Raman spectra and liquid structures of the N,N-Dimethylformamide /carbontetrachloride binary mixtures

The polarized Raman spectra have been calculated for the amide I band of the binary liquid mixture of N,N-dimethylformamide (DMF) and carbon tetrachloride (CCl4) with a time-domain computational method. In this method (the extended MD/TDC/WFP method), the liquid structures and dynamics are calculated by the molecular dynamics (MD) method, and the wave function of the Raman excitation is time-evolved by the vibrational Hamiltonian H(t) evaluated every time step. In constructing H(t), the diagonal frequency modulations (of individual molecules) controlled by the electric field from the surrounding molecules and the off-diagonal (intermolecular) vibrational coupling controlled by the transition dipole coupling (TDC) mechanism are simultaneously taken into account [1,2]. In calculating the electric field from CCl4 molecules, the atomic quadrupolar effect [3] is also taken into account. The magnitude of the noncoincidence effect (the frequency separation between the isotropic and anisotropic Raman bands) is calculated as 14.9, 12.2, 9.5, and 5.5 cm-1 at the volume fraction of DMF = 1.0, 0.7, 0.4, and 0.1, in reasonable agreement with the observed value (14.6, 12.0, 9.1, and 4.2 cm-1 [4]). The shoulder on the high-frequency side of the observed isotropic Raman band at DMF = 0.4 and 0.1 is also reproduced (allthough rather exaggerated) by the calculation. The relation between this spectral feature and the liquid structure will be discussed.