Structure and reactivity of peroxyl and sulphoxyl radicals from measurement of oxygen-17 hyperfine couplings: relationship with taft substituent parameters

Abstract

A series of peroxyl radicals (ROO˙) with substituent groups of varying electron withdrawing power have been investigated using electron paramagnetic resonance spectroscopy. Sixteen carbon-based radicals have been produced, identified and reacted with ¹⁷O-labelled molecular oxygen to produce the corresponding peroxyl radical. Carbon-centred radical precursors include alkyl, aromatic, halocarbon, lipid and alcohol radicals. The oxygen-17 hyperfine couplings are shown to monitor closely the spin density distribution on the oxygen atoms in the peroxyl radicals. The experimental ¹⁷O couplings suggest a value of –5.7 mT for the anisotropic coupling parameter, B, in good agreement with the theoretical value calculated from improved wavefunctions. An increase in the magnitude of the terminal oxygen coupling is shown to correlate with an increase in the reactivity of peroxyl radicals. In addition, the Taft substituent parameter (σ*) is also correlated with the oxygen-17 couplings. The results allow estimation of peroxyl radical reactivity from oxygen-17 couplings and suggest that complexed superoxide ion may be more reactive than free O^˙–₂. The cysteine sulphonyl peroxyl and thiol peroxyl radicals are discussed in light of the results for carbon-based peroxyl radicals. The sulphonyl peroxyl radical is suggested to be highly reactive and the thiyl peroxyl radical has unique properties which suggest it is of a different nature.