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

Second order Mo/ller–Plesset perturbation theory and density functional theory are employed to localize several stationary points on the potential energy surface of the cyclic methanol tetramer. Two cyclic isomers are identified: one of S4 symmetry, with methyl groups in up-down-up-down configuration, and a second one of Ci symmetry, with the methyl groups in up-up-down-down configuration. The latter minimum is 360 cm−1 above the S4 minimum, with a barrier of 475 cm−1 separating them. These isomers give rise to four asymmetric OH modes around 3300 cm−1. A model of the concerted proton transfer, S4→D2d→S4 (D2d is the transition structure), yields an estimate of 0.7 cm−1 for the tunneling splitting of the totally symmetric OH stretch vibrational fundamental. Raman spectra would show evidence of this fundamental and help to identify admixtures of the Ci isomer.