The anisotropic forces acting on the bonds of benzene in liquid crystals

Abstract

The HH and CH dipolar couplings of benzene measured in five different liquid crystal solvents are subjected to an analysis which allows for the correlation between the molecular reorientational and vibrational motions. The number of adjustable parameters is reduced by treating the CH bonds or both the CH and CC bonds as effectively cylindrically symmetric entities. In this way detailed information on the anisotropic forces acting on the bonds of benzene dissolved in liquid crystals is obtained. The behaviour of the CC bonds, but not that of the CH bonds, may be explained by anisotropic dispersion forces. There is an approximately linear relation between the torques acting on the CH bonds of benzene and methane in the same liquid crystal environment. This suggests that these forces stem from a common interaction mechanism, possibly the van der Waals interaction between the atoms of the solute molecule and the liquid crystal surroundings or the interaction between the molecular quadrupole moment and the electric field gradient due to the surrounding medium. A bond additivity model for the molecular quadrupole moment tensor is developed and discussed.