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

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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

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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

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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

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3

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2

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

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

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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

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

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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

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

131

132

FLUORINE-CONTAINING FREE RADICALS

N /N

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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

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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

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

5.

HSU E T A L .

Reactions of CF, CHF, and CF

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 ) :

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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

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

4

134

FLUORINE-CONTAINING FREE RADICALS

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

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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

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

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

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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

> : >

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

136

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FLUORINE-CONTAINING FREE RADICALS

Figure 2.

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

ob-

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

H s u ET AL.

5.

Reactions of CF, CHF, and CF

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

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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

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

2

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FLUORINE-CONTAINING FREE RADICALS

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

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

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

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(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)

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

140

FLUORINE-CONTAINING FREE RADICALS

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

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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

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

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

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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

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

142

FLUORINE-CONTAINING FREE RADICALS

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

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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

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

1

5.

Hsu E T A L .

Reactions of CF, CHF, and CF

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

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

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

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

reaction

144

FLUORINE-CONTAINING FREE RADICALS

2 CF NO

2 CF 0 + N

CF NO + NO

CF 0 + N 0

2

2

2

2

2

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

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

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

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

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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



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

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

146

FLUORINE-CONTAINING FREE RADICALS

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

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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

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

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

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

FLUORINE-CONTAINING FREE RADICALS

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

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

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.

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

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

5.

HSU E T A L .

Reactions of CF, CHF, and CF Radicals 2

149

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