Carbon–13 NMR studies of tautomerism in some 2-substituted imidazoles and benzimidazoles

The ¹³C NMR spectra of several 2-substituted imidazoles and benzimidazoles have been measured. The substituent was CH₃, COOH and CONHR, where R = H, n-Bu, p-tolyl or m-chlorophenyl. Carbons 4 and 5 in the imidazoles and the carbon pairs 8/9, 4/7 and 5/6 become equivalent by proton transfer from N-1 to N-3, possibly through intermolecular association. The rate of this proton exchange increases with concentration and temperature. It decreases with extension of the 2-substituent (rate CH₃?CONH-phenyl > CONH-p-tolyl ? CONH-m-chlorophenyl ? CONH-n-butyl) due to steric hindrance at the site of the (benz)imidazole nitrogen.