Kinetics and Mechanisms of Reactions of CF, CHF, and CF2 Radicals

5 Kinetics and Mechanisms of Reactions of C F ,CHF,and C F Radicals 2

Downloaded by CORNELL UNIV on September 28, 2014 | http://pubs.acs.org Publication Date: June 1, 1978 | doi: 10.1021/bk-1978-0066.ch005

DAVID S. Y. HSU,* M. E. UMSTEAD and M. C. LIN Chemistry Division, Naval Research Laboratory, Washington, DC 20375

This chapter reviews briefly methods for the production of CF, CHF and CF , and in more detail, the reactions of these in teresting and important radicals. Although a considerable, but not extensive, amount of work has been done on the reactions of CF , l i t t l e of the chemistry of CF and CHF is known. This chap ter also includes the preliminary results of some experiments carried out in this Laboratory on the dynamics of some of the reactions involving these radicals. These results were largely arrived at through investigations of the degree of vibrational excitation of the HF and CO reaction products, determined by HF and CO laser emission and CO laser resonance absorption measure ments. The coverage of this review is restricted to the gas phase chemistry of these radicals, and does not include their addition reactions to olefins. 2

2

I.

CF Radical Reactions The presence of the CF radical often has been observed spectroscopically in the dissociation of fluorocarbons in elec tric discharges and in the reactions of F atoms with organic compounds (1-6). Jacox and Milligan (7) produced the radical by the vacuum-ultraviolet (VUV) photolysis of CH F, and were able to stabilize it in a nitrogen or argon matrix at 14°Κ in sufficient concentration to obtain its infrared spectrum. Simons and Yarwood (8,9) reported that the flash photolysis of CHFBr and CFBr in thin-walled quartz tubes (λ ≥160 nm) produced the CF radical. The presence of CF was detected by the transient appearances in absorption of the χ π-->Α Σ transition. They reported that the production of CF was greater from CHFBr2 than from CFBr . In a similar experiment with the former com3

2

3

2

2

3

*NRC/NRL Postdoctoral Research Associate © 0-8412-0339-7/78/47-066-128$10.00/0

In Fluorine-Containing Free Radicals; Root, J.; ACS Symposium Series; American Chemical Society: Washington, DC, 1978.

Hsu E T A L .

5.

Reactions of CF, CHF, and CF

2

Radicals

129

pound c a r r i e d o u t i n a q u a r t z t u b e by M e r e r and T r a v i s ( 1 0 ) , h o w e v e r , CF was n o t d e t e c t e d . A d d i t i o n a l l y , we f o u n d no s i g n i f i c a n t d i f f e r e n c e between the v i b r a t i o n a l p o p u l a t i o n o f the CO formed i n t h e f l a s h p h o t o l y s i s o f a C H F B r - S 0 m i x t u r e i n t h e UV and V U V above 165 nm. The CO formed i n t h e C H F B r - S 0 sys tem was f o u n d t o be c o n s i d e r a b l y c o l d e r t h a n t h a t formed i n t h e C F B r - S0 system e i t h e r i n a S u p r a s i l or i n a q u a r t z tube (see b e l o w ) . V e r y l i t t l e h a s b e e n r e p o r t e d o f t h e c h e m i s t r y o f t h e CF radical. I n a s h o c k t u b e s t u d y , M o d i c a and S i l l e r s (11) m e a s ured the r a t e of the r e a c t i o n 2

2

2


3

2

2

CF + F + M -

CF + M

(1)

2

26 2.8 5 2 —2 "*1 and r e p o r t e d t h a t ^ = 6 . 5 7 χ 10 Τ * m l mole sec when M = A r . S c h a t z and Kaufman ( 1 2 ) , i n a s t u d y o f c h e m i l u m i n e s c e n c e e x c i t e d by a t o m i c f l u o r i n e , o b s e r v e d t h e e m i s s i o n o f bands o f t h e Cameron s y s t e m , a TT -» χ ^ , o f CO up t o ν = 5 . They s p e c u l a t e d t h a t these m i g h t a r i s e from the f o l l o w i n g r e a c t i o n : 3

1

0 + CF -* CO + F

(2)

On t h e b a s i s o f t h e known h e a t o f f o r m a t i o n f o r t h e CF r a d i c a l , ΔΗ/° = 59 ± 2 k c a l / m o l e ( 1 3 - 1 6 ) , t h e e x o t h e r m i c i t y o f r e a c t i o n (2) i s 126 ± 3 k c a l / m o l e , w h i c h i s 36 k c a l / m o l e b e l o w CO (a TT, ν = 5 ) . R e a c t i o n (2) i s b e l i e v e d t o be r e s p o n s i b l e f o r the CO l a s e r e m i s s i o n observed i n the f l a s h p h o t o l y s i s o f a C F B r - S0 m i x t u r e above 165 nm ( 1 7 ) . O v e r 4 0 v i b r a t i o n - r o t a t i o n l i n e s between 2 -* 1 and 14 -» 13 h a v e b e e n i d e n t i f i e d . The l a s e r o u t p u t o f t h i s s y s t e m i n c r e a s e s v e r y r a p i d l y w i t h t h e S 0 / C F B r r a t i o and reaches a peak v a l u e at S 0 / C F B r = 2, s i m i l a r to t h a t observed i n t h e a n a l o g o u s S 0 - C H B r CO l a s e r s y s t e m ( 1 8 ) . H o w e v e r , u n l i k e t h e l a t t e r s y s t e m , t h e power o u t p u t o f t h e f o r m e r d o e s n o t d e c r e a s e s i g n i f i c a n t l y w i t h S 0 / C F B r r a t i o s as h i g h as 2 2 , i n d i c a t i n g t h a t t h e CF + S 0 r e a c t i o n i s n o t as r a p i d . Addition a l l y , t h e l a s e r o u t p u t o f t h e C F B r system i s c o n s i d e r a b l y weaker than t h a t o f the CHBr system, although the e n e r g e t i c s o f the two d i f f e r o n l y s l i g h t l y . I n o r d e r t o u n d e r s t a n d t h e d y n a m i c s o f t h e 0 + CF r e a c t i o n o c c u r r i n g i n t h e S 0 - C F B r c h e m i c a l CO l a s e r s y s t e m , we c a r r i e d o u t CO l a s e r r e s o n a n c e a b s o r p t i o n e x p e r i m e n t s t o measure t h e v i b r a t i o n a l e n e r g y d i s t r i b u t i o n o f t h e CO formed i n t h e reaction. A d e t a i l e d d e s c r i p t i o n o f t h e f l a s h - p h o t o l y t i c CO l a s e r - p r o b i n g s y s t e m c a n be f o u n d i n r e f e r e n c e ( 1 9 ) . Experi ments were c a r r i e d o u t u s i n g b o t h S u p r a s i l (λ ^ 165 nm) and q u a r t z ( λ ^ 200 nm) f l a s h t u b e s f o r m i x t u r e s o f C F B r and S 0 w i t h He as a d i l u e n t . W i t h t h e S u p r a s i l t u b e , 1 0 - t o r r s a m p l e s o f a 1 : 1 : 9 8 / C F B r : S 0 : H e m i x t u r e were u s e d , w h e r e a s w i t h t h e q u a r t z t u b e 5 - t o r r s a m p l e s o f b o t h t h e 1 : 1 : 9 8 and 1 : 1 : 4 8 m i x t u r e s 2 9 8

3

3

2

2

2

2

3

3

3

2

3

2

3

3

2

3

3

3

2


2

130

FLUORINE-CONTAINING FREE RADICALS

were

used. The i n i t i a l r e l a t i v e v i b r a t i o n a l p o p u l a t i o n s were o b t a i n e d f r o m t h e a b s o r p t i o n t r a c e s u s i n g an e x t r a p o l a t i o n method d e scribed i n reference (20). The r e s u l t s a r e shown i n F i g . 1. I n these e x p e r i m e n t s , the appearance time o f the s t r o n g e s t ab s o r p t i o n o c c u r r e d at about 6 μβεο. Flash p r o f i l e s taken w i t h a f a s t p h o t o d i o d e show t h a t t h e f l a s h i s o v e r w i t h i n 5 μ β β ο . The s e c o n d a r y v i b r a t i o n a l e x c i t a t i o n o f t h e CO p r o d u c t t h a t m i g h t r e s u l t f r o m t h e Χ ΊΓ" — a TT a b s o r p t i o n w o u l d t h e r e f o r e n o t o c c u r t o any s i g n i f i c a n t e x t e n t . T h i s i s s u p p o r t e d by t h e f a c t t h a t t h e r e s u l t s o b t a i n e d f r o m b o t h t h e q u a r t z and S u p r a s i l t u b e s agree c l o s e l y . The v i b r a t i o n a l p o p u l a t i o n o f t h e CO d e c r e a s e s m o n o t o n i c a l l y w i t h a s l i g h t b u t d i s t i n c t d r o p i n p o p u l a t i o n n e a r ν = 1 2 , as t h e v i b r a t i o n a l e n e r g y o f t h e CO i n c r e a s e s . T h i s i s i n sharp c o n t r a s t t o t h e b e l l - s h a p e d d i s t r i b u t i o n o b s e r v e d i n t h e 0 + CS r e a c t i o n ( 2 3 - 2 5 ) , w h i c h c o u l d be a c c o u n t e d f o r by an i m p u l s i v e m o d e l (26) i n d i c a t i n g t h e p o s s i b i l i t y o f t h e a b s e n c e o f a s i g n i f i c a n t w e l l i n t h e t r i p l e t OCS i n t e r m e d i a t e . I n t h e 0 + CF r e a c t i o n , h o w e v e r , t h e o b s e r v e d CO p o p u l a t i o n was f o u n d t o l i e c l o s e t o t h a t p r e d i c t e d by a s i m p l e s t a t i s t i c a l m o d e l ( 2 0 ) , t a k i n g t h e t o t a l a v a i l a b l e r e a c t i o n e n e r g y , Ε t o t — 126 + 2 . 5 R T ~ 128 k c a l / m o l e . On t h e b a s i s o f t h i s m o d e l , t h e r e l a t i v e v i b r a t i o n a l p o p u l a t i o n o f CO c a n be e s t i m a t e d f r o m t h e f o l l o w i n g simple expression (20): 3


ι

N —

2p(E

V

- Ev)

t o t

=

(I)

No

2P(E t> t o

where E and N a r e t h e v i b r a t i o n a l e n e r g y and p o p u l a t i o n o f CO a t t h e v l e v e l , and Σ ρ ( Ε ) , t h e t o t a l e n e r g y l e v e l sum o f t h e FCO i n t e r m e d i a t e w i t h b o t h CO and CF s t r e t c h e s e x c l u d e d . The CO s t r e t c h c o r r e s p o n d s t o t h e mode o f v i b r a t i o n l e a d i n g t o t h e p r o d u c t CO v i b r a t i o n a l e x c i t a t i o n , and t h e CF s t r e t c h corresponds to the r e a c t i o n c o o r d i n a t e . E q u a t i o n ( I ) c a n be e v a l u a t e d by e i t h e r t h e d i r e c t - c o u n t method o r t h e a p p r o x i m a t i o n o f W h i t t e n and R a b i n o v i t c h ( 2 7 ) : v

v

t

n

- s* E = [l

- E

V

t

o

t

/(E

+ *E T

O

T

+

(ID

/

Z

I s*

aE )] z

{

where E i s t h e z e r o p o i n t e n e r g y and s i i s t h e e f f e c t i v e number o f v i b r a t i o n s ( s i = 3 N - 7 f o r a l i n e a r c o m p l e x , and 3 N - 8 o t h e r w i s e ) , and " a " i s a c o r r e c t i o n f a c t o r w h i c h h a s a v a l u e b e t w e e n 0 and 1 , d e p e n d i n g on t h e amount o f e n e r g y , E t o t " E . For s i m p l e , v e r y e x o t h e r m i c r e a c t i o n s , s u c h as 0 + C F , E t o t z> and t h e c l a s s i c a l e x p r e s s i o n ( 2 0 ) : z

v

> > a E



5.

HSU E T A L .

Reactions of CF, CHF, and CF Radicals 2

Figure 1. Vibrational distribution of CO formed in the flashed-initiated CFBr -S0 system. Open circles = l:l:98/CFBr :S0 :He mixture in a quartz tube, filled circles = the same mixture in a Suprasil tube; solid and broken curves = statistical distributions based on Equations (II) and (III), respectively. s

s

2

2


131

132


N /N


v

0

= (1 - E / E v

t

o

t

) * s S

(1 -

jf ) * v

(III)

s

i s adequate. T h i s i s shown by t h e r e s u l t s g i v e n i n F i g , 1 f o r t h e 0 + CF r e a c t i o n . I n t h i s c a l c u l a t i o n , s i = 2 was u s e d f o r E q s . ( Ι ) - ( Π Ε ) , assuming a l i n e a r F CO d i s s o c i a t i o n c o m p l e x . The d i r e c t c o u n t method b a s e d on E q . ( I ) p r e d i c t e d e x a c t l y t h e same d i s t r i b u t i o n . The f a c t t h a t t h e o b s e r v e d CO v i b r a t i o n a l d i s t r i b u t i o n i s n e a r l y s t a t i s t i c a l , w i t h p a r t i a l i n v e r s i o n between a l l l e v e l s , i s n o t i n c o n s i s t e n t w i t h t h e p r e s e n c e o f a r a t h e r deep w e l l . The d e p t h o f t h i s w e l l i s a b o u t 30 k c a l / m o l e b a s e d on 4 H / ° ( F C 0 ) — -34 k c a l / m o l e (21) and a b o u t 60 k c a l / m o l e a c c o r d i n g t o t h e v a l u e ΔΗ/° (FCO) Si - 6 6 k c a l / m o l e ( 2 2 ) . The l a t t e r seems t o be more i n l i n e w i t h t h e o b s e r v e d CO d i s t r i b u t i o n . The e f f e c t o f t h e s t a b i l i t y o f t h e FCO r a d i c a l on t h e p r o d u c t CO v i b r a t i o n a l e n e r g y d i s t r i b u t i o n w i l l be e x a m i n e d i n t h e f u t u r e . The c a u s e o f t h e s l i g h t d e c r e a s e i n p o p u l a t i o n n e a r ν = 12 i s not c l e a r . The e x o t h e r m i c i t y o f r e a c t i o n (2) i s i n s u f f i c i e n t t o e x c i t e e i t h e r CO o r F t o h i g h e r e l e c t r o n i c s t a t e s , and t h e s l i g h t e n e r g y d i f f e r e n c e between P ^ and P ^ o f t h e g r o u n d e l e c t r o n i c s t a t e o f t h e F atom i s n o t e x p e c t e d t o a f f e c t dynam i c a l l y t h e p r o d u c t i o n o f CO f r o m t h e r e a c t i o n w h i c h p r o c e e d s through a complex. I t i s thus q u i t e l i k e l y t h a t the appearance o f c o l d e r CO p o p u l a t i o n s a t l o w e r l e v e l s may a r i s e f r o m a s i d e r e a c t i o n t h a t g e n e r a t e s v i b r a t i o n a l l y c o l d e r CO m o l e c u l e s . The most l i k e l y c a n d i d a t e f o r t h i s i s 0 + C F B r - CO + B r F ( o r F + B r ) , due t o t h e i n c o m p l e t e p h o t o d e t a c h m e n t o f t h e B r atoms f r o m CFBr . The e n e r g e t i c s o f t h i s r e a c t i o n a r e n o t k n o w n , b u t a r e p r o b a b l y much h i g h e r t h a n t h o s e o f t h e a n a l o g o u s r e a c t i o n , 0 + C F , t o be d i s c u s s e d l a t e r . Another p o s s i b l e r e a c t i o n , 0 + CFBr , p r o b a b l y d o e s n o t h a v e enough e n e r g y t o p r o d u c e v i b r a t i o n a l l y e x c i t e d CO. The o v e r l a p p i n g o f a v i b r a t i o n a l l y c o l d e r p o p u l a t i o n f r o m 0 + CFBr w i t h t h a t f r o m r e a c t i o n ( 2 ) , a h o t t e r , n e a r s t a t i s t i c a l d i s t r i b u t i o n , c o u l d account for the d e v i a t i o n oc c u r r i n g at the lower l e v e l s of the observed d i s t r i b u t i o n . The a b s e n c e o f a c o m p l e t e p o p u l a t i o n i n v e r s i o n i n t h e f l a s h - i n i t i a t e d C F B r - S 0 s y s t e m a c c o u n t s f o r t h e weak CO l a s e r i n t e n s i t y , compared w i t h t h a t d e t e c t e d i n t h e C H B r - S 0 flash (18). I n t h e C H B r - S 0 s y s t e m , t h e p r i m a r y pumping r e a c t i o n was assumed t o be 0 + C H , w h i c h p r o b a b l y p r o c e e d s t h r o u g h t h e HCO c o m p l e x w i t h a s h a l l o w e r d e p t h o f a b o u t 20 k c a l / mole. The d y n a m i c s o f t h i s h i g h l y e x o t h e r m i c r e a c t i o n (ΔΗ° = - 1 7 6 k c a l / m o l e ) a r e now u n d e r i n v e s t i g a t i o n . 3

2

2

3

2

3

3

II.

CHF R a d i c a l

2

2

Reactions

L i t t l e i s known o f t h e c h e m i s t r y o f t h e CHF r a d i c a l , l a r g e l y b e c a u s e o f t h e l a c k o f c l e a n methods f o r i t s p r o d u c t i o n . M e r e r a n d T r a v i s ( 1 0 ) o b t a i n e d t h e a b s o r p t i o n s p e c t r u m o f CHF, p r o d u c e d by t h e f l a s h p h o t o l y s i s o f C H F B r i n a q u a r t z v e s s e l , 2


5.

HSU E T A L .


2

Radicals

133

and made a r o t a t i o n a l a n a l y s i s o f t h e b a n d s . The s p e c t r u m c o n s i s t e d o f a s i n g l e p r o g r e s s i o n o f c o m p l e x bands i n t h e r e g i o n 430 - 600 nm. They c o n c l u d e d t h a t t h e g r o u n d s t a t e o f CHF was a s i n g l e t , s i m i l a r to that of CHCl. A l t h o u g h Simons and Yarwood ( 8 , j 9 ) r e p o r t e d t h e p r o d u c t i o n o f CF f r o m t h e f l a s h p h o t o l y s i s o f C H F B r b a s e d upon i t s a b s o r p t i o n s p e c t r u m , M e r e r and T r a v i s d i d not observe i t s presence. Other u n i d e n t i f i e d absorption bands were s e e n , b u t none o f t h e s e w e r e a t t r i b u t e d t o C F . Simons and Yarwood h a d no q u a n t i t a t i v e d a t a f o r CF p r o d u c t i o n , and s i n c e t h e a b s o r p t i o n s p e c t r u m o f CHF was n o t known a t t h a t t i m e , i t s p r e s e n c e w o u l d n o t h a v e been o b s e r v e d ( 2 8 ) . It is l i k e l y t h a t CF i s o n l y a m i n o r p r o d u c t o f t h i s d i s s o c i a t i o n . B o t h t h e v i s i b l e - U V and t h e I R a b s o r p t i o n s p e c t r a o f CHF ( a s w e l l as CF a n d C H F ) w e r e o b t a i n e d from t h e VUV p h o t o l y s i s o f C H F i n a n a r g o n o r n i t r o g e n m a t r i x a t 14°Κ by J a c o x and M i l l i g a n (7). They a l s o f o u n d t h a t t h e r e a c t i o n o f c a r b o n a t o m s , p r o d u c e d by t h e p h o t o l y s i s o f cyanogen a z i d e i s o l a t e d i n an a r g o n m a t r i x , w i t h HF t r a p p e d i n t h e m a t r i x , l e d t o t h e f o r m a t i o n o f s u f f i c i e n t amounts o f CHF f o r I R s p e c t r o s c o p i c a n a l y s i s . T h e i r r e s u l t s c o n f i r m e d M e r e r and T r a v i s f i n d i n g t h a t t h e g r o u n d e l e c t r o n i c s t a t e o f t h e CHF r a d i c a l i s s i n g l e t . Tang and R o w l a n d p r o d u c e d CTF r a d i c a l s b y t h e r e a c t i o n o f e n e r g e t i c t r i t i u m atoms f r o m n u c l e a r r e c o i l w i t h f l u o r i n a t e d compounds s u c h as C H F C 1 , C H F , and C H F ( 2 9 , 3 0 ) , and i n v e s t i g a t e d t h e r e a c t i o n o f CTF f r o m C H F w i t h o l e f i n s i n t h e gas phase ( 3 0 ) :


2

2

3

1

3

3

2

2

T* + C H F

2

CHTF * -

CTF + HF

2

2

CTF + C H 2

4

-

2

2

CHTF * + Η 2

- 0 < TF*

The r e a c t i o n o f CTF w i t h o l e f i n s was f o u n d t o be c o m p l e t e l y s t e r e o s p e c i f i c ; the a l k y l groups i n the fluorocyclopropane p r o d u c t s m a i n t a i n e d t h e same o r i e n t a t i o n s t o e a c h o t h e r a s were present i n the r e a c t i n g o l e f i n s . I n a l l c a s e s where t h e f o r m a t i o n o f i s o m e r s was p o s s i b l e b y t h e s i n g l e - s t e p a d d i t i o n o f C T F , t h e s y n and a n t i i s o m e r s w e r e f o u n d i n e q u a l q u a n t i t i e s . This complete s t e r e o s p e c i f i c i t y , along w i t h the i n s e n s i t i v i t y o f t h e p r e s e n c e o f 0 and a l a r g e amount o f i n e r t gas on t h e r e a c t i o n , was t a k e n as e v i d e n c e t h a t t h e r e a c t i n g s p e c i e s was s i n g l e t C T F . From an e x p e r i m e n t i n w h i c h CTF r e a c t e d w i t h C H i n t h e p r e s e n c e o f e x c e s s 0 , i t was f o u n d t h a t t h e r e a c t i v i t y o f 0 w i t h CTF i s < 0 . 2 t i m e s t h a t o f C H . R o w l a n d e t a l . ( 3 1 , 3 2 ) a l s o p r e p a r e d CH F , a l o n g w i t h CF F , by t h e r e a c t i o n o f e n e r g e t i c F atoms f r o m n u c l e a r r e c o i l w i t h C H F , and s t u d i e d i t s r e a c t i o n w i t h h y d r o g e n h a l i d e s ( 3 1 ) . The f o r m a t i o n o f t h e c a r b e n e s p r o c e e d e d v i a a m e c h a n i s m s i m i l a r to the aforementioned T-atom r e a c t i o n . The r e a c t i o n o f CH F 2

2

2

2

2

l

4

i

g

g

2

2


4

134


w i t h HX was f o u n d

t o t a k e p l a c e by a d i r e c t C H F + HX -

CH

1 8

2

1 8

i n s e r t i o n mechanism:

FX

(3)

E v i d e n c e f o r t h e d i r e c t i n s e r t i o n was l a r g e l y b a s e d on t h e f a c t t h a t the y i e l d o f CH FC1 was i n d e p e n d e n t o f t h e p r e s e n c e o f 0 , w h i l e t h e y i e l d s o f p r o d u c t s a r i s i n g f r o m m o n o r a d i c a l s were s t r o n g l y i n f l u e n c e d by i t s p r e s e n c e . I t was e s t i m a t e d t h a t t h e e x o t h e r m i c i t y o f r e a c t i o n (3) i s a b o u t 74 k c a l / m o l e when X = I , and a b o u t 69 k c a l / m o l e when X = CI. These v a l u e s were b a s e d upon t h e e s t i m a t i o n o f àEf° for CHF, w h i c h i s n o t a v a i l a b l e , b y t a k i n g t h e a r i t h m e t i c mean o f t h e h e a t s o f f o r m a t i o n o f C H and C F : 25 k c a l / m o l e . This value i s p r o b a b l y t o o l o w , a s w i l l be d i s c u s s e d l a t e r . A comparison o f t h e s e e x c i t a t i o n e n e r g i e s w i t h t h e assumed C - I and C - C l bond e n e r g i e s i n d i c a t e d t h a t C H F C 1 s h o u l d be c o m p l e t e l y s t a b l e a g a i n s t C - C l bond b r e a k a g e , b u t t h a t t h e e x c i t a t i o n e n e r g y o f C H F I was a b o u t 19 k c a l / m o l e i n e x c e s s o f t h e a c t i v a t i o n e n e r g y f o r C - I bond d i s s o c i a t i o n . T h i s was found t o be c o n s i s t e n t w i t h t h e e x p e r i m e n t a l r e s u l t s b a s e d on t h e p r o d u c t i o n o f C H F (31). The r e a c t i o n o f t h e CHF r a d i c a l w i t h 0( P) ( 3 3 ) , 0 (34), and NO (35) h a s b e e n shown t o g e n e r a t e s t i m u l a t e d HF e m i s s i o n s . I n t h e s e s t u d i e s , t h e CHF r a d i c a l was p r o d u c e d by t h e s u c c e s s i v e p h o t o d e t a c h m e n t o f C I atoms f r o m CHFC1 i n a S u p r a s i l t u b e (33) o r B r atoms f r o m C H F B r i n a q u a r t z t u b e ( 1 0 ) : 2


2

2

2

2

2

3

3

2

2

2

CHFC1

2

+ h u ( X :> 165 ran)—* CHF + 2C1

CHFBr

2

+ h u ( X :> 200 n m ) - ~ CHF + 2 B r

The v i b r a t i o n a l l y e x c i t e d HF m o l e c u l e i s b e l i e v e d t o be formed p r i m a r i l y by t h e f o l l o w i n g f o u r - c e n t e r e d e l i m i n a t i o n p r o c e s s e s involving chemically activated intermediates; 0( P) 3

+ CHF -

HFCO* -

ΔΗ ° as - 1 9 0 2

-

HFC0Ô -

-

HF+ + C 0

ΔΗ ° 5

CHF + NO -

(4)

kcal/mole

4

CHF + 0

H F t + C0+

FCOOH

(5)

2

-198

kcal/mole

HFCNO -

HF+ + CNO

(6)

ΔΗ ° = ? 6

where " t s t a n d s f o r v i b r a t i o n a l and for electronic e x c i t a tion. N e i t h e r t h e k i n e t i c s n o r t h e mechanisms o f t h e s e r e a c t i o n s are known. The a s s u m p t i o n t h a t t h e s e r e a c t i o n s t a k e p l a c e v i a l o n g - l i v e d i n t e r m e d i a t e s was b a s e d s o l e l y on t h e o b s e r v e d HF f l

f H H f


H s u ET AL.

5.

Reactions

of CF,

CHF,

and CF

Radicals

2

135

a n d / o r CO v i b r a t i o n a l e n e r g y d i s t r i b u t i o n s , w i t h t h e a i d o f p r o d u c t gas a n a l y s i s . The e x o t h e r m i c i t y o f r e a c t i o n (6) i s n o t a v a i l a b l e due t o l a c k o f i n f o r m a t i o n on t h e CNO r a d i c a l . Ac c o r d i n g t o t h e o b s e r v e d HF l a s e r i n t e n s i t y and t h e v i b r a t i o n a l p o p u l a t i o n o f HF ( s e e F i g . 2 ) , Δ Η ° i s p r o b a b l y v e r y c l o s e t o Δ Η ° , w h i c h was e s t i m a t e d t o be a b o u t - 1 9 8 k c a l / m o l e . The e x o t h e r m i c i t i e s o f r e a c t i o n s (4) and (5) were c a l c u l a t e d u s i n g AH/°(CHF) = 39 ± 4 k c a l / m o l e , e s t i m a t e d f r o m t h e t h e o r e t i c a l bond d i s s o c i a t i o n e n e r g i e s , D e ( C H - F ) = 120 and D e ( C F - H ) = 83 k c a l / m o l e b y S t a e m m l e r ( 3 6 ) . These v a l u e s a r e p r o b a b l y r e l i a b l e because a s i m i l a r e s t i m a t e w i t h Staemmler s c a l c u l a t e d v a l u e f o r D e ( C F - F ) gave r i s e t o AH/°(CF) = 61 k c a l / m o l e , w h i c h a g r e e s c l o s e l y w i t h t h e e x p e r i m e n t a l v a l u e , 59 ± 2 k c a l / m o l e g i v e n e a r l i e r . A l o w e r v a l u e , Δ Η ^ ° ( C H F ) — 25 k c a l / m o l e , e v a l u a t e d by t a k i n g t h e s i m p l e a r i t h m e t i c mean o f t h e h e a t s o f f o r m a t i o n o f ground s t a t e CH and C F ( 3 1 ) , i s q u e s t i o n a b l e inasmuch as t h e g r o u n d e l e c t r o n i c s t a t e o f C H i s t r i p l e t , w h e r e a s CHF and CF are both s i n g l e t . The u s e o f t h e s i n g l e t e n e r g y , w h i c h h a s r e c e n t l y b e e n e s t a b l i s h e d t o be 8 ± 1 k c a l / m o l e h i g h e r ( 3 7 ) , w o u l d i n c r e a s e t h e v a l u e o f AH/°(CHF) by a b o u t 4 k c a l / m o l e . It s h o u l d be p o i n t e d o u t t h a t t h e u s e o f e i t h e r v a l u e , Δ Η / ° ( C H F ) = 25 o r 39 k c a l / m o l e , v a r i e s t h e o v e r a l l e x o t h e r m i c i t i e s o f r e a c t i o n s ( 4 ) and ( 5 ) by l e s s t h a n 10%, w h i c h does n o t a f f e c t o u r c o n c l u s i o n on the dynamics o f these v e r y e x o t h e r m i c r e a c t i o n s . 6

5


1

2

2

2

2

The HF v i b r a t i o n a l e n e r g y d i s t r i b u t i o n was m e a s u r e d b y means o f t h e a p p e a r a n c e t i m e o f e a c h v i b r a t i o n a l - r o t a t i o n a l laser line selectively oscillating within a cavity consisting o f a g r a t i n g and a h i g h l y r e f l e c t i v e concave m i r r o r ( 3 8 , 3 9 ) . Assuming t h a t ν - ν r e l a x a t i o n b e f o r e the appearance o f l a s e r pulses i s i n s i g n i f i c a n t , the t h r e s h o l d times o f v a r i o u s l a s e r l i n e s c a n be c o r r e l a t e d w i t h t h e g a i n s a n d t h u s t h e r e l a t i v e v i b r a t i o n a l population of the l e v e l s involved (39,40). The m e a s u r e d HF v i b r a t i o n a l p o p u l a t i o n s f o r r e a c t i o n s ( 4 ) - ( 6 ) a r e p r e s e n t e d i n F i g . 2 , e m p l o y i n g C H F B r as t h e CHF r a d i c a l s o u r c e . F o r r e a c t i o n s ( 4 ) and ( 5 ) , whose e x o t h e r m i c i t i e s c o u l d be e s t i m a t e d , s t a t i s t i c a l m o d e l s b a s e d on E q s . ( I I ) and ( I I I ) p r e d i c t HF v i b r a t i o n a l p o p u l a t i o n d i s t r i b u t i o n s t h a t a g r e e c l o s e l y w i t h the experimental ones. T h i s r e s u l t s t r o n g l y supports the p r o p o s e d r e a c t i o n mechanisms i n v o l v i n g l o n g - l i v e d i n t e r m e d i a t e s . I t s h o u l d be e m p h a s i z e d t h a t t h e r e s u l t s o f t h e s e s i m p l e c a l c u l a t i o n s are not s e n s i t i v e to the s t r u c t u r e or v i b r a t i o n a l f r e q u e n c i e s o f t h e i n t e r m e d i a t e assumed b e c a u s e t h e e x o t h e r m i c i t i e s o f t h e s e r e a c t i o n s a r e so l a r g e t h a t E ^ o t aEz* This i s i n d i c a t e d by t h e c l o s e agreement b e t w e e n t h e v a l u e p r e d i c t e d b y E q . ( I I ) and t h a t by E q . ( I l l ) . The number o f a c t i v e v i b r a t i o n s f o r t h e s e n o n - l i n e a r i n t e r m e d i a t e s was t a k e n t o be 3N - 8, w i t h one d e l e t e d f o r t h e r e a c t i o n c o o r d i n a t e and t h e o t h e r f o r t h e HF p r o d u c t v i b r a t i o n a l e x c i t a t i o n . On t h e b a s i s o f t h e s e s t a t i s t i c a l c a l c u l a t i o n s , t h e HF m o l e c u l e c a r r i e s v i b r a t i o n a l l y a b o u t 18% o f t h e e n e r g y a v a i l a b l e i n r e a c t i o n (4) a n d 10% i n 2

> : >


136



Figure 2.

Vibrational distributions of the HF served in reactions (4), (5), and (6)

ob-


H s u ET AL.

5.


2

Radicals

137

reaction (5). These a r e , o f c o u r s e , much l e s s t h a n t h e v a l u e s (>60%) o b s e r v e d i n s i m p l e a b s t r a c t i o n r e a c t i o n s s u c h as F + C H (41). A complex-forming r e a c t i o n i s therefore a l e s s e f f i c i e n t laser-pumping reaction (42-45). I n a d d i t i o n t o t h e m a j o r p r i m a r y p r o d u c t H F , s m a l l amounts o f F atoms were a l s o b e l i e v e d t o be p r o d u c e d f r o m r e a c t i o n s ( 4 ) (6) ( 3 4 , 3 5 , 4 6 ) . T h i s was s u g g e s t e d b y t h e o b s e r v a t i o n o f weak DF s t i m u l a t e d e m i s s i o n s when D was added t o t h e s e s y s t e m s . The p r o d u c t i o n o f F atoms f r o m t h e s e r e a c t i o n s i s n o t u n e x p e c t e d i n view o f the l a r g e excess o f v i b r a t i o n a l e n e r g i e s c a r r i e d by these intermediates. I n r e a c t i o n ( 4 ) , f o r e x a m p l e , t h e HFCO i n t e r m e d i a t e p o s s e s s e s as much as 185 k c a l / m o l e o f i n t e r n a l e n e r g y , w h i c h i s more t h a n s u f f i c i e n t t o d i s s o c i a t e e i t h e r t h e CH (~90 k c a l / m o l e ) o r t h e C - F (~120 k c a l / m o l e ) b o n d . On a c c o u n t o f t h e w e a k n e s s o f t h e DF e m i s s i o n s and t h e o n l y s l i g h t a t t e n u a t i o n o f HF e m i s s i o n s when D was added ( 4 6 ) , t h e c o n t r i b u t i o n o f t h e s e c o n d a r y , b u t r a t h e r e f f e c t i v e r e a c t i o n , F + RH -* HF^ + R (R = C F C 1 o r C F B r ) , t o t h e HF e m i s s i o n i s t h e r e f o r e n o t s i g n i f i c a n t . T h i s i s m a n i f e s t e d by the observed n e a r - s t a t i s t i c a l v i b r a t i o n a l d i s t r i b u t i o n s shown i n F i g . 2 . 4


2

2

2

2

The v i b r a t i o n a l e n e r g y c o n t e n t o f t h e CO formed i n r e a c t i o n ( 4 ) h a s a l s o been m e a s u r e d w i t h t h e CO l a s e r r e s o n a n c e a b s o r p t i o n method d i s c u s s e d e a r l i e r . The r e s u l t s o b t a i n e d f r o m p h o t o l y z i n g 10 t o r r o f 1 : 1 : 9 8 / S 0 : C H F B r : H e and 1 : 1 : 2 0 : 7 8 / S 0 : C H F B r : H : H e m i x t u r e s i n a S u p r a s i l t u b e a r e shown i n F i g . 3 . I n the s e c o n d m i x t u r e , H was added t o t e s t t h e p o s s i b i l i t y o f t h e p r o d u c t i o n o f C0(a π) from r e a c t i o n ( 4 ) . H h a s b e e n shown t o r e l a x C0(a TT, v=0) v e r y e f f e c t i v e l y ( 4 7 ) . I f C0(a TT) i s p r o d u c e d i n i t i a l l y i n s i g n i f i c a n t amount, t h e a d d i t i o n o f H s h o u l d be e x p e c t e d t o change t h e p o p u l a t i o n o f t h e i n i t i a l g r o u n d e l e c t r o n i c s t a t e CO n o t i c e a b l y . The r e s u l t s p r e s e n t e d i n F i g . 3 , h o w e v e r , do n o t show s u c h an e f f e c t ; t h i s r u l e s o u t t h e p o s s i b i l i t y t h a t C 0 ( a TT) i s formed t o any s i g n i f i c a n t e x t e n t i n r e a c t i o n ( 4 ) . The CO formed i n t h e f l a s h - p h o t o l y s i s o f b o t h m i x t u r e s men t i o n e d above was f o u n d t o be v i b r a t i o n a l l y e x c i t e d up t o ν = 2 4 , w h i c h c o r r e s p o n d s t o a b o u t 127 k c a l / m o l e o f v i b r a t i o n a l e n e r g y . A s i m i l a r s t a t i s t i c a l c a l c u l a t i o n b a s e d o n E q . ( I I ) shows t h a t the observed v i b r a t i o n a l p o p u l a t i o n d i s t r i b u t i o n i s c o n s i d e r a b l y colder than the s t a t i s t i c a l . A l t h o u g h t h i s seems t o be i n c o n s i s t e n t w i t h t h e o b s e r v e d n e a r s t a t i s t i c a l HF v i b r a t i o n a l p o p u l a t i o n shown i n F i g . 2 , i t i s , h o w e v e r , n o t u n e x p e c t e d i n v i e w of the presence o f the other r e a c t i o n channel l e a d i n g to the p r o d u c t i o n o f F atoms d i s c u s s e d p r e v i o u s l y . S i n c e the ground e l e c t r o n i c s t a t e o f t h e CHF r a d i c a l i s s i n g l e t , t h e s p i n - c o n s e r v a t i o n r u l e ( i f i t i s v a l i d h e r e ) s u g g e s t s t h e f o l l o w i n g two p o s s i b l e r e a c t i o n p a t h s f o r t h e 0( P) + CHF r e a c t i o n : 2

2

2

2

2

2

3

3

2


2



Figure 3. Vibrational distribution of CO produced from Reaction 4 . Open circles = 1:1:98/CHFBr : S0 :He in a Suprasil tube, filled circles = 1:1:20:78/ CHFBr :S0 :H :He flashed in the same tube; solid and broken curves = statistical distributions based on Equation (III) for Reaction 4a and Reaction 4b , respectively. 2

2

2

2

2


5.

HSU E T A L .

Reactions of CF, CHF, and CF 0( P) 3

3

+ CHF ± 1

3

HFCO t #

HFCO* * * ^ F C O * - HF+ + CO * 1

1

kcal/mole

4 a

HFC0*î ^ H + FC0 *

H + F + CO "

1

ÛH ° -

(4b)

1

-55

4b


(4a)

1

A H ° s= - 1 9 0 3

139

Radicals

2

kcal/mole

B e c a u s e o f t h e l a c k o f i n f o r m a t i o n on t h e e x a c t i d e n t i t y o f t h e HFCO m o l e c u l e a t s u c h a h i g h e n e r g y , we d e n o t e t h e t r i p l e t v i b r o n i c a l l y e x c i t e d HFCO m o l e c u l e by H F C 0 * ^ and t h e g r o u n d e l e c t r o n i c v i b r a t i o n a l l y e x c i t e d HFCO m o l e c u l e by HFC0*^. I f we assume t h a t t h e 0 + CHF r e a c t i o n t a k e s p l a c e e x c l u s i v e l y v i a ( 4 b ) , t h e n E q . ( I l l ) p r e d i c t s a much c o l d e r d i s t r i b u t i o n w i t h v a x = 9 , compared w i t h t h e o b s e r v e d v a x 24. Evid e n t l y , t h e o b s e r v e d d i s t r i b u t i o n , w h i c h l i e s b e t w e e n t h e two s t a t i s t i c a l d i s t r i b u t i o n s a t t r i b u t e d t o r e a c t i o n s (4a) and (4b) as n o t e d i n F i g . 3, may a c t u a l l y r e s u l t from t h e s i m u l t a n e o u s o c c u r r e n c e o f t h e s e two r e a c t i o n c h a n n e l s . This i s consistent w i t h t h e p r e s e n c e o f F atoms i n t h e f l a s h - i n i t i a t e d S 0 - C H F X (X=Cl,Br) systems. I f t h e p r o d u c t i o n o f HF f r o m t h e s e c o n d a r y a b s t r a c t i o n r e a c t i o n , F + C H F X , does n o t t a k e p l a c e s i g n i f i c a n t l y b e f o r e l a s i n g , t h e m e a s u r e d HF p o p u l a t i o n u s i n g t h e l a s e r e m i s s i o n method s h o u l d r e p r e s e n t p r i m a r i l y t h a t o f r e a c t i o n ( 4 a ) . T h i s i s b e l i e v e d t o be t h e c a s e , i n a s m u c h as t h e r a t e o f F + C H F -* HF + C F , w h i c h i s p r o b a b l y c o m p a r a b l e t o t h a t o f t h e a n a l o g o u s F + CR¥X ~* HF + C F X r e a c t i o n , was r e c e n t l y r e p o r t e d to be o n l y a b o u t 10~ t i m e s t h a t o f F + H (48). 3

1

m

=

m

2

2

2

3

3

2mm2

2

2

III.

CF

2

Radical Reactions

F o r m a t i o n o f C F . The C F r a d i c a l was i n i t i a l l y d e t e c t e d by means o f i t s e m i s s i o n (49) and a b s o r p t i o n s p e c t r a ( 5 0 - 5 2 ) when f l u o r o c a r b o n s were p a s s e d t h r o u g h e l e c t r i c a l d i s c h a r g e s o r h i g h temperature furnaces (53). I t i s a l s o formed by t h e p y r o l y s i s a t 600°C (54) o r t h e f l a s h p h o t o l y s i s ( 5 5 , 5 6 ) o f C F 3 C O C F 3 , and i t s t r a n s i e n t a b s o r p t i o n s p e c t r u m h a s b e e n o b s e r v e d i n t h e f l a s h p h o t o l y s i s o f CF C1C0CF C1, CF C1C0CFC1 , and CF3COOH ( 5 6 - 5 8 ) . The f l a s h p h o t o l y s i s i n q u a r t z o f t h e difluorohalomethanes: CF Br (56, 58-60), CF HBr ( 5 6 , 5 8 ) , C F C 1 ( 5 8 , 5 9 ) and CF HC1 (59) a l l l e a d t o t h e e l i m i n a t i o n o f CF . O f t h i s s e r i e s , C F i s p r o d u c e d most e f f i c i e n t l y f r o m CF Br (58,59). The f i r s t s t e p i n t h e p y r o l y s i s o f C F H C 1 ( 6 1 - 6 4 ) and C F H B r ( 6 5 ) i n v o l v e s t h e e l i m i n a t i o n o f HX t o y i e l d C F . The o v e r a l l r e a c t i o n p r o c e e d s as f o l l o w s : 2

2

2

2

2

2

2

2

2

2

2

2

2

2

2

2

2

2

2

CF HX 2

2 CF 2

C F + HX

t

2

C F 2

4

(7,-7) (8)


140


The k i n e t i c s o f t h e r e a c t i o n h a v e b e e n m e a s u r e d b y p r o d u c t a n a l y s e s i n b o t h f l o w and s t a t i c s y s t e m s , and t h e A r r h e n i u s p a r a m e t e r s a r e l i s t e d i n T a b l e I, a l o n g w i t h those from a shock tube study of CF H decomposition (66)« The v a l u e s l i s t e d i n t h e t a b l e from R e f e r e n c e (62) a r e p r o b a b l y too l a r g e ( 6 7 , 6 8 ) . 3

Table I .

A r r h e n i u s Parameters for the R e a c t i o n : C F H X - C F + HX 2


X

Τ

CI CI CI Br F

2

(°K)

log

806-1023 943-1023 727-796 710-796 1200-1600

Ε

A (sec ) 1

55.8 51.4 ± 52.8 ± 55.6 ± 47.0

13.84 12.36 ± 0.56 12.6 ± 0.44 14.33 ± 0.32 11.42

Ref.

(kcal/mole)

62 63 64 65 66

2.5 1.5 1.1

Tetrafluoroethylene d i s s o c i a t e s r e v e r s i b l y i n t o CF at elevated temperatures. The k i n e t i c s o f t h e d i s s o c i a t i o n , as w e l l a s t h e e q u i l i b r i u m c o n s t a n t , h a v e been m e a s u r e d i n s h o c k t u b e s b y M o d i c a a n d L a G r a f f ( 6 9 , 7 0 ) a n d by C a r l s o n ( 7 1 ) . Kushina e t a l . ( 6 7 ) i n v e s t i g a t e d t h e p y r o l y s i s o f C F a t 550* - 670°C and o b t a i n e d an e x p r e s s i o n f o r t h e r a t e c o n s t a n t . The e q u i l i b r i u m c o n s t a n t h a s a l s o b e e n d e t e r m i n e d by mass s p e c t r o s c o p y (72), b u t t h e r e i s c o n s i d e r a b l e d i s a g r e e m e n t among t h e r e p o r t e d v a l u e s (71). The H g - s e n s i t i z e d p h o t o l y s i s ( 7 3 - 7 6 ) o f C F as w e l l as i t s u n s e n s i t i z e d p h o t o l y s i s i n q u a r t z ( 5 6 , 5 9 , 6 8 ) i s an e f f i c i e n t means o f p r o d u c i n g C F . A b s o r p t i o n by C F h a s a l s o b e e n o b s e r v e d i n the f l a s h p h o t o l y s i s i n q u a r t z o f C F C F C l and C F C C l (77). H i g h e r f l u o r o a l k e n e s and d i f l u o r o c y c l o p r o p a n e s a r e a l s o s o u r c e s o f CF · D a l b y ( 5 9 ) o b t a i n e d t h i s r a d i c a l by t h e f l a s h photolysis of perfluoropropylene. The k i n e t i c s o f C F f o r m a t i o n from the t h e r m a l d e c o m p o s i t i o n o f p e r f l u o r o c y c l o p r o p a n e a t 526° - 549°Κ w e r e r e p o r t e d b y A t k i n s o n and McKeagan ( 7 8 ) . B i r c h a l l , H a s z e l d i n e and R o b e r t s ( 7 9 ) i n v e s t i g a t e d t h e t h e r m a l d e c o m p o s i t i o n o f a s e r i e s o f g e m - d i f l u o r o h a l o c y c l o p r o p a n e s as s o u r c e s o f CF · The c y c l o p r o p a n e s s t u d i e d w e r e p l a c e d i n t h e f o l l o w i n g approximate order o f d e c r e a s i n g s t a b i l i t y toward C F elimination: 2

2

4

2

2

4

2

2

2

2

2

2

2

2

H

CI

2

Cl

2

FCl

2

F

Δ>Δ~Δ>Δ~Δ

F

2

F

2

F

2

Cl

2

F

2

FCl

F

F

2

F

2

2

Cl

2

F

2

> Δ F

2

2

F

a l t h o u g h t h e d i f f e r e n c e s b e t w e e n them were n o t g r e a t . The r e a c t i o n o f 0 ( P ) w i t h C F p r o d u c e s C F i n t h e mary step o f the r e a c t i o n ( 7 7 , 8 0 - 8 6 ) : 3

0( P) 3

2

+ C F 2

4

-

CF

4

2

2

+ CF 0 2


2

pri

(9)

H s u ET AL.

5.

Reactions

of CF, CHF,

and CF

141

Radicals

B

I n a d d i t i o n t o c h e m i c a l e v i d e n c e f o r i t s p r e s e n c e a s an i n t e r m e d i a t e i n t h i s r e a c t i o n , b o t h i t s a b s o r p t i o n (77,85) a n d mass s p e c t r a (86) h a v e been d e t e c t e d . A b s o r p t i o n by C F was a l s o o b s e r v e d i n t h e r e a c t i o n o f 0 ( P ) w i t h C F C H ( s ) , CF CHC1 ( w ) , and C F C F C 1 (vw) (85). The s y m b o l s f o l l o w i n g t h e f o r m u l a s r e f e r to the r e l a t i v e strength o f C F absorption. M a h l e r (87) f o u n d t h a t C F c a n be p r o d u c e d c o n v e n i e n t l y b y the d i s s o c i a t i o n o f t r i f l u o r o m e t h y 1 f l u o r o p h o s p h o r a n e s a t about 100° - 120°C. The e l i s i o n o f C F o c c u r s s t e p w i s e a n d r e v e r s i b l y : 2

3

2

2

2

2

2

2

2

(CF ) PF

2

£

(CF ) PF

(CF ) PF

3

t

CF PF

CF PF

4

ï*

PF +CF


3

3

3

2

3

3

3

2

+ CF

3

+ CF

4

5

2

2

2

These p h o s p h o r a n e s c o u l d be decomposed i n t h e p r e s e n c e o f o t h e r reagents to study the r e a c t i o n s o f C F . A n o t h e r v e r y c o n v e n i e n t and e x c e p t i o n a l l y c l e a n s o u r c e o f CF i s the p h o t o l y s i s or p y r o l y s i s o f d i f l u o r o d i a z i r i n e , reported by M i t s c h (88,89): 2

2

CF N z

2

z

> or Δ h V

2

CF + N

A

z

2

2

L

This process i s very s u i t a b l e f o r producing C F f o r the study o f i t s r e a c t i o n s o r f o r s y n t h e t i c p u r p o s e s , as t h e o n l y o t h e r major product o f the decomposition i sN . C a v e l l , D o b b i e and Tyerman ( 9 0 ) f o u n d t h a t t h e s i n g l e s t e p e l i m i n a t i o n o f C F occurs i n the f l a s h p h o t o l y s i s of t r i f l u o r o m e t h y l p h o s p h i n o compounds. The compounds ( C F ) P , ( C F ) P P ( C F ) , ( C F ) P X and C F P X ( X = F , C l o r H) a l l y i e l d e d C F , b a s e d upon t h e i r o b s e r v a t i o n o f i t s a b s o r p t i o n s p e c t r u m . R o w l a n d and c o - w o r k e r s ( 3 1 , 3 2 , 9 1 ) p r e p a r e d C F a t r a d i o a c t i v e t r a c e r l e v e l s by the r e a c t i o n o f e n e r g e t i c F atoms f r o m n u c l e a r r e c o i l w i t h v a r i o u s m o l e c u l e s , i n c l u d i n g C F , C H F , C H F , and C F , f o l l o w e d by secondary decomposition o f t h e v i b r a t i o n a l l y excited F - l a b e l e d precursors. The r e a c t i o n w i t h C H F , w h i c h was u s e f u l f o r t h e p r o d u c t i o n o f b o t h C F and C H F , p r o c e e d s as follows: 2

2

2

3

3

2

3

2

3

3

3

2

2

2

2

1

8

4

2

3

2

2

4

1 8

2

2

2

1 8

F + CH F 2

2

- CHgF^F * 1

+ F

C H F F t - C H F + HF 1 8

2

1 8

1 8

F + CH F 2

CHF

2

1 8

2

Ft

-» C H F

2

1 8

F

t

+ H

- C F F + HF 1 8

E l e c t r o n i c S t a t e o f C F . The C F r a d i c a l i n i t s g r o u n d s t a t e e x h i b i t s a n u n u s u a l l y l o w r e a c t i v i t y compared t o o t h e r c a r b o n d i r a d i c a l s , s u c h a s C H and C C 1 , due t o t h e s t r o n g 2

2

2

2


142


i n t e r a c t i o n o f t h e e l e c t r o n e g a t i v e F atoms w i t h t h e f r e e e l e c t o n pair. I t h a s b e e n shown s p e c t r o s c o p i c a l l y ( 9 2 , 9 3 ) t o h a v e a s i n g l e t e l e c t r o n i c ground s t a t e , which i s c o n s i s t e n t w i t h the s t e r e o s p e c i f i c i t y of i t s a d d i t i o n reactions with o l e f i n s (89). Simons h a s e s t i m a t e d t h a t t h e f i r s t e x c i t e d t r i p l e t s h o u l d l i e a b o u t 45 k c a l / m o l e above t h e g r o u n d s t a t e ( 9 4 ) . Staemmler d e d u c e d a v a l u e o f 47 k c a l / m o l e by ab i n i t i o c a l c u l a t i o n s ( 3 6 ) . No d i r e c t e v i d e n c e f o r t h e f o r m a t i o n o f t r i p l e t C F h a s been r e p o r t e d . A l l the p r e v i o u s l y mentioned processes p r o d u c i n g CF appear to g i v e r i s e to the s i n g l e t , w i t h the probable e x c e p t i o n o f t h e 0( P) + C F r e a c t i o n and p e r h a p s C F p r o d u c t i o n i n a glow d i s c h a r g e c o n t a i n i n g C F ( 9 5 ) . H e i c k l e n and c o w o r k e r s ( 8 0 , 8 3 , 8 4 , 9 5 ) have o b t a i n e d i n d i r e c t e v i d e n c e f o r the p a r t i c i p a t i o n o f t r i p l e t C F i n t h e 0( P) + C F r e a c t i o n , w h i c h w i l l be d i s c u s s e d i n more d e t a i l l a t e r . The H g - s e n s i t i z e d p h o t o l y s i s o f C F a p p a r e n t l y g i v e s r i s e to s i n g l e t CF o n l y ( 7 4 , 7 6 , 9 5 ) . T h i s i s c o n s i s t e n t w i t h the e n e r g e t i c s o f the system. Thus t h e f o l l o w i n g r e a c t i o n s r e p o r t e d f o r C F , w i t h the e x c e p t i o n n o t e d , a r e b e l i e v e d t o be t h o s e o f t h e g r o u n d s t a t e s i n g l e t . R e a c t i o n s o f C F . I t was n o t e d e a r l y i n t h e i n v e s t i g a t i o n o f the c h e m i s t r y o f C F t h a t the r e c o m b i n a t i o n o f t h i s d i r a d i c a l took place extremely slowly (50). The k i n e t i c s o f t h e r e c o m b i n a t i o n r e a c t i o n h a v e b e e n i n v e s t i g a t e d by s e v e r a l w o r k e r s . Dalby ( 5 9 ) and Tyerman (68) m e a s u r e d t h e r e c o m b i n a t i o n r a t e s a t 300° 600°Κ by f o l l o w i n g t h e d e c r e a s e i n a b s o r p t i o n o f C F p r o d u c e d b y f l a s h p h o t o l y s i s as a f u n c t i o n o f t i m e . Dalby o b t a i n e d the rate equation: 2

2

3


2

4

2

4

3

2

2

2

4

4

2

2

2

2

2

and

k

8

= (7.5)10 T lxp(-1200/RT)

k

8

= ( 5 . 0 ± 1 . 0 ) 1 0 T e x p ( - 4 0 0 ± 100/RT)

9

ml mole

V

sec" ,

-1

1

Tyerman: 9

V

m l mole"

1

sec .

T h e i r r e s u l t s were g e n e r a l l y i n good a g r e e m e n t , a l t h o u g h d i f f e r e n t v a l u e s f o r t h e a c t i v a t i o n e n e r g y were o b t a i n e d . The d i s a g r e e m e n t i s a p p a r e n t l y due t o d i f f e r e n c e s i n t h e e v a l u a t i o n o f t h e e x t i n c t i o n c o e f f i c i e n t (e) f o r C F . Tyerman a t t r i b u t e d t h e h i g h e r a c t i v a t i o n e n e r g y o b t a i n e d by D a l b y t o h i s n e g l e c t t o take i n t o account the v a r i a t i o n o f e w i t h temperature. Edwards and S m a l l ( 6 2 ) , b y t h e a n a l y s i s o f p r o d u c t s i n t h e C H F C 1 p y r o l y s i s , r e p o r t e d a r a t e c o n s t a n t o f 1.74 χ 10 m l mole sec , w i t h an assumed z e r o a c t i v a t i o n e n e r g y . T h i s constant appears t o be much t o o l a r g e i n c o m p a r i s o n w i t h o t h e r v a l u e s . M o d i c a and L a G r a f f s t u d i e d t h e C F - C F d i s s o c i a t i o n i n e x c e s s N b e h i n d i n c i d e n t s h o c k waves o v e r t h e t e m p e r a t u r e r a n g e o f 1200° t o 1600°K ( 7 0 ) . They f o u n d l i t t l e t e m p e r a t u r e d e p e n d ence f o r t h e r e c o m b i n a t i o n r e a c t i o n and o b t a i n e d t h e r a t e l a w : 2

i

2

4

2

2

2

k

8

= (4.1)10 T * exp(-18,400/RT) 3 8

6

3 6

ml

2

molé

2

sec . 1


1

5.

Hsu E T A L .


2

The r e a c t i o n o f C F CF

with

2

+ C F

2

2

4

C F : 2

-

4

cyclo-C F 3

(10)

6

was i n v e s t i g a t e d by A t k i n s o n (73) and by Cohen and H e i c k l e n ( 7 5 , 8 2 ) i n t h e H g - s e n s i t i z e d p h o t o l y s i s o f C F , by L e n z i and M e l e (96) i n t h e p y r o l y s i s o f C F 0 , and by A t k i n s o n and McKeagan (78) i n the p y r o l y s i s o f c y c l o - C F . I n these s t u d i e s , the r a t e o f C F a d d i t i o n t o C F was compared w i t h i t s r a t e o f r e c o m b i n a t i o n , and v a l u e s o f ki /k were o b t a i n e d w h i c h a g r e e d w i t h i n a f a c t o r of 2. These v a l u e s have been compared by Tyerman ( 6 8 ) , who, by t h e u s e o f h i s v a l u e f o r k , o b t a i n e d t h e f o l l o w i n g e x pression for k : 2

2

2

4

4

3

2

6

4

0


143

Radicals

8

8

1 0

k

1 0

= 8.7

χ 10 T exp(-6400 7

V

± 1300/RT) m l mole"

sec* .

1

1

M a h l e r n o t e d t h a t C F adds t o HC1 t o g i v e C H F C 1 ( 8 7 ) . The k i n e t i c s o f t h e r e a c t i o n were m e a s u r e d i n s t u d i e s o f t h e p y r o l y s i s o f C H F C 1 ( 6 1 - 6 4 ) , and t h e A r r h e n i u s p a r a m e t e r s o b t a i n e d are l i s t e d i n Table I I . A l s o l i s t e d a r e v a l u e s found f o r the analogous HBr r e a c t i o n , o b t a i n e d from the p y r o l y s i s o f C H F B r (65). The t e m p e r a t u r e r a n g e s i n v e s t i g a t e d 2

2

2

2

Table I I .

A r r h e n i u s Parameters f o r the R e a c t i o n : C F + HX - C F H X 2

2

X

l o g A ( m l / m o l e « sec

)

Cl

11.35

Ε (kcal/mole)

Ref.

6.2

62

15±5

63

Cl Cl

11.33

12.1±2.7

64

Br

11.33

9.6*1.3

65

a r e t h e same a s t h o s e

given i n Table I .

S m a i l and R o w l a n d

(91)

18

""

s t u d i e d t h e r e a c t i o n s o f h v d r o g e n h a l i d e s w i t h CF F p r o d u c e d by r e a c t i o n s o f e n e r g e t i c F atoms. They f o u n d t h a t a t 10° 1 5 ° C , H I was a b o u t 70 t i m e s as e f f i c i e n t as H B r , w h i c h i n t u r n 18

was a b o u t 50 t i m e s more e f f i c i e n t t h a n HC1 i n s c a v e n g i n g CF F from t h e r e a c t i o n s y s t e m . The k i n e t i c s and m e c h a n i s m o f t h e r e a c t i o n o f C F w i t h NO was i n v e s t i g a t e d by M o d i c a (97) i n s h o c k t u b e e x p e r i m e n t s . He r e p o r t e d t h a t the r e v e r s i b l e r e a c t i o n : C F + NO CF N0 (11) 2

2

t o o k p l a c e b e l o w 2500°K, proceeded f a r t h e r by:

2

and above t h i s t e m p e r a t u r e ,

the


reaction

144


2 CF NO

2 CF 0 + N

CF NO + NO

CF 0 + N 0

2

2

2

2

2

2


An e x p r e s s i o n f o r t h e r a t e c o n s t a n t f o r r e a c t i o n (11) was r e ported. H o w e v e r , B u r k s and L i n p o i n t e d o u t t h a t t h e a c t i v a t i o n e n e r g y r e p o r t e d was u n r e a s o n a b l y h i g h , and t h a t t h e r e l a t i v e m a g n i t u d e o f t h e A f a c t o r s f o r t h e f o r w a r d and t h e r e v e r s e r e a c t i o n s were n o t c o n s i s t e n t ( 9 8 ) . They p r o p o s e d t h e f o l l o w i n g m e c h a n i s m , b a s e d upon a HF l a s e r e m i s s i o n s t u d y and mass s p e c t r a l a n a l y s i s of the p r o d u c t s : CF

2

+ NO -

CF 0 + Ν

CF

2

+ Ν

-

FCN + F

CF

2

+

0

-

CO +

2

2F

w i t h 0 b e i n g g e n e r a t e d by t h e w e l l - k n o w n Ν + NO r e a c t i o n . B i o r d i , L a z z a r a , and Papp s t u d i e d t h e r e a c t i o n s o f C F i n C F B r - i n h i b i t e d methane f l a m e s , and r e p o r t e d some k i n e t i c d a t a f o r r a d i c a l r e a c t i o n s a t flame temperatures ( 9 9 ) . The r e a c t i o n o f g r o u n d s t a t e C F w i t h 0 , 2

3

2

CF

+ 0

2

2

-» C F 0

2

2

(12)

2

is slow. D a l b y ( 5 9 ) r e p o r t e d an u p p e r l i m i t f o r k o f 10 m l mole" sec , s i n c e t h e l i f e t i m e o f C F i n h i s e x p e r i m e n t s was i n d e p e n d e n t o f 0 n r e s s u r e s ^ t o 110 t o r r . Tyerman d e d u c e d a v a l u e o f ~ 1 . 3 χ 10 m l mole" sec b a s e d upon t h e c o m p e t i t i o n o f r e a c t i o n s (10) and ( 1 2 ) f o r C F ( 7 7 ) . M o d i c a (69) s t u d i e d t h e C F o x i d a t i o n r e a c t i o n i n a s h o c k t u b e and f o u n d i t t o be f i r s t o r d e r i n b o t h C F and 0 . The r a t e c o n s t a n t was g i v e n b y : 1 2

2

2

2

2

2

k

1 2

2

= 2.92

χ 10 T^exp(-13,300/RT)

m l mole"

1 0

1

sec" . 1

H e i c k l e n and c o - w o r k e r s i n t h e i r i n v e s t i g a t i o n o f the C F + 0 ( P ) ( p r o d u c e d by H g - s e n s i t i z e d N 0 d i s s o c i a t i o n ) r e a c t i o n , o b s e r v e d t h a t t h e y i e l d o f C F 0 was e n h a n c e d when 0 was added t o t h e r e a c t i n g s y s t e m ( 8 0 ) , and t h a t t h e y i e l d o f c y c l o - C F i n c r e a s e d under c o n d i t i o n s i n w h i c h i t s h o u l d have decreased i f s i n g l e t C F ( C F ) were p r o d u c e d ( 9 5 ) . They a t t r i b u t e d b o t h these f a c t s to the f o r m a t i o n o f t r i p l e t C F ( C F ) i n the r e a c t i o n , a s s h o u l d be p r e d i c t e d by t h e s p i n - c o n s e r v a t i o n r u l e . H o w e v e r , i t was a l s o shown t h a t t h e r e a c t i v i t y t o w a r d C F of t h e C F o b t a i n e d was i d e n t i c a l w i t h t h a t o f C F ( 8 2 ) . I t was also postulated that CF c o u l d undergo s e l f - a n n i h i l a t i o n : 2

3

4

2

2

2

3

2

1

6

2

2

2

2

1

2

3

2

2 CF 3

4

2

2

-

C F * 2

4

(13)

2 CF . 1

2

M i t c h e l l and Simons ( 8 5 ) s t u d i e d t h e 0 ( P ) + C F reaction by p r o d u c i n g t h e 0 atoms by t h e f l a s h p h o t o l y s i s o f N 0 i n P y r e x 3

2

4

2


Hsu E T A L .

5.

Reactions

of CF,

CHF,

and CF

2

Radicals

145

and m e a s u r i n g t h e C F a b s o r p t i o n . They f o u n d o n l y bands o f CF i n the ground v i b r a t i o n a l s t a t e . No o t h e r t r a n s i e n t s p e c i e s w e r e d e t e c t e d i n t h e s p e c t r a l r a n g e o f 2 1 0 - 6 0 0 nm. T h e r e was no d e l a y i n t h e a p p e a r a n c e t i m e o f t h e C F bands a s compared w i t h i t s u n s e n s i t i z e d p r o d u c t i o n f r o m C F w i t h l i g h t o f ^ 170 nm, w h i c h i n d i c a t e d t h a t i f C F ^ i s a p r i m a r y p r o d u c t , i t s l i f e t i m e must be n o t g r e a t e r t h a n 10 sec. Tyerman ( 7 7 ) o b t a i n e d r e s u l t s f o r t h i s r e a c t i o n t h a t s u p p o r t e d t h o s e o f M i t c h e l l and S i m o n s . He a l s o o b s e r v e d t h a t i n t h e f l a s h p h o t o l y s i s e x p e r i m e n t s , t h e s i n g l e t C F a p p e a r e d more r a p i d l y than t r i p l e t - t r i p l e t a n n i h i l a t i o n could account f o r . It was p r o p o s e d t h a t t h e r a p i d f o r m a t i o n o f s i n g l e t C F c o u l d be e x p l a i n e d i f C F i s r e l a x e d b y N 0 and NO a t a r a t e g r e a t e r than 2 χ 1 0 ml mole sec ( w h i c h i s e q u i v a l e n t t o 1/10 t h e collision rate). E x p e r i m e n t s w i t h 700 t o r r 0 added gave a n u p p e r l i m i t f o r t h e r e a c t i o n r a t e o f C F w i t h 0 o f 6 χ 10 m l mole* s e c , i f l e s s t h a n 5% o f t h e C F was s c a v e n g e d i n p r o cesses not r e t u r n i n g C F to the system. J o h n s t o n and H e i c k l e n d e d u c e d an u p p e r l i m i t o f 6 χ 10 m l m o l e sec o n t h e assump t i o n t h a t r e a c t i o n (13) o c c u r r e d a t e v e r y c o l l i s i o n ( 8 4 ) , and t h e t r u e r a t e c o n s t a n t may be much s m a l l e r ( 7 7 ) . 2

2

2

2

3

4

2


2

2

3

2

2

1 3

1

1

2

3

1

3

2

2

2

2

We h a v e i n v e s t i g a t e d t h e r e a c t i o n s o f 0( P) atoms w i t h C F by means o f t h e CO l a s e r r e s o n a n c e a b s o r p t i o n method i n an a t tempt t o e l u c i d a t e t h e m e c h a n i s m o f t h i s r e a c t i o n . A l t h o u g h CO h a s n o t b e e n r e p o r t e d t o be a n i m p o r t a n t p r o d u c t o f t h i s r e a c t i o n , we b e l i e v e d , o n t h e b a s i s o f t h e e n e r g e t i c s and t h e a p p e a r a n c e o f s t r o n g HF l a s e r e m i s s i o n f r o m t h e f l a s h - i n i t i a t e d S 0 - C F B r H system, t h a t the 0 + C F r e a c t i o n s h o u l d generate F atoms, a c c o m p a n i e d by t h e p r o d u c t i o n o f CO. S i n c e t h e f o l l o w i n g r e a c tions: 2

2

2

2

4

2

2

0( P) 3

+

1

C¥

AH?

-

2

= -4

4 a

AH? 0( P) 3

+

3

AH?

4 c

2

-

Afl?

4 d

(14b)

2

kcal/mole

CO + 2F

= -51 -

(14a)

kcal/mole

CO + F

= -42

4 D

CF

CO + 2F

kcal/mole

CO + F

= -89

(14c)

(14d)

2

kcal/mole

have l a r g e l y d i f f e r e n t e x o t h e r m i c i t i e s , the v i b r a t i o n a l energy c o n t e n t o f t h e CO m o l e c u l e formed i n t h e 0 + C F r e a c t i o n may p r o v i d e i n f o r m a t i o n on t h e i d e n t i t y o f t h e C F r a d i c a l i n v o l v e d . The e x o t h e r m i c i t i e s o f t h e above r e a c t i o n s w e r e c a l c u l a t e d by 2

2


146


t a k i n g Δ Η | ( C F ) = - 4 4 . 5 k c a l / m o l e ( 7 1 ) a n d ΔΕ ( C F - * C F ) = 47 k c a l / m o l e ( 3 6 ) . I n o u r e x p e r i m e n t s , 1 0 - t o r r samples o f a 2 : 2 . 7 : 4 5 . 3 / C F : N 0 : He m i x t u r e were f l a s h - p h o t o l y z e d i n a P y r e x r e a c t i o n t u b e . CO a b s o r p t i o n was d e t e c t e d up t o ν = 1 2 , c o r r e s p o n d i n g t o a s much a s 69 k c a l / m o l e o f v i b r a t i o n a l e n e r g y . The m e a s u r e d CO v i b r a t i o n a l d i s t r i b u t i o n i s p r e s e n t e d i n F i g . 4 . The d e t e c t i o n o f s u c h a h i g h l y e x c i t e d CO f r o m t h e 0 + C F r e a c t i o n , a l t h o u g h e n e r g e t i c a l l y t e n a b l e , i s s u r p r i s i n g i n view o f M i t c h e l l and S i m o n s ' f a i l u r e t o d e t e c t v i b r a t i o n a l l y e x c i t e d C F i n t h e same s y s t e m ( 8 5 ) . I f t h e C F i s n o t v i b r a t i o n a l l y e x c i t e d , o n l y one r e a c t i o n g i v e n above, namely ( 1 4 d ) , h a s s u f f i c i e n t energy t o e x c i t e CO up t o ν = 1 2 . To examine t h i s p o s s i b i l i t y , we compared t h e o b s e r v e d CO v i b r a t i o n a l d i s t r i b u t i o n w i t h t h o s e p r e d i c t e d b y several s t a t i s t i c a l models. Assuming r e a c t i o n (14d) occurs v i a a C F 0 complex, w i t h E = -AHJ I + 2.5RT = 90 k c a l / m o l e , E q . ( I l l ) p r e d i c t s a d i s t r i b u t i o n w h i c h i s c o l d e r t h a n t h e observed o n e , as i n d i c a t e d by t h e d o t t e d curve I i n F i g . 4 . I n t h i s c a l c u l a t i o n , however, CF was assumed t o be v i b r a t i o n a l l y c o l d . I f one assumes t h a t t h e 53 k c a l / m o l e o f r e a c t i o n e n e r g y from t h e p r i m a r y r e a c t i o n : 3

1

2

2

2

2

4

2

2

4

2


2

2

t

o

t

4C

3

0( P)

+ C F

3

2

- C F 0* - CF 0

4

2

4

2

f

+

3

CF

2

2

t

i s s t a t i s t i c a l l y d i s t r i b u t e d among a l l v i b r a t i o n a l modes o f t h e C F 0 c o m p l e x , t h e n t h e C F r a d i c a l s h o u l d c a r r y 53 χ ( 3 N Q ? - 6 ) / ( 3 N C F 0 ~ 6 ) o r 11 k c a l / m o l e o f v i b r a t i o n a l e n e r g y . This increases t h e t o t a l a v a i l a b l e e n e r g y i n r e a c t i o n ( 1 4 d ) t o a maximum o f 100 kcal/mole. The u s e o f t h i s v a l u e f o r t h e t o t a l a v a i l a b l e e n e r g y i n s t a t i s t i c a l c a l c u l a t i o n s b a s e d on E q s . ( I I ) and ( I I I ) gave r i s e t o CO v i b r a t i o n a l d i s t r i b u t i o n s t h a t a g r e e c l o s e l y w i t h t h e e x p e r i m e n t a l d a t a a s shown b y t h e s o l i d and d a s h e d c u r v e s , r e s p e c t i v e l y , i n F i g . 4 . A s i m i l a r c a l c u l a t i o n assuming t h a t v i b r a t i o n a l l y excited C F i s involved i n reaction (14b), with Etot 63 k c a l / m o l e , l e d t o a d i s t r i b u t i o n t h a t i s much c o l d e r than t h a t observed, as i n d i c a t e d by the dotted curve I I i n F i g . 4. I n t h i s c a s e , t h e C F r a d i c a l c a r r i e d a n a d d i t i o n a l 100 χ (3Ncp^-6)/(3Ne ]? 0-6) = 20 k c a l / m o l e o f v i b r a t i o n a l e n e r g y f r o m reaction (9). On t h e b a s i s o f t h e s e m o d e l c a l c u l a t i o n s ^ t h e r e a c t i o n o f v i b r o n i c a l l y e x c i t e d C F ( i . e . C F t ) w i t h 0 ( P) atoms seems t o be t h e o n l y p r o c e s s w h i c h h a s s u f f i c i e n t e n e r g y t o a c c o u n t f o r t h e e x t e n t o f t h e CO v i b r a t i o n a l e x c i t a t i o n o b s e r v e d . In hind s i g h t , t h i s p r o c e s s a l s o seems t o be t h e most r e a s o n a b l e one i f the r a t e o f t h i s r e a c t i o n and t h a t o f e l e c t r o n i c r e l a x a t i o n ( C F + M -* CF* + M b y NO o r N 0 ) a r e c o m p a r a b l e a n d much f a s t e r t h a n t h a t o f 0( P ) + C F . I n t h i s way o n l y c a n t h e v i b r a t i o n a l as w e l l a s t h e e l e c t r o n i c e n e r g y o f t h e C F r a d i c a l formed i n t h e p r i m a r y 0 + C F r e a c t i o n be e f f e c t i v e l y c h a n n e l e d i n t o t h e p r o d u c t CO. O u r p r e s e n t r e s u l t s t h u s a p p e a r t o s u p p o r t t h e 2

4

2

2

4

2

1

2

=

1

2

2

4

3

2

3

2

2

2

2

3

2

2

4



5.

HSU E T A L .

Reactions

of CF,

CHF,

and CF

2

147

Radicals

Figure 4. Vibrational population of CO observed in the 0( P) + C F reaction. Open circles = experimental data; solid and dashed curves = statistical distributions based on Equations (II) and (HI), respectively, using E = 100 kcal/mol ; dotted curves (I) and (II) ^statistical distributions based on Equation (III) using E = 90 and 63 kcal/mol , respectively. Models employed in these calculations are given in the text. 3

2

i

tot

tot

In Fluorine-Containing Free Radicals; Root, J.; Warlii i:ûÛ36 ACS Symposium Series; American Chemical Society: Washington, DC, 1978.

148


assumption that t r i p l e t CF r a d i c a l s are indeed present i n the 0( P) - C F system. The C F r a d i c a l , w h i c h may a l s o be p r e s e n t o r formed i n i t i a l l y i n a d d i t i o n t o C F , d o e s n o t r e a c t r a p i d l y t o p r o d u c e CO, a c c o r d i n g t o o u r l a s e r a b s o r p t i o n r e s u l t s . The p r o d u c t i o n o f F i n the 0 + C F r e a c t i o n does n o t p r e c l u d e the p r e s e n c e o f F atoms b e c a u s e , e n e r g e t i c a l l y , t h e amount o f a v a i l a b l e e n e r g y ( 1 0 0 k c a l / m o l e ) i s more t h a n s u f f i c i e n t t o d i s s o c i a t e F . The m o l e c u l a r e l i m i n a t i o n o f F i n t h i s r e a c t i o n , t h o u g h r a t h e r uncommon, i s n o t s u r p r i s i n g . The p h o t o d i s s o c i a t i o n o f C l C O a t 125 nm, w h i c h c o r r e s p o n d s c l o s e l y t o t h e amount o f i n t e r n a l e n e r g y c a r r i e d b y t h e F C O i n t e r m e d i a t e , h a s b e e n demon s t r a t e d to e l i m i n a t e C l (100). 2

3

2

4

2

3

3

2

2

2

2

2

2


2

2

Acknowledgement The a u t h o r s a r e g r a t e f u l t o M i s s L a u r a C o l c o r d ( F e d e r a l Junior Fellow) for her v a l u a b l e assistance i n preparing t h i s review. One o f u s ( D . S . Y . H . ) g r a t e f u l l y a c k n o w l e d g e s t h e a w a r d o f an NRC/NRL R e s i d e n t R e s e a r c h A s s o c i a t e s h i p .

Literature Cited 1.

Andrews, E.B. and Barrow, R.F., Proc. Phys. Soc. A (1951), 64, 481. 2. Thrush, B.A. and Zwolenik, J . J . , Trans. Faraday Soc. (1963), 59, 582. 3. Porter, T.L., Mann, D.E. and Acquista, Ν., J . Mol. Spectro scopy (1965), 16, 228. 4. Carroll, P.K. and Grennan, T.P., J . Phys. Β (1970), 3, 865. 5. Carrington, A. and Howard, B . J . , Mol. Phys. (1970), 18, 225. 6. Hall, J.A. and Richards, W.G., ibid. (1972), 23, 331. 7. Jacox, M.E. and Milligan, D.E., J . Chem. Phys. (1969), 50, 3252. 8. Simons, J.P. and Yarwood, A.J., Trans. Faraday Soc. (1963), 59, 90. 9. Yarwood, A . J . and Simons, J.P., Proc. Chem. Soc. (1962), 62. 10. Merer, A . J . and Travis, D.N., Can. J . Phys. (1966), 44, (1541). 11. Modica, A.P. and Sillers, S.J., J . Chem. Phys. (1968), 48, 3283. 12. Schatz, G. and Kaufman, Μ., J . Phys. Chem. (1972), 76, 3586. 13. "JANAF Thermochemical Tables", Stull, D.R., Ed., the DOW Chemical Co., Midland, Mich., 1965. 14. Hildenbrand, D.L., Chem. Phys. Letters (1975), 32, 523. 15. Kuzyakov, Yu. Ya., Vestn. Mosk. Univ. Khim. (1968), 23, 21. 16. Farber, M., Frisch, M.A. and Ko, H.C., Trans. Faraday Soc. (1969), 65, 3202.



5.

HSU E T A L .


149

17. Lin, M.C.,Paper No. 142, 166th Annual ACS Meeting, Chicago, Ill., Aug. 1973. 18. Lin, M.C., Int. J. Chem. Kinet. (1974), 6, 1. 19. Lin, M.C., and Shortridge, R.G., Chem. Phys. Letters (1974), 29, 42. 20. Shortridge, R.G. and Lin, M.C., J . Chem. Phys. (1976), 64, 4076. 21. Henrici, Η., Lin, M.C. and Bauer, S.H., ibid. (1970), 52, 5834. 22. Gangloff, H.J., Milks, D., Maloney, K.L., Adams, T.N., and Matula, R.A., ibid. (1975), 63, 4915. 23. Hancock, G., Morley, C. and Smith, I.W.M., Chem. Phys. Letters (1971), 12, 193. 24. Djeu, N . , J. Chem. Phys. (1974), 60, 4100. 25. Hudgens, J.W., Gleaves, J.T., and McDonald, J.D., ibid. (1976), 64, 2528 26. Shapiro, M. and Halavee, U., Chem. Phys. Letters (1976), 40, 387. 27. Whitten, G.Z., and Rabinovitch, B.S., J . Chem. Phys. (1964), 41, 1883. 28. Simons, J.P., private communication. 29. Tang, Y.-N. and Rowland, F . S . , J. Am. Chem. Soc. (1966), 88, 626. 30. Tang, Y.-N. and Rowland, F . S . , ibid. (1967) 89, 6420. 31. Tang, Y.-N., and Smail, T., and Rowland, F.S., ibid. (1969), 91, 2130. 32. Smail, T. and Rowland, F.S., J. Phys. Chem., (1970), 74, 1866. 33. Lin, M.C., Int. J. Chem. Kinet. (1973), 5, 173. 34. Gordon, R.J. and Lin, M.C., Chem. Phys. Letters (1973), 22, 107. 35. Shortridge, R.G. and Lin, M.C., IEEE J. Quantum Electron. (1974), QE-10, 873. 36. Staemmler, V., Theoret. Chim. Acta (1974), 35, 309. 37. Simons, J.W. and Curry, R., Chem. Phys. Letters (1976), 38, 171. 38. Green, W.H. and Lin, M.C., J. Chem. Phys. (1971), 54, 3222. 39. Berry, M.J., ibid. (1973), 59, 6229. 40. Parker, J.H. and Pimentel, G.C., ibid. (1969), 51, 91; (1971), 55, 857. 41. Bogan, D.J. and Setser, D.W., ibid. (1976), 64, 586 and papers cited therein. 42. Berry, M.J. and Pimentel, G.C., ibid. (1968), 49, 5190. 43. Padrick, T.D. and Pimentel, G.C., J. Chem. Phys. (1971), 54, 720., J. Phys. Chem. (1972), 76, 3125. 44. Lin, M.C., J . Phys. Chem. (1971), 75, 3642; (1972), 76, 811, 1425. 45. Clough, P.N., Polanyi, J.C. and Taguchi, R.T., Can. J. Chem. (1970), 48, 2919. 46. Hsu, D.S.Y., Shortridge, R.G., and Lin, M.C., to be pub lished. In Fluorine-Containing Free Radicals; Root, J.; ACS Symposium Series; American Chemical Society: Washington, DC, 1978.


150


47. Taylor, G.W. and Setser, D.W., J. Chem. Phys. (1973), 58, 4840. 48. Goldberg, I.B. and Schneider, G.R., ibid. (1976), 65, 147. 49. Venkateswarlu, P., Phys. Rev. (1950), 77, 676. 50. Laird, R.K., Andrews, E.B. and Barrow, R.F., Trans. Faraday Soc. (1950), 46, 803. 51. Bass, A.M. and Mann, D.E., J. Chem. Phys. (1962), 36, 3501. 52. Thrush, B.A. and Zwolenik, J.J., Trans. Faraday Soc. (1963), 59, 582. 53. Margrave, J.L. and Wieland, Κ., J. Chem. Phys. (1953), 21, 1552. 54. Batey, W. and Trenwith, A.B., J. Chem. Soc. (1961), 1388. 55. Hertzberg, G., Chemical Institute of Canada Symposium on Free Radicals, Laval University, Quebec (1956). 56. Simons, J.P. and Yarwood, A.J., Nature (1961), 192, 943. 57. Simons, J.P. and Yarwood, A.J., ibid. (1960), 187, 316. 58. Simons, J.P. and Yarwood, A.J., Trans. Faraday Soc. (1961), 57, 2167. 59. Dalby, F.W., J. Chem. Phys. (1964), 41, 2297. 60. Mann, D.E. and Thrush, B.A., J. Chem. Phys. (1960), 33, 1732. 61. Edwards, J.W. and Small, P.Α., Nature (1964), 202, 1329. 62. Edwards, J.W. and Small, P.Α., Ind. Eng. Chem. ( Funda mentals) (1965), 4, 396. 63. Gozzo, F. and Patrick, C.R., Tetrahedron (1966), 22, 3329. 64. Barnes, G.R., Cox, R.A. and Simmons, R.F., J. Chem. Soc. (B) (1971), 1176. 65. Cox, R.A. and Simmons, R.F., J. Chem. Soc. (B) (1971), 1625. 66. Tschuikow-Roux, E. and Marte, J.E., J. Chem. Phys. (1965), 42, 2049. 67. Kushina, D., Politanskii, S.F., Sheuchuk, V.U., Gutor, I.M., Ivashenko, Α.Α., and Nefedov, O.M., Izvest. Akad. Νauk SSSR, Ser. Khim. (1974), 946. 68. Tyerman, W.J.R., Trans. Faraday Soc. (1969), 65, 1188. 69. Modica, A.P. and LaGraff, J.E., J. Chem. Phys. (1966), 43, 3383. 70. Modica, A.P. and LaGraff, J.E., ibid. (1966), 45, 4729. 71. Carlson, G.A., J. Phys. Chem. (1971), 75, 1625. 72. Zmbov, K.F., Uy, O.M. and Margrave, J.L., J. Am. Chem. Soc. (1968), 90, 5090. 73. Atkinson, Β., J. Chem. Soc. (1952), 2684. 74. Heicklen, J., Knight, V. and Greene, S.A., J. Chem. Phys. (1965), 42, 221. 75. Cohen, Ν. and Heicklen, J., ibid. (1965), 43, 871. 76. Heicklen, J. and Knight, V., J. Phys. Chem. (1966), 70, 3901. 77. Tyerman, W.J.R., Trans. Faraday Soc. (1969), 65, 163. 78. Atkinson, B. and McKeagan, D., Chem. Comm. (1966), 189.



5.

Hsu E T A L .


151

79. Birchall, J.M., Haszeldine, R.N. and Roberts, D.W., J.C.S. Perkin I (1973), 1071. 80. Saunders, D. and Heicklen, J., J. Am. Chem. Soc. (1965), 87, 2088. 81. Saunders, D. and Heicklen, J., J. Phys. Chem. (1966), 70, 1950. 82. Cohen, N. and Heicklen, J., ibid. (1966), 70, 3082. 83. Heicklen, J. and Knight, V., ibid. (1966), 70, 3893. 84. Johnson, T. and Heicklen, J., J. Chem. Phys. (1967), 47, 475. 85. Mitchell, R.C. and Simons, J.P., J. Chem. Soc. (Β), (1968), 1005. 86. Gilbert, J.R., Slagle, I.R., Graham, R.E. and Gutman, D. J. Phys. Chem. (1976), 80, 14. 87. Mahler, W., Inorg. Chem. (1963), 2, 230. 88. Mitsch, R.A., J. Heterocyclic Chem. (1964), 1, 59, 223. 89. Mitsch, R.A., J. Am. Chem. Soc. (1965), 87, 758. 90. Cavell, R.G., Dobbie, R.C. and Tyerman, W.J.R., Can. J. Chem. (1967), 45, 2849. 91. Smail, T., Miller, G.E., and Rowland, F.S., J. Phys. Chem. (1970), 74, 3464. 92. Powell, F.X. and Lide, D.R., J. Chem. Phys. (1966), 45, 1067. 93. Mathews, C.W., ibid. (1966), 45, 1068. 94. Simons, J.P., J. Chem. Soc. (1965), 5406. 95. Heicklen, J., Cohen, Ν. and Saunders, D., J. Phys. Chem. (1965), 69, 1774. 96. Lenzi, M. and Mele, Α., J. Chem. Phys. (1965), 43, 1974. 97. Modica, A.P., ibid. (1967), 46, 3663. 98. Burks, T.L. and Lin, M.C., ibid. (1976), 64, 4235. 99. Biordi, J.C., Lazarra, C.P. and Papp, J.F., J. Phys. Chem. (1976), 80, 1040. 100. Okabe, Η., Laufler, A.H. and Ball, J.J., J. Chem. Phys. (1971), 55, 373.


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