13C NMR study of some derivatives of 1,3,4-oxadiazoles, 1,3,4-thiadiazoles and isosydnones

The assignment of all ring carbons of 37 derivatives of 1,3,4-oxa- or thiadiazoles and of seven isosydnones is described. In some cases, ¹³C-labelled compounds were used. The analysis of substituent effects and the calculation of charge densities of some of the derivatives suggest that the two nitrogen atoms form a protective screen against electron perturbation and play the role of an ‘electronic buffer’. A notable variation of chemical shifts could be observed only when oxygen is replaced by sulphur.