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 butane, 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 analysis involves the use of mean-field models to describe the orientational ordering of solutes in a nematic solvent. Several models were found to adequately describe the molecular ordering, including the chord model of Photinos et al. [D. J. Photinos, E. T. Samulski, and H. Toriumi, J. Phys. Chem. 94, 4688 (1990)] and recent versions of a model proposed 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 rotational isomeric states did not significantly improve the quality of the fits or alter the results. Finally the anisotropic component of the solute-solvent interaction was found to perturb only marginally the conformational probabilities from the isotropic values. (C) 1995 American Institute of Physics.