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Sterically hindered amines (2,2,6,6-tetramethyl-substituted piperidines) are easily oxidized (i) by electron transfer to parent cations in n-butyl chloride solution, (ii) by the sulfate radical anion in aqueous solution, and (iii) by sensitized electron transfer to carbonyl triplets. In nonpolar surroundings in the nanosecond time range, the radical cations of the tertiary piperidines have been directly observed by optical spectroscopy to exhibit absorption maxima below λ = 300 nm and around 550 nm. Subsequently, they deprotonate to α-alkylamine radicals, which are also the first observable products of oxidation with sulfate radical anions in water. In the case of secondary piperidines, the amine radical cations deprotonate to aminyl radicals in times < 10 ns. The triplet-sensitized electron transfer to the benzophenone as well as cyclohexanone triplet results in amine-derived and ketyl-type radicals formed at a nearly diffusion-controlled rate, which suggests an electron- and subsequent proton-transfer mechanism. In the presence of oxygen, the amine-derived radicals are oxidized to nitroxyl radicals by different pathways for secondary and tertiary piperidines. For the reaction of the nitroxyl radicals with other radicals, rate constants are found to be quite similar (about 5 × 108 M-1 s-1 ) for several alkyl radicals and for the tert-butyloxyl radical and less than 105 M-1 s-1 for alkylperoxyl radicals. Because of the minor importance of radical reactions with the sterically hindered amines (HALS), the antioxidant effect of these compounds ought to be explained by oxidation, primarily via cationic and subsequently radical intermediates to the persistent nitroxyl radicals, which scavenge other radicals very efficiently.

The photoreduction of 9,10-anthraquinone-1,5-disulfonate and 9,10-anthraquinone-2,6-disulfonate with triethylamine in aqueous solution has been studied by laser photolysis using time-resolved Fourier transform electron spin resonance and optical detection in the nanosecond and microsecond time scale. With FT-ESR the semiquinone radical anions and the triethylamine radical cations could be detected at pH ) 11 in Coulombcoupled radical pairs in addition to the separated semiquinone radical anions. At pH ) 14, the R-aminoalkyl radical could be detected as the decay product of the triethylamine radical cation. From the time dependence of the spin-polarized ESR intensities of the various components the lifetime of the Coulomb-coupled radical ion pair was determined. The radical ion pair decay exhibits a two-component behavior with a fast and a slow decay time. These lifetimes depend on the pH of the solution and are on the order of 10 ns to 2 µs.

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