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 coupl es phenol/phenolate and cyanophenols/cyanophenolates in both gas and soluti on phases. In contrast to the traditional view, the gas phase calculations indicate that the enhanced acidity of the SI state with respect to So arise s mainly from effects in the deprotonated species, the effects of excitatio n 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 importa nt consequences for physically realistic reaction geometries in its neighbo rhood. Solution-phase ab initio calculations on S-0 and S-1 have also been performed, exploiting the polarizable continuum method, and support the con clusions drawn from the gas-phase calculations.