THEORETICAL-STUDIES ON THE EFFECT OF THE MEDIUM ON TAUTOMERIC PHENOMENA IN NEUTRAL AND PROTONATED ACRIDIN-9-AMINE - MECHANISM OF TAUTOMERIZATION IN NEUTRAL ENTITIES

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- an d di-protonated acridin-9-amine. The energy gaps between possible taut omers are only insignificantly influenced by the medium and reveal the coexistence of neutral and diprotonated entities in two forms, and th e 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 betwe en tautomers. The mechanism of bimolecular tautomerization of neutral acridin-9-amine was examinated by the PM3 and PM3-AQ methods. The incl usion of entropy changes accompanying tautomerization in the gaseous p hase enabled high-pressure-limit rate constants for well-defined steps of the process at the RRKM, and transition state level of theory to b e predicted. Thermodynamic and kinetic considerations indicate that ne utral acridin-9-amine should coexist in amino and imino tautomeric for ms, and that both these entities can convert into each other at modera te temperatures.