Spectral, Kinetic, and Theoretical Studies of Sulfur-Centered Reactive Intermediates Derived from Thioethers Containing an Acetyl Group (original) (raw)

2004, The Journal of Physical Chemistry A

The mechanism of the • OH-induced oxidation of S-ethylthioacetate (SETAc) and S-ethylthioacetone (SETA) was investigated in aqueous solution using pulse radiolysis and steady-state γ radiolysis combined with chromatographic and ESR techniques. For each compound, • OH radicals were added, as an initial step, to the sulfur moiety, forming hydroxysulfuranyl radicals. Their subsequent decomposition strongly depended on the availability of Ror -positioned acetyl groups, pH, and the thioether concentration. For SETAc, which contains the R-positioned acetyl group, hydroxysulfuranyl radicals SETAc(> • S-OH) subsequently decay into secondary products, which do not include intermolecularly three-electron-bonded dimeric radical cations, even at high concentrations of SETAc. At low pH, these observations are rationalized in terms of the highly unstable nature of sulfur monomeric radical cations SETAc(>S +• ) because of their rapid conversion via deprotonation to the R-(alkylthio)alkyl radicals H 3 C-• CH-S-C(dO)-CH 3 (λ max ) 420 nm). However, at low proton concentrations, the R-positioned acetyl group destabilizes SETAc(> • S-OH) radicals within a five-membered structure that leads to the formation of alkyl-substituted radicals, H 3 C-CH 2 -S-C(dO)-• -CH 2 . A somewhat different picture is observed for SETA, which contains a -positioned acetyl group. The main pathway involves the formation of hydroxysulfuranyl radicals SETA(> • S-OH) and R-(alkylthio)alkyl radicals H 3 C-CH 2 -S-• CH-C(dO)-CH 3 (λ max ) 380 nm). The latter radicals are highly stabilized through the combined effect of both substituents in terms of the captodative effect. At low pH, SETA(> • S-OH) radicals undergo efficient conversion to intermolecularly three-electron-bonded dimeric radical cations SETA-(>S∴S<) + (λ max ) 500 nm), especially for high SETA concentrations. In contrast, at low proton concentrations, SETA(> • S-OH) radicals decompose via the elimination of water, formed through intramolecular hydrogen transfer within a six-membered structure that leads to the formation of alkyl-substituted radicals, H 3 C-CH 2 -S-CH 2 -C(dO)-• CH 2 . The latter radicals undergo a 1,3-hydrogen shift and intramolecular hydrogen abstraction within the six-membered structure, leading to the R-(alkylthio)alkyl radicals H 3 C-CH 2 -S-• CH-C(dO)-CH 3 and H 3 C-• CH-S-CH 2 -C(dO)-CH 3 , respectively. To support our conclusions, quantum mechanical calculations were performed using density functional theory (DFT-B3LYP) and second-order Møller-Plesset perturbation theory (MP2) to calculate the bond-formation energies of some key transients and the location and strength of their associated optical absorptions. * Corresponding we have extended our studies to the reactions of the • OH radicals with two model thioether compounds, S-ethylthioacetate and S-ethylthioacetone, containing acetyl groups in the R and positions with respect to the sulfur atom, respectively. Furthermore, to support our conclusions, we performed quantum mechanical ab initio calculations by using density functional theory (DFT-B3LYP) and second-order Møller-Plesset perturbation theory (MP2) to calculate the bond formation energies of some of the key transients and the location and strength of their associated optical absorptions.