Vibrational spectra of the methanol tetramer in the OH stretch region. Twocyclic isomers and concerted proton tunneling

Second order Moller-Plesset perturbation theory and density functional theo ry are employed to localize several stationary points on the potential ener gy surface of the cyclic methanol tetramer. Two cyclic isomers are identifi ed: one of S-4 symmetry, with methyl groups in up-down-up-down configuratio n, and a second one of C-i symmetry, with the methyl groups in up-up-down-d own configuration. The latter minimum is 360 cm(-1) above the S-4 minimum, with a barrier of 475 cm(-1) separating them. These isomers give rise to fo ur asymmetric OH modes around 3300 cm(-1). A model of the concerted proton transfer, S-4-->D-2d-->S-4 (D-2d is the transition structure), yields an es timate of 0.7 cm(-1) for the tunneling splitting of the totally symmetric O H stretch vibrational fundamental. Raman spectra would show evidence of thi s fundamental and help to identify admixtures of the C-i isomer. (C) 2001 A merican Institute of Physics.