Far infrared absorption of CH2Cl2 in isotropic and cholesteric solvents

Abstract

The far infrared broad band absorptions of CH₂Cl₂, a highly dipolar molecule, have been measured carefully in isotropic and cholesteric solvents in order to bring out, by direct comparison, unusual dynamical effects on solute (probe) molecules dissolved in a mesophase. Whereas the CH₂Cl₂ band maximum ( _max) shifts by some 30 cm^–1 to lower frequency upon dilution in both CCl₄ and decalin, very little change occurs in cholesteryl linoleate and cholesteryl oleyl carbonate. This is attributed to an unusual persistence of statistical cross-correlation effects which vanish gradually in isotropic solvents. The observed integrated intensity per molecule (A/N) of CH₂Cl₂ is decreased significantly in a cholesteric environment, which may be indicative of an unusually large internal field effect. If this were not so then conservation of 〈µ²〉^½, the apparent root mean square dipole, demands the enhancement of another rotational type absorption at frequencies other than those of the far infrared. The reorientation of CH₂Cl₂ in all environments is characterised approximately by itinerant oscillation in two dimensions.