Modulation of intramolecular proton transfer by electronic excitation and environment: 2-Pyridone as a case study

The kinetics and thermodynamics of lactam/lactim tautomerization in 2-pyridone have been investigated, with special attention to direct and assisted proton transfer mechanisms in the ground and first excited electronic state. 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 comparatively negligible, although the lactam form is further stabilized. Electron excitation strongly destabilizes the saddle point for proton transfer and, especially, the lactim form with respect to the lactam species. As a consequence, the direct reaction barrier is increased, but the reverse barrier is lowered. Nonpotential energy effects are relatively small and do not modify the aforementioned general trends. © 1994 by John Wiley & Sons, Inc.