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 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 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 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.     

烷基环己烷夺氢反应类的动力学研究

应用反应类过渡态理论(RC-TST)研究了氢自由基抽取烷基环己烷氢的反应类,给出了该类反应的动力学参数.通过研究该反应类中的14个代表反应,分析得到了RC-TST方法所需要的各种参数,建立了该类反应的线性能量关系式(LER).应用RC-TST/LER方法计算得到速率常数与TST/Eckart方法计算结果吻合很好,分析表明RC-TST/LER方法对于准确预测夺氢反应类的速率常数十分有效,可以节约大量计算成本.     

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.     

Methyl vinyl ketone+OH and methacrolein+OH oxidation reactions: a master equation analysis of the pressure- and temperature-dependent rate constants

High-level electronic structure calculations and master equation analyses were carried out to obtain the pressure- and temperature-dependent rate constants of the methyl vinyl ketone+OH and methacrolein+OH reactions. The balance between the OH addition reactions at the high-pressure limit, the OH addition reactions in the fall-off region, and the pressure-independent hydrogen abstractions involved in these multiwell and multichannel systems, has been shown to be crucial to understand the pressure and temperature dependence of each global reaction. In particular, the fall-off region of the OH addition reactions contributes to the inverse temperature dependence of the rate constants in the Arrhenius plots, leading to pressure-dependent negative activation energies. The pressure dependence of the methyl vinyl ketone+OH reaction is clearly more important than in the case of the methacrolein+OH reaction owing to the weight of the hydrogen abstraction process in this second system. Comparison of the theoretical rate constants and the experimental measurements shows quite good agreement.     

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.     

Pressure dependence in the methyl vinyl ketone + OH and methacrolein + OH oxidation reactions: an electronic structure study.

High-level electronic structure calculations were carried out for the study of the reaction pathways in the OH-initiated oxidations of methyl vinyl ketone (MVK) and methacrolein (MACR). For the two conformers of MVK (called synperiplanar and antiperiplanar), the addition channels of OH to the terminal and central carbon atom of the double bond dominate the overall rate constant, whereas the abstraction of the methyl hydrogen atoms has no significant kinetic role. In the case of MACR, only the antiperiplanar conformer is important in its reactivity. In addition, the lower Gibbs free energy barrier for MACR corresponds to the aldehydic hydrogen abstraction reaction, which will be somewhat more favorable than the addition processes. The subtle balance between the different pathways (additions versus abstractions) serves to give an understanding of the pressure dependence of the rate constants of these tropospheric oxidation processes.     

Ab initio investigation on the reaction path and rate for the gas-phase reaction of HO + H2O <--> H2O + OH

This article describes an ab initio investigation on the potential surfaces for one of the simplest hydrogen atom abstraction reactions, that is, HO + H2O <--> H2O + OH. In accord with the findings in the previously reported theoretical investigations, two types of the hydrogen-bonding complexes [HOH--OH] and [H2O--HO] were located on the potential energy surface. The water molecule acts as a hydrogen donor in the [HOH--OH] complex, while the OH radical acts as a hydrogen donor in the [H2O--HO] complex. The energy evaluations at the MP2(FC) basis set limit, as well as those through the CBS-APNO procedure, have provided estimates for enthalpies of association for these complexes at 298 K as -2.1 approximately -2.3 and -4.1 approximately -4.3 kcal/mol, respectively. The IRC calculations have suggested that the [H2O--HO] complex should be located along the reaction coordinate for the hydrogen abstraction. Our best estimate for the classical barrier height for the hydrogen abstraction is 7.8 kcal/mol, which was obtained from the CBS-APNO energy evaluations. After fitting the CBS-APNO potential energy curve to a symmetrical Eckart function, the rate constants were calculated by using the transition state theory including the tunneling correction. Our estimates for the Arrhenius parameters in the temperature region from 300 to 420 K show quite reasonable agreement with the experimentally derived values.     

Mechanistic and kinetic study of the O + CH2OH reaction

The mechanism for the O + CH2OH reaction was investigated by various ab initio quantum chemistry methods. For the chemical activation mechanism, that is, the addition/elimination path, the couple-cluster methods including CCSD and CCSD(T) were employed with the cc-pVXZ (X = D, T, Q, 5) basis sets. For the abstraction channels, multireference methods including CASSCF, CASPT2, and MRCISD were used with the cc-pVDZ and cc-pVTZ basis sets. It has been shown that the production of H + HCOOH is the major channel in the chemical activation mechanism. The minor channels include HCO + H2O and OH + CH2O. The hydrogen abstraction by an O atom from the CH2OH radical produces either OH + CH2O or OH + HCOH. Moreover, the two abstraction reactions are essentially barrierless processes. The rate constants for the association of O with CH2OH have been calculated using the flexible transition state theory. A weak negative temperature dependence of the rate constants is found in the range 250-1000 K. Furthermore, it is estimated that the abstraction processes also play an important role in the O + CH2OH reaction. Additionally, the falloff behavior for the OCH2OH --> H + HCOOH reaction has been investigated. The present theoretical results are compared to the experimental measurements to understand the mechanism and kinetic behavior of the O + CH2OH reaction and the unimolecular reaction of the OCH2OH radical.     

Dual-level direct dynamics studies for the reactions of OH radical with bromine-substituted ethanes

The dynamic properties of the multichannel hydrogen abstraction reactions of CH(3)CH(2)Br + OH --> products and CH(3)CHBr(2) + OH --> products are studied by dual-level direct dynamics method. For each reaction, three reaction channels, one for alpha-hydrogen abstraction and two for beta-hydrogen abstractions, have been identified. The minimum energy paths (MEPs) of both the reactions are calculated at the Becke's half-and-half (BH&H)-Lee-Yang-Parr (LYP)/6-311G(d, p) level and the energy profiles along the MEPs are further refined with interpolated single-point energies (ISPE) method at the G2M(RCC5)//BH&H-LYP level. There are complexes with energies less than those of the reactants or products located at the entrance or exit channels, which indicates that the reactions may proceed via an indirect mechanism. By canonical variational transition-state theory (CVT) the rate constants are calculated incorporating the small-curvature tunneling (SCT) correction in the temperature range of 220-2000 K. The agreement of the rate constants with available experimental values for two reactions is good in the measured temperature range. The calculated results show that alpha-hydrogen abstraction channel is the major reaction pathway in the lower temperature for two reactions, while the contribution of beta-hydrogen abstraction will increase with the increase in temperature.     

Mechanistic and kinetic investigations of N2H4 + OH reaction

The reaction of N₂H₄ with OH has been investigated by quantum chemical methods. The results show that hydrogen abstraction mechanism is more feasible than substitution mechanism thermodynamically. The calculated rate constants agree with the available experimental data. The calculated results show that the variational effect is small at lower temperature region, while it becomes significant at higher temperature region. On the other hand, the small‐curvature tunneling effect may play an important role in the temperature range 220?3000 K. Moreover, the calculated rate constants show negative temperature dependence at the temperatures below 500 K, which is in accordance with Vaghjiani's report that slightly negative temperature dependence is found over the temperature range of 258?637 K. The mechanism of the major product (N₂H₃) with OH has also been investigated theoretically to understand the title reaction thoroughly. © 2009 Wiley Periodicals, Inc. J Comput Chem, 2010     

Measurements of the kinetics of the OH + α‐pinene and OH + β‐pinene reactions at low pressure

The rate constants for the OH + α‐pinene and OH + β‐pinene reactions have been measured in 5 Torr of He using discharge‐flow systems coupled with resonance fluorescence and laser‐induced fluorescence detection of the OH radical. At room temperature, the measured effective bimolecular rate constant for the OH + α‐pinene reaction was (6.08 ± 0.24) × 10^?11 cm³ molecule^?1 s^?1. These results are in excellent agreement with previous absolute measurements of this rate constant, but are approximately 13% greater than the value currently recommended for atmospheric modeling. The measured effective bimolecular rate constant for the OH + β‐pinene reaction at room temperature was (7.72 ± 0.44) × 10^?11 cm³ molecule^?1 s^?1, in excellent agreement with previous measurements and current recommendations. Above 300 K, the effective bimolecular rate constants for these reactions display a negative temperature dependence suggesting that OH addition dominates the reaction mechanisms under these conditions. This negative temperature dependence is larger than that observed at higher pressures. The measured rate constants for the OH + α‐pinene and OH + β‐pinene reactions are in good agreement with established reactivity trends relating the rate constant for OH + alkene reactions with the ionization potential of the alkene when ab initio calculated energies for the highest occupied molecular orbital are used as surrogates for the ionization potentials for α‐ and β‐pinene. © 2002 Wiley Periodicals, Inc. Int J Chem Kinet 34: 300–308, 2002     

Canonical variational transition-state theory study of the CF3CH2CH3 + OH reaction

Variational transition-state theory rate constants with multidimensional tunneling contributions using the small curvature method have been calculated for the CF3CH2CH3 (HFC-263fb) + OH reaction over a temperature range from 200 to 373 K. The mPW1B95-41.0 hybrid functional, parametrized by Albu and Swaminathan to generate theoretical rate constants nearly identical to the experimental values for the CH3F + OH reaction, has been used in conjunction with the 6-31+G** basis set to explore the potential energy surface of the title reaction. The good agreement found between theoretical predictions and the experimental data available suggests that the present approach is an excellent option to obtain high-quality results at low computational cost for direct dynamics studies of hydrogen abstraction reactions from complex hydrofluorocarbons. The reliability of the structure activity relationship used to estimate rate constant values for OH reactions with hydrofluorocarbons is also discussed in detail.     

Theoretical Studies of the Reactions CFxH3−xCOOR+Cl and CF3COOCH3+OH

The mechanism and kinetics of the reactions of CF₃COOCH₂CH₃, CF₂HCOOCH₃, and CF₃COOCH₃ with Cl and OH radicals are studied using the B3LYP, MP2, BHandHLYP, and M06‐2X methods with the 6‐311G(d,p) basis set. The study is further refined by using the CCSD(T) and QCISD(T)/6‐311++G(d,p) methods. Seven hydrogen‐abstraction channels are found. All the rate constants, computed by a dual‐level direct method with a small‐curvature tunneling correction, are in good agreement with the experimental data. The tunneling effect is found to be important for the calculated rate constants in the low‐temperature range. For the reaction of CF₃COOCH₂CH₃+Cl, H‐abstraction from the CH₂ group is found to be the dominant reaction channel. The standard enthalpies of formation for the species are also calculated. The Arrhenius expressions are fitted within 200–1000 K as k_T(1)=8.4×10^?20T ^2.63exp(381.28/T), k_T(2)=2.95×10^?21T ^3.13exp(?103.21/T), k_T(3)=1.25×10^?23T ^3.37exp(791.98/T), and k_T(4)=4.53×10^?22T ^3.07exp(465.00/T).     

Hydrogen atom abstraction reactions of the sugar moiety of 2′‐deoxyguanosine with an OH radical: A quantum chemical study

Mechanisms of hydrogen atom abstraction reactions of the sugar moiety of 2′‐deoxyguanosine with an OH radical were investigated using the B3LYP and BHandHLYP functionals of density functional theory and the second order Møller–Plesset Perturbation (MP2) theory in gas phase and aqueous media. The 6‐31+G* and AUG‐cc‐pVDZ basis sets were used. Gibbs free barrier energies and rate constants of the reactions in aqueous media suggest that an OH radical would abstract the hydrogen atoms of the sugar moiety of 2′‐deoxyguanosine in the following order of preference: H5′ ≈ H5″ > H3′ > H4′ > H1′ ≈ H2′ > H2″, the rate constant for H5′ abstraction being 10³–10⁵ times greater than that for H2″ at the different levels of theory. Relative stabilities of the different deoxyribose radicals are also discussed. The most and least favored hydrogen abstraction reactions found here are in agreement with experimental observation. © 2010 Wiley Periodicals, Inc. Int J Quantum Chem, 2011     

Insight into the Mechanism of the Initial Reaction of an OH Radical with DNA/RNA Nucleobases: A Computational Investigation of Radiation Damage

Earlier theoretical investigations of the mechanism of radiation damage to DNA/RNA nucleobases have claimed OH radical addition as the dominating pathway based solely on energetics. In this study we supplement calculations of energies with the kinetics of all possible reactions with the OH radical through hydrogen abstraction and OH radical addition onto carbon sites, using DFT at the ωB97X‐D/6‐311++G(2df,2pd) level with the Eckart tunneling correction. The overall rate constants for the reaction with adenine, guanine, thymine, and uracil are found to be 2.17×10^?12, 5.64×10^?11, 2.01×10^?11, and 5.03×10^?12 cm³ molecules^?1 s^?1, respectively, which agree exceptionally well with experimental values. We conclude that abstraction of the amine group hydrogen atoms competes with addition onto C₈ as the most important reaction pathway for the purine nucleobases, while for the pyrimidine nucleobases addition onto C₅ and C₆ competes with the abstraction of H₁. Thymine shows favourability against abstraction of methyl hydrogens as the dominating pathway based on rate constants. These mechanistic conclusions are partly explained by an analysis of the electrostatic potential together with HOMO and LUMO orbitals of the nucleobases.