【作者】 盛利；

【导师】 李泽生；

【作者基本信息】 吉林大学 ， 物理化学， 2004， 博士

【摘要】 化学反应速率常数的研究和测定工作一直是化学领域主要研究课题之一。在理论上实现对不同条件下化学反应速率常数的预测是目前理论化学领域中的前沿课题。本文利用直接动力学方法，对几类在大气和燃烧化学中起重要作用的自由基和分子反应的微观机理和速率常数进行了理论研究，其中包括：1) H原子与CHnCl4-n (n=0,1,3)、CH3Br和C2H5Cl反应。H原子与卤代烷烃的反应在工业生产和卤代废弃物的焚烧过程中起重要的作用，研究这类反应的微观机理，精确地得到反应速率常数，对保护大气臭氧和环境有重要的意义。目前实验测量的速率常数仅限于较小的温度范围内(T < 1000K)，实际上卤代烷烃的燃烧温度可达2000－2500K。对于个别多通道反应，实验上测量的只是总包反应速率常数，还没有反应分支比的信息。2) HBr与CHnCl3-n (n=1,2,3)和C2H5自由基的反应。得到小分子烷烃自由基、氯化烷烃自由基与溴化氢反应的速率常数就可以精确地计算自由基的标准生成热及相应分子C-H键的键能。最近的实验研究发现CH3 + HBr和C2H5+ HBr反应具有负的反应活化能及较大的反应速率常数，完全不同于早期的实验研究结果，即：两个反应具有正的反应活化能。3) OH自由基与CH3CF3的反应。全F、Cl取代的烷烃化合物(CFCs)广泛应用于制冷剂、清新剂和泡沫灭火剂等，这些物质进入大气层以后，光解产生的Cl原子可以消耗同温层中的臭氧。部分F、Cl取代的烷烃化合物(HCFCs)被认为是CFCs的潜在替代品，包括本文研究的反应物CH3CF3，它们可以减少对臭氧的损耗。在对流层中和OH的反应是<WP=167>HCFCs降解的主要反应通道。因此研究OH和HCFCs反应机理对保护臭氧层有重要意义。本文的目的就是通过探讨这几类反应的反应路径和速率常数等动力学信息，揭示这些反应的微观机理，探索反应的控制条件。为理解大气化学和燃烧化学中有关自由基和分子反应的微观过程和机理提供理论基础，为进一步的实验研究提供理论依据。本文的理论计算主要是利用GAUSSIAN98程序，在BH&HLYP或MP2水平下计算稳定点(反应物、过渡态和产物)的几何和频率，进一步利用内禀反应坐标理论获得反应的最小能量路径，在最小能量路径上选点进行较高水平[QCISD(T)或PMP4(SDTQT)]的单点能量校正。然后结合POLYRATE8.4.1程序，使用变分过渡态理论计算上述化学反应的速率常数。主要研究成果如下：通过对H原子与卤代烷烃反应的研究表明： (1) 在H + CH3X (X= F、Cl、Br)反应中，卤原子迁移反应能垒按CH3F > CH3Cl > CH3Br的顺序降低，而不同卤原子对氢原子迁移反应的影响比较小，其反应的能垒变化不大。这导致了在H＋CH3F反应中，F原子迁移是主要的反应通道；在H + CH3Cl反应中，氯迁移反应与氢迁移存在着竞争，对总反应的贡献在温度1000K时达到最大值31％；在H + CH3Br反应中，溴迁移反应是主要的反应通道。(2) 在H+CHCl3反应中，当温度低于677K时氢迁移反应是主要的反应通道，而温度高于677K时氯迁移反应的速率常数快于氢迁移反应的速率常数。(3) 对于H + CH3CH2Cl反应，在 298 ? 5000K温度区间内，氢迁移反应是主要的反应通道；温度低于3000K时，α-H迁移反应是总的氢迁移反应中的主要通道；温度高于3000K时，β-H氢迁移反应要快于α-H迁移反应。对于本文研究的H原子与卤代烷烃的多通道化学反应，实验上还没有关于反应分支比的报道，而且实验测量得到的总包速率常数分布在较小的温度范围内 (T < 1000K)，卤代烷烃实际的燃烧温度可达2000 ?2500 K。本文计算的结果在实验测量的温度区间与实验值吻合得很好，因此可以为进一步实验测量提供理论线索。 <WP=168>在温度较低时，隧道效应对H原子和卤代烷烃反应的速率常数计算影响较大，在理论计算速率常数时应当考虑。变分效应对计算H + CH3Br ( HBr + CH3反应速率常数有较大的影响，而对其它反应影响较小。产生这种不同变分效应的原因可能是由于这个反应具有相对低平的入口势垒的经典势能曲线与在过渡态附近有较大下降的零点振动能曲线的结合，使振动绝热基态势能曲线在反应物区的势垒相对于在过渡态附近的势垒更高的缘故。本文计算得到的CH3 + HBr和C2H5 + HBr两个反应的速率常数在低温区间(T < 500K)具有负的温度依赖关系，与最近Seetula、Nicovich、Krasnoperov及Seakins等人实验测量值符合得较好，不同于早期实验测量结果，早期的实验研究认为这两个反应具有正的反应活化能。本文研究结果表明CH3 + HBr和C2H5 + HBr两个反应的速率常数表现出强烈的非线性阿仑尼乌斯形式：对于CH3 + HBr反应，在温度低于536 K时，计算的速率常数具有负的温度依赖关系；而在536K以上，计算的速率常数具有正的温度依赖关系。对于C2H5 + HBr反应，在温度低于850K时，计算的速率常数具有负的温度依赖关系；在850K以上时，计算的速率常数具有正的温度依赖关系。理论计算得到的CHnCl3-n + HBr (n = 1,2)反应的速率常数与实验值比较接近，由于在低温实验测量的CHCl2 + HBr反应的速率常数有误差，导致实验的速率常数显出不合理的负的温度依赖关系。本文的理论计算可以弥补实验上的不足，给出了该反应在200 ( 799K的速率常数表达式。通过研究C2H5、CH更多还原

【Abstract】 The study and determination of reaction rate constants has always been one of the main research fields in chemistry. It is one of the most active subjects to predict the rate constants theoretically. In this thesis, ab initio and density functional theory combined with the direct dynamics methods have been used to study the following chemical reactions: 1) Reactions H + CHnCl4-n (n=0,1,3), H + CH3Br, and H + C2H5Cl. Incineration is a cleanest method for disposal of halogenated hydrocarbons, which are potential sources of chlorine or bromine in the stratosphere, where they can catalytically destroy ozone. The current studied reactions play an important role in the incineration of halogenated hydrocarbons. To incinerate halogenated hydrocarbons in a more efficient and less hazardous way, it is necessary to further investigate all aspects of this kind of combustion process. It is especially important to understand the incineration mechanism, specific pathways, and rate constants for important elementary reactions.2) Reactions HBr + CHnCl3-n (n=1,2,3) and HBr +C2H5. The reactions of small alkyl radicals and chlorinated alkyl radicals with HBr have been extensively studied during the past decades. This is due to their importance for fundamental chemical kinetics and thermodynamics. The measured rate constants can be used to determine the heats of formation for these radicals. Early measurements showed that these radical reactions were activated process characterized by small positive activation energies. However, since 1988, a number of experimental kinetic studies using flash photolysis techniques have found out the negative activation energy and high rate constants for these reactions.3) Reaction OH+CH3CF3. It is well known that the chlorofluorocarbons <WP=170>(CFCs) are responsible for greenhouse effect and depletion of ozone layer in the atmosphere; HFCs are currently thought to be the prime candidates as CFC replacements. The hydrogen abstraction reaction of hydroxyl radical with hydrofluorocarbons (HFCs) plays an important role in atmospheric chemistry and combustion process.The above-mentioned reactions play an important role in atmosphere and combustion chemistry. So the studies on reaction mechanism and the rate constants of these reactions are of great benefit to environmental and stratospheric ozone protection. There have many experimental investigations concerning rate constants for these reactions, but the theoretical studies are little. The main object of the current thesis is to provide accurate results of the reaction path and the temperature dependence of rate constants and to explore the reaction mechanism of these reactions. The theoretical results may provide useful information for further experimental studies.By means of the Gaussian98 program, at the BH&HLYP or MP2 level, the geometries and frequencies of the stationary points (reactant, transition state, and products) are calculated. The minimum energy path (MEP) is calculated at the same level by intrinsic reaction coordinate (IRC) theory. Furthermore, with the selected points along the MEP, the force constant matrices as well as the harmonic vibrational frequencies are obtained. In order to gain more accurate energy profile, the energies of the selected points on the MEP are refined at QCISD(T) or PMP4(SDTQ) level. Finally, the canonical variational transition state theory with small-curvature tunneling correction is applied to obtain the reaction rate constants by using POLYRATE-Version 8.4.1 program. The main results can be summarized as follows:The studies on the reactions of halogenated hydrocarbons with H atom indicate that (1) in the reactions H + CH3X (X=F, Cl, Br), for the halogen abstraction reactions, the values of classical barrier heights (ΔEf) rapidly decrease in the order CH3F > CH3Cl > CH3Br. For the hydrogen abstraction reactions, however, the change of barrier heights is very slight. This phenomenon may be attributed to the different electronegativity of <WP=171>halogen atom in methyl halide. As a result, thi更多还原