Local order and transition dipole coupling in liquid methanol and acetone as the origin of the Raman noncoincidence effect

Model calculations are performed on the Raman noncoincidence effect (frequency difference between the isotropic and anisotropic components) observed for the C–O stretching band of liquid methanol and the C=O stretching band of liquid acetone. Microscopic liquid structures are obtained by Monte Carlo simulations, and coupling between molecular vibrations is introduced by the transition dipole coupling mechanism. Ab initio molecular orbital calculations are also performed to check the validity of the assumed direction of the transition dipole for the C–O stretching mode of methanol. The different signs of the Raman noncoincidence between the C–O stretching band of liquid methanol and the C=O stretching band of liquid acetone can be explained by the transition dipole coupling mechanism. The calculated magnitudes of the frequency separations between the isotropic and anisotropic components are in good agreement with the experimental results. Pressure dependence of the Raman noncoincidence is also calculated and compared with the experimental results. In the case of the C–O stretching band of liquid methanol, local anisotropy in the pressure-induced changes of the liquid structure is shown to be important for the pressure dependence of the Raman noncoincidence.