Theoretical Studies on the Effect of the Medium on Tautomeric Phenomena in Neutral and Protonated Acridin-9-amine. Mechanism of Tautomerization in Neutral Entities

Abstract

Semiempirical AM1 and PM3 (gaseous phase) and AM1-COSMO, PM3-COSMO and PM3-AQ (liquid phase) methods were used to examine the structure, as well as the thermodynamic and physicochemical features (dipole moments and LUMO and HOMO energies), of tautomeric forms of neutral, mono- and di-protonated acridin-9-amine. The energy gaps between possible tautomers are only insignificantly influenced by the medium and reveal the coexistence of neutral and diprotonated entities in two forms, and the monoprotonated entity in one form. Entropy and thermal energy, which can be evaluated only for gaseous systems on the basis of statistical mechanics, both increase with temperature and affect equilibria between tautomers. The mechanism of bimolecular tautomerization of neutral acridin-9-amine was examinated by the PM3 and PM3-AQ methods. The inclusion of entropy changes accompanying tautomerization in the gaseous phase enabled high-pressure-limit rate constants for well-defined steps of the process at the RRKM and transition state level of theory to be predicted. Thermodynamic and kinetic considerations indicate that neutral acridin-9-amine should coexist in amino and imino tautomeric forms, and that both these entities can convert into each other at moderate temperatures.