卟吩内氢迁移反应的质子化效应

Theoretical study on protonation effect of transfer reaction of inner hydrogen atoms in porphine

采用B3LYP/6-31G^(**)方法在Gaussian03程序下,优化去质子化卟吩(PH^-)和单质子化卟吩(PH_3^+)的结构后发现,PH^-和卟吩(PH_2)一样具有平面构型,但PH_3^+的3个内氢原子存在明显的静电排斥和空间位阻,构型发生非平面扭曲。在相同的条件下优化得到PH^-和PH_3^+内氢迁移反应的过渡态,通过与卟吩(PH_2)比较,分析质子化对内氢迁移反应速率的影响。结果表明,PH^-的过渡态构型与PH_2过渡态的结构参数基本相同,内氢迁移速率相差不大(PH^-仅为PH_2的一半);但PH_3^+的过渡态构型偏离平面的程度远小于平衡态,从平衡态到过渡态,共轭效应明显增强,PH_3^+的活化焓远小于PH_2,故PH_3^+的内氢迁移速率明显大于PH_2。

A theoretical study of the mechanism and kinetics of the inner hydrogen atom-transfer process in monodeprotonated porphine （PH^-） and monoprotonated porphine（PH_3~＋） is presented.The structures and energies of PH^- and PH_3~＋ are calculated with B3LYP/ 6-31G^（＊＊） method without symmetry restriction on Gaussian 03 programm.The results show that PH^- is planar as well as porphine （PH_2）,but there are notable electrostatic repulsion and steric hindrance between three inner hydrogen atoms in PH_3~＋,thus PH_3~＋ is noplanar. Then the calculations of transition states in IHAT reactions of PH^- and PH_3~＋ indicate that the structural parameters of transition states of PH^- and PH_2 are alike,and the speed of IHAT reaction of PH^- is little smaller than that of PH_2;meanwhile the structural changes between transition state and equilibrium state of PH_3~＋ are so marked that the conjugated effect of equilibrium state is more than its transition state,so the enthalpy of activation of PH_3~＋ is far smaller than that of PH_2,and the speed of IHAT reactions of PH_3~＋ is obviously larger than that of PH_2.