3-Fluoro-2H-azirine: Generation, Characterization, and Photochemistry

The elusive 3-fluoro-2H-azirine, cyclic NCH₂CF, has been generated through the stepwise decomposition of the acryloyl azide CH₂CFC(O)N₃ in an N₂-matrix at 10 K. The characterization of cyclic NCH₂CF with matrix-isolation IR spectroscopy is supported by ¹⁵N isotope labeling and the calculations with density functional theory (DFT) at the B3LYP/6-311++G(3df,3pd) level of theory. Upon irradiation at 193 nm, cyclic NCH₂CF undergoes ring opening by forming the more stable nitrile isomer CH₂FCN. In contrast to the photodecomposition reactions, the high-vacuum flash pyrolysis of CH₂CFC(O)N₃ in the gas phase at 500 °C yields the Curtius rearrangement product CH₂CFNCO along with secondary fragmentation to the atmospherically relevant fluorocarbonyl radical (FCO) and cyanomethyl radical (CH₂CN). Calculations on the potential energy profile for the decomposition reactions of CH₂CFC(O)N₃ demonstrate that the excessive energy, arising from the highly exothermic Curtius rearrangement of the azide, plays a key role in driving further dissociation reactions of CH₂CFNCO by overcoming the formidable barriers (>50 kcal mol^-1) under the pyrolysis conditions.