Molecular Hydrogens Dissolved in Liquid Crystals

The NMR spectra of molecular hydrogen isotopologs dissolved and orientationally ordered in nematic liquid-crystal solvents provide a wealth of information. Because they are quantum rotors, the various isotopologs H2, D2, T2, HD, HT, and DT have different order parameters, with larger internuclear distance having smaller order parameter. The signs and magnitudes of the order parameters are liquid-crystal dependent, this being a direct result of the dominant anisotropic intermolecular interaction being that between the solute quadrupole moment and the mean liquid-crystal electric-field gradient (EFG) felt by the solute. Thus, the mixing of different liquid crystals can produce a solvent where molecular hydrogen feels a zero EFG, called a magic mixture. The remaining, dominant anisotropic interaction (for solutes larger than hydrogen) in these magic mixtures involves short-range anisotropic forces that depend on solute size and shape. Studies involving the very symmetrical methane H, D, and T isotopologs point out the important effect on dipolar couplings of molecular vibrations and interactions between molecular vibration and reorientation, effects which make it impossible to utilize fully the high accuracy with which dipolar couplings in general can be measured.