Trajectory surface-hopping study of 1-pyrazoline photodissociation dynamics

Abstract

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).