Direct dynamics study on the hydrogen abstraction reactions N2H4 + R-->N2H3 + RH (R=NH2, CH3)

We present a direct ab initio dynamics study on the hydrogen abstraction reactions N(2)H(4)+R-->N(2)H(3)+RH (R=NH(2),CH(3)), which are predicted to have six possible reaction channels for NH(2) abstraction and four for CH(3) abstraction caused by the different N(2)H(4) isomers and various attacking orientations of foreign radical to N(2)H(4). The structures and frequencies at the stationary points and the points along the minimum energy paths (MEPs) of all reaction channels are obtained at the UMP2(full)6-31+G(d,p) level of theory. Energetic information of stationary points and the points along the MEPs is further refined by means of MC-QCISD method. The rate constants of these channels are calculated using the improved canonical variational transition-state theory with the small-curvature tunneling correction (ICVT/SCT) method. The calculated results show that the favorable reaction channels are channels (n1) and (n4) as well as (c1) and (c3) (refer to Scheme 1) in the whole temperature range. The total ICVT/SCT rate constants of all channels for the two reactions at the MC-QCISDUMP2(full)6-31+G(d,p) level are both in good agreement with the available experimental data, and corresponding three-parameter expressions of k(ICVTSCT) in 220-3000 K are fitted as 6.46 x 10(-15)(T298)(3.60) exp(-386T) cm(3) mol(-1) s(-1) for NH(2) abstraction and 1.04 x 10(-14)(T298)(4.00) exp(-2037T) cm(3) mol(-1) s(-1) for CH(3) abstraction. Additionally, the long range interaction between the H atom of X-H bond in foreign radicals and the lone pair on the nonreactive N atom of the transition states is further discussed to explain the various transition-state numbers of the two similar hydrogen abstraction reactions.     

CHnF4-n与O3吸氢反应途径和速率常数计算

用量子化学方法, 从理论上探讨了CHnF4-n(n=2,3)与臭氧反应的微观机理, 并计算了各反应在不同温度下的速率常数.     

Direct dynamics study on the reaction of N2H4 with F atom: a hydrogen abstraction reaction?

We present a systematic direct ab initio dynamics investigation of the reaction between N2H4 and F atom, which is predicted to have three possible reaction channels. The structures and frequencies at the stationary points and the points along the minimum energy paths (MEPs) of all reaction channels were calculated at the UB3LYP/6-31+G(d,p) level of theory. Energetic information of stationary points and the points along the MEPs was further refined by means of the CCSD(T)/aug-cc-pVTZ method. The calculated results revealed that the first two primary channels (N2H4+F-->N2H3+HF) are equivalent and occur synchronously via the formation of a pre-reaction complex with Cs symmetry rather than via the direct H abstraction. The pre-reaction complex then evolves into a hydrogen-bonding intermediate through a transition state with nearly no barrier and a high exothermicity, which finally makes the intermediate further decompose into N2H3 and HF. Another reaction channel of minor role (N2H4+F-->NH2F+NH2) was also found during the calculations, which has the same Cs pre-reaction complex but forms NH2F and NH2 via another transition state with high-energy barrier and low exothermicity. The rate constants of these channels were calculated using the improved canonical variational transition state theory with the small-curvature tunneling correction (ICVT/SCT) method. The three-parameter ICVT/SCT rate constant expressions of k(ICVT/SCT) at the CCSD(T)/aug-cc-pVTZ//UB3LYP/6-31+G(d,p) level of theory within 220-3000 K were fitted as (7.64x10(-9))T (-0.87) exp(1180/T) cm3 mole-1 s-1 for N2H4+F-->N2H3+HF and 1.45x10(-12)(T/298)(2.17) exp(-1710/T) cm3 mole-1 s-1 for N2H4+F-->NH2F+NH2.     

Reaction-path dynamics and theoretical rate constants for the CH(n)F(4-n) + O3 --> HOOO + CH(n-1)F(4-n) (n = 2,3) reactions

We present a theoretical study of the hydrogen abstraction reactions from CH(3)F and CH(2)F(2) by an ozone molecule. The geometries and harmonic vibrational frequencies of all stationary points are calculated at the MPW1K, BHandHLYP, and MPWB1K levels of theory. The energies of all of the stationary points were refined by using both higher-level (denoted as HL) energy calculations and QCISD(T)/6-311++G(2df,2pd) calculations based on the optimized geometries at the MPW1K/6-31+G(d,p) level of theory. The minimum energy paths (MEPs) were obtained by the MPW1K/6-31+G(d,p) level of theory. Energetic information of the points along the MEPs is further refined by the HL method. The rate constants were evaluated on the basis of the MEPs from the HL level of theory in the temperature range 200-2500 K by using the conventional transition-state theory (TST), the canonical variational transition-state theory (CVT), the microcanonical variational transition-state theory (microVT), the CVT coupled with the small-curvature tunneling (SCT) correction (CVT/SCT), and the microVT coupled with the Eckart tunneling correction (microVT/Eckart) based on the ab initio calculations. A general agreement was found among the TST, CVT, and microVT theories. The fitted three-parameter Arrhenius expressions of the calculated forward CVT/SCT and microVT/Eckart rate constants of the ozonolysis of fluoromethane are k(CVT/SCT)(T) = 2.76 x 10(-34)T(5.81)e((-13975/)(T)) and k(microVT/Eckart)(T) = 1.15 x 10(-34)T(5.97)e((-14530.7/)(T)), respectively. The fitted three-parameter Arrhenius expressions of the calculated forward CVT/SCT and microVT/Eckart rate constants of the ozonolysis of difluoromethane are k(CVT/SCT)(T) = 2.29 x 10(-36)T(6.42)e((-15451.6/)(T)) and k(microVT/Eckart)(T) = 1.31 x 10(-36)T(6.45)e((-15465.8/)(T)), respectively.     

Theoretical study and rate constant computation on the reaction HFCO + OH --> CFO + H2O

The potential energy surface, including the geometries and frequencies of the stationary points, of the reaction HFCO + OH is calculated using the MP2 method with 6-31+G(d,p) basis set, which shows that the direct hydrogen abstraction route is the most dominating channel with respect to addition and substitution channels. For the hydrogen abstraction reaction, the single-point energies are refined at the QCISD(T) method with 6-311++G(2df,2pd) basis set. The calculated standard reaction enthalpy and barrier height are -17.1 and 4.9 kcal mol(-1), respectively, at the QCISD(T)/6-311++G(2df,2pd)//MP2/6-31+G(d,p) level of theory. The reaction rate constants within 250-2500 K are calculated by the improved canonical variational transition state theory (ICVT) with small-curvature tunneling (SCT) correction at the QCISD(T)/6-311++G(2df,2pd)//MP2/6-31+G(d,p) level of theory. The fitted three-parameter formula is k = 2.875 x 10(-13) (T/1000)1.85 exp(-325.0/T) cm(3) molecule(-1) s(-1). The results indicate that the calculated ICVT/SCT rate constant is in agreement with the experimental data, and the tunneling effect in the lower temperature range plays an important role in computing the reaction rate constants.     

Theoretical study for the reaction of CH3OCl with Cl atom

A direct dynamics method is employed to study the kinetics of the multiple channel reaction CH(3)OCl + Cl. The potential energy surface (PES) information is explored from ab initio calculations. Two reaction channels, Cl- and H-abstractions, have been identified. The optimized geometries and frequencies of the stationary points and the minimum-energy paths (MEPs) are calculated at the MP2 level of theory using the 6-311G(d, p) and cc-pVTZ basis sets, respectively. The single-point energies along the MEPs are further refined at the G3(MP2)//MP2/6-311G(d, p), G3//MP2/6-311G(d, p), as well as by the multicoefficient correlation method based on QCISD (MC-QCISD) using the MP2/cc-pVTZ geometries. The enthalpies of formation for the species CH(3)OCl and CH(2)OCl are calculated via isodesmic reactions. The rate constants of the two reaction channels are evaluated by using the variational transition-state theory over a wide range of temperature, 220-2200 K. The calculated rate constants exhibit the slightly negative temperature dependence and show good agreement with the available experimental data at room temperature at the G3(MP2)//MP2/6-311G(d, p) level. The present calculations indicate that the two channels are competitive at low temperatures while H-abstraction plays a more important role with the increase of temperature. The calculated k(1a)/k(1) ratio of 0.5 at 298 K is in general agreement with the experimental one, 0.8 +/- 0.2. The high rate constant for CH(3)OCl + Cl shows that removal by reaction with Cl atom is a potentially important loss process for CH(3)OCl in the polar stratosphere.     

Water-catalyzed gas-phase hydrogen abstraction reactions of CH3O2 and HO2 with HO2: a computational investigation

The gas-phase hydrogen abstraction reactions of CH(3)O(2) and HO(2) with HO(2) in the presence and absence of a single water molecule have been studied at the CCSD(T)/6-311++G(3d,2p)//B3LYP/6-311G(2d,2p) level of theory. The calculated results show that the process for O(3) formation is much faster than that for (1)O(2) and (3)O(2) formation in the water-catalyzed CH(3)O(2) + HO(2) reaction. This is different from the results for the non-catalytic reaction of CH(3)O(2) + HO(2), in which almost only the process for (3)O(2) formation takes place. Unlike CH(3)O(2) + HO(2) reaction in which the preferred process is different in the catalytic and non-catalytic conditions, the channel for (3)O(2) formation is the dominant in both catalytic and non-catalytic HO(2) + HO(2) reactions. Furthermore, the calculated total CVT/SCT rate constants for water-catalyzed and non-catalytic title reactions show that the water molecule doesn't contribute to the rate of CH(3)O(2) + HO(2) reaction though the channel for O(3) formation in this water-catalyzed reaction is more kinetically favorable than its non-catalytic process. Meanwhile, the water molecule plays an important positive role in increasing the rate of HO(2) + HO(2) reaction. These results are in good agreement with available experiments.     

CH_3SGN与O_2气相反应机理的理论研究

在G3(MP2)水平上,通过对CH_3S与O_2rcyi2rvylce dm (PES)上关键驻点的能 量计算,共找到4种中间体,9个过渡态,6种产物通道,并对这些气相反应机理进 行了讨论,同时应用TST－RRKM理论对主要反应的速率进行计算。结果表明:CH_3S 与O_2反应在低温下以生成CH_3SOO为主,并与实验结果吻合;在中高温下以消去和 抽提反应为主,分别生成CH_3 + SO_2和CH_2S + HO_2,其它产物较少。     

Theoretical study of the reactions CF3CH2OCHF2 + OH/Cl and its product radicals and parent ether(CH3CH2OCH3) with OH

A dual-level direct dynamic method is employed to study the reaction mechanisms of CF3CH2OCHF2 (HFE-245fa2; HFE-245mf) with the OH radicals and Cl atoms. Two hydrogen abstraction channels and two displacement processes are found for each reaction. For further study, the reaction mechanisms of its products (CF3CH2OCF2 and CF3CHOCHF2) and parent ether CH3CH2OCH3 with OH radical are investigated theoretically. The geometries and frequencies of all the stationary points and the minimum energy paths (MEPs) are calculated at the B3LYP/6-311G(d,p) level. The energetic information along the MEPs is further refined at the G3(MP2) level of theory. For reactions CF3CH2OCHF2 + OH/Cl, the calculation indicates that the hydrogen abstraction from --CH2-- group is the dominant reaction channel, and the displacement processes may be negligible because of the high barriers. The standard enthalpies of formation for the reactant CF3CH2OCHF2, and two products CF3CH2OCHF2 and CF3CHOCHF2 are evaluated via group-balanced isodesmic reactions. The rate constants of reactions CF3CH2OCHF2 + OH/Cl and CH3CH2OCH3 + OH are estimated by using the variational transition state theory over a wide range of temperature (200-2000 K). The agreement between the theoretical and experimental rate constants is good in the measured temperature range. From the comparison between the rate constants of the reactions CF3CH2OCHF2 and CH3CH2OCH3 with OH, it is shown that the fluorine substitution decreases the reactivity of the C--H bond.     

Direct dynamics study on the reaction of acetaldehyde with ozone

The hydrogen abstraction reaction of ozone with acetaldehyde has been studied theoretically over the temperature range 250-2500 K. Two different reactive sites of acetaldehyde molecule, CH(3) and CHO groups have been investigated, and results confirm that the CHO group is a highly reactive site. In this study, the geometries and harmonic vibrational frequencies of all stationary points are calculated at the MPW1K, BHandHLYP, and MPWB1K levels of theory. The minimum energy paths (MEPs) were obtained at the MPW1K/6-31+G(d,p) level of theory. To refine the energies along the MEPs of each channel, single-point energy calculations were performed by a higher-level energy calculation method (denoted as HL). The rate constants were evaluated based on the MEPs from the HL method in the temperature range 250-2500 K by using the conventional transition state theory (TST), the canonical variational transition state theory (CVT), the microcanonical variational transition state theory (muVT), the CVT coupled with small-curvature tunneling (SCT) correction (CVT/SCT), and the muVT coupled with Eckart tunneling correction (muVT/Eckart). The fitted three-parameter Arrhenius expressions of the calculated CVT/SCT and muVT/Eckart rate constants of the H abstraction from CHO group are k CVT/SCT(T) = 4.92 x 10(-27).T 3.77.e(-7867.0/T) and k muVT/Eckart(T) = 2.10 x 10(-27).T(3.90).e(-7706.2/T), respectively. The fitted three-parameter Arrhenius expressions of the calculated CVT/SCT and muVT/Eckart rate constants of the H abstraction from CH3 group are k(CVT/SCT)(T) = 1.27 x 10(-27).T(3.94).e(-14554.1/T) and k muVT/Eckart(T) = 1.62 x 10(-26).T(3.66).e(-15459.8/T), respectively.     

Computational study of the reaction mechanism of the methylperoxy self-reaction

To provide insight on the reaction mechanism of the methyperoxy (CH(3)O(2)?) self-reaction, stationary points on both the spin-singlet and the spin-triplet potential energy surfaces of 2(CH(3)O(2)?) have been searched at the B3LYP/6-311++G(2df,2p) level. The relative energies, enthalpies, and free energies of these stationary points are calculated using CCSD(T)/cc-pVTZ. Our theoretical results indicate that reactions on a spin-triplet potential energy surface are kinetically unfavorable due to high free energy barriers, while they are more complicated on the spin-singlet surface. CH(3)OOCH(3) + O(2)(1) can be produced directly from 2(CH(3)O(2)?), while in other channels, three spin-singlet chain-structure intermediates are first formed and subsequently dissociated to produce different products. Besides the dominant channels producing 2CH(3)O? + O(2) and CH(3)OH + CH(2)O + O(2) as determined before, the channels leading to CH(3)OOOH + CH(2)O and CH(3)O? + CH(2)O + HO(2)? are also energetically favorable in the self-reaction of CH(3)O(2)? especially at low temperature according to our results.     

CH2SH与NO2双自由基反应机理的理论研究

在B3LYP/6-311＋＋G(2df,p)水平上优化了标题反应驻点物种的几何构型, 并在相同水平上通过频率计算和内禀反应坐标(IRC)分析对过渡态结构及连接性进行了验证. 采用双水平计算方法HL//B3LYP/6-311＋＋G(2df,p)对所有驻点及部分选择点进行了单点能校正, 构建了CH2SH＋NO2反应体系的单重态反应势能剖面. 研究结果表明, CH2SH与NO2反应体系存在4条主要反应通道, 两个自由基中的C与N首先进行单重态耦合, 形成稳定的中间体HSCH2NO2 (a). 中间体a经过C—N键断裂和H(1)—O(2)形成过程生成主要产物P1 (CH2S＋trans-HONO), 此过程需克服124.1 kJ•mol－1的能垒. 中间体a也可以经过C—N键断裂及C—O键形成转化为中间体HSCH2ONO (b), 此过程的能垒高达238.34 kJ•mol－1. b再经过一系列的重排异构转化得到产物P2 (CH2S＋cis-HONO), P3 (CH2S＋HNO2)和P4 (SCH2OH＋NO). 所有通道均为放热反应, 反应能分别为－150.37, －148.53, －114.42和－131.56 kJ•mol－1. 标题反应主通道R→a→TSa/P1→P1的表观活化能为－91.82 kJ•mol－1, 此通道在200～3000 K温度区间内表观反应速率常数三参数表达式为kCVT/SCT＝8.3×10－40T4.4 exp(12789.3/T) cm3•molecule－1•s－1.     

A systematic computational study of the reactions of HO2 with RO2: the HO2 + CH2ClO2, CHCl2O2, and CCl3O2 reactions

Potential energy surfaces for the reactions of HO(2) with CH(2)ClO(2), CHCl(2)O(2), and CCl(3)O(2) have been calculated using coupled cluster theory and density functional theory (B3LYP). It is revealed that all the reactions take place on both singlet and triplet surfaces. Potential wells exist in the entrance channels for both surfaces. The reaction mechanism on the triplet surface is simple, including hydrogen abstraction and S(N)2-type displacement. The reaction mechanism on the singlet surface is more complicated. Interestingly, the corresponding transition states prefer to be 4-, 5-, or 7-member-ring structures. For the HO(2) + CH(2)ClO(2) reaction, there are two major product channels, viz., the formation of CH(2)ClOOH + O(2) via hydrogen abstraction on the triplet surface and the formation of CHClO + OH + HO(2) via a 5-member-ring transition state. Meanwhile, two O(3)-forming channels, namely, CH(2)O + HCl + O(3) and CH(2)ClOH + O(3) might be competitive at elevated temperatures. The HO(2) + CHCl(2)O(2) reaction has a mechanism similar to that of the HO(2) + CH(2)ClO(2) reaction. For the HO(2) + CCl(3)O(2) reaction, the formation of CCl(3)O(2)H + O(2) is the dominant channel. The Cl-substitution effect on the geometries, barriers, and heats of reaction is discussed. In addition, the unimolecular decomposition of the excited ROOH (e.g., CH(2)ClOOH, CHCl(2)OOH, and CCl(3)OOH) molecules has been investigated. The implication of the present mechanisms in atmospheric chemistry is discussed in comparison with the experimental measurements.     

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.     

Direct ab initio dynamics calculation of the reaction rates of CH3OCl with OH

A direct ab initio dynamics method was carried out for the reaction CH3OCl + OH --> products. Three abstraction channels from chlorine atom, in-plane hydrogen, and out-of-plane hydrogen atoms at the CH3 group have been found. The optimized geometries and frequencies of the stationary points and the minimum-energy paths (MEPs) were calculated at the MP2/6-311G(d,p) level. To improve the reaction enthalpy and potential barrier, single-point calculations were made at three higher levels of theory, the approximate QCISD(T)/6-311++G(3df,2pd), G3, and G3(MP2) levels. Furthermore, the rate constants for three abstraction channels were evaluated using canonical variational transition state theory (CVT) with the small-curvature tunneling correction (SCT) over a wide temperature range of 220-2000 K at above three higher theory levels, respectively. The calculated rate constants as well as branching rates are in reasonable agreement with the experimental values in the temperature region 250-341 K. The present results indicate H-abstraction especially from out-of-plane hydrogen is the main reaction pathway, while Cl-abstraction is much less competitive.     

A systematic computational study on the reactions of HO2 with RO2: The HO2 + CH3O2(CD3O2) and HO2 + CH2FO2 reactions

A systematic theoretical study of the reactions of HO2 with RO2 has been carried out. The major concern of the present work is to gain insight into the reaction mechanism and then to explain experimental observations and to predict new product channels for this class of reactions of importance in the atmosphere. In this paper, the reaction mechanisms for two reactions, namely, HO2 + CH3O2 and HO2 + CH2FO2, are reported. Both singlet and triplet potential energy surfaces are investigated. The complexity of the present system makes it impossible to use a single ab initio method to map out all the reaction paths. Various ab initio methods including MP2, CISD, QCISD(T), CCSD(T), CASSCF, and density function theory (B3LYP) have been employed with the basis sets ranging from 6-31G(d) to an extrapolated complete basis set (CBS) limit. It has been established that the CCSD(T)/cc-pVDZ//B3LYP/6-311G(d,p) scheme represents the most feasible method for our systematic study. For the HO2 + CH3O2 reaction, the production of CH3OOH is determined to be the dominant channel. For the HO2 + CH2FO2 reaction, both CH2FOOH and CHFO are major products, whereas the formation of CHFO is dominant in the overall reaction. The computational findings give a fair explanation for the experimental observation of the products.     

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.     

CH3CH2+O(3P)反应机理的理论研究

The reaction for CH3CH2+O(3P) was studied by ab initio method. The geometries of the reactants, intermediates, transition states and products were optimized at MP2/6-311+G(d,p) level. The corresponding vibration frequencies were calculated at the same level. The single-point calculations for all the stationary points were carried out at the QCISD(T)/6-311+G(d,p) level using the MP2/6-311+G(d,p) optimized geometries. The results of the theoretical study indicate that the major products are the CH2O＋CH3, CH3CHO+H and CH2CH2+OH in the reaction. For the products CH2O＋CH3 and CH3CHO+H, the major production channels are A1: (R)→IM1→TS3→(A) and B1: (R)→IM1→TS4→(B), respectively. The majority of the products CH2CH2+OH are formed via the direct abstraction channels C1 and C2: (R)→TS1(TS2)→(C). In addition, the results suggest that the barrier heights to form the CO reaction channels are very high, so the CO is not a major product in the reaction.     

Theoretical study and rate constant calculation for reaction of CF(3)CH(2)OH with OH

The reaction mechanism of CF(3)CH(2)OH with OH is investigated theoretically and the rate constants are calculated by direct dynamics method. The potential energy surface (PES) information, which is necessary for dynamics calculation, is obtained at the B3LYP/6-311G (d, p) level. The single-point energy calculations are performed at the MC-QCISD level using the B3LYP geometries. Complexes, with the energies being less than corresponding reactants and products, are found at the entrance and exit channels for methylene-H-abstraction channel, while for the hydroxyl-H-abstraction channel only entrance complex is located. By means of isodesmic reactions, the enthalpies of the formation for the species CF(3)CH(2)OH, CF(3)CHOH, and CF(3)CH(2)O are estimated at the MC-QCISD//B3LYP/6-311G (d, p) level of theory. The rate constants for two kinds of H-abstraction channels are evaluated by canonical variational transition state theory with the small-curvature tunneling correction (CVT/SCT) over a wide range of temperature 200-2000 K. The calculated results are in good agreement with the experimental values in the temperature region 250-430 K. The present results indicate that the two channels are competitive. Below 289 K, hydroxyl-H-abstraction channel has more contribution to the total rate constants than methylene-H-abstraction channel, while above 289 K, methylene-H-abstraction channel becomes more important and then becomes the major reaction channel.     

Direct dynamics studies on hydrogen abstraction reactions of CH3CHFCH3 and CH3CH2CH2F with OH radicals

The hydrogen abstraction reactions of CH3CHFCH3 and CH3CH2CH2F with the OH radicals have been studied theoretically by a dual-level direct dynamics method. The geometries and frequencies of all the stationary points are optimized by means of the DFT calculation. There are complexes at the reactant side or exit route, indicating these reactions may proceed via indirect mechanisms. To improve the reaction enthalpy and potential barrier of each reaction channel, the single point energy calculation is performed by the MC-QCISD/3 method. The rate constants are evaluated by canonical variational transition state theory (CVT) with the small-curvature tunneling correction method (SCT) over a wide temperature range 200-2000 K. The canculated CVT/SCT rate constants are consistent with available experimental data. The results show that both the variation effect and the SCT contribution play an important role in the calculation of the rate constants. For reactions CH3CHFCH3 and CH3CH2CH2F with OH radicals, the channels of H-abstraction from -CHF- and -CH2- groups are the major reaction channels, respectively, at lower temperature. Furthermore, to further reveal the thermodynamics properties, the enthalpies of formation of reactants CH3CHFCH3, CH3CH2CH2F, and the product radicals CH3CFCH3, CH3CHFCH2, CH3CH2CHF, CH3CHCH2F, and CH2CH2CH2F are studied using isodesmic reactions.