Anisotropies in carbon-13 chemical shift tensor, H-C-H bond angles and the signs of the coupling constants in13CH3I,13CH3Br,13CH3Cl

A pulsed fast Fourier transform N.M.R. spectrometer is used to investigate the temperature dependence of the 13C chemical shifts in the nematic liquid crystal solutions of the following halides: 13CH3I, 13CH3Br, 13CH3Cl. The H-C-H bond angles are accurately measured in this investigation from proton magnetic resonance studies. The signs of the coupling constants D 13CH , D HH, J 13CH and the ordering parameter have been assigned to be all positive. The 13C magnetic shielding anisotropy is found to become more positive with the increasing electronegativity of the halogen substituents. The values of the anisotropies are: Δσ = σ ∥ - σ ⊥ = - 101 ± 15 p.p.m. (13CH3I), - 10 ± 5 p.p.m. (13CH3Br), 22 ± 5 p.p.m. (13CH3Cl). Results for Δσ obtained from the slope of σn versus S 33 agree with those obtained from the nematic-isotropic phase subtraction method.