Nature of Thiyl Peroxyl Radical: ESR and ab Initio MO Evidence for Intermolecular Stabilization of the Charge Transfer State, RS(+)OO.bul.-

Abstract

The formation, chemistry, and nature of the thiyl peroxyl radicals, RSOO', are investigated by ESR and UV-vis spectroscopy in a variety of organic and aqueous matrices. Experimental evidence suggests that the unusual properties of thiyl peroxyl radicals result from the specific nucleophilic interaction of solvent which stabilizes the charge transfer state, RS+OO'-. Anisotropic oxygen-17 coupling constants derived from ESR spectra of 0-17 labeled species which are proportional to the unpaired spin at the oxygen are used to estimate the spin density distribution in RSOO' radicals. The couplings and spin density distribution are found to vary with the nature of the thiol and the matrix. For example, in freon the thiyl peroxyl I7O couplings for the terminal and inner oxygens (1701, I7O2) differ for primary (79, 62 G), secondary (84, 57 G), and tertiary alkyl thiols (96,5 1 G), whereas in aqueous systems or methanol all thiols yield RSOO' radicals of approximately the same couplings (80, 62 G). All RSOO' species in polar media have a visible absorption (Amax = ca. 540 nm) and are found to undergo photoisomerization to R S O i and subsequent oxygen addition to form RS02-00'. About 5-10% of the spin density is found on the sulfur atom. Results found in neutral or acid aqueous glasses show no pH dependence of either the I7O hyperfine couplings or visible absorption maximum. The degree of charge transfer and the varying oxygen couplings are suggested to be a function of the ability of the medium to act as an electron pair donor. For tertiary thiols solvent access is sterically hindered in freons, which results in 0-17 couplings and spin distribution much like that found for a usual carbon-centered peroxyl radical. The solvent-stabilized charge transfer state, RS+OO'-, is found to be far more thermally stable than the uncomplexed state, which is found to react likely by thermal isomerization to R S O i at 100 K. Ab initio MO calculations are found to mimic the charge transfer state by association of negative ions such as OH-or F-with the sulfur atom.

Activation of molecular oxygen by S-radicals: experimental and computational studies on a novel oxidation of alkynes to α-diketones

“…This process proceeds with high efficiency (F = 0.8) using light with l = 546 nm. 16 However, when the autoxidation reaction (method (ii)) was performed under irradiation,y similar yields of diketone 7 were obtained as in control experiments performed under strict exclusion of light. Therefore, we conclude that 16 is not involved in the conversion of alkyne 6 into diketone 7.…”

Density Functional Theory Investigation of the Electronic Structure and Spin Density Distribution in Peroxyl Radicals

“…Thiyl Peroxyl Radical . Razskazovskii et al performed experimental and theoretical studies of thiyl peroxyl radical (RSOO • ) and showed that RSOO • has a highly variable spin distribution that in aqueous solution differs in nature from carbon-based peroxyl radicals, but that in some nonpolar systems has a similar distribution to carbon-based peroxyl radicals. These experimental findings showed that the spin density of RSOO • in aqueous environment was more evenly distributed in the peroxyl group than is generally found for carbon-based peroxyl radicals.…”

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5 S−OO, S−OH, and O−O bond distances for thiyl peroxyl and hydroxylated thiyl peroxyl radicals from UHF and DFT. UHF structures are from ref . DFT structures are from this work and were optimized with B3LYP/6-31G*.

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One-Electron Oxidation and Reduction of Sulfites and Sulfinic Acids in Oxygenated Media: The Formation of Sulfonyl and Sulfuranyl Peroxyl Radicals

“…Thus such species can be excluded as possible precursors of sulfur-based peroxyl radicals unless further stabilized toward decomposition. Intramolecular hydrogen bonding in sulfuranyl radical formed by hydroxyl radical attack to the sulfur atom of 2-methylthiomethyl acetate has recently been reported to provide such stabilization, making the reaction with oxygen possible …”