Ab initio calculations at the S-0 state of 1-hydroxy-2-acetonaphthone reveal the existence of several conformers that result from the hydroxyl and acetyl groups rotation and a coupled proton-transfer and twisting motion on the potential-energy surface, The most stable structure has an O-H...O = C internal H-bond. The bulk effect on the stability of the structures was studied through a continuum model. Specific interactions with two bridged molecules of water lead to the breaking and making of H-bonds and to the stabilization of rotamers.