Redox and acidity properties of alkyl- and arylamine radical cations and the corresponding aminyl radicals

In this work the pK(a)s of six alkyl- and arylamine radical cations in aqueous solution have been determined by means of laser flash photolysis. The corresponding N-nitrosamines were used as precursors for the aminyl radicals which, upon protonation, formed the amine radical cations. The following pK(a)s were obtained: 3.6 +/- 0.2, 7.6 +/- 0.3, 6.8 +/- 0.5, 5.3 +/- 0.5, 5.5 +/- 0.5, and 5.8 +/- 0.5 for the radical cations of diphenylamine, N-methylaniline, dimethylamine, diethylamine, pyrrolidine, and piperidine, respectively. In addition, the peak oxidation potentials of diphenylamine, N-methylaniline, aniline, diethylamine, pyrrolidine, and piperidine have been measured in aqueous solution and in acetonitrile using cyclic voltammetry. Furthermore, the one-electron reduction potentials of the diphenylaminyl radical and the N-methylanilinyl radical in acetonitrile were measured using photomodulation voltammetry. The results of this study and of previously published studies are discussed in terms of relative substituent and solvent effects.