Hydrogen atom formation by ultrasound in D2O solutions of nitrone spin traps

To elucidate the mechanism of ultrasonically induced H atom formation in D2O solutions of nitrone spin traps, alpha-phenyl-N-tert-butylnitrone (PBN) and alpha-(4-pyridyl-1-oxide)- N-tert-butylnitrone (POBN) were exposed to 50 kHz ultrasound in the presence of Ar, and the ratio of the ESR signal of the H-spin adduct to that of the D-spin adduct (H/D ratio) was examined. The H/D ratio increased with the concentration of the spin traps. The magnitude of the H/D ratios correlates with the hydrophobicity of the spin traps. The H/D ratios for the more hydrophobic spin trap, PBN, are larger than those for POBN at all concentrations. Xenon, which has a lower thermal conductivity than argon, was employed for creating higher final temperatures of the cavitation bubbles. For the less hydrophobic spin trap POBN the H/D ratio is lower for xenon than for argon. A similar result was found for PBN at lower concentrations. These results show that the H adducts of PBN and POBN are formed from the spin traps or their decomposition products by homolytic scission of C-H bonds due to pyrolysis.