Intramolecular proton or hydrogen-atom transfer in the ground and excited states of 2-hydroxybenzoyl compounds

Potential energy surfaces for the intramolecular proton transfer of ground (GSIPT) and excited (ESIPT) states of 2-hydroxybenzoyl compounds were obtained. Based on the results, intramolecular proton transfer in this type of compound is strongly dependent on the distances between the oxygen atoms that bear the intramolecular hydrogen bond (IMHB). Also, the GSIPT curves for these compounds contain a single minimum that is located in the zone for the normal (enol) form. The ESIPT curves also contain a single minimum but lie in the zone for the keto form. There is net correlation between the strength of the IMHB and the proton transfer barrier through it. The energy for the excited singlet 1(n,pi*) for these compounds is strongly dependent on the resonance effect of the substituent, -R, so this state is the first excited singlet only in derivatives with nearly nonresonating R. The ESIPT processes are of the proton transfer type, even though the final form possesses no zwitterionic connotations. Finally, these theoretical features are quite consistent with photophysical experimental evidence for this type of compounds.