Ab initio study on mechanisms and kinetics for reaction of NCS with NO

The mechanisms and kinetics of the reaction of a thiocyanato radical (NCS) with NO were investigated by a high-level ab initio molecular orbital method in conjunction with variational RRKM calculations. The species involved were optimized at the B3LYP/6-311++G(3df,2p) level, and their single-point energies were refined by the CCSD(T)/aug-cc-PVQZ//B3LYP/6-311+G(3df,2p) method. Our calculated results indicate favorable pathways for the formation of several isomers of an NCSNO complex. Formation of OCS + N 2 also is possible, although this pathway involves a substantial energy barrier. The predicted total rate constants, k total, at a 2 torr He pressure can be represented by the following equations: k total = 9.74 x 10 (26) T (-13.88) exp(-6.53 (kcal mol (-1))/ RT) at T = 298-950 K and 1.17 x 10 (-22) T (2.52) exp(-6.86 (kcal mol (-1))/ RT) at T = 960-3000 K, in units of cm (3) molecule (-1) s (-1), and the predicted values are in good agreement with the experimental results in the temperature range of 298-468 K. The calculated results clearly indicate that the branching ratio for R M1 in the temperature range of 298-950 K has the largest value ( R M1 accounts for 0.53-0.39). However, in the higher temperature range (960-3000 K), the formation of OCS + N 2 ( P5) with branching ratio R P5 (0.40-0.79) becomes dominant. The rate constants for key individual product channels are provided for different temperature and pressure conditions.     

用从头算研究Shilov反应的机理

Shilov反应在CH~4活化中占有中心地位,它有氧化加成和σ迁移两种可能的机理。本文用较大基组的从头算研究了这两种机理的反应过程,认为Shilov反应应按氧化加成机理进行。     

Ab initio chemical kinetics for the OH+HNCN reaction

The kinetics and mechanism of the reaction of the cyanomidyl radical (HNCN) with the hydroxyl radical (OH) have been investigated by ab initio calculations with rate constants prediction. The single and triplet potential energy surfaces of this reaction have been calculated by single-point calculations at the CCSD(T)/6-311+G(3df,2p) level based on geometries optimized at the B3LYP/6-311+G(3df,2p) and CCSD/6-311++G(d,p) levels. The rate constants for various product channels in the temperature range of 300-3000 K are predicted by variational transition-state and Rice-Ramsperger-Kassel-Marcus (RRKM) theories. The predicted total rate constants can be represented by the expressions ktotal=2.66 x 10(+2)xT-4.50 exp(-239/T) in which T=300-1000 K and 1.38x10(-20)xT2.78 exp(1578/T) cm3 molecule(-1) s(-1) where T=1000-3000 K. The branching ratios of primary channels are predicted: k1 for forming singlet HON(H)CN accounts for 0.32-0.28, and k4 for forming singlet HONCNH accounts for 0.68-0.17 in the temperature range of 300-800 K. k2+k7 for producing H2O+NCN accounts for 0.55-0.99 in the high-temperature range of 800-3000 K. The branching ratios of k3 for producing HCN+HNO, k6 for producing H2N+NCO, k8 for forming 3HN(OH)CN, k9 for producing CNOH+3NH, and k5+k10 for producing NH2+NCO are negligible. The rate constants for key individual product channels are provided in a table for different temperature and pressure conditions.     

Ab initio study on the structures of fluorinated formates and hydrogen abstraction reaction with OH radical

The conformational potential energy surfaces for mono- and difluoromethyl formate have been determined by using a modified G2(MP2) level of calculations. The structures and vibrational frequencies for the conformers of mono- and difluoromethyl formate have been reported. The hydrogen abstraction reaction channels between these two formates and OH radicals have been studied at the same level of theory. Using the standard transition state theory and taking into account the effect of tunneling across the reaction barrier, we have estimated the rate constant for hydrogen abstraction by OH radical. The effect of successive fluorine substitution for methyl hydrogen on the conformational stability and on the hydrogen abstraction rate has been analyzed.     

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.     

Ab initio study on the reaction mechanism of ozone with bromine atom

Ab initio calculations of the potential energy surface (PES) for the Br+O₃ reaction have been performed using the MP2, CCSD(T), and QCISD(T) methods with 6‐31G(d), 6‐311G(d), and 6‐311+G(3df). The reaction begins with a transition state (TS) when the Br atom attacks a terminal oxygen of ozone, producing an intermediate, the bromine trioxide (M), which immediately dissociates to BrO+O₂. The geometry optimizations of the reactants, products, and intermediate and transition states are carried out at the MP2/6‐31G(d) level. The reaction potential barrier is 3.09 kcal/mol at the CCSD(T)/6‐311+G(3df)//MP2 level, which shows that the bromine atom trends intensively to react with the ozone. The comparison of the Br+O₃ reaction with the F+O₃ and Cl+O₃ reactions indicates that the reactions of ozone with the halogen atoms have the similar reaction mechanism. © 2006 Wiley Periodicals, Inc. Int J Quantum Chem, 2007     

Ab initio chemical kinetics for the ClOO + NO reaction: effects of temperature and pressure on product branching formation

The kinetics and mechanism for the reaction of ClOO with NO have been investigated by ab initio molecular orbital theory calculations based on the CCSD(T)/6-311+G(3df)//PW91PW91∕6-311+G(3df) method, employed to evaluate the energetics for the construction of potential energy surfaces and prediction of reaction rate constants. The results show that the reaction can produce two key low energy products ClNO + (3)O(2) via the direct triplet abstraction path and ClO + NO(2) via the association and decomposition mechanism through long-lived singlet pc-ClOONO and ClONO(2) intermediates. The yield of ClNO + O(2) ((1)△) from any of the singlet intermediates was found to be negligible because of their high barriers and tight transition states. As both key reactions initially occur barrierlessly, their rate constants were evaluated with a canonical variational approach in our transition state theory and Rice-Ramspergen-Kassel-Marcus/master equation calculations. The rate constants for ClNO + (3)O(2) and ClO + NO(2) production from ClOO + NO can be given by 2.66 × 10(-16) T(1.91) exp(341/T) (200-700 K) and 1.48 × 10(-24) T(3.99) exp(1711/T) (200-600 K), respectively, independent of pressure below atmospheric pressure. The predicted total rate constant and the yields of ClNO and NO(2) in the temperature range of 200-700 K at 10-760 Torr pressure are in close agreement with available experimental results.     

Ab initio study of the mechanism of the atmospheric reaction: NO2 + O3 --> NO3 + O2

The atmospheric reaction NO2 + O3 --> NO3 + O2 (1) has been investigated theoretically by using the MP2, G2, G2Q, QCISD, QCISD(T), CCSD(T), CASSCF, and CASPT2 methods with various basis sets. The results show that the reaction pathway can be divided in two different parts at the MP2 level of theory. At this level, the mechanism proceeds along two transition states (TS1 and TS2) separated by an intermediate, designated as A. However, when the single-reference higher correlated QCISD methodology has been employed, the minimum A and the transition state TS2 are not found on the hypersurface of potential energy, which confirms a direct reaction mechanism. Single-reference high correlated and multiconfigurational methods consistently predict the barrier height of reaction (1) to be within the range 2.5-6.1 kcal mol(-1), in reasonable agreement with experimental data. The calculated reaction enthalpy is -24.6 kcal mol(-1) and the reaction rate calculated at the highest CASPT2 level, of k = 6.9 x 10(-18) cm(3) molecule(-1) s(-1). Both results can be regarded also as accurate predictions of the methodology employed in this article.     

Ab initio CI study of the reaction between NH2 and NO

Using ab initio CI calculations we have evaluated the structural, energetic and kinetic parameters of the reaction between NH₂ and NO. In light of the results obtained, it appears that while the formation of molecular nitrogen is highly probable, the reaction pathway leading to N₂H+OH cannot be thermodynamically excluded. The kinetic model based on the RRKM and TST methods leads to a calculated rate constant at 298 K (k = 1.64×10⁻¹¹ cm³ molecule⁻¹ s⁻¹) which is comparable to that determined experimentally and which decreases with temperature in the range 200–700 K.     

Ab initio studies on hydrogen-transfer tunneling for Cl + HCl abstraction hydrogen reaction

This article presents a treatment scheme of the tunneling of hydrogen between two molecular centers (Cl…Cl). The purpose is to calculate the tunneling probabilities of hydrogen atom transfer from the initial (the proceeding complex) to the final-state energy minima (the succeeding complex) in two anharmonic vibrational states (0 → 0 and 1 → 1) in terms of the time-dependent perturbation theory expression and to see whether spectroscopic signatures of tunneling persist in the form of splittings of the vibrational modes. The analysis uses the realistic potential energy function calculated at the HF/6−31 + G** self-consistent-field basis-set level for the interaction between transferred hydrogen and its molecular skeleton (Cl…H…Cl). This potential energy surface is calibrated by comparing its properties with those from sf-POLCI and the LEPS potential-energy surfaces. The anharmonic vibrational state is characterized by the corrected vibrational energy levels and a set of linear combination coefficients obtained via perturbation theory. The tunneling probabilities for two transitions (0 → 0 and 1 → 1) were calculated and compared with those from Gamow's equation. Applicability of the time-dependent perturbation theory expression and Gamow's equation to the [Cl BOND H…Cl] system is discussed. The vibrational splitting energies are obtained, and a spectroscopic signature caused by tunneling is expected and should be observable. © 1996 John Wiley & Sons, Inc.     

Ab initio study on the mechanism of cycloaddition reaction of ketene with methylenimine: A new reaction scheme

The cycloaddition reaction of ketene and methylenimine, leading to 2-azetidinone, has been studied theoretically by RHF /3-21G and IRC. This reaction is believed to be nonsynchronous and concerted, taking place through a twisted transition state. Four π orbitals are involved in this reaction, which is a “2 × [1 + 1]”-type cycloaddition. In the course of the reaction, rotation of the methylene group instead of oxygen in ketene was ascertained. The activated barrier is calculated to be 33.9 kcal/mol. © 1992 John Wiley & Sons, Inc.     

Ab initio quantum chemical studies of reaction mechanism for CH2CO with NCO

An extensive quantum chemical study of the potential energy surface (PES) for all possible isomerization and dissociation reactions of CH₂CO with NCO is reported at the DFT (B3LYP/6-311++G(d,p)) and CCSD(T)/cc-pVDZ//B3LYP/6-311++G(d,p) levels of theory. For the CH₂CO+NCO reaction, the formation of CO+CH₂NCO via an addition–elimination mechanism is the dominant channel on the doublet surface. While the formation of CO+CH₂OCN via bimolecular substitution reaction is in the secondary. Meanwhile, the isomerization and dissociation reactions of the products, CH₂NCO and CH₂OCN, also have been investigated using the same theoretical approach. It can be concluded that these reaction channels are not feasible kinetically at low or fairly high temperatures. On the basis of the ab initio data, the total rate constants for the CH₂CO+NCO reaction in the T=296–560 K range have been computed using conventional transition state theory with Wigner tunneling correction and fitted by a rate expression as k=2.14×10⁻¹² (cm³ molecule⁻¹ s⁻¹) exp(654.29/T). The calculated total rate constants with Wigner tunneling correction for the CH₂CO+NCO reaction are in good agreement with the available experimental values.     

Ab initio study on the mechanism of reaction HNCO+NH_2

Ab initio UMP2 and UQCISD(T) calculations, with 6-311G** basis sets, were performed for the titled reactions. The results show that the reactions have two product channels: NH2+ HNCO→NH3+NCO (1) and NH2+HNCO-N2H3+CO (2), where reaction (1) is a hydrogen abstraction reaction via an H-bonded complex (HBC), lowering the energy by 32.48 kJ/mol relative to reactants. The calculated QCISD(T)//MP2(full) energy barrier is 29.04 kJ/mol, which is in excellent accordance with the experimental value of 29.09 kJ/mol. In the range of reaction temperature 2300-2700 K, transition theory rate constant for reaction (1) is 1.68 × 1011- 3.29 × 1011 mL · mol-1· s-1, which is close to the experimental one of 5.0 ×1011 mL× mol-1· s-1 or less. However, reaction (2) is a stepwise reaction proceeding via two orientation modes, cis and trans, and the energy barriers for the rate-control step at our best calculations are 92.79 kJ/mol (for cis-mode) and 147.43 kJ/mol (for trans-mode), respectively, which is much higher than     

A computational study of the kinetics and mechanism for the reaction of HCO with HNO

The mechanism for the reaction of HCO with HNO has been studied at the G2M level of theory, based on the geometric parameters optimized by the BH&HLYP/6‐311G(d, p) method. There are three direct hydrogen abstraction channels producing (1) H₂CO + NO, (2) H₂NO + CO, and (3) HNOH + CO with barriers of 3.7, 3.9, and 10.4 kcal/mol, respectively. Another important reaction channel, (4), involves an association process forming HN(O)CHO (LM1) with a very small barrier and the subsequent isomerization and decomposition of LM1 producing HNOH + CO as major products. The rate constants of the dominant reaction channels (1), (2), and (4) in the temperature range 200–3000 K have been predicted by the microcanonical RRKM and transition state theory calculations with Eckart tunneling corrections. The theoretical result shows that in the high temperature range ( T > 1500 K), k₁ (H₂CO + NO) and k₂(H₂NO + CO) are preponderant, while in the low temperature range, both k₄(LM1) and k₄(HNOH + CO) appear to be dominant at high and low pressures, respectively. © 2004 Wiley Periodicals, Inc. Int J Chem Kinet 36: 205–215, 2004     

Ab initio reaction kinetics of hydrogen abstraction from methyl formate by hydrogen, methyl, oxygen, hydroxyl, and hydroperoxy radicals

Combustion of renewable biofuels, including energy-dense biodiesel, is expected to contribute significantly toward meeting future energy demands in the transportation sector. Elucidating detailed reaction mechanisms will be crucial to understanding biodiesel combustion, and hydrogen abstraction reactions are expected to dominate biodiesel combustion during ignition. In this work, we investigate hydrogen abstraction by the radicals H·, CH(3)·, O·, HO(2)·, and OH· from methyl formate, the simplest surrogate for complex biodiesels. We evaluate the H abstraction barrier heights and reaction enthalpies, using multireference correlated wave function methods including size-extensivity corrections and extrapolation to the complete basis set limit. The barrier heights predicted for abstraction by H·, CH(3)·, and O· are in excellent agreement with derived experimental values, with errors ≤1 kcal/mol. We also predict the reaction energetics for forming reactant complexes, transition states, and product complexes for reactions involving HO(2)· and OH·. High-pressure-limit rate constants are computed using transition state theory within the separable-hindered-rotor approximation for torsions and the harmonic oscillator approximation for other vibrational modes. The predicted rate constants differ significantly from those appearing in the latest combustion kinetics models of these reactions.     

Ab initio quantum chemical studies of reaction mechanism for CN with CH2CO

Six product channels have been found in the association reaction of CN + CH₂CO, and a variety of possible complexes and saddle points along the minimum energy reaction paths have been characterized at the UMP2(full)/6‐31G(d) level. The dominant reaction channels are the production of CH₂CN + CO and CH₂NC + CO. The isomerization and dissociation reactions of the major products of CH₂CN and CH₂NC have been investigated using the G2MP2 level. © 2005 Wiley Periodicals, Inc. Int J Quantum Chem, 2006     

Ab initio study of the F + CH3NHNH2 reaction mechanism

The F + CH(3)NHNH(2) reaction mechanism is studied based on ab initio quantum chemistry methods as follows: the minimum energy paths (MEPs) are computed at the UMP2/6-311++G(d,p) level; the geometries, harmonic vibrational frequencies, and energies of all stationary points are predicted at the same level of theory; further, the energies of stationary points and the points along the MEPs are refined by UCCSD(T)/6-311++g(3df,2p). The ab initio study shows that, when the F atom approaches CH(3)NHNH(2), the heavy atoms, namely N and C atoms, are the favorable combining points. For the two N atoms, two prereaction complexes with C(s) symmetry are generated and there exists seven possible subsequent reaction routes, of which routes 1, 2, 5, and 7 are the main channels. Routes 1, 2, and 5 are associated with HF elimination, with H from the amino group or imido group, and route 7 involves the N-N bond break. Routes 3 and 6 with relation to HF elimination with H from methyl, and route 4 involved the C-N bond break, are all energetically disfavored. For the C atom, the attack of F results in the break of the C-N bond and the products are CH(3)F + NHNH(2). This route is very competitive.     

Ab initio方法研究CH~3+OClO反应的可能通道

利用abinitio方法研究了CH~3+OClO反应的三个可能通道,首次应用UMP2(full)/6-31G(d,p)方法得到各反应物、产物、中间物及过渡态的优化构型和谐振频率;然后采用G2MP2理论计算各通道反应焓变和势垒高度。理论计算表明产物通道CH~2O+HOCl是最可能发生的途径,反应放热为443.80kJ·mol^-^1。可能的反应过程为：CH~3和OClO自由基先经无垒过程生成了一个富能中间物,继而通过较低的势垒解离成HOCl+H~2CO。     

Ab initio study on the reaction of uranium with oxygen

The primary reaction products and reaction mechanism of uranium with oxygen were discussed from MP2 method with the relativistic core potential of SDD basis set for U and 6-311+G* for O. The molecular geometries, electronic structure and energies of uranium oxides were obtained. The inspection on the three-dimensional potential energy surfaces of the U–O₂ interaction suggested that the abstraction and insertion mechanism were responsible for the studied reactions. The abstraction reaction channel resulting in the formation of UO and O is favored because the energy barrier is remarkably smaller than the one of the insertion channel resulting in the linear OUO product directly.     

Ab initio studies on the mechanism of the cycloaddition reaction between ketene and allene

The mechanism of the cycloaddition reaction between ketene and allene to form methylene–cyclobutanones has been studied theoretically by HF /3–21G and MP2 /3–21G . These two reactions are believed to be unsynchronous and concerted, taking place through the twisted transition states. Four orbitals are mainly involved in each reaction, which is a “2 × [1 + 1]”-type cycloaddition. The activated barrier for the two reactions are 27.2 and 27.1 kcal/mol, respectively, at the level of MP 2/6–31G * based on the MP 2/3–21G geometries, i.e., these two reactions are compatible. © 1994 John Wiley & Sons, Inc.