Radical dinitroalkane dianions from the nitration of nitroalkanes by peroxynitrite

In alkaline solutions at pH >10, peroxynitrite (ONOO-) rapidly and efficiently nitrates aci-nitroalkane anions, RCH=NO2- (R = H, CH3, or CH3CH2), to give the radical dinitrodianions, RC(NO2)(2)(-). These anions have been characterized by EPR and have multiplets based on 1:2:3:2:1 pentets consistent with a hyperfine coupling with two equivalent N-14 nuclei of the nitro groups, and the following hyperfine coupling constants, in gauss: R = H, alpha (N) = 9.96 and alpha (H) = 1.85; R = CH3, alpha (N) = 9.90 and alpha (H) = 3.22; and R = CH(3)C(H)2, alpha (N) = 9.57 and alpha (H) = 4.01. Nitration is attributed to the trapping of nitrogen dioxide, formed by ONO-OCO2- homolysis for the carbon dioxide adduct, of peroxynitrite, by the aci-nitroalkane anion. In air, the radical dinitrodianions are oxidized to the monoanions, and the kinetics of its formation is readily followed by UV/vis spectroscopy of the rise in absorption at 380 nm. This report represents the first successful spin trapping of nitrogen dioxide formed from peroxynitrite, and the method may be a useful one for preparing geminal dinitroalkanes.