Monte Carlo Calculations. IV. Further Studies of Unimolecular Dissociation

Abstract

Distributions of lifetimes with respect to dissociation were determined vs energy for a variety of rotating anharmonic triatomic molecular models. About 235 000 trajectories were included in the study. Results are in adequate agreement with the Rice—Ramsperger—Kassel—Marcus (RRKM) theory as regards high-pressure dissociation rate vs energy. However, some models (especially those with widely separated vibration frequencies) fail to conform to the necessary RRKM assumption that lifetimes are randomly distributed. A theory of the lifetime distributions is developed which accounts for the computed results. Application of this theory to actual molecules indicates that intramolecular energy relaxation is complete in about 10—11 sec regardless of molecular complexity. Nonrandom lifetime distributions are likely only for a few triatomic molecules, or under exceptional experimental conditions. The RRKM theory is then endorsed for all reactions to which it has been applied. Partition of excess energy between product vibration and translation is discussed.