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Carbene radical ions have been postulated repeatedly as intermediates in the (electro)chemical reduction or oxidation of diazo compounds and other precursors. Bethell and Parker have shown that unimolecular loss ofN2 from ionized diazo compounds does not represent a major reaction pathway in solution except in special cases,2 i.e., the radical cations of diarylcarbenes which are formed upon electrochemical oxidation of the corresponding diazo precursors.3 Such carbene species exhibit both electrophilic and radical-like reactivities, as expected for radical cations, but the kinetic experiments do not reveal whether the diaryl carbene cations (e.g., 2,+) are

How far apart can CC bonds be and still interact through space? An ESR investigation on the radical cation of 1,16‐dodecahedradiene (1) gives an answer to this question. On the hyperfine timescale 1•+ has a delocalized structure with D2h symmetry. The π, π distances in 1 and 1•+ are almost identical.

Diphenyldiazomethane (2) and its 15N2, 13C, and D10 isotopomers, as well as the C(Me)2 and CH2CH2 bridged derivatives, 9-diazo-9,10-dihydro-10,10-dimethylanthracene (3) and 5-diazo-10,11-dihydro-5H-dibenzo[a,d]cycloheptene (4), are ionized by electrolytic and chemical oxidation in fluid solution and by γ-irradiation in Freon matrices. Depending on the experimental conditions of these reactions, two distinct species which correspond to π- and σ-radical states of the diazo radical cations are observed by optical and ESR spectroscopy. They drastically differ in their color, hyperfine pattern, and photostability. Theoretical calculations demonstrate that the experimentally found energetic proximity of the two states is not an intrinsic property of the diaryldiazo radical cations, but it must be due to some unidentified solvent (and/or counterion) effects acting to preferentially stabilize the σ-states by about 1 eV.