n-Hexane proton dipolar couplings and the rotational isomeric state approximation

Abstract

The three-state rotational isomeric state (RIS) model of conformer statistics is used to analyse the 16 independent dipole coupling constants measured in a recent proton NMR study of n-hexane in a nematic liquid crystal solvent by Gochin et al. The orientational ordering of the hexane molecule is treated in the context of the modular formulation of the potential of mean torque developed recently by Photinos et al. This formulation gives a very accurate description of alkane solute orientational order and conformer probabilities in the nematic solvent. Consequently, substantially more accurate calculated dipolar couplings are obtained than in previous calculations and, moreover, this is achieved without the need to resort to unconventionally high values of the trans-gauche energy difference E g in the RIS model; an optimized fit to the data wherein the location of the gauche dihedral angle RIS state φg and E g are varied yields the values φg = 112° and E g = 0·67 kcal mol-1. The sensitivity of the dipolar couplings to conformational variations of the interproton distances (1/r 3) provides a critical test of the accuracy of the RIS model. It is found that the standard RIS model itself (in terms of its intrinsic features: three rotamer states, a single position-independent E g value, and rigid geometry), and not its parametrization (φg, E g), is the impediment to an exact reproduction of the experimental dipolar couplings.