An ab initio study of the potential energy surface in the S¹ state of 2-hydroxypyridine

The potential energy (PE) surface of the lowest excited singlet state relevant to the photophysics of interconversion of the 2-hydroxypyridine/2(1H)-pyridinone (2HP/2PY) system is characterized by ab initio calculations. The geometry optimizations in the S₁ state performed at the CIS/6-31G(d,p) level confirm the experimental findings that the 2HP form is planar in this state whereas the 2PY form is significantly out-of-plane distorted. The lowest-energy first-order saddle-points which are relevant to the photophysics of the "mobile" hydrogen atom on the S₁ PE surface lead toward its dissociation from the molecule. There is no saddle-point for a "direct" proton transfer reaction on the S₁ PE surface leading from the 2HP form to the 2PY form. The results confirm the hypothesis that the photo-induced dissociation-association (PIDA) mechanism is probably responsible for the excited-state tautomerization observed in this system.