Singh Nakul, O'Malley Patrick J, Popelier Paul L A
School of Chemistry, Sackville Site, The University of Manchester, Manchester, UK.
Phys Chem Chem Phys. 2005 Feb 21;7(4):614-9. doi: 10.1039/b415075a.
Density functional calculations using the B3LYP functional are used to provide insight into the hydrogen abstraction mechanism of phenolic antioxidants. The energy profiles for 13 ortho, meta, para and di-methyl substituted phenols with hydroperoxyl radical have been determined. An excellent correlation between the enthalpy (DeltaH) and activation energy (DeltaEa) was found, obeying the Evans-Polanyi rule. The effects of hydrogen bonding on DeltaEa are also discussed. Electron donating groups at the ortho and para positions are able to lower the activation energy for hydrogen abstraction. The highly electron withdrawing fluoro substituent increases the activation energies relative to phenol at the meta position but not at the para position. The electron density is studied using the atoms in molecules (AIM) approach. Atomic and bond properties are extracted to describe the hydrogen atom abstraction mechanism. It is found that on going from reactants to transition state, the hydrogen atom experiences a loss in volume, electronic population and dipole moment. These features suggest that the phenol hydroperoxyl reactions proceed according to a proton coupled electron transfer (PCET) as opposed to a hydrogen atom transfer (HAT) mechanism.
使用B3LYP泛函的密度泛函计算用于深入了解酚类抗氧化剂的氢原子提取机制。已确定了13种邻位、间位、对位和二甲基取代酚与氢过氧自由基反应的能量曲线。发现焓（ΔH）和活化能（ΔEa）之间存在良好的相关性，符合埃文斯-波拉尼规则。还讨论了氢键对ΔEa的影响。邻位和对位的供电子基团能够降低氢原子提取的活化能。相对于间位的苯酚，高吸电子性的氟取代基会增加活化能，但对位的情况并非如此。使用分子中的原子（AIM）方法研究电子密度。提取原子和键的性质以描述氢原子提取机制。研究发现，从反应物到过渡态，氢原子的体积、电子布居和偶极矩都会减小。这些特征表明，酚与氢过氧自由基的反应是按照质子耦合电子转移（PCET）机制进行的，而不是氢原子转移（HAT）机制。
