Solute molecular structure determination by N.M.R.

Abstract

The apparent variation of the HCH bond angle in methyl iodide and of the ratios of internuclear H-H distances in benzene was studied in several thermotropic liquid crystals and in their mixtures. In each case, ¹³C-methane was used as an internal ‘deformation reference’ and also as a ¹H chemical shift reference for the determination of chemical shift anisotropies. The results show that structures in agreement with microwave studies are obtained in liquid crystal mixtures in which the dipolar C-H coupling constant of methane vanishes.

The ¹H chemical shift anisotropy, Δσ_H, of methyl iodide was determined by the gradient method in the mixture of two thermotropic liquid crystals, ZLI 1167 and Phase IV. The variation of the relative concentrations of the components led to a wide range of Δσ_H values: from 1·7 p.p.m. to 7·7 p.p.m. when going from Phase IV to ZLI 1167. Also a new means to determine ¹H chemical shift anisotropies is proposed. For methyl iodide and benzene, this method predicts the Δσ_H values of 8·0 p.p.m. and -4·3 p.p.m., respectively, which are in close agreement with theoretical estimates.