THE VIBRATIONAL FREQUENCY OF THE DONOR OH GROUP IN THE H-BONDED DIMERS OF WATER, METHANOL AND SILANOL - AB-INITIO CALCULATIONS INCLUDING ANHARMONICITIES

The equilibrium structures of the three dimers are determined using quantum chemical ab initio techniques. The anharmonic vibrational frequencies are determined by one-dimensional numerical integration. Second-order Moller-Plesset perturbation theory (MP2) is used with valence triple-zeta basis sets augmented by first and second polarization functions, VTZ(2df, 2p), and corrections are made for the basis set superposition error. This level of approximation yields frequencies for the monomers that are too high (by 30 to 40 cm(-1)) and frequency shifts that are too large. Complete fourth-order perturbation theory (MP4-SDTQ) reproduces observed OH frequencies in the monomers and in the dimer with an error of +/- 10 cm(-1), but has been applied to the water dimer only. The MP4 results support a recent assignment of the 3601 cm(-1) band in the IR spectrum of water clusters to the donor OH group of the dimer. Combination of the MP4 results on the water dimer with the MP2 results on all three systems leads to estimated OH frequency shifts of -126 and -175 cm(-1) in the methanol and silanol dimers, respectively.