Ab initio molecular dynamics of excited-state intramolecular proton transfer using multireference perturbation theory

We present the first calculations of excited-state dynamics using ab initio molecular dynamics with a multireference perturbation theory description of the electronic structure. The new AIMS-CASPT2 method is applied to a paradigmatic excited-state intramolecular proton-transfer reaction in methyl salicylate, and the results are compared with previous ultrafast spectroscopic experiments. Agreement of AIMS-CASPT2 and experimental results is quantitative. The results demonstrate that the lack of an observed isotope effect in the reaction is due to multidimensionality of the reaction coordinate, which largely involves heavy-atom bond alternation instead of proton transfer. Using the dynamics results as a guide, we also characterize relevant minima on the ground and first singlet excited state using CASPT2 electronic structure theory. We further locate an S1/S0 minimal energy conical intersection, whose presence explains experimental observations of a sharp decrease in fluorescence quantum yield at excitation energies more than 1,300 cm-1 above the excited-state origin.