多溴二苯并呋喃的热力学性质和稳定性的密度泛函理论研究

在B3LYP/6-31G*水平上对二苯并呋喃和135个多溴二苯并呋喃系列化合物(PBDF)进行了全优化计算和振动分析, 得到各分子的总能量(E_T)、焓(H⁰)、熵(S⁰)、自由能(G⁰)和恒容热容(C_V⁰), 研究了这些参数与溴原子的取代数目及取代位置的关系. 溴原子置换在不同位置时, 使E_T, H⁰和G⁰数值增加大小次序为: 1(9)＞3(7)＞2(8)＞4(6), 即置换在1(9)位最不稳定; 两个溴原子处在同一苯环时, E_T, H⁰和G⁰数值也都增加, 增加的顺序为: 邻>>间＞对, 即相互处于邻位最不稳定. 而且有两个溴原子同时取代在1和9位时, 使H⁰和G⁰的数值的增加比取代在邻位、间位和对位时的增加值都大. 每增加1个溴原子, S⁰增大约40.1 J•mol^－1•K^－1, C_V⁰增大约16.3 J•mol^－1•K^－1. 同时, 设计等键反应, 计算了各异构体的标准生成热(H_f⁰)和标准生成自由能(G_f⁰). 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, 各异构体组中的稳定性顺序与PCDF系列的稳定性顺序基本一致.

Study on the Thermodynamic Properties and Stability of a Series of Polybrominated Dibenzo-furans by Density Functional Theory

The thermodynamic function data, including total energy (E_T), enthalpy (H⁰), entropy (S⁰), free energy (G⁰) and constant volume molar heat capacity (C_V⁰) for dibenzo-furan and 135 polybrominated dibenzo-furans (PBDF), were predicted by fully optimized calculation and frequency analysis at the B3LYP/6-31G* level. In addition, the dependence of these thermodynamic parameters on the number and the position of bromine substitution were discussed. It was suggested that the total energy, enthalpy and free energy of PBDF vary remarkably with the positions of bromine substitution, with the order of decrease of parameter value being 1(9) position＞3(7) position＞2(8) position＞4(6) position. It was also found that the thermodynamic parameters are all increased in the case of the two bromine substitutions on the same aromatic ring, compared with that on two aromatic rings, with the increase effect order being ortho>>meta＞para. Moreover, the increment of the H⁰ and G⁰ values of PBDF congeners with both 1 and 9 positions substituted with bromine is larger than the corresponding values of that with substitution at ortho position, meta position and para position respectively. With addition of each bromine atom to PBDF, the increased values of S⁰ and C_V⁰ are about 40.1 and 16.3 J•mol^－1•K^－1 respectively. The isodesmic reactions were designed for calculating standard formation heat (H_f⁰) and standard free energy (G_f⁰) of PBDF. The relative stability of PBDF congeners was theoretically proposed in this work. The most stable and unstable isomer in different isomer groups of PBDF are similar to those for PCDF.