Orientational order in liquid crystals by combiningH2andC13nuclear magnetic resonance spectroscopy and density functional theory calculations

Abstract

Structural and orientational order properties of the liquid crystal 4,4'-bis-heptyl-azoxybenzene (HAB) have been obtained in its nematic and smectic- A phases by simultaneously analyzing several observables extracted from 2H and 13C nuclear magnetic resonance (NMR) spectra, i.e., 2H quadrupolar, 2H-1H and 13C-2H dipolar couplings, as well as 13C chemical shift anisotropy. 13C experiments required the application of high-resolution solid-state NMR techniques like 1H high-power decoupling and cross polarization, as well as the independent determination of chemical shift tensors by means of density functional theory (DFT) calculations, here performed taking into account the effect of the anisotropic medium by the polarizable continuum model method. The approach, consisting in the simultaneous analysis of all the 2H and 13C experimental data to derive orientational order parameters, and in the use of geometrical parameters determined by DFT methods, allows more detailed and reliable results to be obtained with respect to the traditional approach based on the sole analysis of 2H experiments.