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

The vibrational dephasing of the nu(1) (C-H, C-D stretching) and nu(3) (C-H , C-D bending) symmetric motions of liquid acetonitrile in its light and fu lly 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 bandsha pe approach formulated by analytical Fourier transformation of the Kubo vib rational time correlation functions in order to derive the relaxation param eters in the frequency domain, The effects of the isotopic (CH3CN/CD3CN and vice versa) and chemical (CCl4) dilution on the bandshapes and on the vibr ational relaxation parameters have been studied, It was observed that the d ecay rate of nu(1) mode is insensitive to the isotopic dilution but varies appreciably with chemical (CCl4) dilution. The vibrational dephasing of nu( 3) mode is qualitatively, but not quantitatively, affected in the same way by chemical dilution and shows a slower modulation regime than that exhibit ed by the stretching mode, Unlikely from the latter, the nu(3) mode results are slightly affected by the isotopic dilution, Phase relaxation mechanism s 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. (C) 1999 Elsevier Science B.V. All rights reserved.