THEORETICAL INVESTIGATION OF POTENTIAL-ENERGY SURFACES RELEVANT FOR EXCITED-STATE HYDROGEN-TRANSFER IN O-HYDROXYBENZALDEHYDE

The potential energy functions of the electronic ground state as well as the lowest n pi* and pipi* excited singlet states of o-hydroxybenzaldehyde (OHBA) have been theoretically characterized along the proton transfer (PT) reaction coordinate as well as the reaction coordinate leading to the prefulvenic form. The calculations have been performed with the ab initio complete-active-space self-consistent-field (CASSCF) method and with second-order perturbation theory, employing the CASSCF wave function as the reference state. It is found that the 1pipi* state is almost isoenergetic with the 1npi* state after vertical excitation of OHBA. The 1pipi* potential energy function is found to be barrierless along the PT reaction coordinate, while the 1npi* potential energy exhibits a significant barrier. An efficient non-radiative pathway has been identified resulting from strong non-adiabatic interactions between the ground and the excited singlet states along the reaction coordinate to the prefulvenic form of OHBA. It is argued that the consideration of multi-dimensional vibronic interactions is essential for the understanding of the excited state dynamics of OHBA.