Fast Heavy-Atom Tunneling in Trifluoroacetyl Nitrene.

Chemical reactions involving quantum mechanical tunneling (QMT) increasingly attract the attention of scientists. In contrast to the hydrogen-tunneling as frequently observed in chemistry and biology, tunneling solely by heavy atoms is rare. Herein, we report heavy-atom tunneling in trifluoroacetyl nitrene, CF ₃ C(O)N. The carbonyl nitrene CF ₃ C(O)N in the triplet ground state was generated in cryogenic matrices by laser (193 or 266 nm) photolysis of CF ₃ C(O)N ₃ and characterized by IR and EPR spectroscopy. In contrast to the theoretically predicted activation barriers (>10 kcal mol ^-1 ), CF ₃ C(O)N undergoes rapid rearrangement into CF ₃ NCO with half-life times of less than 10 min and unprecedentedly large ¹⁴ N/ ¹⁵ N kinetic isotope effects (1.18-1.33) in solid Ar, Ne, and N ₂ matrices even at 2.8 K. The tunneling disappearance of CF ₃ C(O)N becomes much slower in the chemically active toluene and in 2-methyltetrahydrofuran at 5 K.
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