甲基取代对分子电离能的影响

Effect of Methyl Substitution on Molecular Ionization Energy

当甲基与苯环的不饱和碳原子相连时,甲基的成键电子受到不饱和碳原子的吸引而具备一定的给电子能力。甲基的给电性导致甲基取代后分子的电离能降低。利用高精度能量计算方法CBS-QB3,计算了多个甲基取代苯分子上氢原子形成的分子的电离能。结果表明,随着取代的甲基数增加分子的电离能会逐渐降低。甲基在邻、间、对不同位置取代的分子电离能满足关系:邻>间>对。与实验值相比,CBS-QB3高估了电离能,其相对误差在0.64%~1.14%范围。

When methyl groups are connected to the unsaturated carbon atoms of the benzene ring,the bonding electrons of methyl groups are attracted by the unsaturated carbon atoms,and the methyl group plays a role of electron-donating.The electron-donating property of the methyl group leads to a decrease in the ionization energy(IE)after substitution.In this paper,the IEs of molecules formed by multiple methyl substitution on benzene were calculated using a high-precision method of CBSQB3.The results show that the IE decreases as the number of substituted methyl increases.The IE of the molecule with methyl group substituted at different positions(ortho,meta and para)meets the relationship:ortho>meta>para.Compared with the experimental values,CBS-QB3overestimates the IE with a relative error of 0.64%~1.14%.