Evaluation of Principal Molecular Polarizabilities

At least three observational equations are necessary (unless, from symmetry, a polarizability ellipsoid of revolution, with two equal semi-axes, can be safely foreseen, e.g. as with HX, CH3X, CHX3, etc.). The first of these is provided by (22), whereby the sum 61 + 62+63 may be extracted from the electronic polarization FP of a molecule  

A second equation, sometimes available, is (23), where A is the depolarization factor of light scattered transversely by the substance under examination  

The third equation involves the so-called Kerr Constant B—the birefringence observed when polarized light of wavelength A traverses a 1-cm path through an otherwise isotropic dielectric subjected to a unit electric field applied at 90° to the light beam and at 45° to the plane of polarization. By the Langevin-Born orientation theory, B can be written as (24), 

Only a minority of distortion polarizations are available from experiment, a fairly extensive study of the dependence of the total dielectric polarization of a gas upon absolute temperature being required to provide a confident value for DP via the Debye equation 

In some cases DP may be estimated as AP + Rm by calculating AP s from spectroscopic data in ways illustrated by Le Fevre and Narayana Rao(1954, 1955). All else failing, it can be noted that DP/EP is commonly ca. 1-1. 

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