Intramolecular dynamics of the overtone-induced isomerization of methyl isocyanide

Abstract

Overtone-induced isomerization of methyl isocyanide to methyl cyanide is studied by using classical trajectories on several potential-energy surfaces. The several potential-energy surfaces are variations of a potential which we have developed based on available experimental and ab initio results. The trajectory results for simple potentials which neglect stretch–bend interactions show that overtone excitation of a CH stretch to the v=6 level at total energies (including overtone excitation energy) of 75, 125, and 150 kcal/mol does not enhance the rate of isomerization. However, at an initial total energy of 200 kcal/mol, the isomerization rate is enhanced by as much as a factor of 3 by selective excitation of a CH stretching overtone. However, the mode specificity is sensitive to the potential-energy surface. When a more realistic potential is used in which the bending force constants are attenuated as a function of the bond lengths or in which nondiagonal quadratic coupling terms are included, the dominant reaction is CH bond dissociation. The rate coefficient for the CH bond dissociation is an order of magnitude greater than the rate of isomerization at 200 kcal/mol. The initial energy flow out of an excited CH stretch is rapid (occurring on a times scale of less than 0.5 ps) and is primarily into the methyl bending modes. The energy that flows into the bending modes does not transfer out over the time period of 5.4 ps that the trajectories were followed. The methyl bending modes act as an energy ‘‘sink’’.