Factors Influencing the Antioxidant Activities of Phenols by an Ab Initio Study.
作者:Shogo Tomiyama、Shogo Sakai、Tomihiro Nishiyama、Fukiko Yamada
DOI:10.1246/bcsj.66.299
日期:——
An ab initio molecular orbital theory has been applied to the elucidation of the hydrogen abstraction mechanism of phenolic antioxidants in the chain process of autoxidations. The optimum structures of o-, m-, and p-substituted phenols, of peroxides, and of those compounds in the transition states were obtained with a Hartree-Fock/STO-3G basis set. From the values of the enthalpy (ΔH), activation (Ea), and OH bond dissociation (D(O–H)) energies obtained, it was found that these three parameters indicate a good relationship with each other; particularly, the relation between the ΔH and Ea values follows the Evans-Polanyi rule. The electron-releasing substituents in the o- and p-positions in phenols decrease the activation parameters for hydrogen abstraction, while those in the m-position increase. The electron densities on the ipso-carbon, oxygen, and hydrogen of the OH substituent and their bond populations are obtained. Variations of the electron densities from the reactants to the transition states lead to a clarification of the reaction mechanism as an antioxidant. Namely, the gain or loss of electrons in the reaction states may be correlated to the experimental data, 13C chemical shifts of the ipso-carbon of the OH substituent, and the values of the induction period as an antioxidant activity.
从头算分子轨道理论已被应用于阐明酚类抗氧化剂在自氧化链式反应中的夺氢机理。通过使用哈特里-福克/STO-3G基组,得到了邻、间、对位取代酚、过氧化物以及过渡态化合物的优化结构。从获得的焓变(ΔH)、活化能(Ea)和羟基键解离能(D(O–H))值来看,这三个参数之间显示出良好的相关性;特别是ΔH和Ea值之间的关系遵循埃文斯-波拉尼规则。酚类化合物中邻位和对位上的供电子取代基会降低夺氢反应的活化参数,而间位上的取代基则会提高这些参数。计算得到了OH取代基的ipso碳、氧、氢上的电子密度及其键的布居数。从反应物到过渡态电子密度的变化有助于阐明抗氧化剂反应机理。即,反应态中电子的增益或损失可能与实验数据、OH取代基的ipso碳的13C化学位移以及抗氧化活性的诱导期值相关联。