Kerr effect ground electronic terms

As a first example consider birefringence in an electric field by a gas of molecules with D3h symmetry and a twofold degenerate ground electronic term belonging to representation E. Using the explicit form of the matrices of the components of the operators of the dipole moment [Eq. (20)] and polarizability and by means of Eqs. (84) and (85a)-(85c), we obtain for the molecular constant in the Kerr effect in the E term case under consideration the expression... 

The Kerr constant is related to the difference between the refractive indices of the medium parallel and perpendicular to an applied field, and these may be calculated for the assumed conformations. On this basis a 50 50 A-Meax lV-Meeq equilibrium was estimated for iV-methylpiperidine and an 80% N-Hai preference for piperidine in benzene solution.116 Some theoretical support117 was produced for this latter preference in terms of a more effective overall electron bonding in the iV-Hax conformer. These results are very much out of line with those estimates based on other methods and the Kerr-constant work has been criticized3 on the grounds of insufficient accuracy in the bond polarizabilities used in the calculations.118... 

The Kerr Effect in Molecules with Ground Electronic G3/2 Terms... 

It follows from the preceding results that the electro-optical properties of molecules in degenerate electronic states should have unusual temperature dependence, which is absent in the case of nondegenerate states. Even for nondipolar degenerate electronic states (e.g., for states in which the reduced matrix elements of the dipole moment are zero) for certain relationships between the vibronic constant and the temperature, there may be a quadratic dependence of the Kerr effect on p, similar to that observed in the case of molecules that are simultaneously anisotropic polarizable and possess a proper dipole moment. The nonlinear dependence on p under consideration is due exclusively to the vibronic interaction that redetermines the vibronic spectrum and leads to different polarizability in different vibronic states. This dependence on p has to be distinguished from that which arises due to the nonzero value of the dipole moment in the initial ground electronic state (e.g., as in the case of the E term in molecules with D3h symmetry). The two sources of the... 

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