烷基环戊烷被羟基自由基夺氢反应类的动力学研究

基于燃烧详细反应机理构建的需要, 采用反应类过渡态理论(RC-TST)研究了OH自由基夺取烷基环戊烷环上和侧链上氢原子的动力学. 在考察侧链氢提取反应类的16个代表反应的基础上, 本工作首次将该方法推广到环上α氢提取反应类的10个代表反应的研究, 分别建立了两类反应的线性自由能(LER)关系式. 计算结果表明, 采用RC-TST/LER方法预测的这两类反应的速率常数与直接应用TST/Eckart方法得到的结果接近, 说明RC-TST/LER方法对预测这两类反应的速率常数非常有效, 且节约了大量计算成本. 而且, 无论是侧链还是五元环, OH·夺取叔碳上的氢原子最易发生.     

过氧烷基自由基分子内氢迁移反应类速率常数的计算

过氧烷基自由基分子内氢迁移是低温燃烧反应中的一类重要基元反应. 本文用等键反应方法计算了该类反应的动力学参数. 所有反应物、过渡态、产物的几何结构均在B3LYP/6-311+G(d,p)水平下优化得到. 本文提出了用过渡态反应中心几何结构守恒作为反应类判据, 并将该分子内氢迁移反应分为四类, 包括(1,3)、(1,4)、(1,5)、(1,n) (n=6, 7, 8)氢迁移类. 分别将这4 类反应类中最小反应体系作为类反应的主反应, 并分别在B3LYP/6-311+G(d,p)低水平和CBS-QB3 高水平下得到其近似能垒和精确能垒. 其余氢迁移反应作为目标反应, 在B3LYP/6-311+G(d,p)低水下计算得到其近似能垒, 再采用等键反应方法校正得到目标反应的精确反应势垒和精确速率常数. 研究表明, 采用等键反应方法只需在低水平用从头算计算就可以得到大分子反应体系的高精度能垒和速率常数值, 且本文按等键反应本质的分类方法更能揭示反应类的本质, 并对反应类的定义给出了客观标准. 本文的研究为碳氢化合物低温燃烧模拟中重要的过氧烷基分子内氢迁移反应提供了准确的动力学参数.     

烷基过氧化氢中氢提取反应类大分子体系的反应能垒与速率常数的精确计算

自由基与分子反应是一类具有负活化能的非基元反应,通常认为是通过反应复合物的两步过程,在大气化学和碳氢燃料燃烧机理中广泛存在,且在理论计算和实验上广泛研究.以碳氢燃料燃烧机理中重要反应类羟基自由基提取烷基过氧化氢α位氢的反应为研究对象,通过量化计算揭示其反应规律,计算得到其精确动力学参数.在所研究反应类中,定义第一步反应复合物的生成反应的标准摩尔吉布斯自由能变化等于零时所对应的温度为其转折温度T_c,并表明了当T >> T_c时可采用稳态近似法处理该类反应体系,得到总包反应速率常数.所有反应涉及的物种几何结构优化和频率分析均在BHandHLYP/6-311G（d,p）水平下得到,并在所研究反应类中选取了5个代表反应,通过CCSD（T）/CBS单点能计算,得到其最高转折温度为195.17 K,远远低于碳氢燃料燃烧模拟通常关注温度范围的最低温度650 K,表明用稳态近似法处理该类负活化能反应体系是合理的.计算还表明,该类反应的过渡态反应中心几何结构守恒,因此可将等键反应方法引入类反应,通过对低水平从头算得到的反应能垒进行校正,以得到高精度的结果.为了验证等键反应方法的可靠性,选取5个反应作为研究对象,将低水平BHandHLYP/6-311G（d,p）的校正结果和高水平CCSD（T）/CBS直接计算的结果进行比较,反应能垒最大绝对偏差由校正前的19.99 kJ·mol^－1降到校正后的1.47 kJ·mol^－1,表明用等键反应方法,只需在低水平从头算水平下就可以得到高水平的计算结果,从而可解决大分子体系精确动力学参数缺乏的问题.利用等键反应方法计算了20个反应的反应能垒,并结合过渡态理论计算得到了总包反应的速率常数,并揭示了该类反应只在低温段呈现负活化能关系.     

多环芳烃与不饱和自由基氢提取反应类的动力学研究

对2~6个环的多环芳烃的氢提取反应类进行了系统研究, 提取氢原子的不饱和自由基包括丙炔基自由基（C₃H₃）、 烯丙基自由基（C₃H₅）、 丁二烯基自由基（nC₄H₅, iC₄H₅）、 环戊二烯基自由基（C₅H₅）以及苯基自由基（C₆H₅）. 采用M06-2X/cc-pVTZ方法得到了多环芳烃的电子结构信息, 利用过渡态理论并结合Eckart隧道校正, 计算了所有反应在500~2500 K范围内的反应速率常数.考察了多环芳烃的大小、 结构对反应速率常数的影响, 对比了不同氢提取自由基及不同氢提取反应类型的速率常数. 结果表明, 多环芳烃的大小对反应速率常数影响不大, 但是多环芳烃的环结构对反应速率常数影响较大. 将不同的氢提取反应类简化为发生在五元环上的C₅类和发生在六元环上的C₆类两类, 结果表明, C₆类的反应活性高于C₅类. 研究了nC₄H₅, iC₄H₅以及C₆H₅自由基与多环芳烃的氢提取反应, 它们的氢提取反应活性大小顺序为C₆H₅>nC₄H₅>iC₄H₅. 通过对每类典型反应的速率常数取平均值, 总结出相应类型的速率规则, 可用于构建多环芳烃和碳烟机理.     

CF_2ClC(O)OCH_2CH_3+OH的反应机理及动力学性质的理论研究

利用双水平直接动力学方法,在MCG3-MPWB//M06-2X/aug-cc-pVDZ水平上研究了CF_2ClC(0)OCH_2CH_3+OH的微观反应机理.得到了反应物CF_2ClC(O)OCH_2CH_3的5种稳定构象(RCl~RC5),并对每一构象考察了发生在-CH_3-和-CH_2-基团上的所有可能氢提取反应通道.利用改进的变分过渡态理论(ICVT)结合小曲率隧道效应校正(SCT)计算了各反应通道的速率常数,分析了各构象反应位点选择性.结果表明,对于构象RCl和RC2,低温时氢提取反应主要发生在-CH_2-基团上;而对于构象RC3RC4和RC5,发生在-CH_3基团上的氢提取反应通道在整个温度区间内占绝对优势.根据Boltzmann配分函数计算总包反应速率常数,在298 K温度下计算的体系总包反应速率常数与实验值相符,进而给出200~1000 K温度范围内拟合了速率常数的三参数Arrhenius表达式:k_(overall)=5.45×10~(25)T~(4.54)exp(-685/T).     

十氢化萘低温燃烧反应的动力学机理

采用量子化学方法研究了十氢化萘低温燃烧的动力学机理,获得了脱氢反应、自由基加氧反应及1,5氢迁移反应等反应的动力学参数,并在CBS-QB3水平下获得了相关物种的热力学参数,通过过渡态理论计算获得了具有紧致过渡态反应的高压极限速率常数,而无能垒反应的速率常数则由变分过渡态理论得到.基于此机理分析了十氢化萘低温反应的动力学规律和热力学机制.相比于链烷烃和单环烷烃,十氢化萘自由基加氧反应的速率常数随温度变化较快,1,5-氢迁移反应的能垒较高,揭示了物质结构对反应动力学的影响.热力学平衡常数分析结果表明,在低温下十氢化萘自由基加氧反应起主导作用.通过拟合获得了所有反应Arrhenius形式的速率常数,这些参数可用于双环烷烃低温燃烧机理的构建和优化.     

烷基自由基β位裂解反应类反应势垒与速率常数的精确计算

提出反应类等键方法并用于高温燃烧机理中一类重要反应——烷基自由基β位裂解反应的反应势垒和速率常数的精确校正计算. 通过10种不同从头算水平对类反应中5个代表反应的反应势垒的计算发现, 用反应类等键反应方法和直接从头算方法获得的5 个代表反应的反应势垒最大绝对偏差的平均值分别为5.32 和16.16 kJ·mol^-1, 表明反应类等键反应方法计算的反应势垒对不同水平从头算方法的依赖性小, 可在较低从头算水平计算得到精确的反应势垒, 解决大分子体系反应势垒的精确计算问题. 此外应用反应类等键反应方法在BHandHLYP/cc-pVDZ 从头算水平计算了3 个代表反应的速率常数, 并与文献报道的实验值进行了比较, 其在500-2000 K温度区间内计算速率常数与实验速率常数中较大值与较小值的比值k_max/k_min的平均值为1.67, 最大值也仅有2.49. 表明应用反应类等键反应方法在较低从头算水平即可对同类反应的速率常数进行精确计算.最后在BHandHLYP/cc-pVDZ从头算水平用反应类等键反应方法计算了13个烷基自由基β位裂解反应的速率常数.     

H原子和SiH2Cl2抽提反应的理论研究

对H+SiH2Cl2反应进行了详细的理论研究,理论证明了抽提氢的通道是唯一可行的反应通道。并在从头算给出的电子结构信息基础上,用变分过渡态理论(CVT)加小曲率隧道效应校正(SCT)等方法对该反应进行了直接的动力学研究,得到该反应的理论速率常数,并详细讨论了各动力学参数沿反应坐标的变化。在较宽的温度范围内,反应速率常数表现出非Arrhenius行为,用三参数公式似合了速-温关系式,为k(T)=(1.32×10^-22)T^3.67exp(-26/T)。理论计算的速率常数与实验数值符合得很好。     

烷烃与氢过氧自由基氢提取反应类反应能垒与速率常数的精确计算

氢过氧自由基从烷烃分子中提取氢的反应是碳氢燃料中低温燃烧化学中非常重要的一类反应。本文用等键反应方法计算了这一类反应的动力学参数。所有反应物、过渡态、产物的几何结构均在HF/6-31+G(d)水平下优化得到。以反应中的过渡态反应中心的几何结构守恒为判据,该反应类可用等键反应处理。本文选取了乙烷和氢过氧自由基的氢提取反应为参考反应,其它反应作为目标反应,用等键反应方法对目标反应在HF/6-31+G(d)水平的近似能垒和反应速率常数进行了校正。为了验证方法的可靠性,选取C5以下的烷烃分子体系,对等键反应方法校正结果和高精度CCSD(T)/CBS直接计算结果进行了比较,最大绝对误差为5.58k J?mol~(-1),因此,采用等键反应方法只需用低水平HF从头算方法就可以再现高精度CCSD(T)/CBS计算结果,从而解决了该反应类中大分子体系的能垒的精确计算。本文的研究为碳氢化合物中低温燃烧模拟中重要的烷烃与氢过氧自由基氢提取反应提供了准确的动力学参数。     

CF3CH2CF2CH3（HFC-365mfc）与Cl原子反应的微观机理及动力学性质

采用密度泛函理论方法 M06-2X结合6-31+G(d,p)基组研究了CF3CH2CF2CH3与Cl原子反应的反应机理.计算获得了CF3CH2CF2CH3的两种可区分的稳定几何构象RC1和RC2以及与它们相对应的8条氢提取反应通道和2条取代反应通道.运用改进的正则变分过渡态理论(ICVT)并结合小曲率隧道效应校正(SCT),在M06-2X/6-31+G(d,p)水平上计算了各氢提取通道的速率常数,并由Boltzmann配分函数得到总包反应的速率常数kT(cm3.molecule-1.s-1).计算结果表明,体系的总反应速率常数与已有实验值相吻合,进而给出了该反应在200～1000 K温度区间内反应速率常数kT的三参数表达式kT=1.88×10-22T3.76.exp(-1780.69/T),并讨论了两种构象RC1和RC2对总反应的贡献及各构象中氢提取发生在—CH3或—CH2—基团上的位置选择性.此外,由于缺少相关反应物及产物自由基标准生成焓ΔHf,298 K的数据,利用等化学键法估算了在上述物种的标准生成焓.     

Kinetics of the hydrogen abstraction OH + alkane --> H2O + alkyl reaction class: an application of the reaction class transition state theory

This paper presents an application of the reaction class transition state theory (RC-TST) to predict thermal rate constants for hydrogen abstraction reactions of the type OH + alkane --> HOH + alkyl. We have derived all parameters for the RC-TST method for this reaction class from rate constants of 19 representative reactions, coupling with linear energy relationships (LERs), so that rate constants for any reaction in this class can be predicted from its reaction energy calculated at either the AM1 semiempirical or BH&HLYP/cc-pVDZ level of theory. The RC-TST/LER thermal rate constants for selected reactions are in good agreement with those available in the literature. Detailed analyses of the results show that the RC-TST/LER method is an efficient method for accurately estimating rate constants for a large number of reactions in this class. Analysis of the LERs leads to the discovery of the beta-carbon radical stabilization effect that stabilizes the transition state of any reaction in this class that yields products having one or more beta-carbons, and thus leads to the lower barrier for such a reaction.     

Kinetics of the C-C bond beta scission reactions in alkyl radical reaction class

Kinetics of the β-scission in alkyl radical reaction class was studied using the reaction class transition state theory (RC-TST) combined with the linear energy relationship (LER) and the barrier height grouping (BHG) approach. All necessary parameters were derived from first-principle density functional calculations for a representative set of 21 reactions. Different error analyses and comparisons with available literature data were made. Direct comparison with available experimental data indicates that the RC-TST/LER, where only reaction energy is needed, can predict rate constants for any reaction in this reaction class with excellent accuracy. Specifically for this reaction class, the RC-TST/LER method has less than 60% systematic errors on average in the predicted rate constants when compared to explicit rate calculations.     

Kinetics of the hydrogen abstraction R−OH + H → R•−OH + H2 reaction class: An application of the reaction class transition state theory

This paper presents an application of the reaction class transition state theory (RC‐TST) to predict thermal rate constants for hydrogen abstraction reactions of the type R‐OH + H → R^?‐OH + H₂. We have derived all parameters for the RC‐TST method with linear energy relationships (LERs) and the barrier height grouping (BHG) approach for this reaction class from rate constants of 37 representative reactions divided in two types of hydrogen abstraction, namely from α carbon sites and non‐α carbon sites two training sets. Error analyses indicate that the RC‐TST/LER, where only reaction energy is needed, and RC‐TST/ BHG, where no other information is needed, can predict rate constants for any reaction in this reaction class with satisfactory accuracy for combustion modeling. Specifically for this reaction class, the RC‐TST/LER and RC‐TST/BHG methods have, respectively, less than 40% and 90% systematic errors in the predicted rate constants, when compared to the explicit full TST/Eckart method. The branching ratio analysis shows that in the low‐temperature regime α abstractions are dominant, whereas, for T > 1500 K, abstractions at other sites become more important. © 2010 Wiley Periodicals, Inc. Int J Chem Kinet 43: 78–98, 2011     

Kinetics study of the OH + alkene --> H2O + alkenyl reaction class

In this paper we report on the kinetics of hydrogen abstraction for the OH + alkene reaction class, using the reaction class transition state theory (RC-TST) combined with the linear energy relationship (LER) and the barrier height grouping (BHG) approaches. Parameters for the RC-TST were derived from theoretical calculations using a set of 15 reactions representing the hydrogen abstractions from the terminal and nonterminal carbon sites of the double bond of alkene compounds. Both the RC-TST/LER, where only reaction energy is needed at either density functional theory BH&HLYP or semiempirical AM1 levels, and RC-TST/BHG, where no additional information is required, are found to be promising methods for predicting rate constants for a large number of reactions in this reaction class. Detailed error analyses show that, when compared to explicit theoretical calculations, the averaged systematic errors in the calculated rate constants using both the RC-TST/LER and RC-TST/BHG methods are less than 25% in the temperature range 300-3000 K. The estimated rate constants using these approaches are in good agreement with available data in the literature.     

Ab initio study on the kinetics of hydrogen abstraction for the H+alkene-->H2+alkenyl reaction class

Kinetics of the hydrogen abstraction reaction class of the H+alkene has been studied using the reaction class transition state theory (RC-TST) combined with the linear energy relationship (LER) and the barrier height grouping (BHG) approach. The rate constants for the reference reaction, H+C2H4, were obtained by the canonical variational transition state theory (CVT) with the small curvature tunneling (SCT) correction in the temperature range of 300-3000 K. Combined with these data, both the RC-TST/LER, where only reaction energy is needed, and RC-TST/BHG, where no other information is needed, are found to be promising methods for predicting rate constants for a large number of reactions in this reaction class. Our analysis indicates that less than 50% systematic errors on the average exist in the predicted rate constants using the RC-TST/LER or RC-TST/BHG method while in comparison to explicit rate calculations the differences are less than 100% or a factor of 2 on the average.     

Kinetics of the hydrogen abstraction R?OH + H → R?O• + H2 reaction class

This paper presents an application of the reaction class transition state theory (RC‐TST) to predict thermal rate constants for the hydrogen abstraction R? OH + H → R? O^? + H₂ reaction class, where R is an alkyl group. We have derived all parameters for the RC‐TST method for this reaction class from rate constants of 19 representative reactions, coupling with linear energy relationships (LERs) and the barrier height grouping (BHG) approach. Error analyses indicate that the RC‐TST/LER, where only reaction energy is needed, and RC‐TST/BHG, where no other information is needed, can predict rate constants for any reaction in this reaction class with satisfactory accuracy for combustion modeling. Specifically for this reaction class, the RC‐TST/LER method has less than 25% systematic errors in the predicted rate constants, whereas the RC‐TST/BHG method has less than 35% error when compared to explicit rate calculations. © 2010 Wiley Periodicals, Inc. Int J Chem Kinet 42: 414–429, 2010     

Kinetics of 1,5-hydrogen migration in alkyl radical reaction class

Kinetics of the 1,5-intramolecular hydrogen migration in the alkyl radicals reaction class has been studied using the reaction class transition state theory combined with the linear energy relationship (LER) and the barrier height grouping (BHG) approach. The high pressure limits of the rate constants for the reference reaction of 1-pentyl → 1-pentyl, calculated by the Canonical Variational Transition State Theory (CVT) with the Small Curvature Tunneling (SCT), are taken from the literature. Direct comparison with available experimental data indicates that the RC-TST/LER, where only reaction energy is needed, can predict rate constants for any reaction in this reaction class with excellent accuracy. Specifically for this reaction class, the RC-TST/LER method has less than 65% systematic errors in the predicted rate constants when compared to explicit rate calculations.     

Kinetics of the hydrogen abstraction *CH3 + alkane --> CH4 + alkyl reaction class: an application of the reaction class transition state theory

Kinetics of the hydrogen abstraction reaction (*)CH(3) + CH(4) --> CH(4) + (*)CH(3) is studied by a direct dynamics method. Thermal rate constants in the temperature range of 300-2500 K are evaluated by the canonical variational transition state theory (CVT) incorporating corrections from tunneling using the multidimensional semiclassical small-curvature tunneling (SCT) method and from the hindered rotations. These results are used in conjunction with the Reaction Class Transition State Theory/Linear Energy Relationship (RC-TST/LER) to predict thermal rate constants of any reaction in the hydrogen abstraction class of (*)CH(3) + alkanes. Our analyses indicate that less than 40% systematic errors on the average exist in the predicted rate constants using the RC-TST/LER method while comparing to explicit rate calculations the differences are less than 100% or a factor of 2 on the average.     

Kinetics of the hydrogen abstraction CHO + Alkane → HCHO + Alkyl reaction class: an application of the reaction class transition state theory

The kinetics of the hydrogen abstraction at alkanes by formyl radicals is investigated using the reaction class transition state theory (RC-TST) approach combined with the linear energy relationship (LER) or the barrier height grouping (BHG). The rate constants of a reaction in this class can be estimated through those of the reference reaction, CHO + C₂H₆, which are obtained from rate constants of the reaction that involves the smallest species, namely CHO + CH₄, using the explicit RC-TST scaling. The thermal rate constants of this smallest reaction are evaluated at the canonical variational transition state theory (CVT) with the corrections from the small-curvature tunneling (SCT) and hindered rotation (HR) treatments. Our analyses indicate that less than 40% systematic errors, on the average, exist in the predicted rate constants using both the LER approach, where only reaction energy is needed, and the BHG approach, where no additional information is needed; while comparing to explicit rate calculations the differences are less than 60%. Contribution to Mark S. Gordon 65th Birthday Festschrift Issue.     

Computational studies of intramolecular hydrogen atom transfers in the beta-hydroxyethylperoxy and beta-hydroxyethoxy radicals

The beta-hydroxyethylperoxy (I) and beta-hydroxyethoxy (III) radicals are prototypes of species that can undergo hydrogen atom transfer across their intramolecular hydrogen bonds. These reactions may play an important role in both the atmosphere and in combustion systems. We have used density functional theory and composite electronic structure methods to predict the energetics of these reactions, RRKM/master equation simulations to model the kinetics of chemically activated I, and variational transition state theory (TST) to predict thermal rate constants for the 1,5-hydrogen shift in I (Reaction 1) and the 1,4-hydrogen shift in III (Reaction 2). Our multi-coefficient Gaussian-3 calculations predict that Reaction 1 has a barrier of 23.59 kcal/mol, and that Reaction 2 has a barrier of 22.71 kcal/mol. These predictions agree rather well with the MPW1K and BB1K density functional theory predictions but disagree with predictions based on B3LYP energies or geometries. Our RRKM/master equation simulations suggest that almost 50% of I undergoes a prompt hydrogen shift reaction at pressures up to 10 Torr, but the extent to which I is chemically activated is uncertain. For Reaction 1 at 298 K, the variational TST rate constant is approximately 30% lower than the conventional TST result, and the microcanonical optimized multidimensional tunneling (muOMT) method predicts that tunneling accelerates the reaction by a factor of 3. TST calculations on Reaction 2 reveal no variational effect and a 298 K muOMT transmission coefficient of 10(5). The Eckart method overestimates transmission coefficients for both reactions.