Cotton-Mouton effect measurements

Note 3 The divergence temperature for nematogens can be measured by using the Kerr effect or Cotton-Mouton effect or by light-scattering experiments. 

The second high-frequency term involves a sum over all discrete states and an integration over the continuum states the difficulties involved have been outlined before. Little is known about the continuum states, but what few calculations there are for simple systems92 suggest that they may be at least as important as the discrete states. For this reason early calculations were done in the closure approximation, notably by Van Vleck in the 1930 s. The difficulties of calculating xHF have been reviewed by Weltner.93 Experimentally xHF may be obtained from rotational magnetic moments. For linear molecules these can be obtained from molecular-beam experiments, which also measure the anisotropy x Xi- directly. The anisotropies may also be derived from crystal data, the Cotton-Mouton effect and, recently, Zeeman microwave studies principally by Flygare et al.9i... 

Computational analysis has been limited, on the other hand to the gas phase [40, 184, 185, 189, 190], However, optical apparata designed for measurements in the gas phase of other birefringences, as the Kerr and Cotton-Mouton effects, may in principle be adapted to measure JB and MEB. It is thus probably useful to provide reliable predictions of the magnitude of the effect. Table 19 shows a comparison from [40] of predicted birefringences and corresponding retardances for the two birefringences discussed in this section, i.e. CME and JB (or MEB). 

The Kerr and Cotton-Mouton experiments stand apart from the others in the sense that it is the electric-field-induced anisotropy in the refractive index which is measured (i.e. the difference in two quantities) rather than a single quantity such as an intensity. This means that, e.g., for the Cotton-Mouton effect, the experimentally-useful quantity is... 

The Cotton-Mouton effect, magnetic anisotropy, and charge localization of 2,4,6-tris(dimethyl-amino)-1,3,5-triazine compared with 1,3,5-triazine have been examined <91JST(248)20l>. Dilute solution Kerr and Cotton-Mouton constants were measured for both 1,3,5-triazine and 2,4,6-tris(dimethylamino)-l,3,5-triazine. These constants were then used to derive experimental values for the optical-frequency polarizability anisotropies, and more importantly the magnetic anisotropies of... 

Information concerning orientational correlations between anisotropic molecules in the liquid state can be obtained from measurements of the Cotton-Mouton effect and of depolarized Rayleigh light scattering. Le Fevre, Murthy, and Stiles studied C F, + CeH, using the former technique and showed that... 

The collision-induced Cotton-Mouton effect is thus negligibly small and the effective magnetizability anisotropy is equal to the isolated molecule value. g2 and Ygff values for CSo and a number of benzenoids ) have been determined by combining and values with measurements of the gas-phase magnetizability. 

Finally, let us mention the experimental methods that use other physical effects to measure molecular polarizability. These methods use the birefringence effects [15] in any magnetic field (Cotton-Mouton effect) and flow (dynamic optical effect of Maxwell), the acoustic birefiingence effect, absorption spectra induced by the electric field [16] and so on. It should be noted that last group of methods have the greater errors compared to the methods discussed above. 

This approach is based on the introduction of molecular effective polarizabilities, i.e. molecular properties which have been modified by the combination of the two different environment effects represented in terms of cavity and reaction fields. In terms of these properties the outcome of quantum mechanical calculations can be directly compared with the outcome of the experimental measurements of the various NLO processes. The explicit expressions reported here refer to the first-order refractometric measurements and to the third-order EFISH processes, but the PCM methodology maps all the other NLO processes such as the electro-optical Kerr effect (OKE), intensity-dependent refractive index (IDRI), and others. More recently, the approach has been extended to the case of linear birefringences such as the Cotton-Mouton [21] and the Kerr effects [22] (see also the contribution to this book specifically devoted to birefringences). 

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