Depolarized light scattering in dilute solutions of alkanes: A comparison of the bond additive and interacting atom approximations to the molecular polarizability

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Abstract
The molecular polarizability of a few small alkane (4–10 bond) chains has been represented by (1) an interacting atom model (IAM), wherein the atoms are treated as isotropic point polarizabilitiesinteracting by the dipole tensor; and (2) the bond additive approximation (BAA) in which each bond is assigned an axially symmetric polarizabilitytensor, and the total molecular polarizability is the sum of the individual bond values. For selected values of the g a u c h e–t r a n s energy difference (0.3 kcal/mole), the calculated mean anisotropy per backbone atom 〈g2〉/N increases linearly with N for the IAM and is essentially independent of N in the BAA. Orientational correlation functions have been determined for several second rank tensors characterizing the flexible chains using a modified version of Fixman’s Brownian dynamics programs. The orientational correlation functions displayed an effective nonanalytic decay for short times merging into an exponential for long times. Single particle correlation times for the IAM increased more rapidly with N than did those of the BAA. Relaxation of the end‐to‐end vector (actually its second rank analog) was found to be the slowest process, followed by the IAM and the BAA polarizabilities, and finally the fastest was a local rotational mode.