Protomeric equilibria in the ground and excited states of 2-pyridone. A semiempirical study including solvent effects

Abstract

Interconversion between lactim and lactam forms of 2-pyridone has been investigated for the ground and first excited electronic states by the AM1 semiempirical method. Only non-dissociative processes have been considered, namely direct intramolecular transfer, interconversion within a self-associated dimer, and a mechanism assisted by one water molecule. The role of bulk solvent has also been investigated by means of the polarizable continuum model. The results for the ground electronic state are generally comparable with those obtained by refined ab initio computations, except for a significant overestimation of energy barriers to intermolecular proton transfer. The results for the first excited electronic state are very similar, thus ruling out any interpretation of spectral shifts in terms of excited state proton transfer. The strong Stokes shift observed in fluorescence spectra can rather be ascribed to the significant skeletal modifications connected to electronic excitation. A nice correspondence has been found between structural characteristics and composition of frontier orbitals.