Continuous bond rotation models for the conformational analysis of the methoxy groups in 1,2-dimethoxy- and 1,2,3-trimethoxy-benzene using dipolar couplings obtained from the NMR spectra of oriented samples in nematic liquid crystalline solutions

Abstract

The conformational probabilities generated by rotations about the ring–oxygen bonds in 1,2-dimethoxy- (DMB) and 1,2,3-trimethoxy-benzene (TMB) are determined by comparing observed and calculated ¹H–¹H dipolar couplings, D_ij. The D_ij values are obtained by the analysis of the ¹H NMR spectra of samples of DMB and TMB dissolved in nematic liquid crystalline solvents. The calculated D_ij are obtained by averaging over the conformations which are obtained by taking 5° steps in bond rotational angles, with the severely sterically hindered conformers being eliminated by including a steric term in the bond rotational energies. It is shown that the conformational distributions obtained are strongly dependent on the form adopted for this steric term. The results obtained are compared with conformational distributions obtained by analysing the same data with a rotational isomeric state (RIS) model in which the molecules are assumed to exist in a small number of discrete conformations. It is concluded that the RIS model produces a reasonable approximation to the correct distribution for DMB, but not for the much more sterically hindered TMB.