Double-proton-transfer in [2,2′-bipyridine]-3,3′-diol: An ab initio study

The intramolecular two-proton transfer reaction in the ground and lowest excited states has been characterized for the model molecule of [2,2′-bipyridine]-3,3′-diol (BP(OH)₂) with the use of ab initio techniques. In accord with the experimental findings it has been determined that only a single tautomeric form of BP(OH)₂ exists in the ground state (di-enol form) and in the lowest excited single state (di-keto form), respectively. The excited-state proton transfer reaction occurs on an essentially barrierless potential energy surface. There is a significant barrier for the proton transfer reaction on the lowest excited triplet state surface.