NMR in liquid crystalline solvents. Vibrational corrections on nuclear dipole-dipole interactions in the methylhalides

Abstract

Evidence of large anisotropic indirect spin-spin couplings has been obtained by Krugh and Bemheim from NMR measurements of methyl fluoride in a liquid crystal. In the present study the effects of vibrational motions on the anisotropic couplings in the methyl halides are investigated. The corrections due to vibrational motions are based on a Urey-Bradley force field, the anharmonicity of the force field being estimated from semiempirical intramolecular potential functions. In the case of methyl fluoride two additional calculations based on a general force field but constrained by assumptions of the Urey-Bradley model, have been made. The direct dipole-dipole interactions in methyl fluoride, calculated while making allowance for the vibrational corrections, show a large discrepancy with respect to the total anisotropic couplings measured by Krugh and Bernheim. This discrepancy, which is interpreted by these authors in terms of anisotropic indirect couplings, is even larger than that calculated with neglect of vibrational effects. It is concluded that solvent effects on isotropic J couplings seem to be the most obvious cause of the observed discrepancy.