Recombination of 1,1-dimethylpropyl peroxy radicals in polar solvents

Abstract

The kinetics of 1,1-dimethylpropyl peroxy radicals recombination in polar solvents—water, methanol, and their mixtures—was studied by EPR spectroscopy in combination with the stopped-flow method, and the rate constants of this reaction were determined. Peroxyl radicals were generated by mixing solutions of Ce⁴⁺ sulfate and 1,1-dimethylpropyl hydroperoxide. The observed EPR signal of the peroxyl radical is a singlet with a g-factor of 2.015 ± 0.001, and a line width of ΔH = (1.36 ± 0.02) × 10⁻³ T for methanol and ΔH = (9.7 ± 0.2) × 10⁻⁴ T for water. The measured rate constants of (CH₃)₂C(O₂^·)CH₂CH₃ radical recombination at 298 K are 2k_t = (3.9 ± 0.4) × 10⁴ L mol⁻¹ s⁻¹ for water and 2k_t = (5.2 ± 0.5) × 10³ L mol⁻¹ s⁻¹ for methanol. A linear relationship between ln(2k_t) and the Kirkwood function (ε−1)/(2ε + 1), where e is the dielectric constant of the medium, has been established, indicating an important role of nonspecific solvation in the recombination of tertiary peroxyl radicals.