Thomas Bally,* Claudio Carra, Stephan Matzinger, Leo Truttmann, Fabian Gerson,*
Reto Schmidlin, Matthew S. Platz,* and Atnaf Admasu

Contribution from the Institut de Chimie Physique, UniVersitéde Fribourg, Pérolles, CH-1700 Fribourg,
Switzerland, Institut für Physikalische Chemie der UniVersitätBasel, Klingelbergstrasse 80, CH-4056
Basel, Switzerland, and Department of Chemistry, The Ohio State UniVersity, Columbus, Ohio 43210
ReceiVed February 2, 1999

Abstract: Diphenyldiazomethane (2) and its ¹⁵ N₂, ¹³ C, and D₁₀ isotopomers, as well as the C(Me)₂ and CH₂-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
g-irradiation in Freon matrices. Depending on the experimental conditions of these reactions, two distinct
species which correspond to s- and p-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 s-states by about 1 eV.