Study of photoinduced N-hydroxy-arylnitroxide radicals (ArNO*OH) by time-resolved EPR.
The photoreduction of aromatic nitro compounds by alcohols is a well-known reaction; however, the first stages of its mechanism remain controversial. This study aims at characterizing the "primary" radicalar transients involved in this reaction by EPR spectroscopy. Laser flash photolysis (lambda = 266 nm) of nitrobenzene, 5-nitrouracil, p-nitroacetophenone, o-propylnitrobenzene, and 2-nitroresorcinol in ethylene glycol was followed by time-resolved EPR spectroscopy. In all reported TR-EPR spectra, except those obtained from the photolysis of 2-nitroresorcinol, the key intermediate N-hydroxy-arylnitroxide radicals (ArNO*OH, 1-4) could be identified unambiguously. In 2-nitroresorcinol, the radical anion (ArNO*O(-), 5) and a sigma iminoxy radical (6) were observed, and a third radical (7) remains unidentified. These observations indicate that two radicalar mechanisms (by H* transfer and by electron transfer) are competing in the photoreduction mechanism. The attribution of the EPR spectra was helped by DFT calculations of the hyperfine coupling constants (hcc's).
Maurel
Vincent
V
Département de Recherche Fondamentale sur la Matière Condensée, Service de Chimie Inorganique et Biologique, CEA-Grenoble, 17 Avenue des Martyrs, 38054 Grenoble CEDEX 9, France.
Mouesca
Jean-Marie
JM
Desfonds
Gérard
G
Gambarelli
Serge
S
D004578:Electron Spin Resonance Spectroscopy; D004583:Electrons; D006900:Hydroxylation; D015394:Molecular Structure; D009589:Nitrogen Oxides / Q000737:chemistry*; D010777:Photochemistry; D013997:Time Factors
D009589:Nitrogen Oxides; C039900:nitroxyl