采用密度泛函理论对以偶氮二异丁腈为引发剂形成的丙烯腈单体自由基(CH3)2(CN)C—CH2—(CN)CH的终止方式反应机理进行研究。 先用B3LYP/6-31G(d)基组对反应物、 偶合中间体、 过渡态、 歧化产物的几何构型进行优化, 并计算了各物种经过零点能校正后的总能量, 红外频率和电子结构, 同时对过渡态结构进行了验证。 研究结果表明, 反应物先通过自由基的偶合终止方式形成偶合中间体a, 再由此经过氢迁移、 裂解等机理得到歧化终止产物P［p1(CH3)2(CN)C〖: ＪＹ〗CH〖: ＪＬＸ, Ｙ〗CHCN+p2(CH3)2(CN)C—CH2—CH2CN］。 双自由基发生偶合和歧化终止反应分别形成偶合中间体a和歧化终止产物P均为放热反应, 且偶合产物有着更低的能量。 反应过程的速控步骤a→TS→P在常温下的速率常数表示为k(298.15 K)=2.71 10－59。 歧化终止反应随着反应温度的升高易于进行, 歧化产物所占的比例也增加。 通过对反应过程各物种红外谱图的分析, 说明自由基在整个终止反应的化学变化, 同时分析了HOMO-LUMO电子密度分布的信息, 验证丙烯腈双自由基偶合终止方式和各物种结构的准确性。

By using the density functional theory, the study of reaction termination mechanism of two (CH3)2(CN)C—CH2—(CN)CH was carried out at the B3LYP/6-31G(d) level. The initiator AIBN was used. Reactants, coupled intermediates, transition states and disproportionation products were optimized at the B3LYP/6-31G(d) level. Then the total energies corrected by zero-point energy, vibrational frequencies and electronic structures were calculated, the transition states structure was also verified. The results show that it forms the energy-rich adducts a through the coupling termination. Then, the disproportionation product P［p1(CH3)2(CN)C〖: JＹ〗CH〖: ＪＬＸ, Ｙ〗CHCN+p2(CH3)2(CN)C—CH2—CH2CN］formed via hydrogen shift and dissociation. The reactions of coupling termination and disproportionation termination are all exothermic reactions, and the coupled product has lower energy. The rate constant of step a→TS→P is k(298.15 K)=2.71 10－59 at the normal atmospheric temperature. Disproportionation termination occurs more easily with the reaction temperature rising, so the proportion of disproportionation products is increasing. Also, the analysis of infrared spectrogram of each species in reaction process shows chemical change of free radicals in the whole termination reaction. The authors give the HOMO-LUMO in this paper to verify the accuracy of biradical coupling termination and structures. It has important guiding significance to controlling the free radicals termination methods of acrylonitrile monomer.