Spectroscopic detection and kinetic analysis of short-lived aromatic aminecation radicals using an electron transfer stopped-flow method

Abstract

Using an electron transfer stopped-flow (ETSF) method, the reactions of short-lived methyldiphenylamine and diphenylamine cation radicals (MDPA˙⁺ and HDPA˙⁺) in acetonitrile (AN) were analysed by generating them quantitatively by electron transfer with tris(p-bromophenyl)amine cation radical (TBPA˙⁺). The absorption spectra of short-lived MDPA˙⁺ and HDPA˙⁺ could be successfully observed in a time frame of less than a few tens of milliseconds using a rapid-scan spectrophotometer, and as a result, the kinetics of the dimerization reaction could be analyzed in spite of the follow-up oxidation reactions of the dimer compounds, which are affected by the concentration of the neutral monomer. Both reactions are found to be second order in concentration of cation radicals, and the dimerization rate constants were determined to be 1.4 × 10⁶ M⁻¹ s⁻¹ for MDPA˙⁺, and 1.0 × 10⁶ M⁻¹ s⁻¹ for HDPA˙⁺. Compared with electrochemical methods, the ETSF method permits direct detection and straightforward kinetic analysis of short-lived cation radicals, as well as the elucidation of the effect of precursor molecules on subsequent reactions.