A theoretical investigation of excited-state acidity of phenol and cyanophenols

present an ab initio study of the first few singlets of the acid/base couples phenol/phenolate and cyanophenols/cyanophenolates in both gas and solution phases. In contrast to the traditional view, the gas phase calculations indicate that the enhanced acidity of the SI state with respect to So arises mainly from effects in the deprotonated species, the effects of excitation on conjugated acids being of minor importance. Evidence for the presence of a conical intersection in the excited state of phenol and p-cyanophenol, following the proton dissociation coordinate, has been found, with important consequences for physically realistic reaction geometries in its neighborhood. Solution-phase ab initio calculations on S-0 and S-1 have also been performed, exploiting the polarizable continuum method, and support the conclusions drawn from the gas-phase calculations.