RAMAN NONCOINCIDENCE EFFECT AND INTERMOLECULAR INTERACTIONS IN LIQUIDDIMETHYL-SULFOXIDE - SIMULATIONS BASED ON THE TRANSITION DIPOLE COUPLING MECHANISM AND LIQUID STRUCTURES DERIVED BY MONTE-CARLO METHOD

Model calculations are performed on the Raman noncoincidence effect (f requency difference between the isotropic and anisotropic components) observed in the region of the S=O stretching band of liquid dimethyl s ulfoxide (DMSO). Liquid structures are obtained by Monte Carlo simulat ions, by using potential functions without specific interactions invol ving the hydrogen atoms of the methyl groups. Intermolecular coupling between the S=O stretches is evaluated by the transition dipole coupli ng mechanism. The magnitude and the direction of the transition dipole are calculated by ab initio molecular orbital calculations at the sec ond-order Moller-Plesset perturbation level with a basis set extended from 6-31G(2d,p). The calculated sign and magnitude of the Raman nonco incidence are in reasonable agreement with the observed, indicating th at the observed large Raman noncoincidence can be mostly explained by the liquid structures obtained without specific interactions involving the hydrogen atoms of the methyl groups. The pair distribution functi on of liquid DMSO is also calculated to the second order in the orient ation correlation of molecules. The origin of the Raman noncoincidence effect is discussed by comparing the cases of liquid DMSO, acetone, a nd methanol.