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Triplet States of Tetrazoles, Nitrenes, and Carbenes from Matrix Photolysis of Tetrazoles, and Phenylcyanamide as a Source of Phenylnitrene

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journal contribution
posted on 30.08.2018 by Manabu Abe, Didier Bégué, Hugo Santos Silva, Alain Dargelos, Curt Wentrup
Photolysis of 1- and 5-aryltetrazoles at 5–10 K using a 266 nm laser immediately generates their triplet excited states, which are characterized by their electron spin resonance (ESR) spectra with zero-field splitting parameters D = 0.12–0.13 cm–1 and E = 0.002–0.008 cm–1. Further photolysis of all of the aryltetrazoles affords arylnitrenes (D ≅ 1 cm–1), and in the case of 5-aryltetrazoles also arylcarbenes (D ≅ 0.5 cm–1). The formation of arylnitrenes from 5-aryltetrazoles, where no aryl–N bond is present, is explained by the photochemical rearrangement of initially formed nitrile imines ArCN+NR to carbodiimides. The monosubstituted carbodiimide PhNCNH isomerizes to phenylcyanamide, PhNH–CN, and photolysis of the latter causes rapid elimination of HCN and formation of phenylnitrene. When N-methyl groups are present in the tetrazoles, methylnitrene, CH3–N, is formed too. In the case of 5-phenyltetrazole, additional hydrogen shift and fragmentation afford cyano- and isocyanonitrenes, NCN and CNN.