Proton transfer in excited electronic states: environmental effects on the tautomerization of 2-pyridone

The modulation of proton-transfer reactions by environmental effects has been investigated for the case of lactam—lactim isomerization in several electronic states to 2-pyridone. Specific interactions with a single water molecule strongly enhance the reaction rate and shift the equilibrium toward the lactam form. The effect of bulk solvent is smaller, although the lactam form is further stabilized. In the lowest singlet and triplet excited electronic states the lactim form is noticeably destabilized. In the latter case the planarity of the pyrimidinic ring is destroyed. Zero-point and entropy effects on the thermodynamics and kinetics of the reaction are generally small and do not modify the above general trends. Absorption and flourescence spectra are well reproduced by the semiempirical method of intermediate neglect of differential overlap for spectroscopy. As is always the case for π-π^* transitions solvent-induced spectral shifts are negligible.