Light scattering studies of rotational and vibrational relaxations of acetonitrile in carbon tetrachloride

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Abstract
The rotational and vibrational relaxation times of acetonitrile–carbon tetrachloride solutions were investigated as a function of concentration, viscosity, and temperature using depolarized Rayleigh and Raman scattering. Using a Fabry‐Perot interferometer and single frequency laser source, we have shown that reliable results for the single particle orientational correlation times (τs) for CH3CN can be obtained by carrying out a concentration dependent depolarized Rayleigh scattering study. Raman scattering was shown to yield inconsistent results for τs in CH3CN. At constant viscosity, it was found that the Rayleigh scattering relaxation time (τRay) of CH3CN in CCl4 does not change with CH3CN concentration, indicating that the orientational pair correlation factor of liquid CH3CN is close to unity. This result suggests that the dynamic pair correlation in CH3CN is just as important as the static pair correlation. The experimental data were also compared with the predictions of the hydrodynamic stick and slip models for a rotational diffusion. The CH3CN data were found to be close to the prediction of the slip model. The isotropic relaxation time (τiso) of the C≡N stretching mode was also studied as a function of concentration and viscosity using Raman spectroscopy. This viscosity dependence of τiso also decreases with decreasing number density of CH3CN, suggesting that pair correlations are also important in the Raman scattering of CH3CN.