QUANTUM, SEMICLASSICAL AND CLASSICAL DYNAMICS OF THE BENDING MODES OFACETYLENE

The dynamics and spectroscopy of an adiabatic Hamiltonian, derived by McCoy and Sibert [J. Chem. Phys. 105, 459 (1996)], describing the bend ing motions of acetylene are presented and discussed. The resulting ei genfunctions of this model are interpreted using classical, semiclassi cal, and quantum mechanical descriptions of the vibrations. Using pert urbation theory, the Hamiltonian describing the bends is reduced to tw o coupled hindered rotor Hamiltonians. This simple Hamiltonian predict s that local mode dynamics of the bending motion first occurs at about 6000 cm(-1) of excitation. This prediction is confirmed by the quantu m mechanical studies. The hindered rotor Hamiltonian also predicts tha t l-type doubling leads classically to stable periodic orbits correspo nding to planar motion. The extent of planar type motion is quantified using both semiclassical and quantum mechanical models. (C) 1996 Amer ican Institute of Physics.