Application of chemical shifts in 1H and 13C NMR spectra to configuration assignment of Schiff bases in the liquid phase

In 1H and 13C NMR spectra of N-substituted dimethylketimines chemical shifts of protons and carbon atoms of the methyl groups in the cis-position with respect to the unshared electron pair of the nitrogen are larger than those of the CH3 groups in the trans-position by 0.2–0.4 and 8–11 ppm respectively. This effect is accompanied by the reduction of the corresponding direct spin-spin coupling constant 13C-13C by 10 Hz. The experimental trends in the variation of the spectral parameters are well reproduced by ab initio quantum-chemical calculations. The discovered stereochemical dependence of the chemical shifts of 1H and 13C may underlie a simple and efficient method of the configuration assignment in various compounds with a C=N bond.