Vibration-rotation coupling in anisotropic environments: NMR of methanes in liquid crystals

It is shown that the interaction potential which describes the orientation of methane and its isotopic derivatives in a liquid–crystal environment has a simple second-order tensorial form. The dipolar couplings previously observed in the methanes arise from a vibration-rotation coupling mechanism and a rigid-molecule effect present in the nonsymmetrically substituted methanes. These effects are calculated from the methane force field. A large part of the interaction can be identified with the coupling between the anisotropic liquid–crystal electric field and the (vibrationally induced) solute anisotropic polarizability. Relative sign information is obtained on the derivatives of the methane polarizability with respect to the symmetry modes, quantities which are crucial in the determination of absolute Raman intensities.