Radiolysis of 4-benzylideneamino-2,2,6,6-tetramethylpiperidine-1-oxyl (BAPO) and 3-carboxamido-2,2,5,5-tetramethyl-pyrrolidine-1-oxyl (CATPO) in aqueous solutions.

The mechanism of the gamma-radiolysis of the free nitroxyl radicals BAPO and CATPO in neutral deaerated as well as in N2O-saturated aqueous solutions was investigated. The decomposition of BAPO and CATPO was followed spectrophotometrically by measuring the absorbance at 249 and 233 nm, respectively. The yields G(-BAPO) and G(-CATPO) observed under several conditions are reported and discussed in relation to the yield of OH radicals, H atoms and e-aq, produced in irradiated aqueous solutions. It was found, on the analogy of other nitroxyls, that the corresponding hydroxylamines were formed when BAPO and CATPO react with H atoms, e-aq and hydroxyalkyl radicals. The H atoms and e-aq also react partly with BAPO and CATPO by addition to the aromatic nucleus and by elimination of NH3, respectively. The OH radicals react with BAPO by addition to the aromatic nucleus and by abstraction of H atoms from the 3 or 5 positions or from the methyl groups of the 2 or 6 positions. The reaction of OH radicals with CATPO consists in the abstraction of H atoms from the 3 or 4 positions or from the methyl groups of the 2 or 5 positions. The EPR spectra suggest that in both nitroxyls the final radiolysis products are stable biradicals and other paramagnetic compounds.