2 H -Imidazol-2-one O -Oxide: A Criegee Intermediate from a σ 0 π 2 Singlet Ground-State Carbene.

Triplet carbenes and triplet molecular oxygen can combine to form singlet carbonyl O -oxide Criegee intermediates in an overall spin-allowed transformation. While this reaction runs at the diffusion limit in the case of triplet carbenes, singlet carbenes are commonly more reluctant to bind 3 O 2 . In contradiction to this customarily encountered spin selectivity, the σ 0 π 2 singlet ground-state carbene 2 H -imidazol-2-ylidene 1 reacts extremely rapidly with 3 O 2 at temperatures as low as 30 K. The product of this cryogenic reaction is singlet 2 H -imidazol-2-one O -oxide 7 , an N-heterocyclic Criegee intermediate. The addition reaction becomes possible through the electrophilic activation of dioxygen in the triplet state, in which O 2 binding can initially proceed without a barrier. Criegee intermediate 7 exhibits an unprecedented high O-O stretching vibration at 1105 cm -1 , which can be explained by a resonance structure with an O 2 double bond. Furthermore, 2 H -imidazol-2-one 5 and spiro-dioxirane 6 could be identified as the photodecomposition products of the herein-reported carbonyl oxide.