Contributions of the interaction of internal rotation with other vibrations to the effective potential energy for internal rotation in molecules with symmetric internal rotors

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Abstract
The theory of vibration–large amplitude internal motion interaction in molecules is used to calculate these contributions to the effective Hamiltonian for internal rotation when the internal rotor has a threefold axis of symmetry. Calculations are made for four deuterium isotopic species of methyl alcohol, two isotopic species of acetaldehyde and trifluoroacetaldehyde, and perfluoroacetyl fluoride. The dynamic contribution to the potential energy coefficient V6 ranges from −3 to −6 cm−1. The potential energy coefficient V3 shows a dependence of a few tens of cm−1 upon the state of the perpendicular vibrations. A cos6τ dependence of the reduced torsional coefficient is of the order of 0.05% of Gττ. The mixing of internal rotation with other vibrations cannot account for the decrease in barrier for methyl alcohol with deuterium substitution; actually the calculation gives an increase in barrier of comparable magnitude to the decrease observed for CD3OH compared to CH3OH.