Molecular dynamics in liquid cyclopropane. III. Depolarized Rayleigh scattering

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Abstract
Depolarized Rayleigh scattering profiles of pure liquid cyclopropanes H 6 and D6 were recorded in the temperature range 155-298 K and at room temperature for dilution at different concentrations in carbon tetrachloride. The analysis of these profiles shows that they cannot be interpreted by assuming only orientational fluctuations of the molecules. The symmetrized spectral densities Î(v) and v 2.Î(v) are well reproduced if one assumes that they result from a weighted sum of two components. The former, nearly Lorentzian, whose width at room temperature and different dilutions in carbon tetrachloride, is close to the rotational Raman one indicates the effect of molecular reorientations. At lower temperature this component becomes wider than the rotational Raman profile, which indicates an organization of the liquid in which the molecules tend to be oriented orthogonally to one other. The second component, much wider, is a damped oscillator profile. The characteristic relaxation time of this oscillator, which is shorter than the orientational correlation time, could be that of a fast variable governing the temporal evolution of the induced polarizabilities. The effect of the temperature on this relaxation time is discussed within the framework of the theories of induced spectra. The intensity of this component, which represents the major part of the spectrum, is of the order of magnitude normally expected for a spectrum produced by a dipole-induced dipole mechanism. It decreases with temperature, probably as a result of an effect of symmetrization of the molecular environment. Nevertheless, the eventual coupling of the orientational slow variable with the fast variable can have an influence on the ratio of the intensities of both components. Finally, the values of the correlation time and of the second and fourth moments associated with the fast variable are weakly temperature dependent and are such that the profile associated with this variable presents an exponential shape in a large frequency range