Molecular hydrogen in nematic liquid crystals. Unusual isotope effects

Abstract

Dipolar and quadrupolar couplings measured from n.m.r. experiments are reported for H₂, HD and D₂ dissolved in several nematic liquid-crystal mixtures. These results are discussed in terms of the solute–solvent interactions. It has been shown that the interaction between the solute molecular quadrupole moment and a mean external electric field gradient due to the liquid crystal accounts for most but not all of the ordering of these solutes. In some of the liquid-crystal mixtures used the external field gradient is very small. The results indicate that at least one other mechanism is involved in the solute ordering. This mechanism makes a negative contribution to the order parameter of molecular hydrogen and has an unusual isotope dependence. This second contribution is discussed in terms of van der Waals forces which include both attractive and repulsive interactions between the solute and liquid crystal. It is also shown that this additional contribution is observed in the experimental couplings obtained in other nematic liquid crystals.