Theoretical investigations of the excited-state intramolecular proton transfer reaction in N-substituted-3-hydroxypyridinones

The potential energy functions of the electronic ground state, as well as the lowest nπ^* and ππ^* excited singlet states of 3-hydroxy-4-pyridinone and 3-hydroxy-2-methyl-4-pyridinone, have been theoretically investigated along the proton transfer (PT) reaction coordinate. The full geometry optimization has been performed along the PT reaction path. In the geometry optimization, the Hartree-Fock approximation and the configuration interaction scheme with single excitations have been employed. The energy calculations at the optimized geometries have been performed with the complete-active space self-consistent field (CASSCF) method followed by second-order perturbation theory calculations, employing the CASSCF wave function as the reference. We found that the near-degeneracy between the ππ^* and nπ^* excited singlet states, which is removed upon methyl substitution, might be the factor which differentiates the two systems with respect to the excited state intramolecular PT reaction. A simple vibrational model has been proposed for investigation of the dynamics of the process.