Mechanistic
Insight into Decomposition of 2<i>H</i>‑Azirines:
Electronic Structure Calculations and
Dynamics Simulations
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Abstract
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<sup>–1</sup>). On the other hand, photochemical cleavage reactions starting from
the <sup>1</sup>nπ* 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 S<sub>1</sub>/S<sub>0</sub> 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 S<sub>1</sub>(<sup>1</sup>ππ*)
state is another decay pathway for the S<sub>2</sub>(<sup>1</sup>nπ*)
state. The C–N bond cleavage upon photoexcitation was usually
considered to take place in T<sub>1</sub> 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 S<sub>1</sub> state.
The present work provides new insights into photocleavage mechanism
of 2<i>H</i>-azirine and the related derivatives