Trajectory surface-hopping study of 1-pyrazoline photodissociation dynamics

The photodissociation dynamics of 1-pyrazoline has been studied from its first excited electronic state (S₁) using the Direct Dynamics Trajectory Surface-Hopping method in conjunction with Tully's fewest switches algorithm at the CASSCF(8,8)/6-31G* level of theory. After excitation of the molecule into the Franck-Condon region of the first excited state, S₁, the molecule hops to the ground (S₀) state quickly. The dissociation of one of the C-N bonds initially starts in the first excited state. Then, the molecule comes to the ground state (S₀) via S₁/S₀ conical intersections, followed by complete dissociation in the ground state. Two different conical intersections are identified between the first excited singlet (S₁) and the ground (S₀) electronic states. One primary and three secondary dissociation channels are observed from our dynamics calculations of photodissociation of 1-pyrazoline that are in accord with the experimentally observed channels. After internal conversion to the ground electronic state (S₀), the molecule dissociates to N₂ and trimethylene biradical as the primary dissociation products. The trimethylene biradical then rearranges, leading to three secondary dissociation channels, N₂ + cyclopropane, N₂ + CH₂ + C₂H₄, and N₂ + CH₃CHCH₂. The major products formed from the trimethylene biradical in the secondary process is cyclopropane contributing about 78% of the overall products formation along with ∼12% propene and the rest ∼10% methylene (CH₂) with ethene (C₂H₄).