Depolarized rayleigh scattering and the mean-squared optical anisotropies of n-alkanes in solution

Abstract

A procedure has been developed whereby the mean-squared optical anisotropy 〈γ²〉 associated with isolated solute molecules can be obtained from measurements of the depolarized Rayleigh scattering by their solutions in optically isotropic and geometrically symmetric solvents. Depolarized Rayleigh ratios R_HV for light scattered at an angle of 90° by carbon tetrachloride solutions of 7 n-alkanes from C₅ to C₂₂ have been measured using a He-Ne laser (λ= 632.8 nm) as the light source. Intensity due to Raman scattering was eliminated by a narrow bandpass filter of 3 nm width. The scattered intensity due to collision induced anisotropy was estimated from the decrease in R_HV when a 0.3 nm filter was used, and a procedure is described for suppressing the contribution to R_HV from this source for the n-alkane solutions. Values of 〈γ²〉 for the n-alkanes obtained by extrapolating the present results to infinite dilution are considerably smaller than those reported by other workers. The dependence of 〈γ²〉 on chain length is well reproduced by calculations carried out using the valence optical scheme and rotational isomeric state theory, gauche states being assigned an energy of 500 ± 100 cal mol^–1 relative to trans in conformity with other evidence. Coincidence between calculated and observed optical anisotropies requires assignment of a value of 0.54 ų to the parameter Γ, which is related to the bond anisotropies by Γ=Δα_CC–1.95 Δα_CH according to numerical calculations for a ∠ CCC angle of 112°.