MechanisticInsight into Decomposition of 2H-Azirines:Electronic Structure Calculations andDynamics Simulations.

In the present work, the combinedelectronic structure calculationsand dynamics simulations have been performed to explore the decompositionreactions of 2H-azirine, 2-phenyl-2H-azirine, and 3-phenyl-2H-azirine in gas phase.Thermal cleavage of the C–C single bond yields nitrile ylidesas products with a high barrier (>50.0 kcal mol–1). On the other hand, photochemical cleavage reactions starting fromthe 1nπ* state of 2H-azirine and3-phenyl-2H-azirine are ultrafast nonadiabatic processes(<100 fs), leading to nitrile ylides through the S1/S0conical intersection. The fast formation of ylides in experimentswas well reproduced by present dynamics simulations. For 3-phenyl-2H-azirine, population of the S1(1ππ*)state is another decay pathway for the S2(1nπ*)state. The C–N bond cleavage upon photoexcitation was usuallyconsidered to take place in T1state. However, our calculationsreveal that photocleavage of the C–N single bond for 2-phenyl-2H-azirine is very likely to take place in S1state.The present work provides new insights into photocleavage mechanismof 2H-azirine and the related derivatives. [ABSTRACT FROM AUTHOR]