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首页> 外文期刊>The journal of physical chemistry, A. Molecules, spectroscopy, kinetics, environment, & general theory >DFT Calculation on the Thermodynamic Properties of Polychlorinated Dibenzo-p-dioxins: Intramolecular Cl-Cl Repulsion Effects and Their Thermochemical Implications
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DFT Calculation on the Thermodynamic Properties of Polychlorinated Dibenzo-p-dioxins: Intramolecular Cl-Cl Repulsion Effects and Their Thermochemical Implications

机译：DFT计算多氯二苯并-对-二恶英的热力学性质：分子内Cl-Cl排斥作用及其热化学意义

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摘要

Polychlorinated dibenzo-p-dioxins (PCDDs) are one of the most intensively studied chemical pollutants. However, the absence of reliable thermodynamic data on PCDDs seriously limits quantitative understanding of their formation and distribution at combustion sources. By carrying out a DFT calculation (B3LYP/6-31G~(**)) for thermodynamic properties of 75 PCDD congeners, we find that previous thermodynamic estimations on the gas-phase enthalpies of formation (ΔH_f) and Gibbs free energy of PCDDs were seriously misestimated, particularly for highly chlorinated congeners, largely as a result of underestimating the intramolecular Cl-Cl repulsion energy. The disagreement between the DFT calculation and other methods becomes progressively larger with increasing chlorine substitution. Octachlorodibenzo-p-dioxin (OCDD) that has been considered as the most thermodynamically stable congener in previous calculations turns out to be much less stable. The differences in calculated ΔH_f values between OCDD and the most toxic congener, 2,3,7,8-tetrachlorodibenzo-p-dioxin, or other laterally chlorinated (2,3,7,8-substituted) toxic congeners are within 1 kcal/mol. Most ΔH_f values for congeners with five to eight chlorines differ by only 1-2 kcal/mol, since the decreasing electronic energies with increasing the number of chlorines in PCDDs are counterbalanced by increasing Cl-Cl repulsion energy. The intramolecular chlorine repulsion effects in PCDDs are systematically analyzed by using isodesmic reactions.
机译：多氯二苯并对二恶英（PCDDs）是研究最深入的化学污染物之一。但是，在PCDDs上缺乏可靠的热力学数据，严重限制了对其在燃烧源处的形成和分布的定量理解。通过对75种PCDD同源物的热力学性质进行DFT计算（B3LYP / 6-31G〜（**）），我们发现先前对PCDDs的气相生成焓（ΔH_f）和吉布斯自由能的热力学估计为严重错误地估计了这一点，尤其是对于高度氯化的同类物，主要是因为低估了分子内Cl-Cl排斥能。 DFT计算与其他方法之间的分歧随着氯取代的增加而逐渐变大。在以前的计算中，八氯二苯并对二恶英（OCDD）被认为是热力学最稳定的同类物，但稳定性却差得多。 OCDD与毒性最高的同类物，2,3,7,8-四氯二苯并-对-二恶英或其他侧向氯化的（2,3,7,8-取代的）毒性同类物之间的计算得出的ΔH_f值差异在1 kcal /摩尔对于具有五到八个氯的同类物，大多数ΔH_f值仅相差1-2 kcal / mol，因为随着PCDDs中氯数的增加而降低的电子能通过增加Cl-Cl排斥能来抵消。使用等渗反应系统地分析了PCDDs中的分子内氯排斥作用。

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