pi- and sigma-diazo radical cations: Electronic and molecular structure ofa chemical chameleon

Diphenyldiazomethane (2) and its N-15(2), C-13, and D-10 isotopomers, as we ll 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]cycloh eptene (4), are ionized by electrolytic and chemical oxidation in fluid sol ution and by gamma-irradiation in Freon matrices. Depending on the experime ntal conditions of these reactions, two distinct species which correspond t o pi- and sigma-radical states of the diazo radical cations are observed by optical and ESR spectroscopy. They drastically differ in their color, hype rfine pattern, and photostability. Theoretical calculations demonstrate tha t the experimentally found energetic proximity of the two states is not an intrinsic property of the diaryldiazo radical cations, but it must be due t o some unidentified solvent (and/or counterion) effects acting to preferent ially stabilize the sigma-states by about 1 eV.