The 2-pyridone/2-hydroxypyridine tautomerism in gas phase: An excited state roaming reaction

Multiconfigurational methods (CASSCF and CASPT2) were employed to gain a new understanding of the mechanism of the gas-phase phototautomerization of 2-pyridone/2-hydroxypyridine. Potential energy curves and crossing points of the low-lying excited states were analyzed. The results show that the tautomerization only occurs from 2-pyridone to 2-hydroxypyridine after electronic excitation to the S1 (ππ*) state. From this state, the system would be able to reach a conical intersection between S1 and the dissociative S2 (πσ*) due to vibrational effects. Then, it can evolve to the hydroxy form in its ground state by reaching an intersection seam between the S0 and the πσ* states. For this to happen, a roaming process responsible for the hydrogen atom migration would be required; otherwise, the system would revert to the 2-pyridone tautomer. The unfeasibility of the reverse process after optical excitation from the lactim to the lactam form is explained by the great amount of energy needed to reach the conical intersection between the ππ* and πσ* states. These findings would provide new insights into the understanding of the photophysics and photochemistry of a primordial heterocycle, considered a prebiotic model known to be found in interstellar clouds.