Full dimensional theoretical study of the torsion-vibration eigenstates and torsional splittings of CH3OH

The vibrations of methanol are studied theoretically via a fully coupled treatment of the small amplitude motions and the large amplitude torsional mode. Van Vleck perturbation theory is used to find a representation in which the coupling between the vibrational states is reduced. The full vibration-torsion eigenstates are obtained subsequently via matrix diagonalization. The convergence of the perturbation theory and variational calculations are discussed. The torsion-vibration energies and the torsional tunneling splittings are compared with available spectroscopic data for all the fundamentals. The unusual inverted E–A torsional tunneling splitting pattern observed spectroscopically for rotationless torsion-vibrational states is predicted by our calculation.