Photodissociation dynamics of CH₃C(O)SH in argon matrix: A QM/MM nonadiabatic dynamics simulation

In this work, we have first employed the combined quantum mechanics/molecular mechanics (QM/MM) method to study the photodissociation mechanism of thioacetic acid CH₃C(O)SH in the S₁, T₁, and S₀ states in argon matrix. CH₃C(O)SH is treated quantum mechanically using the complete active space self-consistent field and complete active space second-order perturbation theory methods; argon matrix is described classically using Lennard-Jones potentials. We find that the C-S bond fission is predominant due to its small barriers of ca. 3.0 and 1.0 kcal/mol in the S₁ and T₁ states. It completely suppresses the nearby C—C bond fission. After the bond fission, the S₁ radical pair of CH₃CO and SH can decay to the S₀ and T₁ states via internal conversion and intersystem crossing, respectively. In the S₀ state, the radical pair can either recombine to form CH₃C(O)SH or proceed to form molecular products of CH₂CO and H₂S. We have further employed our recently developed QM/MM generalized trajectory-based surface-hopping method to simulate the photodissociation dynamics of CH₃C(O)SH. In 1 ps dynamics simulation, 56% trajectories stay at the Franck-Condon region; the S₁ C—S bond fission takes place in the remaining 44% trajectories. Among all nonadiabatic transitions, the S₁ → S₀ internal conversion is major (55%) but the S₁ → T₁ intersystem crossing is still comparable and cannot be ignored, which accounts for 28%. Finally, we have found a radical channel generating the molecular products of CH₂CO and H₂S, which is complementary to the concerted molecular channel. The present work sets the stage for simulating photodissociation dynamics of similar thio-carbonyl systems in matrix