12 CO Product Vibrational Energy Distribution in the 3
O( P) + C O Reaction State-to-State Chemistry Downloaded from pubs.acs.org by UNIV OF TEXAS SW MEDICAL CTR on 10/02/18. For personal use only.
3
2
D. S. Y. HSU and M . C. LIN Chemistry Division, Naval Research Laboratory, Washington, D. C. 20375
3
The reaction of the O( P) atom with C O (carbon suboxide) was proposed to be one of several possible laser pumping reactions in an electrically initiated C O /O /He pulsed chemical CO laser system (1). The O + C O reaction has now been shown to occur mainly via the spin-forbidden path producing three ground electronic state CO molecules (2-4) 3
3
2
3
3
1
+
0( P) + C 0 ( z ) 3
2
g
+
2
2
2
2
+
3C0(X Σ )
(1)
ΔΗ^ = -115 kcal/mole. The spin-conserved route associated with the formation of CO and C O, however, was found to be less than 0.2% of the overall rate (2). In order to understand the dynamics of this unique process, we employed a cw CO laser to measure the vibrational population of the CO formed in the initial stage of the reaction. A detailed description of the laser-probing apparatus can be found elsewhere (5,6). A Pyrex flash tube was used in the present study to avoid the photodissociation of C O below 300 nm. The O( P) atom was generated by the photodissociation of NO . In all runs, 10 torr mixtures of C O and NO , diluted with He or SF , were flash-photolyzed at 290°K. C O was prepared by pyro lyzing the vapor of diacetyltartaric anhydride (7) at about 900°K, using a 10 mm ID x 500 mm length quartz tube. The products of decomposition were purified by trap-to-trap distilla tion employing appropriate slush baths. He, SF and NO were obtained in lecture bottles of the highest purity available from the Matheson Gas Products Company. The CO vibrational population distribution was determined by analyzing the initial portion of the time-resolved absorption curves according to the method described previously (5,6). The initial distribution was evaluated by least-square extrapolating (5) the relative population ratios N /N to the appearance time of absorption, 6.8 ysec after flash initiation. A typical mixture used in the experiments was N0 :C 02:SF = 3:1:46, flashed at 2
2
3
2
3
2
3
2
2
6
3
2
6
v
2
3
121
2
6
2
122
STATE-TO-STATE
CHEMISTRY
0.64 k J . The CO v i b r a t i o n a l energy d i s t r i b u t i o n s presented i n Table 1 were normalized to v=2 because of the presence of s l i g h t C3O2 d i s s o c i a t i o n near 300 nm producing a small amount of CO at v=0 and 1. However, c o r r e c t i o n s were made by p h o t o l y z i n g the mixtures of C 0 and S F under the same experimental c o n d i t i o n s employed i n the r e a c t i o n . 3
Table 1.
2
6
CO V i b r a t i o n a l Energy D i s t r i b u t i o n i n the 0 ( P ) + C 0 Reaction 3
V
0
3
2
1
2
3
4
5
6
7
δ
9
.56
.35
.23
.15
.09
.06
.03
.01
.21
.12
.06
.03
.02
.01
Expt
4.0
1.7
1.0
Calc
2.5
1.6
1.0
.61
.36
10
To e l u c i d a t e the dynamics of t h i s r a t h e r unique r e a c t i o n , we compared the observed d i s t r i b u t i o n with t h a t p r e d i c t e d by a simple s t a t i s t i c a l model based on the method employed p r e v i o u s l y (6^,8). Assuming t h a t the C 0 complex c o n s i s t s of three i n d i s t i n g u i s h a b l e C 0 - s t r e t c h i n g modes, each having an average v i b r a t i o n a l energy E , then the population of the CO molecules formed a t the vth l e v e l i n the 0 + C 0 r e a c t i o n can be estimated by the f o l l o w i n g e x p r e s s i o n , 3
2
v
3
N
I g(n,v) ( E . - n E n>v>0
v
t
l
u
+ aE )
t
L
2
(3)
s
v
where g(n,v) = 3(n-v+l) represents the number of ways one can populate the vth l e v e l of the three CO modes with η v i b r a t i o n a l quanta. ( E t o t ~ ^ v ^ z ) i s q u a n t i t y which i s p r o p o r t i o n a l to the t o t a l number of ways (9) the remaining energy ( E ^ - n E ) can be randomly d i s t r i b u t e d among the r e s t of v i b r a t i o n a l modes, excluding the one which becomes the r e a c t i o n c o o r d i n a t e . P r a c t i c a l l y , the choice of the r e a c t i o n coordinate and the assignments o f the v i b r a t i o n a l frequencies of the a c t i v a t e d complex are not c r i t i c a l i n these c a l c u l a t i o n s because the z e r o p o i n t energy c o r r e c t i o n term, aEz = aEhv/2 i s unimportant a t lower n's (or v ' s ) , which are more densely populated. The p r e d i c t e d s t a t i s t i c a l d i s t r i b u t i o n based on E t o t = -ΔΗ-|° + E + 5RT/2 = 118.3 kcal/mole ( t a k i n g E = 2.2 kcal/mole (2) and Τ = 2 9 0 ° K ) , E = 5.8 kcal/mole and s = 3N-6-4 = 8, agrees c l o s e l y with the observed d i s t r i b u t i o n as shown i n the Table. This seems to i n d i c a t e t h a t the 0 + C3O2 r e a c t i o n probably occurs v i a a C3O3 complex which may s u r v i v e several v i b r a t i o n s before breaking i n t o three CO's. In a p r e l i m i n a r y experiment c a r r i e d out w i t h N 0 , however, i t was found t h a t the C 0 formed i n the r e a c t i o n i s n o t i c e a b l y c o o l e r . This f i n d i n g could be explained by the f o l l o w i n g mechanism, using the l a b e l e d r e a c t i o n as an example: n
+a
s
a
o t
a
a
v
1 8
2
1 6
v
12.
Hsu AND LIN
123
CO Product Vibrational Energy
*0
*0
*0( P) + 0=C=C=C=0 -> 0=C-C=C=0
0=C-C-C=0 ->-
*0C + ( C 0 ) -> *0C + 2 C 0 . f
2
2
f
+
+
Since the attack o f 0 atoms occurs sideways, the most l i k e l y r e a c t i o n coordinate i s the s k e l e t a l 0=C-C-C=0 bending motion. This bending v i b r a t i o n , aided by the overlapping of the unpaired e l e c t r o n s , may lead to the concerted decomposition of * 0 C 0 i n t o a h o t t e r C*0 and t r a n s i e n t ( C 0 ) t h a t immediately d i s s o c i a t e s i n t o two CO's each c a r r y i n g equal amount of energy. Further study on t h i s r e a c t i o n i s underway; more d e t a i l e d r e s u l t s w i l l be reported i n a f u t u r e p u b l i c a t i o n . 3
2
2
Literature Cited 1. 2. 3. 4. 5. 6. 7. 8. 9.
L i n , M. C. and Bauer, S. H . , Chem. Phys. L e t t . , (1970), 7, 223. P i l z , C. and Wagner, H. G r . , Z. Physik. Chem., N. F., (1974), 92, 323. L i u t i , G . , Kunz, C. and Dondes, S., J . Amer. Chem. Soc., (1967), 89, 5542. Williamson, D. G. and Bayes, K. D . , J. Amer. Chem. Soc., (1967), 89, 3390. L i n , M. C. and Shortridge, R. G . , Chem. Phys. L e t t . , (1974), 29, 42. Shortridge, R. G. and L i n , M. C., J . Chem. Phys., (1976), 64, 4076. Melville, H. and Gowenlock, B. G . , "Experimental Methods in Gas Reactions", MacMillan and Co. L t d . , London, 1964, p. 197. L i n , M. C., Shortridge, R. G . , and Umstead, M. E., Chem. Phys. L e t t . , (1976), 37, 279. Whitten, G. Z. and Rabinovitch, B. S., J. Chem. Phys., (1964), 41, 1883.
