Bergès Jacqueline, Varmenot Nicolas, Scemama Anthony, Abedinzadeh Zohreh, Bobrowski Krzysztof
LCT, UMR 7616, Universite Pierre et Marie Curie, 4 Place Jussieu, 75230 Paris Cedex 5 France.
J Phys Chem A. 2008 Jul 31;112(30):7015-26. doi: 10.1021/jp711944v. Epub 2008 Jul 9.
The mutual location of the sulfur atom and the acetyl group was found to affect significantly the ()OH-induced oxidation mechanism of the organic sulfides containing either an alpha- or beta-positioned acetyl group. This phenomenon was reflected in formation of different intermediate products observed in pulse radiolysis experiments (Varmenot et al. J. Phys. Chem. A. 2004, 108, 6331-6346). In order to obtain a better support for the earlier interpretation of the experimental data, quantum mechanical calculations were performed using a density functional theory method (DFT-B3LYP) and the ab initio method (Møller-Plesset perturbation theory MP2) for optimizations and energy calculations of the parent molecules and radicals and radical cations derived from them. In accordance with experiments, it was found that the alpha-positioned acetyl group in S-ethylthioacetate (SETAc) destabilizes hydroxysulfuranyl radicals and monomeric sulfur radical cations. Instead, formation of stable C-centered radicals of the alpha-(alkylthio)alkyl-type was found energetically favorable, the H3C-()CH-S-C(=O)CH3 radical, in particular. On the other hand, the beta-positioned acetyl group in S-ethylthioacetone (SETA) does not destabilize hydroxysulfuranyl radicals, monomeric sulfur radical cations, and dimeric sulfur radical cations. Moreover, the alpha-(alkylthio)alkyl radicals of the type -S-(*)CH-C(=O)- were found to be particularly stabilized. The calculated transition states pointed toward the efficient direct conversion of the hydroxysulfuranyl radicals derived from SETAC and SETA radicals into the respective C-centered radicals. This reaction pathway, important in neutral solutions, is responsible for the absence of the dimeric radical cations of SETAc at low and high concentrations and of the dimeric radical cations of SETA at relatively low concentrations of the solute.
研究发现，硫原子与乙酰基的相对位置对含有α-或β-位乙酰基的有机硫化物的（）OH诱导氧化机制有显著影响。这一现象体现在脉冲辐解实验中观察到的不同中间产物的形成上（瓦尔梅诺特等人，《物理化学杂志A》，2004年，108卷，6331 - 6346页）。为了更好地支持对实验数据的早期解释，使用密度泛函理论方法（DFT - B3LYP）和从头算方法（莫勒 - 普列塞特微扰理论MP2）进行了量子力学计算，以优化母体分子、自由基以及由它们衍生的自由基阳离子的结构并计算其能量。与实验结果一致，研究发现硫代乙酸乙酯（SETAc）中的α-位乙酰基会使羟基硫烷基自由基和单体硫自由基阳离子不稳定。相反，发现形成α-（烷硫基）烷基型的稳定碳中心自由基在能量上是有利的，特别是H3C - （）CH - S - C（=O）CH3自由基。另一方面，硫代乙酮（SETA）中的β-位乙酰基不会使羟基硫烷基自由基、单体硫自由基阳离子和二聚体硫自由基阳离子不稳定。此外，发现 - S - （*）CH - C（=O）- 型的α-（烷硫基）烷基自由基特别稳定。计算得到的过渡态表明，源自SETAc和SETA自由基的羟基硫烷基自由基能有效地直接转化为各自的碳中心自由基。这条在中性溶液中很重要的反应途径，导致了在低浓度和高浓度下SETAc的二聚自由基阳离子以及在相对低浓度溶质时SETA的二聚自由基阳离子不存在。
