Centre of Radiation Research and Technology, Institute of Nuclear Chemistry and Technology, 03-195 Warsaw, Poland.
J Phys Chem A. 2010 Jan 14;114(1):105-16. doi: 10.1021/jp9071026.
Pulse radiolysis with UV-vis/ESR detection and steady-state gamma-radiolysis, combined with chromatographic techniques, were used to investigate the detailed mechanism of the ()OH-induced oxidation of alpha-(methylthio)acetamide (alpha-MTA) in aqueous solution. The main pathway involves the formation of hydroxysulfuranyl radicals alpha-MTA-(>S()-OH) and alpha-(alkylthio)alkyl radicals H(3)C-S-()CH-C( horizontal lineO)-NH(2) (lambda(max) </= 260 and 340 nm). The latter radicals are highly stabilized through the combined effect of both substituents in terms of the captodative effect. At low pH, alpha-MTA-(>S()-OH) radicals undergo efficient conversion to intermolecularly three-electron-bonded dimeric radical cations of alpha-MTA-(>S thereforeS<)(+) (lambda(max) = 480 nm), especially for high alpha-MTA concentrations. In contrast, at low proton concentrations, alpha-MTA-(>S()-OH) radicals decompose via the elimination of water, formed through intramolecular hydrogen (attached to the nitrogen atom) transfer to the hydroxysulfuranyl moiety within a six-membered structure. This process leads to the formation of the imine radical H(3)C-S-CH(2)-C( horizontal lineO)()NH, which subsequently decays in three independent channels. The first decay channel begins with a beta-scission followed by hydrolysis and a subsequent Hofmann rearrangement. One of the end products of this first decay channel is CO(2), which was detected. The second decay channel involves an intramolecular hydrogen transfer from the deltaC carbon atom to the radical imine site producing the alpha-(alkylthio)alkyl radical H(2)C()-S-CH(2)-C( horizontal lineO)-NH(2). In the third decay channel there is a 1,3-hydrogen shift in the imine radical which forms the radical H(3)C-S-()CH-C( horizontal lineO)-NH(2). The presence of the amide group induces more complex radical chemistry that leads unexpectedly to the degradation of the CH(3)SCH(2)CONH(2) molecule into gaseous products, CO(2) and NH(3). These features of the mechanism of the (*)OH-induced oxidation of alpha-MTA are quite different from those seen in other organic sulfides in neutral solutions.
利用脉冲辐解结合紫外可见/电子顺磁共振波谱（ESR）和稳态辐解技术，并结合色谱技术，研究了α-（甲硫基）乙酰胺（α-MTA）在水溶液中被·OH 诱导氧化的详细机制。主要途径涉及羟硫基自由基α-MTA-（>S()-OH）和α-（烷基硫基）烷基自由基 H(3)C-S-（>CH-C( 水平线O）-NH(2)（λ(max) ≤ 260nm 和 340nm）的形成。后者自由基通过两个取代基的加电子诱导效应得到高度稳定。在低 pH 值下，α-MTA-（>S()-OH）自由基通过分子间三电子键合转化为α-MTA-（>S 因此 S<）（+）的二聚自由基阳离子（λ(max) = 480nm），特别是在高α-MTA 浓度下。相比之下，在低质子浓度下，α-MTA-（>S()-OH）自由基通过分子内氢键转移到六元结构中环硫基团内分解，形成水。这个过程导致亚胺自由基 H(3)C-S-CH(2)-C( 水平线O)()NH 的形成，随后它在三个独立的通道中衰减。第一个衰减通道始于β断裂，然后水解，随后霍夫曼重排。这个第一个衰减通道的一个终产物是 CO(2)，它被检测到。第二个衰减通道涉及从δC 碳原子到自由基亚胺位点的分子内氢键转移，产生 H(2)C()-S-CH(2)-C( 水平线O)-NH(2)的α-（烷基硫基）烷基自由基。在第三个衰减通道中，亚胺自由基发生 1,3-氢转移，形成 H(3)C-S-（>*CH-C( 水平线O）-NH(2)的自由基。酰胺基团的存在诱导了更复杂的自由基化学，导致 CH(3)SCH(2)CONH(2)分子出乎意料地降解为气态产物，CO(2)和 NH(3)。这些机制特征与中性溶液中其他有机硫化物的·OH 诱导氧化机制非常不同。
