Unimolecular reactions in isolated and collisional systems: is the transition-state rate an upper limit for the isomerization of stilbene?
The Journal of Physical Chemistry
The effect of few-body (e.g., binary) collisions on photochemical trans-cis-stilbene isomerization was studied by measuring the transient fluorescence decay from isolated-molecule conditions up to 5 atm of methane gas pressure at room temperature. Simple statistical considerations were found to account for the thermal isolated-molecule decay using as a basis the energy-dependent rates k(Ex) measured in supersonic jet experiments. The isomerization rate was found to increase with methane
... ith methane pressure throughout the 0-5-atm range. In statistical theories of barrier crossing the transition-state rate at temperature T, kmT(T), represents an upper limit for the reaction rate and is expected to be equal to k( T), the thermal average of k(Ex). However, contrary to expectation, for pressures greater than about 2 atm the isomerization rate was found to exceed k(n by over a factor of 2 at 5 atrn and by as much as a factor of 10 at pressures up to 100 atm. Relations between the various types of experimentally observed rate constants are explored, including a method for the direct measurement of k(T). Possible origins for the nonstatistical behavior of the stilbene-methane collision dynamics are also discussed. (6) Felker, P. M.; Zewail, A. H. J. Phys. Chem. 1985, 89, 5402. (7) Lee, M.; Holtom, G. R.; Hochstrasser, R. M. Chem. Phys. Lett. 1985, ( 8 ) Rothenberger, G.; Negus, D. K.; Hochstrasser, R. M. J. Chem. Phys. 83.