Nonadiabatic ab initio chemical reaction dynamics for the photoisomerization reaction of 3,5-dimethylisoxazole via the S1 electronic state

Nonadiabatic ab initio molecular dynamics simulations were performed to explore the photoisomerization pathway from isoxazole (iso-OXA) to oxazole (OXA), considering four electronic states. The XMS-CASPT2 and SA4-CASSCF theories were employed to describe these electronic structures, which were caused by 12 electrons in 11 orbitals with the cc-pVDZ + sp diffuse basis set; the Gaussian s- and p-type diffuse functions were extracted from Dunning's aug-cc-pVDZ function. The potential energy and its gradient at each time step were computed on-the-fly at these levels in the time evolution of the classical trajectory. When the two electronic states were close to each other, the trajectory surface hopping (TSH) judgment between the two adjacent states was carried out by the anteater procedure based on the Zhu–Nakamura formula (ZN-TSH). The two different excited state lifetimes were found to exist in the first electronic state (S₁), estimated at 10.77 and 119.81 fs. Upon photoexcitation, the N–O bond breaks and energetically relaxes to the ground state (S₀). In the pathway leading to the main product, azirine formation, the 5-membered ring retains a planar structure while undergoing a non-adiabatic transition with an increasing N–O bond distance. Furthermore, it was verified that a 1,2-shift takes place in the pathway that results in the production of ketenimine, causing a nonadiabatic transition.