J. Phys. Chem. 1988, 92, 178-182
178
Collision- Induced Dissociation of Cyclohexadiene by a Vibrationally Hot Coilider A. Pashutski and I. Oref* Department of Chemistry, Technion-Israel Institute of Technology, Haifa 32000, Israel (Received: June 8 , 1987)
A hot jet of 1,3,5-trimethyl-l,l,3,5,5-pentaphenyltrisiloxane (TTP)is allowed to collide with bulk cold cyclohexadiene(CHD) molecules. A CHD molecule decomposes ( E , = 42.5 kcal/mol) when collisionally excited by a hot TTP molecule. The probability of transferring 43 kcal/mol or more is found to be - 5 X When the experimental results are fitted to the exponential collisional energy transfer probability model, it is found that the average energy transferred per collision is 3.2
-
kcal/mol.
Introduction Energy transfer between molecules in the gas phase takes place on every collision.'-4 The energy evolution can be of the type vibration to vibration (V-V), vibration to rotation (V-R), or vibration to translation (V-T). These are all important in the excitation and deexcitation of reacting molecules where the internal vibrational-rotational energy content plays a role in the decomposition process. There are two regions in the energy spectrum of polyatomic molecules where the transfer behavior is experimentally recorded to a large extent. At low levels of excitation short-range Landau-Zener-type transfer takes place, and resonance V-V transfer is Lhe predominant route.%" At higher levels of excitation, the long-range forces play the major r 0 1 e , ~ J ~and . ' ~ the energy transfer process is more efficient. The noble gases are 2-3 orders of magnitude more effective in deactivating a highly excited polyatomic molecule than ,~~ bath molecules a molecule excited to its u = 1 l e ~ e l . ~Polyatomic are very effective and can deactivate in one collision an excited molecule to below its threshold energy for decomposition. In spite of the fact that there are many experimental facts concerning vibrational energy transfer at a high level of excitation, the situation is not very satisfying as far as detailed models are concerned. There are empirical models such as the step ladder, ~ ~ ~ ~are ' ~used to correlate the exexponential, or P o i s s ~ n . 'which periments. They determine the value of ( AE),the average energy transferred per collision or ( AE),,the average energy transferred in a down collision. ( AE) is the parameter whose value is adjusted to fit the model to the data. Its values range between 1 and 6 kcal/mol depending on the efficiency of the collider. Since ( A E ) defines a probability distribution function, there are low-probability
(1) Tardy, D. C.; Rabinovitch, B. S. Chem. Reu. 1977, 77, 369. (2) Zellweger, J. M.; Brown, T. C.; Barker, J. R. J . Phys. Chem. 1986,90, 461. ( 3 ) Richmond, G.; Setser, D. W. J . Phys. Chem. 1980, 84, 2699. (4) The same effects occur in gas-surface collisions: see, for example, Kelley, D. F.; Kasai, T.; Rabinovitch, B. S. J . Chem. Phys. 1980, 73, 5611; J. Phys. Chem. 1981, 85, 1100. (5) Earl, B.;G a m s , L. A,; Ronn, A. M . Acc. Chem. Res. 1978, 11, 183. (6) Lambert, J. D. Vibrational and Rotational Relaxation in Gases; Clarendon: Oxford, 1977. (7) Oref, I.; Rabinovitch, B. S. Acc. Chem. Res. 1979, 12, 166. (8) Mandich, M. L.; Flynn, G. W. J . Chem. Phys. 1980, 73, 1265. (9) Cravens, D.; Shields, F. D.; Bass, H . E . J . Chem. Phys. 1979, 71, 2797. (10) Apkarian, V. A,; Weitz, E. J . Chem. Phys. 1979, 71, 4349 (11) Allen, D. C.; Price, T. J.; Simpson, C. J. S. M . Chem. Phys. 1979, 41, 449. (12) Forst, W.; Bhattacharjee, B. C. Chem. Phys. 1978, 30, 217; Chem. Phys. 1979, 37, 343. ( 1 3 ) Oref, I.; Rabinovitch, B. S. Chem. Phys. 1977, 26, 385. (14) Troe, J. J . Phys. Chem. 1979, 83, 114. ( I 5 ) Quack, M.; Troe, J. In Gas Kinetics and Energy Transfer; Chemical Society Specialist Periodical Reports; Chemical Society: London, 1977; Vol. 11, p 175. (16) Tardy, D. C.; Rabinovitch, B. S. J . Chem. Phys. 1966, 45, 3720; J . Chem. Phys 1968, 48, 1282.
collisions which are energetic and can transfer a large quantity of energy. It is the purpose of this work to report the probability of collisions in which large quantities of energy are transferred in a binary collision between a hot bath and a cold reactant molecule.
Method Thermally excited "bath" molecules (B) in a jet are allowed to collide with cold reactant molecules. The jet molecules are high molecular weight 1,3,5-trimethyl- 1,1,3,5,5-pentaphenyItrisiloxane (TTP) : Ph
Ph
I
I
the
the
Ph
I I Ph-Si-0-Si-0-Si-Ph
I
I
he
These molecules are stable at high temperatures and with 210 normal modes are highly excited internally at moderate temperatures. When a highly excited B molecule collides with a reactant molecule (R), there is a finite probability that a large quantity of energy AE will be transferred from B* to R. The latter will decompose when AE is above its threshold energy for decomposition. The measure of decomposition then is a measure of the collisional energy transferred. 1,4-CycIohexadiene (CHD; Eo = 42.5 kcal/mol) was chosen as R. Its one-step unimolecular decomposition is well-known and does not involve radical intermediate~.'~-~~
CHD
Theory The processes that take place in the reactor are given by the kinetic scheme activation R
+ B* -ki-,R* + B
deactivation
-+ -
R* + M deactivation at the wall
R*
dissociation R*
kl
k2*
R
M
R
products
(17). Robinson, P. J.; Holbrook, K. A. Unimolecular Reactions; WileyInterscience: New York, 1972. (18) Ellis, R. J.; Frey, H . M . J . Chem. SOC.A 1966, 553. (19) Benson, S. W.; Shaw, R. Trans. Faraday SOC.1967, 63, 985. (20) Orchard, S. W.; Ramsden, J. Int. J . Chem. Kinet. 1982. 14. 43.
0022-3654/88/2092-0178$01.50/00 1988 American Chemical Society
The Journal of Physical Chemistry, Vol. 92, No. I, 1988 179
Collision-Induced Dissociation of Cyclohexadiene TABLE I: Summary of the Results init press. CHD, lower oven run mTorr f 5% temp, K f 3 1 2 3 4 5 6 7
3 0 0 f 15 82 f 4 58 f 3 110 f 5 60 f 3 80 f 4 58 f 3
upper oven temp, K
498 498 498 473 473 463 468
603 603 603 787 831 787 831
f 10 f 10 f 10
f5 f 17
f 13 f 10
bath press., mTorr
from GLC
e, h-l,
[CHD1/[CHDln
190 f 37 190 f 37 190 f 37 39 f 8 39 f 9 20 f 4 28 f 6
0.59 f 0.06 0.44 f 0.06
(2.3 f 0.2) X lo-' (3.1 f 0.3) X IO-'
0.57 f 0.45 f 0.66 f 0.45 f
(3.6 (3.0 (3.5 (4.2
where * indicates an excited molecule, M is a collision partner (R, B, or products), and kz is the deactivation rate coefficient specific to each member of M. Steady-state treatment of R* and allowing for the fact that H2 is a weak collider compared with R and benzene, Le., k2(H2)
