Ab initio study of the vibrational properties of acetic acid monomers and dimers

The vibrational spectra of CH3COOH, CH3COOD, CD3COOH and CD3COOD, either mo nomeric or in two dimeric structures (centrosymmetric or side-on linear, wi th two bent hydrogen bonds), were studied by ab initio calculations at the DFT/B3LYP/6-31++G** level. The force field, the potential energy distributi on and the derivatives of the electric dipole moment with respect to intern al coordinates were analysed in addition to the vibration wavenumbers. A co mprehensive discussion of the infrared intensities between 1150 and 1500 cm (-1) is made for the twelve structures studied. The calculated infrared int ensities of monomers are in close agreement with the experimental spectra. In particular, the puzzling wavenumber increase that had been observed in a rgon matrices for the intense band assigned to the COH angle bending delta COH, by substituting OD for OH, is explained without invoking the previous hypothesis of the tunnelling of the H atom between the two O atoms. It is r elated to the decoupling of delta COH from a mode involving the stretching of the C-C and C-O bonds with opposite phases.