碳前驱体热解机理的量子化学理论研究——几何结构、反应焓变、化学键和热反应活性

Quantum Chemistry Theoretical Research of Pyrolysis Mechanism of Carbon Matrix Precursor ——Geometry,Enthalpy Change, Chemical Bond & Thermoreactive Activity

运用 Gaussian98程序包中半经验的量子化学 ( AM1)方法 ,对间二甲苯系列化合物(苯环上引入不同取代基 - OH、- SH、- CN时的化合物 )的热裂解机理进行了研究 ,对设定的 8种有自由基形成的热裂解过程中的反应物、产物的全优化几何构型和反应的热力学量的计算结果表明 :无论苯环上联接的是吸电子基团还是供电子基团 ,这些化合物的热裂解键 ,都是首先从苯环上甲基碳氢键开始 ,也就是说甲基碳氢键是这类化合物最易断裂的键 ,也是热裂解的最初反应过程 ,该结论与实验结果一致 ;间二甲苯上若联有供电子取代基 ( - OH,- SH) ,反应物的热反应活性将增大 ,若联有吸电子取代基 ( - CN) ,反应物的热反应活性将减小。间二甲苯系列化合物的热反应活性的顺序为 :d1>d2 >a>d3。

Pyrolysis mechanism about a series of m\|xylene compounds(the compounds with different substituent on the benzene ring, including\|OH,\|SH,\|CN)is studied by using the AM1 method in Gaussian 98 program package, and 8 types of pyrolysis processes are established. The calculation results about equilibrium geometries and thermodynamics energies of these compounds show that no matter the groups connected with the benzene rings are electron acceptor groups or electron donor groups, the pyrolysis process of these compounds firstly starts from the C\|H bonds of the methyl's on the benzene rings. In another word, the C\|H bonds of the methyl are the easiest to break, and also the step that controls the pyrolysis temperature. This conclusion is in accord with the experimental result. The studies show that the thermoreactive activity of the reactants will be increased if there are electron donor groups on the mxylenes(\|OH,\|SH), and the thermoreactive activity of the reactants will be decreased if there are electron acceptor groups on the m\|xylenes(\|CN). The activity sequence of the series of m\|xylene compounds is:d 1>d 2>a>d 3.