EPR study of free radicals induced by ultrasound in organic liquids II. Probing the temperatures of cavitation regions

The spin trap nitrosodurene was used for the detection of radical intermediates formed by 50kHz ultrasound in argon-saturated n-alcohols, n-alkanes, cyclic ethers, toluene, N,N-dimethylformamide (DMF) and dimethylacetamide (DMA). Radicals, produced by pyrolysis in collapsing cavitation bubbles, such as .CH2R in n-alkanes, .CH2R and .CH2OH in n-alcohols, .CH2-phenyl in toluene, .CH2OR and .CH2R in cyclic ethers and .CH3 and .N(CH3)R in DMF and DMA were spin trapped. Secondary radicals formed by hydrogen abstraction from organic liquids (such as .CHRR′ radicals in n-alcohols and n-alkanes and .CH2N-type radicals in DMF and DMA) by the primary pyrolysis radicals were also spin trapped. For a series of n-alcohols, the logarithm of the rate of radical formation decreases linearly with the vapour pressures of the n-alcohols. The kinetic isotope effect, kH/kD, for the ultrasound-induced production of .CHRR′ and .CDRR′ radicals in mixtures of n-dodecane and n-dodecane-d26 was found to be 2.6. For the .CH2-phenyl and .CD2-phenyl-d5 radicals from toluene-toluene-d8, the ratio kH/kD = 1.09 was obtained. From the temperature dependence of the kinetic isotope effect, the temperature region of hydrogen abstraction radical formation (.CHRR′) in n-dodecane was estimated to be 750±150 K; the effective temperature of the region where benzyl radicals are formed from toluene by sonochemical pyrolysis was estimated to be about 6000 K. This method appears to be a promising tool for probing the temperatures to different sonochemical regions.