Redox chemistry of selenenic acids and the insight it brings on transition state geometry in the reactions of peroxyl radicals.

The redox chemistry of selenenic acids has been explored for the first time using a persistent selenenic acid, 9-triptyceneselenenic acid (RSeOH), and the results have been compared with those we recently obtained with its lighter chalcogen analogue, 9-triptycenesulfenic acid (RSOH). Specifically, the selenenyl radical was characterized by EPR spectroscopy and equilibrated with a phenoxyl radical of known stability in order to determine the O-H bond dissociation enthalpy of RSeOH (80.9 ± 0.8 kcal/mol): ca. 9 kcal/mol stronger than in RSOH. Kinetic measurements of the reactions of RSeOH with peroxyl radicals demonstrate that it readily undergoes H-atom transfer reactions (e.g., k = 1.7 × 10(5) M(-1) s(-1) in PhCl), which are subject to kinetic solvent effects and kinetic isotope effects similar to RSOH and other good H-atom donors. Interestingly, the rate constants for these reactions are only 18- and 5-fold smaller than those measured for RSOH in PhCl and CH3CN, respectively, despite being 9 kcal/mol less exothermic for RSeOH. IR spectroscopic studies demonstrate that RSeOH is less H-bond acidic than RSOH, accounting for these solvent effects and enabling estimates of the pKas in RSeOH and RSOH of ca. 15 and 10, respectively. Calculations suggest that the TS structures for these reactions have significant charge transfer between the chalcogen atom and the internal oxygen atom of the peroxyl radical, which is nominally better for the more polarizable selenenic acid. The higher than expected reactivity of RSeOH toward peroxyl radicals is the strongest experimental evidence to date for charge transfer/secondary orbital interactions in the reactions of peroxyl radicals with good H-atom donors.

Oxidation of copper nanoparticles in water: mechanistic insights revealed by oxygen uptake and spectroscopic methods.

Oxidation of aqueous ∼8 nm unprotected copper nanoparticles takes place under air in approximately 2 hours at 30 °C to give Cu(2+) as a final product through an intermediate Cu(+) species. At 5 °C the process is about 5 times slower; similarly, vitamin C, which plays a sacrificial role, also slows down the oxidation, while CuNP catalyses the oxidation. In this work, we present a detailed analysis of the oxidation mechanism of colloidal CuNP inferred through spectroscopic methods (UV-visible and EPR) combined with oxygen uptake measurements, with emphasis on factors affecting the oxidative process.