Intersystem crossing-branched excited-state intramolecular proton transfer for o-nitrophenol: An ab initio on-the-fly nonadiabatic molecular dynamic simulation

The 6SA-CASSCF(10, 10)/6-31G (d, p) quantum chemistry method has been applied to perform on-the-fly trajectory surface hopping simulation with global switching algorithm and to explore excited-state intramolecular proton transfer reactions for the o-nitrophenol molecule within low-lying electronic singlet states (S(0) and S(1)) and triplet states (T(1) and T(2)). The decisive photoisomerization mechanisms of o-nitrophenol upon S(1) excitation are found by three intersystem crossings and one conical intersection between two triplet states, in which T(1) state plays an essential role. The present simulation shows branch ratios and timescales of three key processes via T(1) state, non-hydrogen transfer with ratio 48% and timescale 300 fs, the tunneling hydrogen transfer with ratios 36% and timescale 10 ps, and the direct hydrogen transfer with ratios 13% and timescale 40 fs. The present simulated timescales might be close to low limit of the recent experiment results.