Separation of collision-induced from intrinsic molecular depolarized Rayleigh scattering optical anisotropy of the C—Cl bond

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Abstract
A procedure is demonstrated for separating the intrinsic molecular component of the depolarized Rayleigh scattering (DRS) from the transient, collision-induced contribution. It depends on the use of two interference filters of differing breadths (18 and 53 cm–1, respectively) to distinguish the narrow reorientational component of –1 half-width from the much broader collisional scattering. The procedure is applied to CCl4 and four other spherically symmetric molecules in the liquid state. All exhibit small residual scattering (Rayleigh ratios RHV≈ 0.01–0.02 × 10–6 cm–1) at low frequencies ω from the central line, in addition to copious induced scattering over a broader range of ω. Approximately half of the scattering by neat liquid CHCl3, (CH3)2CHCl, (CH3)3CCl and CH3CCl3 is collision induced. Measurements on CCl4 solutions of each of these anisotropic molecules, corrected for collisional scattering and extrapolated to infinite dilution, yield molecular anisotropies γ2= 2.6, 1.6, 2.2 and 2.1 Å6, respectively. These results give ΓCCl=ΔαCCl–ΔαCH= 1.6, 1.5, 2.0, 2.0 Å3, respectively, where ΔαCCl and ΔαCH are the indicated bond anisotropies. They are in good agreement with values obtained from electric birefringence and absolute Raman intensities. Discordant values of γ2, and hence of ΓCCl, found by Bothorel and co-workers are in error due to their failure to eliminate collisional scattering. Conformational averages 〈γ2〉 for ClCH2CH2Cl and for meso and racemic CH3CHClCH2CHClCH3 calculated by rotational isomeric analysis using ΓCCl= 1.5 Å3, as found for (CH3)2CHCl, are in good agreement with values measured in CCl4, viz., 5.5, 2.2 and 0.5 Å6, respectively.

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