PROTOMERIC EQUILIBRIA IN THE GROUND AND EXCITED-STATES OF 2-PYRIDONE - A SEMIEMPIRICAL STUDY INCLUDING SOLVENT EFFECTS

Interconversion between lactim and lactam forms of 2-pyridone has been investigated for the ground and first excited electronic states by th e AM1 semiempirical method. Only non-dissociative processes have been considered, namely direct intramolecular transfer, interconversion wit hin a self-associated dimer, and a mechanism assisted by one water mol ecule. The role of bulk solvent has also been investigated by means of the polarizable continuum model. The results for the ground electroni c state are generally comparable with those obtained by refined ab ini tio computations, except for a significant overestimation of energy ba rriers to intermolecular proton transfer. The results for the first ex cited electronic state are very similar, thus ruling out any interpret ation of spectral shifts in terms of excited state proton transfer. Th e strong Stokes shift observed in fluorescence spectra can rather be a scribed to the significant skeletal modifications connected to electro nic excitation. A nice correspondence has been found between structura l characteristics and composition of frontier orbitals.