Application of semiempirical method (AM1) to the study of tautomeric equilibria in the gas phase for simple compounds containing the amidine group: 4(5)-substituted imidazoles

Semiempirical method (AM1) has been used to predict the tautomeric equilibrium constants (pKT) in the gas phase for 4(5)-substituted imidazoles. The pKT values have been calculated on the basis of heats of formation of individual tautomers. In calculations it has been assumed that the TΔS term has the same value for both tautomers. For comparison, the pKT values have also been estimated on the basis of the calculated (by AM1) proton affinities of N-methyl-4- and 5-substituted imidazoles. Both estimations give almost the same pKT values. Obtained results are compared with those found in solution. Comparison shows that the gas-phase substituent effects do not reproduce well those in solution. To find an explanation of this observation, the influence of rotational isomerism of substituent on the tautomeric equilibrium constant has been studied. Proton affinities and deprotonation enthalpies of 4- and 5-substituted imidazoles have also been calculated. For unsubstituted imidazole, 4(5)-methyl-imidazole and their N-methyl derivatives they are compared with those experimentally obtained. Errors are considerably smaller than the average errors of AM1 method for nitrogen acids and bases.