Capture and dissociation in the complex-forming CH + H2 → CH2 + H, CH + H2 reactions

The rate coefficients for the capture process CH + H(2)→ CH(3) and the reactions CH + H(2)→ CH(2) + H (abstraction), CH + H(2) (exchange) have been calculated in the 200-800 K temperature range, using the quasiclassical trajectory (QCT) method and the most recent global potential energy surface. The reactions, which are of interest in combustion and in astrochemistry, proceed via the formation of long-lived CH(3) collision complexes, and the three H atoms become equivalent. QCT rate coefficients for capture are in quite good agreement with experiments. However, an important zero point energy (ZPE) leakage problem occurs in the QCT calculations for the abstraction, exchange and inelastic exit channels. To account for this issue, a pragmatic but accurate approach has been applied, leading to a good agreement with experimental abstraction rate coefficients. Exchange rate coefficients have also been calculated using this approach. Finally, calculations employing QCT capture/phase space theory (PST) models have been carried out, leading to similar values for the abstraction rate coefficients as the QCT and previous quantum mechanical capture/PST methods. This suggests that QCT capture/PST models are a good alternative to the QCT method for this and similar systems.     

The distonic ion ·CH2CH2CH+OH,keto ion CH3CH2CH=O +·, enol ion CH3CH=CHOH+·, and related C3H6O+· radical cations. Stabilities and isomerization proclivities studied by dissociation and neutralization-reionization

Metastable ion decompositions, collision-activated dissociation (CAD), and neutralization-reionization mass spectrometry are utilized to study the unimolecular chemistry of distonic ion ^·CH₂CH₂CH^?OH (2^+·) and its enol-keto tautomers CH₃CH=CHOH^?· (1 ^+·) and CH₃CH₂CH=O ^+· (3^+·). The major fragmentation of metastable 1^+·–3^+· is H^· loss to yield the propanoyl cation, CH₃CH₂C≡O⁺. This reaction remains dominant upon collisional activation, although now some isomeric CH₂=CH-CH⁺ OH is coproduced from all three precursors. The CAD and neutralization-reionization (⁺NR⁺) spectra of keto ion 3 ^+· are substantially different from those of tautomers 2^+· and 1^+·. Hence, 3^+· without sufficient energy for decomposition (i. e. , “stable” 3^+·) does not isomerize to the ther-modynamically more stable ions 2^+· or 1^+·, and the 1,4-H rearrangement H-CH₂CH₂CH=O^+·(3 ^+·) → CH₂CH₂CH⁺ O-H (2 ^+·) must require an appreciable critical energy. Although the fragment ion abundances in the ⁺ NR ⁺ (and CAD) spectra of 1 ^+· and 2 ^+· are similar, the relative and absolute intensities of the survivor ions (recovered C₃H₆O^+· ions in the ⁺NR⁺ spectra) are markedly distinct and independent of the internal energy of 1 ^+· and 2 ^+·. Furthermore, 1 ^+· and 2 ^+· show different MI spectra. Based on these data, distonic ion 2 ^+· does not spontaneously rearrange to enol ion 1 ^+· (which is the most stable C₃H₆O^+· of CCCO connectivity) and, therefore, is separated from it by an appreciable barrier. In contrast, the molecular ions of cyclopropanol (4 ^+·) and allyl alcohol (5 ^+·) isomerize readily to 2 ^+·, via ring opening and 1,2-H^? shift, respectively. The sample found to generate the purest 2 ^+· is α-hydroxy-γ-butyrolactone. Several other precursors that would yield 2 ^+· by a least-motion reaction cogenerate detectable quantities of enol ion 1 ^+·, or the enol ion of acetone (CH₂=C(CH₃)OH^+·, 6 ^+·), or methyl vinyl ether ion (CH₃OCH=CH ₂ ^+· , 7 ^+·). Ion 6 ^+· is coproduced from samples that contain the —CH₂—CH(OH)—CH₂— substructure, whereas 7 ^+· is coproduced from compounds with methoxy substituents. Compared to CAD, metastable ion characteristics combined with neutralization-reionization allow for a superior differentiation of the ions studied.     

Semiempirical calculation of the CH4 + CH3 · → CH3 · + CH4 radical-reaction potential surfaces in a basis of frontier orbitals

A semiempirical approach is suggested to describe potential-energy surfaces (PESs) of some radical reactions in the zero-differential overlap (ZDO) approximation. An incomplete basis set is used including only frontier (single-filled) radical molecular orbitals (MOs) depending on geometrical parameters. All possible configurations are taken into account. The parameter selection techniques are analyzed. The approach is applied to PES calculation of the CH₄ + CH₃ · CH₃ · + CH₄ reaction. Some points of the PES are verified by a nonempirical method using the perturbation theory and taking into account the correlation energy. The relaxation energies are calculated. The one-center parameters are determined nonempirically from the CH₃ + CH₃ ·, and CH₃ ^– energetics. The two-center parameters are found by modeling the CH₄ CH₃ · + H and C₂H₆ 2CH₃ reactions in the same single-orbital approximation. The energy parameters of the reactions considered are overestimated by 10%, whereas the geometrical parameters are under-estimated by 15%. Further, a comparative analysis of the Hartree-Fock solutions and those including correlation interactions (CIs) is given. The variations in the spin and charge densities on the reaction centers are considered.Institute of Chemical Kinetics and Combustion, Academy of Sciences of the USSR, Siberian Branch, Novosibirsk. Translated from Teoreticheskaya i Éksperimental'naya Khimiya, Vol. 27, No. 4, pp. 499–506, July–August, 1991. Original article submitted February 10, 1989.     

F-+CH3Cl→CH3F+Cl-反应过程中的分子形貌变化

双分子亲核(SN2)反应是重要的基本有机反应之一,其中电子从亲核基团向离去基团的转移发挥着关键作用.利用从头计算方法CCSD(T)/aug-cc-pVDZ和我们发展的分子形貌理论,对反应F-+CH3Cl→CH3F+Cl-进行了研究,给出了反应过程中分子形状和电子转移的动态变化图像.结果表明,沿内禀反应坐标,从反应开始到生成反应前复合物,亲核试剂F-的分子内禀特征轮廓在缓慢收缩,而其上的电子密度在缓慢增大.此后,F的轮廓迅速膨胀,电子密度急剧下降,尤其是从过渡态到产物复合物的过程中.而在反应过程中,离去基团Cl的轮廓一直在收缩,其上的电子密度一直在增大.对反应过程中电子所受到作用势的研究表明,随着反应的进行,电子在F与C间受到的作用势逐渐降低,而在C与Cl间受到的作用势逐渐升高,清楚地展现反应过程中F与C间化学键生成和C与Cl间化学键断裂的动态过程.     

CH3CH2O+HCHO反应机理及主通道速率常数

采用CCSD(T)/cc-pVDZ//B3LYP/6-311++G(d,p)双水平计算方法研究了CH₃CH₂O+HCHO反应的微观反应机理. 结果表明, 标题反应主要存在5个抽氢和3个氢迁移异构化反应通道, 其中抽氢通道R→ IMa(CH₃CH₂O…CH₂O)→TS1→ IM1b→P1(CH₃CH₂OH+CHO)为优势通道, 其表观活化能为14.65 kJ/mol. 利用变分过渡态理论(CVT)并结合小曲率隧道效应模型计算了主通道R1在275~1000 K温度范围内的速率常数k^TST, k^CVT和k^CVT/SCT, 在此温度区间内表观反应速率常数三参数表达式为k^CVT/SCT=2.26×10^-17 T^0.57 exp(-1004/T), 显示具有正温度系数效应.     

Quasi-classical trajectory study of the dynamics of the Cl + CH4→ HCl + CH3 reaction

We present an on-the-fly classical trajectory study of the Cl + CH(4)→ HCl + CH(3) reaction using a specific reaction parameter (SRP) AM1 Hamiltonian that was previously optimized for the Cl + ethane reaction [S. J. Greaves et al., J. Phys Chem A, 2008, 112, 9387]. The SRP-AM1 Hamiltonian is shown to be a good model for the potential energy surface of the title reaction. Calculated differential cross sections, obtained from trajectories propagated with the SRP-AM1 Hamiltonian compare favourably with experimental results for this system. Analysis of the vibrational modes of the methyl radical shows different scattering distributions for ground and vibrationally excited products.     

Ten-Dimensional Quantum Dynamics Study of H+CH3D→H2+CH2D Reaction

The mode selectivity of the H+CH3D→H2+CH2D reaction was studied using a recently developed ten-dimensional time-dependent wave packet method.The reac-tion dynam...     

Reduced dimensionality spin-orbit dynamics of CH3 + HCl ⇌ CH4 + Cl on ab initio surfaces

A reduced dimensionality quantum scattering method is extended to the study of spin-orbit nonadiabatic transitions in the CH(3) + HCl ? CH(4) + Cl((2)P(J)) reaction. Three two-dimensional potential energy surfaces are developed by fitting a 29 parameter double-Morse function to CCSD(T)/IB//MP2/cc-pV(T+d)Z-dk ab initio data; interaction between surfaces is described by geometry-dependent spin-orbit coupling functions fit to MCSCF/cc-pV(T+d)Z-dk ab initio data. Spectator modes are treated adiabatically via inclusion of curvilinear projected frequencies. The total scattering wave function is expanded in a vibronic basis set and close-coupled equations are solved via R-matrix propagation. Ground state thermal rate constants for forward and reverse reactions agree well with experiment. Multi-surface reaction probabilities, integral cross sections, and initial-state selected branching ratios all highlight the importance of vibrational energy in mediating nonadiabatic transition. Electronically excited state dynamics are seen to play a small but significant role as consistent with experimental conclusions.     

A photoionization study of the ion-neutral complexes CH3CH +CH 3 · CH2CH3] and CH3CH2CH+CH 3 · CH3 in the gas phase: Formation,H-transfer and C—C bond formation between partners,and channeling of energy into dissociation

Photoionization mass spectrometry was used to investigate the dynamics of ion-neutral complex-mediated dissociations of the n-pentane ion (1). Reinterpretation of previous data demonstrates that a fraction of ions 1 isomerizes to the 2-methylbutane ion (2) through the complex CH₃CH⁺CH ₃ ^· CH₂CH₃ (3), but not through CH₃CH⁺CH₂CH ₃ ^· CH₃ (4). The appearance energy for C₃Hin ₇ ⁺ formation from 1 is 66 kJ mol^?1 below that expected for the formation of n-C₃H ₇ ⁺ and just above that expected for formation of i-C₃H ₇ ⁺ . This demonstrates that the H shift that isomerizes C₃H ₇ ⁺ is synchronized with bond cleavage at the threshold for dissociation to that product. It is suggested that ions that contain n-alkyl chains generally dissociate directly to more stable rearranged carbenium ions. Ethane elimination from 3 is estimated to be about seven times more frequent than is C-C bond formation between the partners in that complex to form 2, which demonstrates a substantial preference in 3 for H abstraction over C-C bond formation. In 1 → CH₃CH⁺CH₂CH₃ + CH₃ by direct cleavage of the C1–C2 bond, the fragments part rapidly enough to prevent any reaction between them. However, 1 → 2 → 4 → C4H ₈ ⁺ + CH₄ occurs in this same energy range. Thus some of the potential energy made available by the isomerization of n-C₄H₉ in 1 is specifically channeled into the coordinate for dissociation. In contrast, analogous formation of 3 by 1 → 3 is predominantly followed by reaction between the electrostatically bound partners.     

CH2+O2反应的反应机理

CH2与 O2的反应是不饱和碳氢化合物燃烧过程中的一个十分重要的反应 .CH2（基态,三重态）与 O2反应的实验研究工作已有不少报导 [1－ 5],其主要产物有两 组 :H2O＋ CO和 CO2＋ H2,这些产物表明反应在反应体系的单重态位能面上发生 .至今还未见到关于 CH2＋ O2反应机理的完整的理论研究报导 .本文用 CASSCF方法详细研究了 CH2＋ O2反应的机理,给出从反应物至两组不同最终产物的完整的反应途经的描写 .1计算方法 　　计算使用量子化学高斯 98 W软件 .用从头算 CASSCF(8,8)/6-31G(d,p)方法在 CH2＋ O2单重态位能面上以优化…     

Mechanistic and kinetic study of CF3CH═CH2 + OH reaction

The potential energy surfaces of the CF(3)CH═CH(2) + OH reaction have been investigated at the BMC-CCSD level based on the geometric parameters optimized at the MP2/6-311++G(d,p) level. Various possible H (or F)-abstraction and addition/elimination pathways are considered. Temperature- and pressure-dependent rate constants have been determined using Rice-Ramsperger-Kassel-Marcus theory with tunneling correction. It is shown that IM1 (CF(3)CHCH(2)OH) and IM2 (CF(3)CHOHCH(2)) formed by collisional stabilization are major products at 100 Torr pressure of Ar and in the temperature range of T < 700 K (at P = 700 Torr with N(2) as bath gas, T ≤ 900 K), whereas CH(2)═CHOH and CF(3) produced by the addition/elimination pathway are the dominant end products at 700-2000 K. The production of CF(3)CHCH and CF(3)CCH(2) produced by hydrogen abstractions become important at T ≥ 2000 K. The calculated results are in good agreement with available experimental data. The present theoretical study is helpful for the understanding the characteristics of the reaction of CF(3)CH═CH(2) + OH.     

CH3 CH2 +N(4S)反应机理的理论研究

The reaction for CH3CH2+N(4S) 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 CH2CH2+3NH and H2CN＋CH3, and the minor products are the CH3CHN+H in the reaction. The majority of the products CH2CH2+3NH are formed via a direct hydrogen abstraction channel. The products H2CN＋CH3 are produced via an addition/dissociation channel. The products CH3CHN+H are produced via an addition/dissociation channel.     

CH3+HNCO反应机理的理论研究

异氰酸(HNCO)分解引发的一系列自由基反应是氮氧化物快速消除机理[1,2](RAPRENOX)所研究的领域,该反应涉及到燃烧化学中氮氧化物NOX的消除,所以获得这些反应准确的位垒就成为实验化学和理论化学所要解决的问题。本文中我们重点研究CH3+HNCO反应机理,探讨CH3自由基是否也能象氮氢自由基一样,在异氰酸(HNCO)分解反应中起作用。1　计算方法用量子化学MP2方法,在6 311++G 水平上计算了CH3自由基与HNCO反应的反应物、产物、中间体和过渡态的几何构型,用QCISD(T)方法在6 311++G 水平上计算了它们的能量。通过振动分析确定…     

CH2+O2反应的反应机理

The mechanisms of the CH2＋ O2→ H2O＋ CO and CH2＋ O2→ H2＋ CO2 reactions have been studied by performing ab initio CAS(8,8)/6-31G(d,p) calculations, and five intermediates(IMn) and eight transitions(TSn) have been located along the reaction paths. The predicted path for the CH2＋ O2→ H2O＋ CO is: CH2＋ O2→ TS1→ IM1→ TS2→ IM2→ TS3→ IM3→ TS4→ IM4a→ TS5→ H2O＋ CO. For the CH2＋ O2→ H2＋ CO2 reaction, there are two paths: (i) CH2＋ O2→ TS1→ IM1→ TS2→ IM2→ TS3→ IM3→ TS6→ H2＋ CO2 and (ii) CH2＋ O2→ TS1→ IM1→ TS2→ IM2→ TS3→ IM3→ TS4→ IM4a→ TS7→ IM4b→ TS8→ H2＋ CO2, with the latter path more favorable energetically.     

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.     

Ab initio potential energy surface and quantum dynamics for the H + CH4 → H2 + CH3 reaction

A new full-dimensional potential energy surface for the title reaction has been constructed using the modified Shepard interpolation scheme. Energies and derivatives were calculated using the UCCSD(T) method with aug-cc-pVTZ and 6-311++G(3df,2pd) basis sets, respectively. A total number of 30,000 data points were selected from a huge number of molecular configurations sampled by trajectory method. Quantum dynamical calculations showed that the potential energy surface is well converged for the number of data points for collision energy up to 2.5 eV. Total reaction probabilities and integral cross sections were calculated on the present surface, as well as on the ZBB3 and EG-2008 surfaces for the title reaction. Satisfactory agreements were achieved between the present and the ZBB3 potential energy surfaces, indicating we are approaching the final stage to obtain a global potential energy surface of quantitative accuracy for this benchmark polyatomic system. Our calculations also showed that the EG-2008 surface is less accurate than the present and ZBB3 surfaces, particularly in high energy region.     

用TR-FTIR研究CH+NO2的反应

CH自由基是烃类燃烧过程中反应活性很高的重要的中间产物[1,2].CH自由基与氮氧化物的反应被认为是通过二次燃烧过程减少氮氧化物的主要反应之一,也是对火焰中氮化物的化学行为建立模型的关键步骤[3].但是,对于CH与NO2反应的研究还不是很深入,到目前为止,只有两篇论文报道了该反应在298 K时的总包反应速率常数[4,5],Taeg和Hershberger用红外二极管激光吸收法研究了该反应[6].他们在实验中只观测到了产物CO和NO,但一些较低能量的产物,如NH+CO2、OH+NCO等却没有被观察到.为了更深一步了解CH与NO2的反应产物及反应通道,我们用时间分辨傅立叶变换红外发射光谱(TR-FTIR)法研究了该反应.