMHERST on September 7, 2012 | http://pubs.acs.org Publication Date: January 1, 1968 | doi: 10.1021/ba-1968-0077.ch070

1

W e shall

see later that t h e reverse is p r o b a b l y also true.

Radiationless Non-Resonance Relaxation W h e n a n external p e r t u r b a t i o n s u c h as that c a u s e d b y a c o l l i d i n g m o l e c u l e is sufficiently great, t h e electronic e x c i t a t i o n m a y b e d e g r a d e d into nuclear motion w i t h i n the collision complex.

H o w e v e r , very little

i n f o r m a t i o n is a v a i l a b l e a b o u t t h e efficiency of s u c h processes, a n d conse q u e n t l y n o c o m p l e t e t h e o r e t i c a l m o d e l exists w h i c h w e c o u l d use to p r e d i c t q u e n c h i n g rates. T h e e x p e r i m e n t a l d e t e r m i n a t i o n of A q u e n c h i n g is difficult because X

of its great s t a b i l i t y . I n most flow systems t h e d e c a y is l a r g e l y o n t h e w a l l s of t h e vessel w h e r e i t c a n suffer a b o u t 2 X 10

r>

collisions before

d e a c t i v a t i o n . C o l l i s i o n s w i t h most other m o l e c u l e s are e v e n less effective. A t t h e m o m e n t i t c a n o n l y b e s a i d that m o r e t h a n 1 0

8

collision w i t h

0 ( 2 ) are necessary to deactivate * A . O t h e r , n o n r e a c t i v e gases cannot 2

3

be tested s i m p l y because so m u c h m u s t b e a d d e d that i t r a d i c a l l y affects the

flow

system a n d discharge, m a k i n g t h e measurements

difficult t o

interpret. T h e q u e n c h i n g of S o x y g e n is s o m e w h a t easier to s t u d y because i t J

is m o r e easily d e a c t i v a t e d . I n t h e absence of a n y q u e n c h i n g gas, a steadystate c o n c e n t r a t i o n of S is m a i n t a i n e d i n t h e flow system b y t h e f o l l o w i n g X

reactions. *A + *A - »

(8)

*2 + S 3

(9) hence, D

PA]

(10)

2

a n d t h e emission i n t e n s i t y f r o m S is g i v e n b y X

I =

* [

1

S ] = * ^ [

1

A ]

2


138

OXIDATION

O F ORGANIC

COMPOUNDS

III

I n the presence of a q u e n c h i n g gas ( Q ) w e m u s t a d d R e a c t i o n 1 1 :

2

S + Q

(11)

products

a n d therefore the steady state c o n c e n t r a t i o n is g i v e n b y ri

S l

-

k

°

W

(12)


a n d t h e emission intensity i n t h e presence of Q :

_ kkD [ * A ]

(13) *w + * [ Q ] T h e ratio of t h e e m i s s i o n w i t h a n d w i t h o u t t h e q u e n c h e r is therefore 2

Q

given b y : (14) A p l o t o f I /I 0

fc /fc . Q

w

Q

against Q s h o u l d y i e l d a straight l i n e w i t h a slope of

S u c h a p l o t is g i v e n i n F i g u r e 3 f o r 15 different gases.

[q]

Figure

3.

Stern-Volmer

x I O

6

M o l e s / 1

plot of 7619-A. emission intensity quenching gases

for a series of


70.

ARNOLD

Relaxation

ET AL.

Table I.

and

139

Reactivity

kq/kv, f r o m I / / Q vs. Q 0

k x

k /k Q

w

He N , Ar, C O CH HBr

0.01 0.02 0.11 0.21

CHCI3

0.31 0.39 0.56 0.78 2.3 3.7 5.0 6.8 8.5

2

4

co H S DME NH Methanol Heptane D 0 H 0


2

2

3

2

2

T h e values of fc /fc Q

i n T a b l e I.

w

io«

X

1

X

S a n d * A concentrations i n the absence

-

are l i s t e d i n T a b l e II together w i t h values of =

D

3

3 X

10

4

c a l c u l a t e d earlier w e o b t a i n fc

w

o n a c l e a n b o r o s i l i c a t e glass surface.

of Zc w i t h the values of fc /fc w

Q

w

the values of k

Q

w

=

C o m b i n i n g this v a l u e

i n T a b l e I, w e o b t a i n e d the values of

l i s t e d i n the s e c o n d c o l u m n of the same table. k

0.07 0.15 0.8 1.5 8.8 2.8 4.0 10 16 26 36 49 60

v

F r o m the v a l u e of k 10

k

F r o m E q u a t i o n 10,

V a l u e s of [ A ] a n d f ^ ] ^= 1.3 X

*/ ^ =

w e r e q u i r e /

w

1,300

7

1.5 3 16 30 45 56 81 110 330 530 720 980 1200

measurements of the steady-state

fci)/fc .

=

w

10~

o b t a i n e d f r o m the slopes of these lines are g i v e n

To obtain k ,

of a n y q u e n c h e r .

if k

X

0

k

Q

T h e t h i r d c o l u m n gives

c a l c u l a t e d w i t h the p r e v i o u s l y d e t e r m i n e d ( 3 )

v a l u e of

65. T a b l e II. [O/'VJ]

P, tort

X

Values of ^ A ] a n d 10\

Moles/Liter

2.4 3.1 3.8 5.1

[0 ('A )] X Moles/Liter 2

g

PS] 10\ Liters/Mole

20.5 21.5 20.8 20.3

9.0 11.0 14.2 17.5

1.75 2.54 4.20 6.24

I n this process % m a y b e r e l a x e d into either the A or 2 state. x

1

3

the t r a n s i t i o n to the 2 state r e q u i r e s the c o n v e r s i o n of m o r e 3

energy i n t o n u c l e a r m o t i o n a n d also r e q u i r e s a " s p i n expect the t r a n s i t i o n to the

1

flip,"

Since

electronic we

A state to be m u c h m o r e p r o b a b l e .

would T h i s is

c o n f i r m e d b y the o b s e r v a t i o n that the q u e n c h i n g b y p a r a m a g n e t i c 0


2

is,

140

OXIDATION

OF

ORGANIC

COMPOUNDS

III

if a n y t h i n g , less effective t h a n species l i k e A r a n d N . N o n e of

the

2

molecules i n c l u d e d i n this s t u d y d i s p l a y a n y s p e c i a l resonance effect. T h e r e is, h o w e v e r , a r o u g h c o r r e l a t i o n b e t w e e n q u e n c h i n g efficiency a n d b o i l i n g p o i n t . T h i s is a reasonable c o r r e l a t i o n since one m i g h t expect t h e p r o b a b i l i t y of s u c h a n i n d u c e d t r a n s i t i o n to b e r e l a t e d to the m a g n i t u d e a n d d u r a t i o n of the p e r t u r b a t i o n . W e w i l l not a t t e m p t a d e t a i l e d analysis of this c o r r e l a t i o n here a n d w i l l s i m p l y observe that b o i l i n g p o i n t s reflect b o t h these quantities i n a s o m e w h a t i n d i r e c t m a n n e r a n d c a n

therefore


b e u s e d to estimate q u e n c h i n g rates.

Physical Quenching

"Processes

F r o m E q u a t i o n 16 i t f o l l o w s that i n the presence of a q u e n c h i n g species Q the r a t i o of 2 to A concentrations is g i v e n b y x

1

Ps] PA]

_ -

ftp l > ] *g[Q]

W h e n Q is w a t e r or a h y d r o c a r b o n w i t h a s i m i l a r b o i l i n g p o i n t , the equation becomes: PS] _ o P A ] 2

y

l

X

1

Q

-

B

°

P A ] [Q]

I n most c h e m i c a l a n d p h o t o c h e m i c a l o x i d a t i o n systems the r a t i o of to [ Q ]

is extremely s m a l l , a n d therefore the

smaller t h a n 10" . 6

1

[ A] 1

2 / A ratio is v e r y m u c h 1

T h e o n l y s u c h system i n w h i c h this ratio m i g h t be

a p p r o a c h e d is the CI2-H2O2 r e a c t i o n w h e r e the p a r t i a l pressure of * A p r o b a b l y exceeds 70 torr

(6).

H o w e v e r , w e m u s t also consider

the

q u e n c h i n g of * A . T h e relevant processes are t h e n the f o l l o w i n g i n the presence of quenchers s u c h as w a t e r : k ^

10

9

[Q]

k~

I k=1.3

X 10

3

X

10 -

10 ^

1

2

A

(

3

2

[Q]

( S i n c e w e are most interested i n the p o s s i b i l i t y that S contributes to the X

r e a c t i v i t y of singlet o x y g e n , w e h a v e u s e d the l o w e r q u e n c h i n g constants for X c a l c u l a t e d f r o m A r n o l d a n d O g r y z l o ' s v a l u e of k X

(3).)

w

one c o l l i s i o n i n 100 ( ~ 1 0 ~

9

sec. i n s o l u t i o n )

X

S is r e l a x e d to

I n about X

A by Q.

O n l y if A is «^ 0 . 1 % of Q is S efficiently r e f o r m e d f r o m A . O t h e r w i s e , 1

1

X

it is r e l a x e d to 2 i n a b o u t one c o l l i s i o n i n 1 0 - 1 0 3

9

(1-10 msec, i n s o l u

1 0

t i o n ) w i t h Q . U n d e r these c o n d i t i o n s w e o b t a i n a A / 2 ratio of about 1

10 . 8

1

C o n s e q u e n t l y , i n s u c h systems w h e n a steady state is established,

the rate constant f o r r e a c t i o n w i t h X w o u l d h a v e to b e about 1 0 X

t h a n that f o r A to b e c o m p e t i t i v e . J


8

faster

70.

ARNOLD

ET

Relaxation

AL.

and

141

Reactivity

Chemical Reactions N o t e c h n i q u e f o r m e a s u r i n g the absolute values of rate constants f o r singlet o x y g e n reactions i n s o l u t i o n has yet b e e n r e p o r t e d . H o w e v e r , s u c h a m e a s u r e m e n t is possible i n the gas phase w i t h the t e c h n i q u e d e s c r i b e d here, p r o v i d e d the species is v o l a t i l e a n d h i g h l y reactive. W e h a v e s t u d i e d the r e a c t i o n of singlet o x y g e n w i t h t e t r a m e t h y l e t h y l e n e ( T M E ) , w h o s e r e a c t i o n w i t h singlet o x y g e n i n the gas phase w a s first d e s c r i b e d b y B a y e s Downloaded by UNIV MASSACHUSETTS AMHERST on September 7, 2012 | http://pubs.acs.org Publication Date: January 1, 1968 | doi: 10.1021/ba-1968-0077.ch070

and W i n e r (5).

T h e r e a c t i o n w a s f o l l o w e d b y m o n i t o r i n g the * A a n d *2

concentrations u n d e r v a r i o u s c o n d i t i o n s . W e h a v e f o u n d , h o w e v e r , that k i n e t i c a l l y the process is not as s i m p l e as the p r e l i m i n a r y studies sug gested.

It is possible that this m a y b e a characteristic of

exothermic

association processes i n l o w d e n s i t y systems w h e r e there are a n insuffi cient n u m b e r of collisions w h i c h u n r e a c t i v e m o l e c u l e s to p r e v e n t c h a i n reactions f r o m d e v e l o p i n g . I n contrast to the s i t u a t i o n i n c o n d e n s e d m e d i a , i t is h i g h l y p r o b a b l e that the e n e r g y - r i c h p r o d u c t of the i n i t i a l r e a c t i o n w i l l c o l l i d e w i t h another energetic m o l e c u l e rather t h a n w i t h a n inert species w h i c h c o u l d relax it to a stable p r o d u c t . W e are a t t e m p t i n g to s t u d y the r e a c t i o n u n d e r c o n d i t i o n s w h i c h are m o r e c o m p a r a b l e w i t h those i n the s o l u t i o n r e a c t i o n , w i t h the h o p e that the k i n e t i c s w i l l b e c o m e s o m e w h a t s i m p l e r . I g n o r i n g the c o m p l e x i t y of the system w e c a n m a k e a preliminary

estimate

(TME- A) We

of

10

8

liters/mole/sec.

for

this

rate

constant

f r o m the i n i t i a l slope of the d e c a y c u r v e .

1

h a v e n o e v i d e n c e for a d i r e c t r e a c t i o n b e t w e e n T M E a n d

T h e effect of T M E o n the steady-state % c o n c e n t r a t i o n is consistent w i t h x

its b o i l i n g point—i.e., it quenches ( o r reacts w i t h ) it w i t h a rate constant s o m e w h a t smaller t h a n 10° liters/mole/sec.

I n the last section w e c o n

c l u d e d that i n o r d e r to m a k e a significant c o n t r i b u t i o n to the reactions of singlet o x y g e n i n systems w h e r e steady-state concentrations of *S a n d A are established, the rate constant f o r the S r e a c t i o n w o u l d h a v e to b e X

1

b e t w e e n 1 0 a n d 1 0 times faster t h a n that f o r A . T h i s c l e a r l y cannot b e 5

8

X

the case f o r the T M E since c o l l i s i o n f r e q u e n c y w o u l d be exceeded.

How

ever, i n "non-steady-state" systems w h e r e the i n i t i a l c o n c e n t r a t i o n of *S is c o m p a r a b l e w i t h A, the d i r e c t i m p o r t a n c e of the f o r m e r species d e p e n d s 1

o n the r a t i o of q u e n c h i n g species to reactive species.

However, 2 1

is

p r o b a b l y r e l a x e d to A , a n d e v e n if it does not react d i r e c t l y c a n i n d i r e c t l y X

l e a d to o x i d a t i o n .

Acknowledgment T h e research for this p a p e r w a s s u p p o r t e d b y the D e f e n c e R e s e a r c h B o a r d of C a n a d a , G r a n t n u m b e r 9530-31 a n d p a r t l y b y the U n i t e d States A i r F o r c e A F O S R , G r a n t n u m b e r 158-65.


142

OXIDATION OF ORGANIC COMPOUNDS III


Literature Cited

(1) Arnold, S. J., Browne, R. J., Ogryzlo, E. A., Photochem. Photobiol. 4, 963 (1965). (2) Arnold, S. T., Finlayson, N., Ogryzlo, E. A , J. Chem. Phys. 44, 2529 (1966). (3) Arnold, S. T., Ogryzlo, E. A., Can. J. Phys. 45, 2053 (1967). (4) Bader, L. W., Ogryzlo, E. A., Discussions Faraday Soc. 37, 46 (1964). (5) Bayes, K. D., Winer, A. M., J. Phys. Chem. 70, 302 (1966). (6) Browne, R. J., Ogryzlo, E. A., Can. J. Chem. 43, 2915 (1965). (7) Ogryzlo, E. A., Pearson, A. E., J. Phys. Chem. 72, 2913 (1968). (8) Young, R. A., Black, G., J. Chem. Phys. 42, 3740 (1965). RECEIVED

October 9,

1967.


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