In this work, the 2s + 2s (face-to-face) prototypical example of a photochemical reaction has been re-examined to characterize the evolution of chemical bonding. The analysis of the electron localization function (as an indirect measure of the Pauli principle) along the minimum energy path provides strong evidence supporting that CC bond formation occurs not in the excited state but in the ground electronic state after crossing the rhombohedral S₁/S₀ conical intersection.

In this study, we revealed the significance of chemical bonding for the photochemically induced mechanism of 2‐phenyl tetrazole derivatives generating nitrile imines. The correlated electron localization function shows that the formation of imine nitrile involves two key bond events: (i) the heterolytic C−N breakage taking place in the T₁ state and (ii) the homolytic N−N rupture occurring in the T₂ excited state. In particular, a cation‐radical specie results from the C−N cleavage, whereas the N−N rupture creates a biradical resonant form of imine nitrile. Additionally, we noticed that the substantial pair delocalization of the C−C‐N bonded structure could play a significant role in the conversion of the biradical imine nitrile into both the propargylic and allenic forms via the T₁→S₀ deactivation.

We present a comprehensive reaction mechanism for the benzyne formation through the 1,2-di-iodobenzene photolysis, a topic of major interest in organic synthesis. Our findings firmly support the experiment-based hypothesis of Kharasch and Sharm [Chem Comm, 1967, 10, 492–493], revealing that C–I bonds break homolytically. Upon photolysis, benzyne and molecular iodine are the ground-state products resulting from a non-adiabatic deactivation, whereas the 2-iodo phenyl radical is produced from the triplet excited state. The confluence of both funnels at the same conical point sharpens the competition, significantly limiting the organic synthesis of specific products. Results further indicate that benzyne forms via a two-step process involving the elimination of both iodine atoms without a radical intermediate.