Electron paramagnetic resonance studies of electron-capture processes. p-Nitrobenzyl and p-nitrobenzylidene systems

Abstract

Electron paramagnetic resonance spectroscopy has been used to probe the anion radicals formed by electron capture by p-nitrobenzyl and p-nitrobenzylidene halides and their subsequent decomposition to yield radicals. In the case of the monohalogenated compounds (ArCH2X; Ar =p-NO2C6H4–; X = F, Cl, Br, I) the anion radicals of the fluoride, chloride and bromide are essentially long-lived over the temperature range used (77 K to ca. 160 K) while for the iodide significant decomposition of the anion radical is evident at 77 K. Our results show that the p-nitrobenzyl fluoride and chloride anion radicals adopt a conformation in which the halogen atoms are away from the perpendicular site while the p-nitrobenzyl bromide and iodide anion radicals prefer a conformation where the halogens are close to the perpendicular or maximum overlap site. In the case of the dihalogenated compounds (ArCHXY; Ar =p-NO2C6H4–; X = Y = Cl; X = Cl, Y = Br; X = Y = Br; X = F, Y = Br), it is found that increasing halogen substitution destabilizes the anions and facilitates their decomposition to radicals. In the case of ArCHCl2 and ArCHClBr, both the parent anion radicals and the radicals arising from their dissociation are detected at 77 K. In the case of ArCHCl2, the anion radical is the major product and is shown to prefer a conformation in which the lone hydrogen atom is in the perpendicular position with respect to the aromatic ring. In the case of ArCHClBr, the anion radical is detected as the minor product with a conformation in which π–σ overlap involving the NO2/aromatic π-orbitals and the C–Br σ-orbital has been reduced. The anion radicals of ArCHBr2 and ArCHFBr are not detected at 77 K; only the radicals from bromide ion departure were observed.