Ab initio study of excited-state intramolecular proton dislocation in salicylic acid

The potential-energy (PE) function of the S₁(ππ^*) state along the reaction path for the excited-state intramolecular proton-transfer process in salicylic acid (SA) has been calculated by ab initio electronic-stucture theory based on multi-reference perturbation theory (CASPT2 method). The CI-singles (CIS) method has been employed to construct the minimum-energy reaction path on the S₁ PE surface. The results provide a particularly clear-cut demonstration of the importance of proper inclusion of dynamical electron correlation effects in the calculation of photochemical PE functions. The calculated absorption and emission wavelengths are in excellent agreement with experiment. The calculations predict a single minimum of the S₁(ππ^*) surface, rather than two (enol and keto) minima found in previous semiempirical calculations. The excited-state minimum corresponds to a shift of the H atom by only about 0.15 Å along the OH bond with respect to the equilibrium in the ground state. The photophysics of SA is thus better described as an excited-state vibrational relaxation process than the transfer of a hydrogen atom. It is found that most of the Stokes shift of the fluorescence arises from the rearrangement of the H-chelate ring.