Carbon-13?proton coupling constants in N-substituted imidazoles. A 13C NMR study and MO calculations

The ¹³C NMR data of imidazole, and N-methyl-, N-acetyl-, N-trifluoroacetyl-, N-heptafluorobutyryl- and 2-methyl-imidazoles, were obtained from proton-noise decoupled and single frequency NOE ¹³C NMR spectra. The considerable changes in the directly-bonded (C, H) coupling constants upon N-acetylation of the imidazole ring, which are comparable to those noted when going from the neutral to the cationic form of imidazole, are primarily due to the large π-electron transfer ability of the N-acetyl- and N-trifluoroacetyl-substituents. The differences in magnitude of the three-bond (C, H) coupling constants along different paths in the imidazole ring may be attributed to the larger π-contributions across a pyridine-like nitrogen than along a pyrrole-like nitrogen.