An investigation of the structure and bond rotational potential of some fluorinated ethanes by NMR spectroscopy of solutions in nematic liquid crystalline solvents

Abstract

NMR spectra of 1,2-dibromo-1,1-difluoroethane and 1-bromo-2-iodo-tetrafluoroethane dissolved in nematic liquid crystalline solvents have been analysed to yield the magnitudes and signs of the scalar couplings, J ij , and total anisotropic couplings, T ij , between all the 1H, 19F, and 13C nuclei, except for those between two 13C nuclei. The values obtained for T ij in principle contain a contribution from J ij aniso , the component along the static applied magnetic field of the anisotropic part of the electron-mediated spin–spin coupling. Neglecting this contribution allows partially averaged dipolar couplings, D ij , to be extracted from the T ij , and these were used to determine the structure, orientational order, and the conformational distribution generated by rotation about the C–C bond. The values obtained are compared with the results of calculations by ab initio and density functional methods. The differences found are no greater than those obtained for similar compounds which do not contain fluorine, so that there is no definitive evidence for significant contributions from J CF aniso or J FF aniso in the two compounds studied.