Infrared intensities of liquids .2. integrated absorption intensities of CH3OH, CH3OD, CD3OH and CD3OD and dipole moment derivatives of methanol

This paper presents the analysis of the complete set of vibrational intensities of four isotopomers of methanol, The absolute infrared absorption intensities of liquid methanol in four isotopic forms have been reported recently. In that work, spectral intensities were separated into the integrated intensities of different transitions by comparing the spectra of different isotopomers, and dipole moment derivatives with respect to valence displacements were calculated under the simplest approximations. For many bands it was not possible to determine the integrated intensity in this way because of overlap of several bands, and for others it was clear that the determination was too subjective. This paper first describes an attempt to improve this situation by using a more objective separation of the contributions to the intensity from different bands, by fitting the imaginary molar polarizability spectra with classical damped harmonic oscillator bands or Gaussian bands and calculating the entire area under each component Sand. The integrated intensities so obtained are compared with those reported previously, and a set of accepted integrated intensities for all vibrations is presented. These accepted intensities are then converted to transition moments and analyzed to obtain the dipole moment derivatives with respect to symmetry coordinates, partial derivative mu/partial derivative S. The analysis uses the eigenvectors from a normal coordinate calculation that fits the reliably known fundamental wavenumbers of CH3OH, CH3OD, CD3OH and CD3OD, corrected for anharmonicity where possible, to better than +/- 1.5 cm(-1) on average, and that also fits the experimental near-identity of the wavenumbers and intensities of the CO stretching bands of CM3OH and CH3OD. These calculations were guided by literature ab initio calculations on isolated CH3OH, but an empirical normal coordinate calculation was preferred because the experimental data show clearly that some of the vibrations are not properties of isolated molecules. For lack of other evidence, the directions of the dipole moment derivatives of the A' modes were taken from Torii and Tasumi's recent ab initio calculation. Dipole moment derivatives with respect to internal coordinates, partial derivative mu/partial derivative R, were calculated from the partial derivative mu/partial derivative S. The resulting values for liquid methanol are compared with values for the isolated molecule calculated with an MP2/6-31G err basis set by Torii and Tasumi. For the stronger fundamentals the agreement is good except for the OH and OD stretching vibrations. This suggests that the only hydrogen vibration whose intensity is strongly affected by the hydrogen bonding is the stretching vibration. This in turn implies that it is the charge flux, not the effective charge on the hydrogen atom, that is sensitive to hydrogen bonding. The results of this and other work from this laboratory suggest that most vibrational intensities may not be strongly dependent on phase. (C) 1997 Elsevier Science B.V.