Prediction of tautomeric equilibrium constant in the gas phase for simple compounds containing the amidine group

Prototropic tautomerism together with conformational and configurational isomerism with respect to the C-N single and C=N double bond have been considered for simple compounds containing the amidine group, -NH-C(R)=N-. General methods, based on the theoretical (AM1) calculations for prediction of the tautomeric equilibrium constant (as pK-T) in the gas phase have been discussed and illustrated by their application to cyclic and acyclic N,N'-disubstituted amidines containing substituent(s) without heteroatoms. The results obtained for simple cyclic amides are similar to those reported in solution. The pK-T values calculated for simple acyclic amidines are in good agreement with those found on the basis of the experimental substituent effects observed for non tautomerizing amidines in the gas phase.