The reactiono fd ioxygen with nitrenes can have significant energy barriers, although both reactants are triplet diradicals and the formationof nitroso-O-oxides is spin-allowed.B ym eans of matrix-isolation infrared spectroscopy in solid argon, nitrogen, and neon,a nd through high-level computational quantum chemistry,i ti ss hownh erein that a 3-nitreno-1,3,2-benzodioxaborole CatBN (Cat = catecholato) reacts with dioxygenu nder cryogenicc onditions thermally at temperatures as low as 7K to produce two distinct products, an anti-nitroso-O-oxide and an itritoborane CatBONO. The computedb arriers for the formation of nitroso-O-oxide isomersa re very low.W hereas anti-nitroso-O-oxide is kinetically trapped,i ts bisected isomer has av ery low barrierf or metathesis, yieldingt he CatBO + NO radicals in as trongly exothermic reaction; these radicals can combineu nder matrix-isolation conditions to give nitritoborane CatBONO. The trapped isomer, anti-nitroso-O-oxide, can form the nitri-toboraneC atBONO only after photoexcitation, possibly involvingi somerization to the bisectedi somer of anti-nitroso-O-oxide.
The reaction of a borylnitrene with carbon dioxide is studied under cryogenic matrix isolation conditions. Photogenerated CatBN (Cat=catecholato) reacts with CO2 under formation of the cycloaddition product CatBNCO2, a 3‐oxaziridinone derivative, after photoexcitation (>550 nm). The product shows Fermi resonances between the CO stretching and ring deformation modes that cause unusual 13C and 18O isotopic shifts. A computational analysis of the 3‐oxaziridinone shows this cyclic carbamate to be less strained than an α‐lactone or an α‐lactame.
The boryl nitrene CatBN (Cat = catecholato) turns highly reactive toward small inert molecules upon irradiation of its triplet ground state X ̃3A 2 with light of wavelength λ > 550 nm. A computational study of a model boryl nitrene using complete active space self-consistent field (CASSCF) theory provides evidence for the population of the highly reactive electronic state ã1A 1 upon irradiation. Potential energy scans connecting different critical points (minima, minimum energy crossing points, and conical intersections) reveal two possible pathways that could relax photoexcited boryl nitrene from the Franck−Condon region of A ̃3B 1 to the ã1A 1 state minimum. Considering the energy barriers to relaxation from one electronic state to another and the magnitude of spin−orbit couplings, the energetically most favorable pathway involves photoexcitation to A ̃3B 2 , followed by intersystem crossing to the open-shell singlet state (b 1A 2 ) and internal conversion to ã1A 1 . The relevant minimum energy crossing point is about 7−8 kcal mol −1 higher in energy than the Franck−Condon region.
The reaction of a borylnitrene with carbon dioxide is studied under cryogenic matrix isolation conditions. Photogenerated CatBN (Cat=catecholato) reacts with CO2 under formation of the cycloaddition product CatBNCO2, a 3‐oxaziridinone derivative, after photoexcitation (>550 nm). The product shows Fermi resonances between the CO stretching and ring deformation modes that cause unusual 13C and 18O isotopic shifts. A computational analysis of the 3‐oxaziridinone shows this cyclic carbamate to be less strained than an α‐lactone or an α‐lactame.
The unpaired electron impacts the binding between radicals and ordinary closed-shell molecules in noncovalent complexes. Conversely, the complexation partner can enhance, decrease, or even control the reactivity of the interacting radical. Previously, such radical−molecule (and especially radical−water) complexes were studied by controlled assembly of the interacting partners which mostly leads to formation of the thermodynamically most stable species. Here, we show that UV photolysis of the resonance-stabilized carboxymethyl radical isolated in a cryogenic argon matrix at 4 K leads to the intermediary formation of a metastable, noncovalent complex of the ketenyl radical with a water molecule. In this complex, the ketenyl radical binds water at its terminal carbon atom, although a more stable isomer exists in which water interacts with the C−H bond of the radical. Rigorous W1 theory computations confirm that the ketenyl radical is a stronger donor in C−H•••O interactions than ketene itself, while it performs comparably well as an acceptor. We propose that complex formation proceeds via an initial excited-state C−O bond breaking reaction in carboxymethyl under release of an OH radical, which is supported by multireference QD-NEVPT2 computations.
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