Effects of repulsion and dispersion forces in liquid crystals: Alignment and deformation of H2 solute

The roles of steric repulsions and dispersional attractions in determining the behavior of solute molecules in liquid crystals are investigated by computer simulations of simple model systems. These systems involve a hydrogen molecule (H2) dissolved in a rigid lattice consisting of parallel, infinitely long cylinders, which interact pairwise additively with the H atoms of the solute. The thermal motion of the H2 molecule (at 300 K) is simulated by Monte Carlo and molecular-dynamics methods, which are shown to lead to identical results for all the studied properties, including the centrifugal distortion. The resulting orientational order parameters S are invariably positive at all densities and structures of the lattice, irrespective of the details of the functional form of the interaction potential. The main role of the attractive forces is to pull the solute against the hard cores of the solvent molecules, thus enhancing the effects of the repulsions (increasing S). The implication is that the negative experimental S values observed in several liquid crystals cannot be attributed to the effects of repulsion and dispersion forces; the presence of other interactions, such as electrostatic forces, appears to be essential. The repulsive forces lead to slight orientation-dependent compression of H2, with the result that the nuclear dipolar and quadrupolar couplings DHH and BD are changed by -0.7% and -1.1%, respectively (regardless of the lattice structure and the detailed form of the interaction potential). This points to the inference that the stretching-mode contributions to the orientation-dependent molecular deformations in liquid crystals are generally small, and gives support to the frequently made assumption that NMR spectral parameters of partially oriented molecules can normally be analyzed in terms of a model that includes only the bending-mode deformations.