Proton Magnetic Resonance Study of Mercury Dimethyl in a Nematic Liquid Crystal

The P.M.R.-spectrum of mercury dimethyl partially oriented in a nematic liquid crystal has been studied. From the ratio of the direct magnetic dipole-dipole interactions of the protons the ratio ξ0/ρ is calculated. ξ0 being the distance of the two parallel planes containing the methyl protons, and ρ being the distance of the methyl protons from the molecular symmetry axis. From this ratio the splitting due to the direct coupling of the protons with 199Hg of spin 1/2, present in about 17% natural abundance, is calculated and compared with the experimentally observed one. Exact agreement is obtained under the assumption that the methyl groups are rapidly rotating and that the direct and indirect 199Hg-H-couplings have like signs. Since the preferred orientation of the long axis of mercury dimethyl is probably parallel to the direction of the magnetic field, the absolute sign of J199Hg, H is found to be negative. Correspondingly it is shown that the sign of JCH3, CH3 is positive. No evidence is found for a possible contribution of the anisotropic part of the indirect couplings.