||The dynamics and spectroscopy of an adiabatic Hamiltonian, derived by McCoy and Sibert [J. Chem. Phys. 105, 459 (1996)], describing the bending motions of acetylene are presented and discussed. The resulting eigenfunctions of this model are interpreted using classical, semiclassical, and quantum mechanical descriptions of the vibrations. Using perturbation theory, the Hamiltonian describing the bends is reduced to two coupled hindered rotor Hamiltonians. This simple Hamiltonian predicts that local mode dynamics of the bending motion first occurs at about 6000 cm(-1) of excitation. This prediction is confirmed by the quantum mechanical studies. The hindered rotor Hamiltonian also predicts that l-type doubling leads classically to stable periodic orbits corresponding to planar motion. The extent of planar type motion is quantified using both semiclassical and quantum mechanical models. (C) 1996 American Institute of Physics.