Study of the effect of N‐protonation and N‐methylation on the 1H and 13C chemical shifts of the six‐membered ring in benzazoles and 2‐substituted N,N‐dimethylamino derivatives

The 1H and 13C NMR spectral data of benzimidazole, benzoxazole, benzothiazole and N‐methylbenzimida‐zole and of their 2‐N,N‐dimethylamino derivatives are reported. The spectra were recorded in acetone‐water solution. The chemical shifts of these molecules are compared with those of the corresponding cations obtained by N‐protonation and N‐methylation of the neutral molecules. The carbon atoms C‐4, C‐5, C‐6 and C‐4a show a qualitatively identical behaviour, moving to higher field when the cations are formed from the benzazoles examined, but a different behaviour is found for C‐2, C‐7 and C‐7a, which depends on the heterocyclic ring, on the presence of the substituent in position 2 and on the type of cation formed (N‐protonation and N‐methylation cause different effects). The conversion of benzazoles into the corresponding cations causes all 1H chemical shifts to move to lower field.