F+CH_3OH碰撞反应机机理和反应势能面

以MP2（full）/6-311 + +g(d,p)水平上详细研究了氟原子与甲醇抽氢反应的 多通道反应机理，得到了各条通道中涉及的驻点的构型和振动频率及其能量，给出 了两张完整的反应势能面，结果表明，氟原子从C原子上抽氢时有一条明显的最低 能量通道，而从氧原子上抽氢时要涉及多条分支通道和多个驻点构型，给出了各分 支通道势能面示意图，结果表明以形成五元环状过渡态通道为优势通道，计算得到 经途径1生成CH_2OH时反应放热170.62kJ/mol，经分支途径6生成CH_3O自由基时反 应放热119.4 kJ/mol，此结果与实验值一致。     

<Emphasis Type="Italic">Ab initio</Emphasis> study of the reaction path of the reaction HNCO+CN

We report ab initio UMP2 calculations of the reaction of CN with HNCO using 6-311G(d,p) basis sets. The obtained results show that the reaction has two product channels: HNCO+CN→HCN+NCO (1) and HNCO+CN→HNCN+CO (2). Channel (1) is a hydrogen abstraction reaction, which is a concerted process. The calculated potential energy barrier is 20.80 kJ/mol at UMP2(full)/6-311G(d,p) level. In the range of reaction temperature (1000-2100 K), the conventional transition theory rate constant for channel (1) ranges from 0.32×10⁻¹¹ to 6.9×10⁻¹¹cm³· mol⁻¹· s⁻¹, which is close to the experimental value. Channel (2) is a stepwise reaction involving an intermediate during the process of reaction. The UMP2(full)/6-311G(d,p) potential energy barrier is 83.42 kJ/mol for the rate-controlling step, which is much higher than that of channel (1).     

An investigation on the reaction mechanism of the F2+Cl2→2ClF using the B3LYP method

The reaction mechanism of F₂+Cl₂→2ClF has been investigated with the density functional theory at the B3LYP/6‐311G* level. Six transition states have been found for the three possible reaction paths and verified by the normal mode vibrational and IRC analyses. Ab initio MP2/6‐311G* geometry optimizations and CCSD(T)/6‐311G(2df)//MP2/6‐311G* single‐point energy calculations have been performed for comparison. It is found that when the F₂ (or Cl₂) molecule decomposes into atoms first and then the F (or Cl) atom reacts with the molecule Cl₂ (or F₂) nearly along the molecular axis, the energy barrier is very low. The calculated energy barrier of F attacking Cl₂ is zero and that of Cl attacking F₂ is only 15.57 kJ?mol^?1 at the B3LYP level. However, the calculated dissociation energies of F₂ and Cl₂ are as high as 145.40 and 192.48 kJ?mol^?1, respectively. When the reaction proceeds through a bimolecular reaction mechanism, two four‐center transition states are obtained and the lower energy barrier is 218.69 kJ?mol^?1. Therefore, the title reaction F₂+Cl₂→2ClF is most probably initiated from the atomization of the F₂ molecule and terminated by the reaction of F attacking Cl₂ nearly along the Cl? Cl bond. MP2 calculations lead to the same conclusion, but the geometry of TS and the energy barrier are somewhat different. © 2002 John Wiley & Sons, Inc. Int J Quantum Chem, 2002     

Ab initio study of the reaction path of the reaction HNCO+CN

HNCO is a convenient photolytic source of NCO and NH radicals for laboratory kinetics studies of elementary reaction[1] and plays an important role in the combustion and atmosphere chemistry. It can re- move deleterious compounds rapidly from exhausted ga…     

Barrier heights for H‐atom abstraction by HȮ2 from n‐butanol—A simple yet exacting test for model chemistries?

The barrier heights involved in the abstraction of a hydrogen atom from n‐butanol by the hydroperoxyl radical have been computed with both compound (CBS‐QB3, CBS‐APNO, G3) and coupled cluster methods. In particular, the benchmark computations CCSD(T)/cc‐pVTZ//MP2/6‐311G(d,p) were used to determine that the barrier heights increase in the order α <γ < β < δ < OH. Two prereaction hydrogen‐bonded complexes are formed, one of which connects the TGt conformer of n‐butanol to the α and β transition states and the other connects to the γ and OH channels from the TGg conformer. Four postreaction complexes were also found which link the transition states to the products, hydrogen peroxide + C₄H₉O radical. Abstraction from the terminal δ carbon atom does not involve either a pre or postreaction complex. A number of DFT functionals—B3LYP, BMK, MPWB1K, BB1K, MPW1K, and M05‐2X—were tested to see whether the correct ranking could be obtained with computationally less expensive methods. Only the later functional predicts the correct order but requires a basis set of 6‐311++G(df,pd) to achieve this. However, the absolute values obtained do not agree that well with the benchmarks; the composite G3 method predicts the correct order and comes closest (≤ 2 kJ, mol ^?1) in absolute numerical terms for H‐abstraction from carbon. © 2009 Wiley Periodicals, Inc. J Comput Chem, 2010     

Ab initio study on the mechanism of reaction HNCO+NH2

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 5.0×10¹¹mL·mol^-1·s^-1or less. However, reaction (2) is a stepwise reaction proceeding via two orientation modes,cis andtrans, and the energy barriers for the rate-control step at our best calculations are 92.79 kJ/mol (forcis-mode) and 147.43 kJ/mol (fortrans-mode), respectively, which is much higher than reaction (1). So reaction (1) is the main channel for the titled reaction.     

Theoretical Study on the Reaction Mechanism of Ketene CH2CO with Isocyanate NCO Radical

The bimolecular single collision reaction potential energy surface of an isocyanate NCO radical with a ketene CH2CO molecule was investigated by means of B3LYP and QCISD（T） methods. The computed results indicate that two possible reaction channels exist on the surface. One is an addition-elimination reaction process, in which the CH2CO molecule is attacked by the nitrogen atom at its methylene carbon atom to lead to the formation of the intermediate OCNCH2CO followed by a C-C rupture channel to the products CH2NCO＋CO. The other is a direct hydrogen abstraction channel from CHzCO by the NCO radical to afford the products HCCO＋HNCO. Because of a higher barrier in the hydrogen abstraction reaction than in the addition-elimination reaction, the direct hydrogen abstraction pathway can only be considered as a secondary reaction channel in the reaction kinetics of NCO＋ CH2CO. The predicted results are in good agreement with previous experimental and theoretical investigations.     

Deamination of the Radical Cation of the Base Moiety of 2′‐Deoxycytidine: A Theoretical Study

Five pathways leading to the deamination of cytosine (to uracil) after formation of its deprotonated radical cation are investigated in the gas phase, at the UB3LYP/6‐311G(d,p) level of theory, and in bulk aqueous solvent. The most favorable pathway involves hydrogen‐atom transfer from a water molecule to the N3 nitrogen of the deprotonated radical cation, followed by addition of the resulting hydroxyl radical to the C4 carbon of the cytosine derivative. Following protonation of the amino group (N4), the C4? N4 bond is broken with elimination of the NH₃^?+ radical and formation of a protonated uracil. The rate‐determining step of this mechanism is hydrogen‐atom transfer from a water molecule to the cytosine derivative. The associated free energy barrier is 70.2 kJ mol^?1.     

A fully ab initio potential energy surface for ClH2 reactive system

Anab initio analytical potential energy surface called BW3 for the ClH₂ reactive system is presented. The fit of this surface is based on about 1 200ab initio energy points, computed with multi-reference configuration interaction(MRCI) and scaling external correlation (SEC) method and a very large basis set. The precision in the fit is very high. The BW3 surface could reproduce correctly the dissociation energy of H₂ and HCl, and the endothermicity of the Cl + H₂ abstraction reaction. For the Cl + H₂ abstraction reaction, the saddle point of BW3 lies in collinear geometries, and the barrier height is 32.84 kJ/mol; for the H + ClH exchange reaction, the barrier of BW3 is also linear, with a height of 77.40 kJ/mol.     

Different Catalytic Effects of a Single Water Molecule: The Gas‐Phase Reaction of Formic Acid with Hydroxyl Radical in Water Vapor

The effect of a single water molecule on the reaction mechanism of the gas‐phase reaction between formic acid and the hydroxyl radical was investigated with high‐level quantum mechanical calculations using DFT–B3LYP, MP2 and CCSD(T) theoretical approaches in concert with the 6‐311+G(2df,2p) and aug‐cc‐pVTZ basis sets. The reaction between HCOOH and HO has a very complex mechanism involving a proton‐coupled electron transfer process (pcet), two hydrogen‐atom transfer reactions (hat) and a double proton transfer process (dpt). The hydroxyl radical predominantly abstracts the acidic hydrogen of formic acid through a pcet mechanism. A single water molecule affects each one of these reaction mechanisms in different ways, depending on the way the water interacts. Very interesting is also the fact that our calculations predict that the participation of a single water molecule results in the abstraction of the formyl hydrogen of formic acid through a hydrogen atom transfer process (hat).     

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     

Theoretical Study of the Reaction Mechanism and Kinetics of HO2 with XCHO (X = F,Cl)

The reactions of HO₂ with FCHO and ClCHO have been theoretically investigated by combining beyond‐CCSD(T) electronic structure benchmarks, validated density functional theory, and canonical variational transition state theory with small‐curvature tunneling, coupled‐torsions anharmonicity, and high‐frequency anharmonicity. This investigation explores three different reaction mechanisms: radical addition plus a hydrogen transfer, radical addition, and hydrogen abstraction. The calculated results show that the dominant reaction pathway is the terminal oxygen atom of HO₂ added to the carbon atom of XCHO (X = F, Cl) and simultaneously the hydrogen atom of HO₂ transferred to the oxygen atom of the C=O group in XCHO. The reaction barriers of the other reaction pathways are so high that these processes are negligible in the atmosphere. Although the barrier height of the dominant reaction pathway in the HO₂ + FCHO reaction is 0.61 kcal/mol higher than that of the corresponding HO₂ + ClCHO reaction, the HO₂ + FCHO reaction is faster than the HO₂ + ClCHO reaction because the variational effects of HO₂ + ClCHO is more obvious than that of the HO₂ + FCHO. The present results show that the HO₂ + FCHO reaction may be important in the atmosphere. The present results should be useful in evaluating the atmospheric fate of XCHO (X = F, Cl).     

Gas‐phase radical–radical recombination reactions of nitroxides with substituted phenyl radicals

Fourier‐transform ion cyclotron resonance mass spectrometry has been used to examine gas‐phase reactions of four different nitroxide free radicals with eight positively charged pyridyl and phenyl radicals (some containing a Cl, F, or CF₃ substituent). All the radicals reacted rapidly (near collision rate) with nitroxides by radical–radical recombination. However, some of the radicals were also able to abstract a hydrogen atom from the nitroxide. The results establish that the efficiency (k_reaction/k_collision) of hydrogen atom abstraction varies with the electrophilicity of the radical, and hence is attributable to polar effects (a lowering of the transition‐state energy by an increase in its polar character). The efficiency of the recombination reaction is not sensitive to substituents, presumably due to a very low reaction barrier. Even so, after radical–radical recombination has occurred, the nitroxide adduct was found to fragment in different ways depending on the structure of the radical. For example, a cationic fragment was eliminated from the adducts of the more electrophilic radicals via oxygen anion abstraction by the radical (i.e., the nitroxide adduct cleaves heterolytically), whereas adducts of the less electrophilic radicals predominantly fragmented via homolytic cleavage (oxygen atom abstraction). Therefore, differences in the product branching ratios were found to be attributable to polar factors. © 2004 Wiley Periodicals, Inc. Int J Chem Kinet 36: 216–229 2004     

Kinetics of the hydrogen abstraction reactions of 1,1‐ and 1,2‐difluoroethane with hydroxyl radical: an ab initio study

The hydrogen abstraction reactions of 1,1‐ and 1,2‐difluoroethane with the OH radical have been investigated by the ab initio molecular orbital theory. The geometries of the reactants, products, and transition states have been optimized at the (U)MP2=full level of theory in conjunction with 6‐311G(d,p) basis functions. Single‐point (U)MP2=full with larger basis set, such as 6‐311G(3d,2p), and QCISD(T)=full/6‐311G(d,p) calculations have also been carried out to observe the effects of basis sets utilized and higher order electron correlation. Three and four reaction channels have been identified for 1,1‐ and 1,2‐difluoroethane, respectively. In the case of 1,1‐difluoroethane, hydrogen abstraction from the α‐carbon has been found to be easier than that from the β‐carbon. The barriers of the four reaction channels for 1,2‐difluoroethane are close to each other. Weak hydrogen bonding interactions have been observed between hydroxyl hydrogen and a fluorine atom in the transition states. Rate constants for the reactions of 1,1‐ and 1,2‐difluoroethane with the OH radical have been calculated using the standard transition state theory and found to be in good agreement with the experimental results. © 2000 John Wiley & Sons, Inc. J Comput Chem 21: 1305–1318, 2000     

Theoretical study of thermal rearrangements of α‐silylalcohols: Effects of substituents attached to the silicon atom on the reactions

To investigate the effects of substituents attached to the silicon atom on the thermal rearrangement reactions of α‐silyl alcohols, the thermal rearrangement reactions of dimethylsilyl methanol (CH₃)₂SiHCH₂OH and vinylsilyl methanol CH₂?CHSiH₂CH₂OH were studied by ab initio calculations at the G3 level. Geometries of various stationary points were fully optimized at the MP2(full)/6‐31G(d) and MP2(full)/6‐311G(d,p) levels, and harmonic vibrational frequencies were calculated at the same levels. The reaction paths were investigated and confirmed by intrinsic reaction coordinate (IRC) calculations at the MP2(full)/6‐31G(d) level. The results show that two dyotropic reactions could occur when (CH₃)₂SiHCH₂OH or CH₂?CHSiH₂CH₂OH is heated. One is Brook rearrangement reaction (reaction A), and the dimethylsilyl or vinylsilyl groups migrates from carbon atom to oxygen atom coupled with a simultaneous migration of a hydrogen atom from oxygen atom to carbon atom passing through a double three‐membered ring transition state, forming dimethylmethoxylsilane (CH₃)₂SiHOCH₃ or methoxylvinylsilane CH₂?CHSiH₂OCH₃; the other is a hydroxyl group migration (reaction B) from carbon atom to silicon atom, coupled with a simultaneous migration of a hydrogen atom from silicon atom to carbon atom, via a double three‐membered ring transition state, forming trimethylsilanol (CH₃)₃SiOH or methylvinylsilanol CH₃SiH(OH)CH?CH₂. The G3 barriers of the reactions A and B were computed to be 312.8 and 241.4 kJ/mol for (CH₃)₂SiHCH₂OH, and 317.6 and 233.7 kJ/mol for CH₂?CHSiH₂CH₂OH, respectively. On the basis of the MP2(full)/6‐31G(d) optimized parameters, vibrational frequencies, and G3 energies, the reaction rate constants k(T) and equilibrium constants K(T) were calculated using canonical variational transition state theory (CVT) with centrifugal‐dominant small‐curvature tunneling (SCT) approximation over a temperature range of 400–1800 K. The influences of methyl and vinyl groups attached to the silicon atom on reactions are discussed. © 2006 Wiley Periodicals, Inc. Int J Quantum Chem, 2007     

A fullyab initio potential energy surface for ClH2 reactive system

Anab initio analytical potential energy surface called BW3 for the ClH₂ reactive system is presented. The fit of this surface is based on about 1 200ab initio energy points, computed with multi-reference configuration interaction(MRCI) and scaling external correlation (SEC) method and a very large basis set. The precision in the fit is very high. The BW3 surface could reproduce correctly the dissociation energy of H₂ and HCl, and the endothermicity of the Cl + H₂ abstraction reaction. For the Cl + H₂ abstraction reaction, the saddle point of BW3 lies in collinear geometries, and the barrier height is 32.84 kJ/mol; for the H + ClH exchange reaction, the barrier of BW3 is also linear, with a height of 77.40 kJ/mol.     

(H_2O)_n(n=0～2)催化HS和HOCl反应机理的理论研究

在CCSD(T)/6-311+G(3df,2p)//M06-2X/6-311+G(3df,2p)水平上研究了(H_2O)n(n=0~2)催化HS和HOCl的反应机理.结果表明,HS与HOCl反应中HS夺取HOCl上的H原子形成产物H_2S和ClO.在无水催化时,该反应存在2种不同的路径(分别经过过渡态TS1和TS2,二者互为顺反结构),对应的能垒分别为100.28和100.91kJ/mol,到达产物(H_2S+ClO)需吸收18.99kJ/mol能量,反应不易发生;在单个水分子参与时,水分子可通过形成弱相互作用或者作为H原子转移桥梁影响反应机理,获得了4种水催化路径,能垒(间于53.97~92.39kJ/mol之间)均低于无水催化过程.同时发现,在反应到达产物前,水分子可以与产物形成中间体IM,IM相对能仅为0.46kJ/mol,有利于产物形成;有2个水分子参与反应时,找到了3条催化路径,最优反应路径过渡态TS7的能垒为45.05kJ/mol,低于无水催化过程,相比单个水分子最优路径能垒(53.97kJ/mol)并无显著降低.     

A Possible Reaction Channel from BrONO to BrNO2

A possible isomeriztion channel from BrONO( bromine nitrite) to BrNO2 (nitryl bromide) is predicted by means of MP2 and QCISD(T) (single-point) methods.The channel is a direct bromine abstraction reaction from BrONO molecule by NO2 in which the forward reaction barrier is 89.30 kJ/mol at final UQCISD(T)/6-311 G(2df)//UMP2/6-311G(d) level of theory with zero-point energies included,The result can explian the available experiments very well.     

Quantum study on the electrocyclic reactions of CH2CHCHCHX (XH,F, Cl)

The microscopic mechanisms of the electrocyclic reactions for cis‐1,3‐butadiene and its monofluoro‐, monochloroderivatives have been studied by density functional theory (DFT), using the B3LYP method and 6‐311++G** basis sets. We optimized the geometric configurations of reactants, transition states, and products; verified all the probable transition states through vibrational analysis; and calculated the relative single‐point energies at the QCISD(T)/6‐311++G**//B3LYP/6‐311++G**. The results show that the monofluoro‐, monochloroderivatives of cis‐1,3‐butadiene both have two conformers; the reactant favors the electrocyclic reaction when one outboard hydrogen atom of the CH₂ groups is substituted by the fluorine or chlorine atom. © 2004 Wiley Periodicals, Inc. Int J Quantum Chem, 2004