IR AND UV MATRIX PHOTOCHEMISTRY AND SOLVENT EFFECTS - THE ISOMERIZATION OF DIAZOCYCLOHEXADIENONES (ORTHO QUINONE DIAZIDES) DETECTION OF MOLECULES WITH THE 1,2,3-BENZOXADIAZOLE STRUCTURE - A UV VIS AND IR ABSORPTION AND UV PHOTOELECTRON SPECTROSCOPIC INVESTIGATION

6-Diazo-2,4-cyclohexadienone and derivatives with fluoro, chloro, methyl, tert-butyl and methoxy substituents have been investigated by UV/Vis absorption, IR absorption and UV photoelectron spectroscopy. Spectral results obtained in the gas phase, in an argon matrix at 10 K and in n-hexane solution at room temperature reveal an equilibrium with the respective 1,2,3-benzoxadiazole isomers, thus disproving current textbook opinions. The 1,2,3-benzoxadiazole structure is derived from the agreement of observed and calculated vertical ionization energies, characteristic IR and UV/Vis absorption bands as well as selective IR and UV photochemical transformations. The relative concentration of the respective 1,2,3-benzoxadiazole in equilibrium with the diazoketone isomer strongly depends on the substituents and on solvent effects. The diazoketone structure is stabilized by hydrogen bonding and polar interactions. The most stable 1,2,3-benzoxadiazole in this study, the 5,7-di-tert-butyl derivative, is at least 6.3 kJ mol-1 more stable than its diazocyclohexadienone valence isomer, whereas 2,3,4,5-tetrafluoro-6-diazo-2,4-cyclohexadienone and 3-methoxy-6-diazo-2,4-cyclohexadienone did not isomerize to a notable extent. Energetic considerations for the stabilization of 1,2,3-oxadiazoles are discussed and compared with experimental and theoretical findings.