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 (f requency difference between the isotropic and anisotropic components) observed for the C-O stretching band of liquid methanol and the C=O st retching band of liquid acetone. Microscopic liquid structures are obt ained by Monte Carlo simulations, and coupling between molecular vibra tions is introduced by the transition dipole coupling mechanism. 4b in itio molecular orbital calculations are also performed to check the va lidity of the assumed direction of the transition dipole for the C-O s tretching mode of methanol. The different signs of the Raman noncoinci dence between the C-O stretching band of liquid methanol and the C=O s tretching band of liquid acetone can be explained by the transition di pole coupling mechanism. The calculated magnitudes of the frequency se parations between the isotropic and anisotropic components are in good agreement with the experimental results. Pressure dependence of the R aman noncoincidence is also calculated and compared with the experimen tal results. In the case of the C-O stretching band of liquid methanol , local anisotropy in the pressure-induced changes of the liquid struc ture is shown to be important for the pressure dependence of the Raman noncoincidence.