Intramolecular energy transfer in the HNC/HCN isomerization reaction: Quasiclassical state specific isomerization rates controlled by localized potential features

A model of the HCN molecule with two degrees of freedom, consisting of the H–NC stretch and the H–NC bend with the C≡N distance held fixed, was studied. The semiclassical technique of adiabatic switching was used to obtain trajectories at eigenvalues of metastable states of the system. Quasiclassical results for the isomerization rate from HNC to HCN at various stretch and bend excitations are given. Results show that the isomerization rate is highly dependent on the bend excitation and less so on the total energy of the system in contradiction to predictions of statistical theories. Examination of individual trajectories and surfaces of section reveal the development of chaos near the Nbend =6 level. The development of chaos is apparently responsible for the energy transfer between the modes and thus the isomerization rate. Examination of the potential surface reveals a distinct topological change near the Nbend =6 level. The region responsible for energy transfer is identified, and the possible mechanisms for this transfer are enumerated.