Classical dynamics studies of the unimolecular decomposition of nitromethane

Classical trajectories are used to investigate the unimolecular decomposition of nitromethane on three model potential energy surfaces. The surfaces differ mainly in the barrier height for the isomerization of nitromethane to methyl nitrite. The energies at the barrier to isomerization for the three surfaces are 216.4, 55.1, and 47.6 kcal/mol. Three primary decomposition pathways are observed: CH3NO2 →CH3NO°2 →CH3+NO2, CH3NO°2 →CH3ONO° →CH3O+NO, and CH3NO°2 →CH3ONO° →CH3+NO2. The dynamics results also show that there are two mechanisms for isomerization of nitromethane to methyl nitrite: (a) A two-step process of dissociation of nitromethane to CH3+NO2 and subsequent recombination of these radicals to form methyl nitrite and (b) a one-step process of concerted C–N bond breaking and C–O bond formation. Product energy distributions indicate that the products formed from C–N bond scission in nitromethane and C–O bond scission in methyl nitrite are indistinguishable. The branching ratio for the potential energy surface with the barrier height of 47.6 kcal/mol is close to the experimentally determined value near the threshold energy of the reactions, while the branching ratio for the potential energy surface with a barrier of 55.1 kcal/mol is an order of magnitude smaller than the experimental value. Isomerization via the dissociation–recombination mechanism occurs only on the surface with a large barrier height (216.4 kcal/mol) that prevents direct isomerization. Although isomerization on this surface is rare, the results of trajectories calculated on this surface indicate that this type of isomerization mechanism is feasible, even under collision-free conditions.