Sonochemistry of nitrone spin traps in aqueous solutions. Evidence for pyrolysis radicals from spin traps

When argon-saturated aqueous solutions of alpha-phenyl-N-tert-butylnitrone (PBN) were sonicated, the spin adducts PBN-Phenyl (Ph), PBN-X, and PBN-H were observed. It can be inferred that PBN-Ph and -X arise from spin adducts of thermal decomposition products of PBN induced by the high temperature due to ultrasonic cavitation. The ESR signal of PBN-H was observed at a lower PBN concentration than those of PBN-Ph and PBN-X. The ratios of ESR intensity of PBN-H to those of PBN-Ph and PBN-X increased with the final temperatures of the cavitation bubbles created by different rare gases. The spin adducts of methyl and tert-butyl radicals from the pyrolysis of PBN, induced by the high temperatures due to cavitation, were found from spin trapping experiments in which 3,5-dibromo-2,6-dideuterio-4-nitrosobenzene sulfonate was used as a spin trap. Similar spin adducts induced by pyrolysis were also observed in sonicated aqueous solutions of other nitrone spin traps, such as alpha-(4-pyridyl-1-oxide)-N-tert-butylnitrone, and alpha-(4-nitrophenol) N-tert-butylnitrone. The greater the hydrophobicity of the spin traps, as measured by the 2-octanol/water partition coefficients, the lower the concentration of spin trap at which methyl radicals generated by thermal decomposition of the spin trap can be observed. The present results indicate that the nonvolatile, highly hydrophobic spin traps accumulate preferentially in the interfacial region of cavitation bubbles where they undergo thermal decomposition during cavitation to produce the radicals.