Time-resolved study on the reactions of organic selenides with hydroxyl and oxide radicals, hydrated electrons, and H-atoms in aqueous solution, and DFT calculations of transients in comparison with sulfur analogues

Abstract

A complementary experimental and quantum chemical study has been undertaken on the reactivity, formation and properties of transients generated in the reaction of selected organic selenides with hydroxyl radicals, oxide radical ions, hydrated electrons and hydrogen atoms in aqueous solution. A detailed study of the OH and O (-) reactions with Me(2)Se revealed the formation of the respective adduct-radicals as precursors of (Me(2)Se thereforeSeMe(2))(+) radical cations. In case of the neutral adduct radical Me(2)Se (OH) the conversion into the three-electron bonded dimer species proceeds, in part, via the molecular (Me(2)Se thereforeOH(2))(+) radical cation. Absolute rate constants have been determined for all the underlying processes. The respective reactions with hydrated electrons and hydrogen atoms indicate that selenides exhibit a higher reactivity towards redox-active species than sulfides. A most interesting finding is that the reaction of Me(2)Se with H atoms is faster (k = 4.1 x 10(9) M(-1) s(-1)) than the reduction by hydrated electrons (k = 2.1 x 10(8) M(-1) s(-1)), precluding an electron transfer as mechanistic background. The rationale is rather an effective dissociative attack of the hydrogen atom on the selenium. Both, the e(aq)(-)- and H -induced reductions of Me(2)Se and Me(2)S lead, under cleavage of CH(3) radicals, to the direct formation of selenol and thiol, respectively. Complementary quantum chemical studies, performed with Density Functional Theory (DFT) BHandHLYP methods, confirm this mechanism. They also reveal a generally higher thermodynamic stability of the Se-centered radicals relative to the S-centered ones, e.g., for the molecular radical anions (Me(2)Se) (-) (DeltaH-27 kJ mol(-1)) and (Me(2)S) (-) (DeltaH-16 kJ mol(-1)). Despite of these stabilization energies the calculations indicate an instantaneous Se/S-CH(3) bond lengthening in the respective molecular radical anions. The same applies for the reaction of Me(2)S and Me(2)Se with H atoms. Here the calculations indicate, in fact, no thermodynamic stability of a tentative H-adduct which, therefore, is only a fictional transition state in the H -induced CH(3)-displacement process.

Selenium–Selenium Bond Cleavage of Diaryl Diselenide Radical Anions During Pulse Radiolysis

“…Asmus et al. reported that methyl selenolate (MeSe − ) has an absorption band at 260 nm,20 whereas the absorption of methylselenyl radical (MeSe . ) has not yet been reported.…”

“…The absorption band at 440 nm disappeared with k d of 2.8 10 7 s À1 (Figure 4, inset), and no spectral change such as a new band formation was observed. Asmus et al reported that methyl selenolate (MeSe À ) has an absorption band at 260 nm, [20] whereas the absorption of methylselenyl radical (MeSeC) has not yet been reported. In the case of HSeSeH, HSe-SeHC À and HSeC were reported to have absorption bands at 410 and 350 nm, respectively.…”

Photoinduced One-Electron Oxidation of Aromatic Selenides: Effect of the Structure on the Reversible Dimerization Reaction

“…To a lesser extent, a number of water-soluble aliphatic selenide compounds were also studied by radiolytic methods . A comprehensive comparison between parent alkyl sulfide and selenide radical cations have been reported by Asmus et al, together with selenomethionine in comparison with methionine . However, to the best of our knowledge only two reports on diarylselenides and aryl methyl selenides radical cations have been described by pulse radiolysis, which is an excellent technique to induced short-lived free radical species formation, although limited to relatively small, water-soluble selenides. , …”

Effect of alkyl chain length on one-electron oxidation of bis(alkyl carboxylic acid) selenides: implication on their antioxidant ability

“…2 Such GPx active compounds also exhibiting free radical scavenging activity, can be considered as a novel class of antioxidants/radioprotectors. [4][5][6][7] As selenium belongs to the same family i.e. 3 Radiation chemistry of sulfur compounds has been extensively explored but similar studies with selenium compounds are scant.…”

“…3 Radiation chemistry of sulfur compounds has been extensively explored but similar studies with selenium compounds are scant. [4][5][6][7] As selenium belongs to the same family i.e. group 16 of the periodic table, selenium compounds are expected to participate in chemical reactions in a similar manner as sulfur compounds.…”