Statistical models for product energy distributions in bimolecular reactions with metastable intermediates

Abstract

Detailed statistical model (phase space and adiabatic channel) calculations of product translational energy distributions P( E t) are presented for a typical example of the substitution reactions of halogen atoms with unsaturated hydro- (halo-) carbons. The influence of the reaction geometry and of potential parameters for the angular and radial potential is investigated as well as the possibility of incomplete vibrational excitation during the short (calculated) lifetime of the intermediate. A simple statistcal formula with one free parameter is proposed to be useful for fitting experimental data. Comparison with published P( E t) from molecular beam experiments suggests for the class of reactions in general and in particular also for the recent example Cl + C 2H 3Br → C 2H 3Cl + Br that there is no conclusive evidence against complete (“statistical”) vibrational excitation in the intermediate on time scales between 0.1 and 1 ps. Although no proof is possible that vibrational excitation actually is complete on that time scale, the major trends of the experimental P( E t) with molecular size, for example, can be predicted on the basis of that assumption and good fits of experimental P( E t) are possible with both detailed statistical calculations including the effects of the radial and the angular potential and with the simple statistical formula.