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

Article abstract of DOI:10.1016/0022-2860(91)87070-X

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 IRand 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.