CONFORMATIONAL EQUILIBRIUM AND ORIENTATIONAL ORDERING - H-1-NUCLEAR MAGNETIC-RESONANCE OF BUTANE IN A NEMATIC LIQUID-CRYSTAL

In this study we use multiple-quantum H-1-NMR spectroscopy to study bu tane, the simplest flexible alkane, dissolved in a nematic solvent. An analysis of the highly accurate H-1 dipolar coupling constants gives important information about conformational and orientational behavior, including the hans-gauche energy difference, E(tg), and the conformer probabilities and order parameters. An essential component of the ana lysis involves the use of mean-field models to describe the orientatio nal ordering of solutes in a nematic solvent. Several models were foun d to adequately describe the molecular ordering, including the chord m odel of Photinos et al. [D. J. Photinos, E. T. Samulski, and H. Torium i, J. Phys. Chem. 94, 4688 (1990)] and recent versions of a model prop osed by Burnell and co-workers [D. S. Zimmerman and E. E. Burnell, Mel . Phys. 78, 687 (1993)]. It was found that E(tg) lies in the range 2.1 -3.0 kJ/mol, which is significantly below most experimental estimates of the gas-phase value. An attempt to describe more realistically the conformational states by including torsional fluctuations about the ro tational isomeric states did not significantly improve the quality of the fits or alter the results. Finally the anisotropic component of th e solute-solvent interaction was found to perturb only marginally the conformational probabilities from the isotropic values. (C) 1995 Ameri can Institute of Physics.