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

The elusive 3-fluoro-2 H -azirine, cyclic NCH 2 CF, has been generated through the stepwise decomposition of the acryloyl azide CH 2 CFC(O)N 3 in an N 2 -matrix at 10 K. The characterization of cyclic NCH 2 CF with matrix-isolation IR spectroscopy is supported by 15 N 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 NCH 2 CF undergoes ring opening by forming the more stable nitrile isomer CH 2 FCN. In contrast to the photodecomposition reactions, the high-vacuum flash pyrolysis of CH 2 CFC(O)N 3 in the gas phase at 500 °C yields the Curtius rearrangement product CH 2 CFNCO along with secondary fragmentation to the atmospherically relevant fluorocarbonyl radical (FCO) and cyanomethyl radical (CH 2 CN). Calculations on the potential energy profile for the decomposition reactions of CH 2 CFC(O)N 3 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 CH 2 CFNCO by overcoming the formidable barriers (>50 kcal mol -1 ) under the pyrolysis conditions.