Assignment of the infrared spectra of the methanol trimer

Second-order Moller-Plesset perturbation theory (MP2) and density functional theory (DFT) were employed to optimize two cyclic conformations of the methanol trimer, denoted "bowl" and "chair," as well as a third noncyclic, "open-chain" conformation. This is the first time all three methanol trimer conformations have been studied at the same level of theory. Harmonic vibrational frequencies confirm that all stationary points are minima on the intermolecular potential energy hypersurface. The chair is consistently predicted to be the most stable conformer. The bowl and open-chain conformers are only slightly less stable and lie within 3 kcal/mol. Results indicate that the bowl structure contains a threefold rotational axis of symmetry which resolves a discrepancy over its symmetry in previous theoretical investigations. Further, this finding precludes the possibility that only the cyclic conformers of (CH3OH)(3) are the source of the five bands observed in the OH stretching spectrum of the methanol trimer recently obtained by infrared cavity ringdown laser absorption spectroscopy. From the ab initio frequencies, it is concluded that the bowl, chair, and open-chain conformations of (CH3OH)(3) must be present in the supersonic expansion in order to account for all five observed bands. (C) 1999 American Institute of Physics. [S0021-9606(99)30430-X].