Theoretical calculations and experimental data on spectral, kinetic and thermodynamic properties of Se∴N and S∴N three-electron-bonded, structurally stabilized σ2σ* radicals

Abstract

A complementary quantum mechanical and experimental study has been undertaken on the reactivity, formation and properties of Se;N and S;N r 2 /r* three-electron-bonded radical species, generated upon one-electron oxidation of selenomethionine, methionine and structurally related compounds. The quantum chemical calculations were based on density functional theory (DFT) hybrid B3LYP and BHandHLYP methods with basis sets ranging from 6-31G(d) to 6-311?G(d,p). Solvent effects, which play an important role concerning structure and energy of ground and excited states, were taken into account as dielectric continuum as well as explicit water molecules. They fully confirm new and previously obtained experimental results concerning the Vis/near-UV absorptions and thermodynamic stability. Special emphasis was put on a comparison between selenium and sulfur. The calculations clearly confirm the higher thermodynamic stability of the Se;N radical species relative to the S;N ones, and also corroborate the observed much higher kinetic stability of the former. Concerning optical absorptions, the calculations predict the Se;N transients to exhibit a blue-shift by about 20 nm relative to the S-based analogues, confirming the few experimental data available so far. The theoretical study includes a comparison of various calculation levels and the influence of the solvent environment, by comparison with vacuum. New experimental data within the scope of this study have been obtained on intramolecularlyformed S;N radical cation moieties, structurally stabilized by a rigid norbornane backbone. The methionine-related species, with an endo-2-amino, exo-2-carboxyl, and endo-6 methylthio substitution, for example, exhibits almost identical optical and kinetic stability properties as the corresponding species from free methionine. Its optical absorption depends on the protonation state of the carboxyl group, with k max at 410 nm for the carboxylate (zwitterionic) form and at 390 nm for the overall cationic form with the protonated carboxyl group. The fast exponential decays with t 1/2 of 490 ns and 2 ls pertain to the decarboxylation of the respective species. A much longer-lived S;N species with t 1/2 [ 500 ls and second order decay kinetics (k max 465 nm) was obtained from an endo-2-cyclohexylamino norbornane analogue which does not carry a carboxyl group. The methionine-based S;N species is not stable anymore in vacuum and in low polarity solvents. This is explained by a decrease in stabilization energy of the 3-e-bond and a faster electron transfer from the carboxylate into the cationic 3-e-center. In conclusion, selenium enhances the thermodynamic and kinetic stability of its radical transients, relative to the sulfur analogues.

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

“…The above numbers are fully in line with two trends observed in our present and earlier experimental and theoretical studies, namely, that (i) the CÀSe bond energy is lower relative to that of CÀS, and (ii) that the thermodynamic stabilization energy of selenium-centered radicals is higher than in their sulfur-centered analogues. 16 The results also exhibit significantly lower bond dissociation energies in the radical anions and H-adducts as compared to the CÀS and CÀSe energies in the parent compounds (CÀS 284 kJ mol À1 , CÀSe 254 kJ mol À1 ). This can be taken as a reflection of an antibonding contribution in these bonds due to the extra electron in these species.…”

“…Another recent investigations of ours was dealing with experimental and theoretical data on structurally stabilized Se'N and S'N three-electron bonded s 2 s* radicals, revealing a generally higher thermodynamic and kinetic stability of the selenium-based species. 16 Concerning rate constants, some earlier studies have reported on the reaction of OH with diphenyl selenide, selenomethionine and selenocysteine, all being close to 1 Â 10 10 M À1 s À1 . [17][18][19][20][21][22] In particular, the detailed time-resolved studies conducted by the Priyadarsini group [20][21][22] on selenomethionine, selenocysteine and derivatives have provided informative kinetic and spectral data which allowed meaningful comparison with corresponding sulfur-containing compounds.…”

“…To investigate the influence of a solvent on the molecular structure of the cation radicals, geometry optimisations were carried out using the self-consistent reaction field polarized continuum (SCRF = CPCM 46,47 ) model. The BHandHLYP/6-311+G(d,p)/CPCM level of theory has been shown 16,22 to give the most reasonable results for calculations of s 2 s* 3-e-bonded systems with S and Se. as more reactive towards redox-and radical-initiated processes than their sulfur analogues.…”

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. , …”

Thermodynamics and Dynamic Kinetics of the Oxidation of Selenomethionine to Methionine Selenoxide: A Dft Study

“…Naumov et al [15] has calculated the spectral, kinetic and thermodynamic properties of the Se : N and S : N structures using the DFT hybrid B3LYP and BHandHLYP methods with basis sets ranging from 6-31G(d) to 6-311 þ G(d,p). However, there exist few studies to establish the oxidative thermodynamic and dynamic kinetic data of SeMet.…”