The thermodynamic properties of 76 polyfluorinated dibenzo-p-dioxins (PFDDs) in the ideal gas state at 298.15 K and 1.013×105 Pa have been calculated at the B3LYP/6-311G** and B3LYP/6-31G* levels using Gaussian 03 program, and it was found that values of standard enthalpy of formation (△fH-θ), standard free energy of formation (△fGθ) and standard entropy (Sθ) were broadly consistent under the two base groups. According to the relative magnitude of their △fGθ at the B3LYP/6-311G** level, the relative stability order of PFDD congeners was theoretically proposed. Octanol-water partition coefficients (lg Kow) of polybrominated dibenzofurans (PBDDs), polychlorinated dibenzofurans (PCDDs) and PFDDs were calculated by group contributions. Comparison of the properties (including lg Kow, Sθ, △fH-θ and △fGθ) between PFDDs and PCDDs, PBDDs was carried out. Moreover, the relative rate constants of formation reactions of PFDDs and the values of molar heat capacity at constant pressure (Cp,m) from 200 to 1800 K for PFDDs congeners were calculated using a statistical thermodynamics calculation program. The temperature dependence relation of Cp,m was obtained using the least-squares method. The result shows that there exists very good relationship between Cp.m and temperature (T, T-1 and T-2) for almost all PFDD congeners.%采用密度泛函理论(DFT)方法,在B3LYP/6-311G**和B3LYP/6-31G**两种水平上,对76种多氟代二苯并对二噁英系列化合物(PFDDs)进行了几何构型的全优化,并计算了各分子在298.15 K,1.013×105 Pa的标准状态下的热力学参数.基组从6-31G*增大到6-311G**没有显著改变标准生成热(△fHθ)、标准生成自由能(△fGθ)和标准熵(Sθ)数值.根据B3LYP/6-311G**水平计算得到的△fGθ的相对大小,求得PFDDs同数目取代氟原子的各异构体的相对稳定性的顺序.采用基团贡献法计算了多溴代二苯并对二噁英(PBDDs)、多氯代二苯并对二噁英(PCDDs)和PFDDs的辛醇-水分配系数(1gKow).并将PFDDs的△fHθ,△fGθ,Sθ和lgKow的计算结果与PBDDs和PCDDs的相关数值进行了比较.同时,计算了PFDDs各组异构体化合物的生成反应相对速率常数,采用统计热力学程序计算了这些化合物在200至1800K的恒压摩尔热容(Cp,m),并用最小二乘法求得Cp,m与温度之间的相关方程,发现Cp,m与T,T-1和T-2之间有着很好的相关性.
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