Theoretical calculations on pyrazole derivatives. Part 2. Effect of cationic C-substituents (NH3+ and N2+) on the basicity and tautomerism of pyrazoles

Abstract

Semiempirical and ab initio calculations have been performed on pyrazoles with 3- or 5-cationic substituents, e.g. NH₃⁺ and N₂⁺, with the aim of discovering if these substituents were able to shift the tautomeric equilibrium towards one of the tautomers. The ammonium substituent is not amenable to the experiment since 3(5)-aminopyrazoles protonate on the pyrazole ring; however, pyrazole-3(5)-diazonium is stable and has been studied. The conclusion is that these equilibria are strongly shifted towards the 3-substituted tautomer. The azide substituent behaves normally in the sense that both tautomers are of similar stability.