Abstract
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 (S0 and S1) and triplet states (T1 and T2). The decisive photoisomerization mechanisms of o-nitrophenol upon S1 excitation are found by three intersystem crossings and one conical intersection between two triplet states, in which T1 state plays an essential role. The present simulation shows branch ratios and timescales of three key processes via T1 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.
Highlights
The 6SA-CASSCF(10, 10)/6-31G (d, p) quantum chemistry method has been applied to perform on-thefly 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 (S0 and S1) and triplet states (T1 and T2)
The excited-state intramolecular proton transfer (ESIPT) reactions have been studied from both ab initio quantum chemistry calculations[13,14,15,16,17] and trajectory-based nonadiabatic molecular dynamic simulations[18,19,20,21,22]
By means of the radiative or nonradiative interaction with target molecule, the redistribution of electron density is essential for the transfer of a hydroxyl proton to an oxygen or nitrogen acceptor atom on the excited states within a hydrogen bond already formed in the electronic ground state
Summary
The present dynamic simulation shows that kinetic energy from the four vibartional normal modes responding for hydrogen transfer dissipates to the other vibrational modes on S1 state much faster than on T1 state. The real lifetimes on excited states from experimental observation might be longer than the present estimation due to some extra kinetic energy being initially put in four vibrational modes which are enhancing hydrogen transfer dynamics. The present simulation shows if this out-of plane synchronous motion form coherent motion before trajectories reach S0T1-ICX (we checked those trajectories from 500 fs to 1000 fs), they stay on T1 state as resonance trajectories This is the second branch as the tunneling hydrogen transfer that counts for 36% sampling trajectories. The present simulation shows that the (O)H-O(NO) stretch related vibrational modes can enhance ESIPT and the torsion motion of nitro group would hinder the hydrogen transfer reaction
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