Nonadiabatic ab initio molecular dynamics of photoisomerization reaction between 1,3-cyclohexadiene and 1,3,5-cis-hexatriene

- The photoisomerization between cyclohexadiene and hexatriene was simulated.
- Nonadiabatic ab initio MD simulations were employed to elucidate the mechanism.
- Each excitations to S1 and S2 were simulated using full-dimensional model.
- Specific molecular motions at CoIns and molecular vibrations on S1 PES were found.
- The one-sided product branching ratio was obtained at the photoexcitation to S2.

The photoisomerization process between 1,3-cyclohexadiene (CHD) and 1,3,5-cis-hexatriene (HT) has been studied by nonadiabatic ab initio molecular dynamics based on trajectory surface-hopping approach with a full-dimensional reaction model. The quantum chemical calculations were treated at MS-MR-CASPT2 level for 8 electrons in 8 orbitals with the cc-pVDZ basis set. The Zhu-Nakamura formula was employed to evaluate nonadiabatic transition probabilities. S1 and S2 states were included in the photoisomerization dynamics. Lifetimes and CHD:HT branching ratios were computationally estimated on the basis of statistical analysis of multiple executed trajectories. The analysis of trajectories suggested that the nonadiabatic transitions at the S0/S1 and S1/S2 conical intersections (CoIn) are correlated to the Kekulé-type vibration and the C3-C4-C5 bending motion, respectively. The one-sided branching ratio was obtained by excitations to the S2 state; 70:30. The critical branching process was found to be dominated by the location of CoIn in potential energy hypersurface of the excited state.