The ¹³C chemical shifts and some ¹H–¹³C coupling constants of 23 pyrazoles and indazoles are reported and discussed. The spectra were recorded in CDCl₃ and in DMSO-d₆ solutions and, in several cases, also in the solid state (CP/MAS technique). These data allow us to determine the tautomerism of these compounds in the solid state (usually only one tautomer) and in solution. The position of the tautomeric equilibrium is related to the Hammett σ_m value of the 3(5)-substituent.

The tautomerism of 5(6)‐methoxy‐2‐{[(4‐methoxy‐3,5‐dimethyl‐2‐pyridinyl)methyl] sulfinyl}‐1H‐benzimidazole (omeprazole) was determined in solution, K_T = 0.59 in THF at 195 K, in favor of the 6‐methoxy tautomer. The assignment of the signals was made by comparison with its two N‐methyl derivatives in acetone‐d₆ and through theoretical calculations of the absolute shieldings (GIAO/DFT/6‐311++G**). Copyright © 2004 John Wiley & Sons, Ltd.

The X-ray molecular structures of two halogenotriazoles, 3-chloro- and 3-bromo-1H-1,2,4-triazole, have been determined, thus ending a controversy that made these compounds an exception to a general rule concerning their annular tautomerism. ¹³C and ¹⁵N NMR experiments in the solid state are complex because of the dipolar couplings with the halogens. The complex signal of the carbon atom bearing the halogen has been analysed and the residual ¹³C–X dipolar couplings determined. However, since the tautomeric structures are known, the spectra can be analysed. Solution NMR, at low temperatures (178 K) at which the tautomerism of triazoles is blocked, and using as models 1,2,4-triazole itself and its 1-methyl derivative, allowed us to determine that the same tautomers, 3-halo-1H-1,2,4-triazole, are present in methanol.