【摘要】 采用密度泛函理论、微扰理论和前线分子轨道分析,对酸环境和无酸环境下的2,2,6,6-四甲基哌啶氮氧化物(TEMPO)捕获过氧自由基的机理进行了理论研究.研究结果表明,与B3LYP泛函相比,M06-2x泛函的计算结果更接近实验值,更适合于所研究的体系;在酸环境下,质子化的TEMPO经过质子耦合电子转移和电子转移两个反应过程清除过氧自由基,并获得再生;与之相比,在无酸条件下TEMPO清除过氧自由基的反应过程则较为复杂,包括了烷氧胺的生成、β-氢原子转移、碳中心自由基的解离和TEMPO的再生4个反应步骤.通过能量计算,酸性条件下反应活化能为35.6 kJ/mol,明显低于无酸环境下的96.8 kJ/mol,说明酸性条件更有利于TEMPO清除过氧自由基,并对这一现象的化学本质给予了理论解释.更多还原

【Abstract】 Density functional theory( DFT),Mller-Plesset( MP) and frontier molecular orbital methods were employed to investigate the fundamental reaction mechanism of the elimination of peroxyl radical by 2,2,6,6-tetramethylpiperidin-1-oxyl( TEMPO) with and without weak acid environment. The results showed that the calculated result from M06-2 x method is in more agreement with the relative experimental results than that from B3 LYP method,suggesting that the former is more suitable for the present system. With weak acid environment,the protonated TEMPO undergoes proton coupled electron transfer and electron transfer to eliminate peroxyl radical and get the regeneration. Without acid environment,the elimination of peroxyl radical by TEMPO consists of four steps,the formation of alkoxyamine,β-hydrogen atom transfer,the decomposition of carbon radical,and the regeneration of TEMPO. After the energy calculations,the activation free energy of the elimination of peroxyl radical by TEMPO with weak acid environment is calculated to be 35.6 kJ/mol,much lower than 96.8 kJ/mol without weak acid environment,showing that weak acid environment is more favorable for the reaction and this phenomenon has also been reasonably explained.更多还原