摘要
在大气同温层中所含有的卤族元素中,Br自由基在参与对臭氧层的循环破坏中是最活泼的物种。科学研究表明,含溴小分子对臭氧层的破坏要负25%-40%的责任。因此研究BrO与其它气相小分子的反应对于了解臭氧的损耗具有重要的现实意义。本文用密度泛函理论(DFT)方法对BrO与ClO的反应机理以及BrO与NO2间的反应机理和BrO与OH间的反应机理进行了较为系统的研究。
首先,采用密度泛函理论(DFT)中的B3LYP方法下,采用多种基组所计算的结果的精确度相比较,通过将所得的键长与相应实验值作比较,结果表明,采用基组6-31G(3df)计算该类反应的精确度最高。同时表明采用B3LYP/6-31G(3df)方法对研究该体系较为理想。
运用DFT-B3LYP方法,对含氢元素的反应采用6-31G(3df,p)基组,其它反应均采用6-31G(3df)基组,计算研究了气相小分子的反应,获得了如下结果和结论:
1.BrO与ClO反应机理的研究。通过计算我们优化得到了八个反应中间体,其中三个中间体为首次报道,最容易生成的中间体是非共面的中间体BrOOCl即M7。通过中间体M2分解成产物Br自由基与OClO是主要的反应途径,反应所需要的能垒是113.27 kJ/mol,主要产物是Br自由基与OClO。
2.BrO与NO2反应机理的研究。优化得到六个反应中间体,其中有四个为首次报道,计算结果表明共面的中间体BrONO_2(即M3)最稳定。反应物分解成主要产物Br自由基与NO_3的反应途径有三条,其中经中间体M5、M3的分解反应途径是主要的反应途径,此过程仅需要跨越能垒23.26 kJ/mol。
3.BrO与OH反应机理的研究。优化得到了单重态和三重态的各三个稳定结构。单重态中间体SM1最易生成,三重态中间体TM2较易生成。两种不同的多重度的反应主要产物均是HOO与Br自由基;单重态反应经中间体SM1直接分解为产物HOO与Br自由基;三重态反应仅需要跨越能垒17.29 kJ/mol即可直接分解为产物HOO与Br自由基,因此单重态的反应比三重态相对容易进行。
Among all the halogens that are present in suffient amounts in the stratosphere, bromine radical is the most effective species that participates in efficient catalytic cycles leading to destruction of ozone layer. In spite of the fact that bromine compounds in the stratosphere, it has been estimated that the chemistry involving bromine species is responsible for 25% of the ozone loss obsevered in Antarctica and up to 40% of ozone loss during winter in the Arctic region. The object of this work is the mechanism of BrO with other radicals (ClO,NO2,OH) reaction. The purpose of this work is to study the intermediate geometry configurations and the possible reaction path and provide the help for the experiment. In this dissertation, density functional theory (DFT) has been employed to investigate the reaction mechanism systematically. Firstly, many basis sets of B3LYP of DFT have been used to study BrO radical and other radicals, and tested comparisons of the calculated bond length with the experimental ones show that the DFT-B3LYP/6-31G(3df ) method is more convenient than other basis sets.
Subsequently, the DFT-B3LYP method has been used to study the reactions between BrO radical and other radicals. The 6-31G(3df,p) basis set has been employed for hydrogen-containing reactions and the 6-31G(3df ) basis set for other reactions. The following results and conclusions have been reached:
1. The reaction mechanism between BrO and ClO. In this work, we report eight conformeric forms of ClOOBr peroxide,and we fistly report three conformeric forms. Among the eight conformeric forms, the most stable is non-conplanar conformeic forms BrOOCl, namely M7. The main reaction pathway is that intermediate M2 decompound to Br radical and OClO, this barrier is 113.27 kJ/mol. Main production is Br radical and OClO.
2. The reaction mechanism between BrO and NO2. In this work, we report six conformeric forms of BrONO_2 peroxide. The most stable is conplanar conformeic forms BrONO_2. The main production is Br radical and NO_2, and main reaction pathway is M5→M3→TS1→Br+NO_3. This reaction barrier is 23.26 kJ/mol, which is the most energetically favored pathway.
3. The reaction mechanism between BrO and OH. In this work, we present DFT quantum mechanical calculations of both singlet and triplet of the structures. Both singlet and triplet potential energy surfaces predicted by B3LYP/6-31G(3df,p). As for energy, the singlet is more lower than the triplet. The reaction was shown to take place primarily over the singlet surface by two main channels producing HOO+Br. The triplet main channels produing HOO+Br need pass higher barrier which are 22.04 kJ/mol and 17.29 kJ/mol.
引文
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