Classical trajectory calculations on the methyl isocyanide molecule at energies above 25,000 cm-l confirm that the rate of reaction to methyl cyanide is bimodal, with a very fast rate before 0.1 ps, and a slower rate from then on. We conclude that before 0.1 ps, the reacting molecules are unrandomized, but thereafter, they are essentially randomized, with decay to products being, to a good approximation, pure exponential. We estimate that the time for randomization is roughly 0.3 ps at energies near the reaction threshold of 13,500 em-'. 0 1994 John Wiley & Sons, Ine.
Computational Procedure
The potential energy surface for the isomerization of CHsNC was that of Sumpter and Thompson 141, slightly modified so as to allow for nonequivalent motions of the three hydrogen atoms [5,61. The random starting conditions for the trajectories were chosen in the usual way [3,71: the molecule was considered to be in its equilibrium configuration and to each atom was assigned a random velocity in each Cartesian direction; then the overall translational and rotational motions were removed, and the residual internal energy was scaled to the required starting value for the vibrational energy. The methods for integrating the trajectories have been described at length elsewhere [8,9], and for this study, because of CPU-time limitations, we relaxed the error criterion [61 for conservation of the total energy from 1 part in 1OI2 to 1 part in lo6; only rotationless trajectories are considered in this article, at energies near 25,000,33,000,41,000, and 49,000 cm-l above the potential minimum for the CH3NC molecule, (i.e., at 1.0 e.v. separations).