${}_{}{}^1{}_{}{}^{}\mathrm{H}$ and ${}_{}{}^2{}_{}{}^{}\mathrm{H}$ NMR measurements are reported for ${\mathrm{H}}_2$, HD, and ${\mathrm{D}}_2$ dissolved in various nematic phases. The main results are the following: Signs of order parameters are liquid-crystal dependent; the molecules with larger average bond lengths show smaller order parameters; the order parameters for ortho- and para-deuterium are equal within experimental error; the ratios of quadrupolar to dipolar couplings in HD and ${\mathrm{D}}_2$ are about seven percent lower than in the gas phase. These results are rationalized in terms of environmental effects of the liquid-crystal field on the dissolved molecules. This field decreases the quadrupolar coupling in HD and ${\mathrm{D}}_2$ and causes extensive mixing of different rotational states of the hydrogens in the liquid phase. The observed isotope dependence of the average orientation is a quantum-mechanical effect which can be explained by the influence of a simple one-parameter mean liquid-crystal field on the rotational energies and rotational wave functions of the hydrogens.

• Received 28 September 1981

DOI:https://doi.org/10.1103/PhysRevA.25.2339