不同取代基对酚羟基抗氧化活性影响的计算研究

Theoretical study on the effect of different substituted radicles on antioxidant activity of phenolic hydroxyl group

目的:研究不同取代苯酚类化合物的抗氧化活性。方法:用杂化密度泛函B3LYP方法在6-31+G(d,p)水平上,分别对20种苯酚类化合物及其自由基的几何结构、电子结构、键解离能等性质进行计算比较研究。结果:①所有自由基中C-O键长较分子状态都变短,但其键长与具有真正C=O双键的对苯醌和环己酮相比较又稍长。②由Mulliken原子电荷分析,酚羟基抽氢形成自由基后,羟基氧原子电荷数都减少,同时本位碳上的净电荷数值比抽氢前增加。③由偶极矩数值显示,苯环上取代基分别位于邻位、间位和对位,其偶极矩依次减小;随着对位取代基上氯原子的增多极性减小。④前线轨道组成表明,在抽氢形成自由基后,氧的p-π共轭作用增强,使其自旋电子密度降低,但吸电子基团使氧原子上自旋密度增加。中性分子中当苯环上有供电子的基团时,HOMO和LUMO的能差降低,但相差不大,均比环己醇低,而自由基的能差均比环己酮、对苯醌要低。⑤取代苯酚类化合物抗氧化能力大小直接体现于O-H键强度,当取代基为吸电子基团时,所需解离能大,抗氧化活性相对减弱;相反,当取代基团为供电子基团时其抗氧活性加强。结论:烷烃基类取代物易于抗氧化,卤代烃基或硝基类取代物抗氧化活性差。

Objective：To explore antioxidant activity of the different substituted phenols. Methods： The geometrical structures, electronic structures, and O-H bond dissociation energies of neutral and free radicals of 20 phenol and substituted phenols were calculated with hybrid density functional theory B3LYP method at 6-31＋G（d,p） level. Results： ①The bond length of C-O bond in all free radicals was shorter than that in their neutral compounds but was longer than that in p-benzoquinone and cyclohexanone with real C=O double bonds. ②The Mulliken population showed that the net charges of O atom decreased in the free radicals, but the charges of connected C atom increased. And different alkanyls had little affected on spin density of O atom in free radicals,however, electron-withdrawing groups such as-CHCl2 led to increase. ③The dipole moments of different alkanyl-phenol compounds were ortho〉meta〉para order, and nearly all the dipole moments of free radicals of different substituted phenols were bigger than that of the neutral substituted phenols, especially the dipole moments of β-naphthol were always bigger than that of α-naphthol. ④The frontier molecular orbitals HOMO and LUMO were comprise of p orbital of O atom, p orbital of benzene, and s or p orbital of substitutes, the gap energies difference values for HOMO and LUMO of free radicals were nearly half of their neutral compounds＇. ⑤The more electron-withdrawing and p-conjugating interaction the smaller the gap energies difference was, and the bigger the O-H bond dissociation energies was, so that these substituted phenols have lower antioxidant activity. Conclusions： The alkanyls substituted phenols have higher antioxidant activity than that of electron-withdrawing groups substituted phenols.