Monte Carlo rate constants for model CH3NC isomerization, determined at 200, 100, and 70 kcal/mole, disagree with theoretical predictions. Also, three different approximate methods of generating initial conditions at 200 kcal lead to divergent results. The molecule does not appear to us to obey the random lifetime assumption of conventional unimolecular rate theory at any of these energies. A discussion is given of the systematics of this kind of effect, and comments are made on the relationship between our results and those obtained in the laboratory.

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The effects of reactant vibrational, fine structure, and collision energy on the reactions of OCS+ with C2H2: Complementary studies of reactions in the [C2H2+OCS]+ system J. Chem. Phys. 94, 6459 (1991); 10.1063/1.460275The bendingcorrectedrotatinglinearmodel calculations of the rate constants for the H+H2 reaction and its isotopic variants at low temperatures: The effect of van der Waals well An empirically calibrated potential energy surface, obtained previously [I. Chem. Phys. 61, 21 (1974)], was used in a trajectory study of the effect of reactant energy partitioning on cross sectionfor CH] + H2 -> CH. + H. Artificial variations in the barrier position, and artificial and naturally occuring changes in the H masses, were also made. For natural CH) + H 2 , H2 vibration enhances and CH 3 out-of-plane bending depresses the cross section, at constant (25 kcal) translational energy of approach. The isotope substitution and barrier position effects are more complex and not predictable from triatomic A + BC generalizations. The relationship of these results to experimental ones is discussed.

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.

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