Orientational Order of Two Fluoro- and Isothiocyanate-Substituted Nematogens by Combination of C-13 NMR Spectroscopy and DFT Calculations

Orientational order properties of two nematogens containing a fluoro- and isothiocyanate-substituted biphenyl moiety have been investigated by means of C-13 NMR spectroscopy. C-13 NMR spectra acquired on static samples under high-power H-1-decoupling allowed both C-13 chemical shift anisotropies and C-13-F-19 couplings to be measured. These data were used to determine the local principal order parameter and biaxiality for the different rigid fragments of the molecules. To this aim, advanced DFT methods for the calculation of geometrical parameters and chemical shift tensors were used. The orientational order parameters obtained by NMR have been critically compared with those obtained by dielectric spectroscopy. Trends of order parameters with temperature have been analyzed in terms of both mean field theory and the empirical Haller equation.