Sonolysis of dimethyl sulfoxide-water mixtures: A spin-trapping study

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The 50-kHz sonolysis of argon-saturated dimethyl sulfoxide (DMSO)-water mixtures was investigated by ESR and spin trapping with 3,5-dibromo-4-nitrosobenzenesulfonate (DBNBS). Two distinct product regions are observed. At low DMSO concentrations, methyl radicals are formed due to the reaction of hydroxyl radicals, generated by the thermal decomposition of water vapor in collapsing cavitation bubbles, which escape into the bulk of the solution and react with DMSO. The maximum CH3-DBNBS yield is observed near 0.1% DMSO (v/v). An apparent maximum in the yield of the sulfur trioxide anion radical spin adduct of DBNBS (SO3-DBNBS) was found at 75% DMSO (v/v). Sulfur dioxide has previously been identified as a major product of the pyrolysis of DMSO vapor. In the presence of water, the SO2 produced in the collapsing cavitation bubbles is oxidized by DBNBS in the bulk of the solution to SO3•-, which subsequently forms stable SO3-DBNBS adducts. Whereas for previous sonolysis studies with aqueous mixtures of solvents more volatile than water, spin-adduct yields decrease at high mole fractions, because of decreasing effective γ = Cp/Cv in the imploding argon cavitation bubbles, the thermolysis product of DMSO formed by sonolysis increases with increasing DMSO concentration.

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Journal of Physical Chemistry