多氯联苯系列化合物的热力学性质和稳定性的密度泛函理论研究

在B3LYP/6-31G*水平上计算了218个多氯联苯系列化合物(PCB), 得到分子总能量(E_T)、焓(H°)、熵(S°)、自由能(G°)、零点振动能(E_ZPV)、恒容热容(C_V°)和热能校正值(E_th), 研究了这些参数与氯原子的取代数目及取代位置的关系.氯原子置换在不同的位置使E_T, H°和G°数值增加的顺序为: 2(6)位>>3(5)位＞4位, 即置换在2(6)位最不稳定; 两个氯原子在同一苯环比分别处于两个苯环的E_T, H°和G°增大, 增加的顺序为: 邻位>>间位＞对位, 即相互处于邻位最不稳定. 每增加1个氯原子, S°增大约30 J•(mol•K)^－1, E_th减小约22.2 kJ•mol^－1, E_ZPV减小约25.5 kJ•mol^－1, C_V°增大约16 J•(mol•K)^－1. 根据异构体自由能的相对大小, 从理论上求得异构体的相对稳定性顺序.

Study on the thermodynamic Property and Relative Stability of a Series of Polychlorinated Biphenyls by Density Functional Theory

The thermodynamic function data, including total energy E_T, enthalpy H°, entropy S°, free energy G°, zero-point vibration energy E_ZPV, constant volume molar heat capacity C_V° and thermal energy correction E_th for 218 polychlorinated biphenyls (PCB), were predicted by fully optimized calculation at the B3LYP/6-31G* level. In addition, the dependence of these thermodynamic parameters on the number and the position of chlorine substitute was discussed. It was suggested that the total energy, enthalpy and free energy of PCB vary remarkably with the positions of chlorine substitutes, with the order of increase of parameter value being position 2(6)>>3(5)＞4. It was also found that the thermodynamic parameters were all increased in the case of the two chlorine substitutes on the same aromatic ring, compared with that on two aromatic rings, with the increase effect order being ortho>>meta＞para. With addition of each chlorine atom to PCB, the increased values of S° and C_V° were about 30 and 16 J•(mol•K)^－1, the decreased ones of E_th and E_ZPV about 22.2 and 25.5 kJ•mol^－1, respectively. Also, the relative stability of PCB congeners was theoretically proposed in this work.