Chemical-shift anisotropy and nuclear quadrupole coupling constant of 14N hi methyl isocyanide

Abstract

H-(14N) spin tickling has been used to determine for the first time a 14N chemical-shift anisotropy (ΔσN). The measurement was made on C1H314NC partially oriented in a nematic liquid-crystal solvent, and the result is +360±73 ppm. The 14N nuclear quadrupole coupling constant was also measured and is +272±2 kHz, only 57% of the gas-phase value. Using a simple valence model of CH2NC to supply the orbital parameters, Pople's chemical-shift theory yields a value of ΔσN = +675 ppm. e 2qNQn/h was calculated to be +880 kHz using the results of an all-electron SCF calculation in a Townes-Dailey treatment modified to reflect expansion of the nitrogen atomic orbitals in the molecule. Considering the level of approximation used in both calculations, the agreement with experiment is satisfactory. The problem of knowing the molecular geometry of CH3NC in the nematic phase is discussed along with the related difficulty of determining a reliable value for the anisotropy in the NH indirect coupling tensor.