π- and σ-diazo radical cations:: Electronic and molecular structure of a chemical chameleon

Diphenyldiazomethane (2) and its N-15(2), C-13, and D-10 isotopomers, as well as the C(Me)(2) and CH-CH2 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 gamma-irradiation in Freon matrices. Depending on the experimental conditions of these reactions, two distinct species which correspond to pi- and sigma-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 sigma-states by about 1 eV.