Anisotropies of Polarizability ofn-Alkanes

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Abstract
Anisotropies of polarizability, or optical anisotropies, of n‐alkanes have been calculated by the matrix method, on the basis of the additivity principle of bond polarizabilities. The geometrical model employed is the familiar GG′‐eliminated three‐state [trans (θ(T)=0°), gauche (θ(G)=120°), and gauche prime (θ(G′)=−120°)] rotational—isomeric model. For this model, expressions derived involve two parameters, EG the energy of the gauche bond relative to that of the trans, and Δαe=(α1—α2)C–C −2(α1—α2)C–H the effective anisotropy of the C–C bond, where (α1—α2)C–C and (α1—α2)C–H are the bond anisotropies of the C–C and C–H bonds. Numerical results are compared with experimental data in the pure liquid and in various solvents, determined by Clément and Bothorel from measurements of depolarizations of Rayleigh scattering. Data in CCl4 are fairly well fitted with theory by using a reasonable value of EG=800 cal/mole. Data in n‐pentane, n‐heptane, and in the pure liquid are greater than predicted by using acceptable values of EG, the difference being much more pronounced with longer chains. This effect is ascribed to the orientational order existing among geometrically asymmetric molecules in condensed phase.