2-DIMENSIONAL PROTON NMR-STUDIES OF THE CONFORMATIONS AND ORIENTATIONS OF N-ALKANES IN A LIQUID-CRYSTAL SOLVENT

Magnetic dipole couplings between pairs of protons in a series of n-alkanes (hexane through decane) dissolved in a nematic liquid crystal were measured by two-dimensional NMR spectroscopy. The dependence of dipole couplings on the distance between pairs of protons and on the orientation of the molecule with respect to the applied magnetic field make them a potentially rich source of information about molecular conformation and orientation. Interpretation of the dipole couplings, however, is complicated by the fact that the measured couplings are averaged over all molecular conformations and orientations sampled by the molecule. Thus, the couplings do not directly provide internuclear distances and orientations, but instead provide constraints on the time-averaged conformations and orientations of the n-alkane molecules that can be used as a rigorous test of models for intermolecular interactions in liquid crystals. Three models for solute-liquid crystal interactions were examined; the model proposed by Photinos et al.1 was found to be superior to the others in describing this system. The results of the modeling, including order parameters and conformer probabilities for the alkanes, are discussed and compared to n-alkanes in the liquid state.