The orientation of molecules with internal rotation in nematic phases

Abstract

The anisotropic couplings obtained from the ¹H nuclear magnetic resonance (NMR) spectra of partially oriented molecules with internal rotation are difficult to analyse because the order parameters for each conformer cannot be deduced without assumptions. The purpose of this paper is to show how a model for the anisotropic short-range potential can be useful in the interpretation of the high-resolution ¹H NMR spectra of the flexible molecules furfural, 2-chlorobenzaldehyde, and 2,2′-dithiophene dissolved in a 55% Merck ZLI 1132-45% N-(p-ethoxybenzylidene)-2,6-dideutero-p'-n-butylaniline (w/w) mixture. In this nematic mixture there is no contribution to the anisotropic potential from the interaction between the solute molecular quadrupole moment and the average electric field gradient of the liquid crystal solvent; the dominant anisotropic intermolecular potential in this mixture is taken to result solely from short-range interactions. We find good agreement between experiment and the order parameters calculated by the model of short-range interactions. The ratios of order parameters for the s-cis and s-trans conformers predicted by the model are used to calculate the statistical weights of the s-trans conformation for the solutes: at 304 K, p^trans (furfural) = 0·45 ± 0·02, p^trans (2-chlorobenzaldehyde) = 0·901 ± 0·006 and p^trans (2,2′-dithiophene) = 0·6094 ± 0·0008.