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The photolysis of aryl azides is widely used to produce highly reactive aryl nitrenes, an active intermediate in organic and inorganic synthesis. Phenyl azide (Ph-N$_3$) is considered a model system for a class of aryl azides. The photoproducts of phenyl azide have applications in various fields, e.g., photoaffinity labeling of enzymes, and preparation of electrically conducting polymers$^{[1]}$, while the involved mechanism is of inherent interest from the perspective of quantum chemistry. This piece of work is focused on simulating the ultrafast dynamics involved in the photo-induced N2 dissociation of phenyl azide (Ph-N$_a$-N$_b$-N$_c$), leading to the formation of phenyl nitrene (Ph-N) and Nitrogen molecule (N$_2$).
The excited states and decay dynamics are studied with Complete Active Space Self Consistent Field (CASSCF) and complementary N-Electron Valence State Perturbation Theory (NEVPT2) calculations. Initial investigations from the potential energy surface scans motivated us towards large-scale Molecular Dynamics (MD) simulations to generate a statistical overview of the excited state dynamics. Excited state MD simulations are carried out within the SHARC package from the singlet excited states S$_1$, S$_2$, S$_3$, S$_4$, and S$_5$, acting as initial states. The studies of the simulated trajectories reveal an average dissociation time of 20-40 fs depending on the state from which the excited state simulation initiates. The electronic structure analysis is complemented with investigations on structural dynamics, for a complete illustration of the dissociation procedure. The population analysis of the trajectories presents that irrespective of the initial state, it is the S$_2$ state (a π/π* state) in which the N$_a$-N$_b$ bond splits followed by Nitrene formation. Analysis of the trajectories reveals a direct correlation between the splitting of the N$_a$-N$_b$ bond and the N$_a$-N$_b$-N$_c$ bond angle which takes the dynamics beyond the single degree of freedom framework.
[1] Nina P. Gritsan, Zhendong Zhu, Christopher M. Hadad, and Matthew S. Platz, $\textit{J. Am. Chem. Soc.}$, 3118 (2014) 1999, 121, 6, 1202-1207.
[2] Weston Thatcher Border, Nina P. Gritsan, Christopher M. Hadad, William L. Karney, Carl R. Kemnitz, and Matthew S. Platz, $\textit{Acc. Chem. Res.}$, 2000, 33,11, 765-771
[3] Juan Soto, and Juan C. Otero, $\textit{J. Phys. Chem. A}$, 2019, 123, 9053-9060
