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

Abstract

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 13_CH , D _HH, J 13_CH 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 σ_n versus S ₃₃ agree with those obtained from the nematic-isotropic phase subtraction method.