Isotopic and chemical dilution effects on the vibrational relaxation rate of some totally symmetric motions of liquid acetonitrile Original Research Article

The vibrational dephasing of the ν1 (C–H, C–D stretching) and ν3 (C–H, C–D bending) symmetric motions of liquid acetonitrile in its light and fully deuterated forms has been studied in the frame of the vibrational time correlation functions obtained as Fourier transforms of the isotropic Raman spectral distributions and interpreted within the Kubo theory. In addition, the experimental isotropic profiles have been analysed within the bandshape approach formulated by analytical Fourier transformation of the Kubo vibrational time correlation functions in order to derive the relaxation parameters in the frequency domain. The effects of the isotopic (CH3CN/CD3CN and vice versa) and chemical (CCl4) dilution on the bandshapes and on the vibrational relaxation parameters have been studied. It was observed that the decay rate of ν1 mode is insensitive to the isotopic dilution but varies appreciably with chemical (CCl4) dilution. The vibrational dephasing of ν3 mode is qualitatively, but not quantitatively, affected in the same way by chemical dilution and shows a slower modulation regime than that exhibited by the stretching mode. Unlikely from the latter, the ν3 mode results are slightly affected by the isotopic dilution. Phase relaxation mechanisms of these two motions of acetonitrile in the liquid state are proposed on the basis of these data, and a comparison is made with the results earlier published.