吡啶气相光氯化反应理论研究──加成反应机理的过渡态和IRC解析

Theoretical Study of Photochemical Chlorination of Pyridine in Gas Phase──Transition States of Addition Mechanism and IRC Analysis

通过PM3方法研究氯自由基与吡啶分子加成反应的结果表明，生成不同产物2－氯吡啶、3－氯吡啶、4-氯吡啶的每一个反应通道都存在两个过渡态，生成2－氯吡啶反应路径主过渡态的能量及活化能最低，分别为-110 293．6和 139．2 kJ／mol，反应优先生成2－氯吡啶，与实验结果一致．生成2－氰吡啶反应过程（IRC）相关的键长、键级和原子净电荷变化表明，吡啶环反应部位C原子与Cl 加成形成C—Cl键主要与共轭双键断裂同步，而C—H键的断裂主要与共轭双键的重新形成同步．反应进程中氯原子净电荷从增加到减少的变化是氯原子诱导效应吸引电子和p－π共轭电荷平均分布等相互作用的结果．

Abstract The addition mechanism of pyridine reacting with chlorine radical was studied theoretically bysemi-empirical method PM3. The results show that there exist two transition states on each reaction pathof obtaining 2-chloropyridine, 3-chloropyridineopyr and 4-chloropyridine. The energies of chief transitionstate and activation barrier for leading to 2-chloridine are the lowest among the three reaction paths, whichare -110 293. 6 and 139. 2 kJ/mol respectively, so 2-chloropyridine is obtained priorly. On getting2-chloropyridine, from the variation of bond length, bond order and net charge, we can concludde that theformation of C－Cl is concerted with the breaking of conjugated double bond, and the break of C-H isconcerted with the reconformation of conjugated double bond Both inductive and resonance effects can in-fluence the net charge of chlorine atom.