Mechanistic Insight into Decomposition of 2<i>H</i>‑Azirines: Electronic Structure Calculations and Dynamics Simulations

In the present work, the combined electronic structure calculations and dynamics simulations have been performed to explore the decomposition reactions of 2<i>H</i>-azirine, 2-phenyl-2<i>H</i>-azirine, and 3-phenyl-2<i>H</i>-azirine in gas phase. Thermal cleavage of the C–C single bond yields nitrile ylides as products with a high barrier (>50.0 kcal mol^–1). On the other hand, photochemical cleavage reactions starting from the ¹nπ* state of 2<i>H</i>-azirine and 3-phenyl-2<i>H</i>-azirine are ultrafast nonadiabatic processes (<100 fs), leading to nitrile ylides through the S₁/S₀ conical intersection. The fast formation of ylides in experiments was well reproduced by present dynamics simulations. For 3-phenyl-2<i>H</i>-azirine, population of the S₁(¹ππ*) state is another decay pathway for the S₂(¹nπ*) state. The C–N bond cleavage upon photoexcitation was usually considered to take place in T₁ state. However, our calculations reveal that photocleavage of the C–N single bond for 2-phenyl-2<i>H</i>-azirine is very likely to take place in S₁ state. The present work provides new insights into photocleavage mechanism of 2<i>H</i>-azirine and the related derivatives