Theoretical study of the mechanism of CH2CO + CN reaction

The potential energy surface information of the CH₂CO + CN reaction is obtained at the B3LYP/6‐311+G(d,p) level. To gain further mechanistic knowledge, higher‐level single‐point calculations for the stationary points are performed at the QCISD(T)/6‐311++G(d,p) level. The CH₂CO + CN reaction proceeds through four possible mechanisms: direct hydrogen abstraction, olefinic carbon addition–elimination, carbonyl carbon addition–elimination, and side oxygen addition–elimination. Our calculations demonstrate that R→IM1→TS3→P3: CH₂CN + CO is the energetically favorable channel; however, channel R→IM2→TS4→P4: CH₂NC + CO is considerably competitive, especially as the temperature increases (R, IM, TS, and P represent reactant, intermediate, transition state, and product, respectively). The present study may be helpful in probing the mechanism of the CH₂CO + CN reaction. © 2005 Wiley Periodicals, Inc. Int J Quantum Chem, 2006     

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.     

The rate constant of the reaction HO2 + COCO2 + OH

The high-temperature oxidation of formaldehyde in the presence of carbon monoxide was investigated to determine the rate constant of the reaction HO₂ + CO ? CO₂ + OH (10). In the temperature range of 878–952°K from the initial parts of the kinetic curves of the HO₂ radicals and CO₂ accumulation at small extents of the reaction, when the quantity of the reacted formaldehyde does not exceed 10%, it was determined that the rate constant k₁₀ is A computer program was used to solve the system of differential equations which correspond to the high-temperature oxidation of formaldehyde in the presence of carbon monoxide. The computation confirmed the experimental results. Also discussed are existing experimental data related to the reaction of HO₂ with CO.     

Theoretical study on the reaction mechanism of(CH_3)_3CO+CO

The global environment pollution includes pho-tochemical smog, acid rain and stratospheric ozonedepletion. The short-lived species/radicals in atmos-phere are closely related to these phenomena. Theshort-lived species/radicals bring the photochemicalsmog,…     

Ca+催化的N2O+CO反应机理的理论研究

采用B3LYP/cc-pVTZ理论水平系统研究了Ca+离子催化N2O+CO→N2+CO2反应的微观机理.反应分两步进行:第一步Ca+夺取N2O中的O原子有两条反应通道,其中优势通道为Ca+金属离子与N2O分子中O作用,形成线性分子复合物,活化N2O分子中的N-O键,之后的反应路径为O-N键断裂机理;第二步为CaO+金属...     

N_2O与氧负离子微观反应机理的理论研究

采用G3B3//B3LYP理论水平对反应O-+N2O的双重电子态势能面反应机理进行了详细的理论研究.该反应涉及的各个稳定点的构型、振动频率是在B3LYP/6-311++G(d,p)理论水平下计算的.计算结果表明,得到的反应焓变与已有实验值相吻合,该反应主反应通道是O-+N2O→NO+NO-,而生成O2-+N2的反应通道是次反应通道.     

Theoretical study on the mechanism of the CH2F + NO2 reaction

Despite the importance of the Fluoromethyl radicals in combustion chemistry, very little experimental information on their reactions toward stable molecules is available in the literature. Motivated by recent laboratory characterization about the reaction kinetics of Chloromethyl radicals with NO2, we carried out a detailed potential energy survey on the CH2F + NO2 reaction at the B3LYP/6-311G(d,p) and MC-QCISD (single-point) levels as an attempt toward understanding the CH2F + NO2 reaction mechanism. It is shown that the CH2F radical can react with NO2 to barrierlessly generate adduct a (H2FCNO2), followed by isomerization to b1 (H2FCONO-trans) which can easily interconvert to b2 (H2FCONO-cis). Subsequently, Starting from b (b1, b2), the most feasible pathway is the C--F and N--O1 bonds cleavage along with N--F bond formation of b (b1, b2) leading to P1 (CH2O + FNO), or the direct N--O1 weak-bond fission of b (b1, b2) to give P2 (CH2FO + NO), or the 1,3-H-shift associated with N--O1 bond rupture of b1 to form P3 (CHFO + HNO), all of which may have comparable contribution to the reaction CH2F + NO2. Much less competitively, b2 either take the 1,4-H-shift and O1--N bond cleavage to form product P4 (CHFO + HON) or undergo a concerted H-shift to isomer c2 (HFCONOH), followed by dissociation to P4. Because the rate-determining transition state (TSab1) in the most competitive channels is only 0.3 kcal/mol higher than the reactants in energy, the CH2F + NO2 reaction is expected to be rapid, and may thus be expected to significantly contribute to elimination of nitrogen dioxide pollutants. The similarities and discrepancies among the CH2X + NO2 (X = H, F, and Cl) reactions are discussed in terms of the electronegativity of halogen atom. The present article may assist in future experimental identification of the product distributions for the title reaction, and may be helpful for understanding the halogenated methyl chemistry.     

Theoretical study on the mechanism of the NCO + HCNO reaction

The complex doublet potential surface of the NCO + HCNO reaction has been investigated at the QCISD(T)/6-311g(d,p)//UB3LYP/6-31G(d,p) level. We have found 29 isomers on the potential surface, which are connected by 38 transition states. The single-point energy calculations are performed at the high-level QCISD(T)/6-311G(d,p) for more accurate energy values. In various possible initial association ways, the end-N attack leading to HC2N2O2 a1 and a2 is the most favorable association way through a barrierless process. Through the thermodynamic and kinetic analyses, the product NO + CO + HCN should be the major product in both the low- and high-temperature conditions for its low-energy determination transition state. Our calculation is consistent with the available data in low-temperature condition and expected to be confirmed in the high-temperature condition.     

The pressure dependence of the rate constant for the reaction: HO + CO → H + CO2

Relative rate measurements of the reactions of the HO-radical with CO [HO + CO → H + CO₂ (1)] and with isobutane [HO + iso-C₄H₁₀ → H₂O + t-(or iso-)C₄H₉ (3)] have been made through the photolysis of dilute mixtures of HONO with CO, iso-C₄H₁₀, NO₂, and NO in simulated air at 700 and 100 torr pressure and 25 ± 2°C. In situ, long path, FT-IR analysis of the reactants and products provided essentially continuous monitoring of the reactions during the course of the experiments. The kinetic analysis of the data coupled with Greiner's estimate of k₃ give new estimates of k₁ = 439 ± 24 ppm^?1 min^?1 in air at 700 torr and k₁ = 203 ± 29 ppm^?1 in air at 100 torr. The results confirm the recent conclusions of Cox and Sie and their co-workers that the rate constant for reaction (1) is pressure dependent. Modeliers of the chemical changes which occur in the troposphere should adopt a new value for the rate constant k₁ which is about a factor of two larger than that in current use by most groups.     

HNCO+OH——NH~2+CO~2反应理论研究

用从头算UHF/6-31G基组研究了异氰酸和羟基生成氨基和二氧化碳即HNCO+OH--NH~2+CO~2的反应机理.优化得到了反应途径上的过渡态和中间体,并通过振动分析对过渡态和中间体进行了确认.在UMP4/6-31G水平上计算了它们的能量,同时对零点能进行了较正.计算结果表明:此反应是多步反应,先后通过3个过渡态(TS1,TS2,TS3),2个内旋转位垒(TSI,TSII),4个中间体(IM1,IM2,IM3,IM4),其中,IM3--TS2这一步为整个反应的决速步骤,速控步的活化能为202.388kJ/mol.与异氰酸和羟基作用的另一反应通道(即HNCO+OH--H~2O+NCO)的活化能(69.038kJ/mol)比较,可看出所研究反应通道为次要反应通道,这与实验结果是一致的。     

Theoretical study on the mechanism of the 1CHCl + NO reaction

The complex doublet potential energy surface of the CHClNO system, including 31 minimum isomers and 84 transition states, is investigated at the QCISD(T)/6-311G(d, p)//B3LYP/6-31G(d, p) level in order to explore the possible reaction mechanism of the singlet CHCl with NO. Various possible isomerization and dissociation channels are probed. The initial association between 1CHCl and NO at the terminal N-site can almost barrierlessly lead to the chainlike adducts HClCNO a (a1, a2) followed by the direct Cl-extrusion to product P9 Cl + HCNO, which is the most feasible channel. Much less competitively, a (a1, a2) undergoes a ring-closure leading to the cyclic isomer c-C(HCl)NO d followed by a concerted Cl-shift and N-O cleavage of d to form the branched isomers ClNC(H)O f (f1, f2). Eventually, f (f1, f2) may take a direct H-extrusion to produce P7 H + ClNCO or a concerted 1,2-H-shift and Cl-extrusion to form P1 Cl + HNCO. The low-lying products P2 HCl + NCO, P3 Cl + HOCN, P14 HCO + 3NCl, P6 ClO + HCN, and P13 ClNC + OH may have the lowest yields observed. Our calculations show that the product distributions of the title reaction are quite different from those of the analogous 1CHF + NO reaction, yet are similar to those of another analogous 3CH2 + NO reaction. The similarities and discrepancies among the three reactions are discussed in terms of the substitution effect. The present article may assist in future experimental identification of the product distributions for the title reaction and may be helpful for understanding the halogenated carbene chemistry.     

Theoretical study on the reaction mechanism of VO2+ with propyne in gas phase

Possible molecular mechanisms of the gas-phase ion/molecule reaction of VO2+ in its lowest singlet and triplet states (1A1/3A' ') with propyne have been investigated theoretically by density functional theory (DFT) methods. The geometries, energetic values, and bonding features of all stationary and intersystem crossing points involved in the five different reaction pathways (paths 1-5), in both high-spin (triplet) and low-spin (singlet) surfaces, are reported and analyzed. The oxidation reaction starts by a hydrogen transfer from propyne molecule to the vanadyl complex, followed by oxygen migration to the hydrocarbon moiety. A hydride transfer process to the vanadium atom opens four different reaction courses, paths 1-4, while path 5 arises from a hydrogen transfer process to the hydroxyl group. Five crossing points between high- and low-spin states are found: one of them takes place before the first branching point, while the others occur along path 1. Four different exit channels are found: elimination of hydrogen molecule to yield propynaldehyde and VO+ (1Sigma/3Sigma); formation of propynaldehyde and the moiety V-(OH2)+; and two elimination processes of water molecule to yield cationic products, Prod-fc+ and Prod-dc+ where the vanadium atom adopts a four- and di-coordinate structure, respectively.     

Theoretical study on the reaction mechanism of NO and CO catalyzed by Rh atom

The gas-phase reaction mechanism of NO and CO catalyzed by Rh atom has been systematically investigated on the ground and first excited states at CCSD(T)//B3LYP/6-311+G(2d), SDD level. This reaction is mainly divided into two reaction stages, NO deoxygenation to generate N₂O and then the deoxygenation of N₂O with CO to form N₂ and CO₂. The crucial reaction step deals with the NO deoxygenation to generate N₂O catalyzed by Rh atom, in which the self-deoxygenation of NO reaction pathway is kinetically more preferable than that in the presence of CO. The minimal energy reaction pathway includes the rate-determining step about N–N bond formation. Once the NO deoxygenation with CO catalyzed by rhodium atom takes place, the reaction results in the intermediate RhN. Then, the reaction of RhN with CO is kinetically more favorable than that with NO, while both of them are thermodynamically preferable. These results can qualitatively explain the experimental finding of N₂O, NCO, and CN species in the NO + CO reaction. For the N₂O deoxygenation with CO catalyzed by rhodium atom, the reaction goes facilely forward, which involves the rate-determining step concerning CO₂ formation. CO plays a dominating role in the RhO reduction to regenerate Rh atom. The complexes, OCRhNO, RhON₂, RhNNO, ORhN₂, RhCO₂, RhNCO, and ORhCN, are thermodynamically preferred. Rh atom possesses stronger capability for the N₂O deoxygenation than Rh⁺ cation.     

CH3S与CO反应机理的理论研究

在G3(MP2)//B3LYP/6-311 G(d,p)水平上,对CH3S自由基与CO气相反应的微观机理进行了理论研究.结果表明:该反应共存在3个反应通道,产物分别为CH3 OCS,CH2S HCO和CH2S HOC.由于形成产物CH3 OCS的活化势垒较低,因此为主要反应通道,这与实验观察到的结果是一致的.     

CH_3SH+CN微观反应机理的理论研究

采用BMC-CCSD//B3LYP/6-311G(d,p)方法对CH3SH+CN反应机理进行了详细的理论研究.反应中涉及的各稳定点的构型、振动频率和零点能在B3LYP/6-311G(d,p)水平下计算得到,计算结果表明,该反应存在两种反应机理,5条可能的反应通道.SN2机理由于能垒太高,与直接氢抽提机理相比可以忽略.该反应的最可行通道为CN中的C原子进攻SH中的H原子经由一个前期和一个后期分子络合物生成产物CH3S和HCN.计算得到的反应焓变与已有实验值非常吻合.     

Temperature dependence of the rate constant for the reaction OH + H2S

Absolute rate constants for the reaction of OH with H₂S have been measured over the temperature range of 239–425 K using the flash photolysis–resonance fluorescence technique. The results showed that the rate constants deviate slightly from Arrhenius behavior but can still be represented adequately by the following Arrhenius equation: Comparisons with recent literature values are presented.     

Computational study on the mechanism and rate constant for the C6H5 + C6H5NO reaction

The reaction mechanism of C6H5 + C6H5NO involving four product channels on the doublet-state potential energy surface has been studied at the B3LYP/6-31+G(d, p) level of theory. The first reaction channel occurs by barrierless association forming (C6H5)2NO (biphenyl nitroxide), which can undergo isomerization and decomposition. The second channel takes place by substitution reaction producing C12H10 (biphenyl) and NO. The third and fourth channels involve direct hydrogen abstraction reactions producing C6H4NO + C6H6 and C6H5NOH + C6H4, respectively. Bimolecular rate constants of the above four product channels have been calculated in the temperature range 300-2000 K by the microcanonical Rice-Ramsperger-Kassel-Marcus theory and/or variational transition-state theory. The result shows the dominant reactions are channel 1 at lower temperatures (T < 800 K) and channel 3 at higher temperatures (T > 800 K). The total rate constant at 7 Torr He is predicted to be k(t) = 3.94 x 10(21) T(-3.09) exp(-699/T) for 300-500 K, 2.09 x 10(20) T(-3.56) exp(2315/T) for 500-1000 K, and 1.51 x 10(2) T(3.30) exp(-3043/T) for 1000-2000 K (in units of cm3 mol(-1) s(-1)), agreeing reasonably with the experimental data within their reported errors. The heats of formation of key products including biphenyl nitroxide, hydroxyl phenyl amino radical, and N-hydroxyl carbazole have been estimated.     

Theoretical study of the phosphotriesterase reaction mechanism

Phosphotriesterase (PTE) is a binuclear zinc enzyme that catalyzes the hydrolysis of extremely toxic organophosphate triesters. In the present work, we have investigated the reaction mechanism of PTE using the hybrid density functional theory method B3LYP. We present a potential energy surface for the reaction and provide characterization of the transition states and intermediates. We used the high resolution crystal structure to construct a model of the active site of PTE, containing the two zinc ions and their first shell ligands. The calculations provide strong support to an associative mechanism for the hydrolysis of phosphotriesters by PTE. No protonation of the leaving group was found to be necessary. In particular, the calculations demonstrate that the nucleophilicity of the bridging hydroxide is sufficient to be utilized in the hydrolysis reaction, a feature that is of importance for a number of other di-zinc enzymes.     

Enhancing the rate of the Diels-Alder reaction using CO2 + ethanol and CO2 + n-hexane mixed solvents of different phase regions

The reaction rate of the Diels-Alder reaction between N-ethylmaleimide and 9-hydroxymethylanthrance in CO2 + ethanol and CO2 + hexane mixed solvents of different compositions were determined by in situ UV/vis spectroscopy at 318.15 K and different pressures. The density of the mixed solvents at different pressures was also determined and the isothermal compressibility was calculated using the density data. The activation volume of the reaction was calculated based on the dependence of rate constant (kc) on pressure. It was demonstrated that the kc was very sensitive to the pressure in the mixed solvents near the critical region and the kc increased dramatically as pressure approached dew points, critical point, and bubble points of the mixed solvents. However, the kc in the mixed solvents outside the critical region or in pure CO2 was not sensitive to pressure. At suitable conditions, kc could be 40 times larger than that in acetonitrile. The activation volume of the reaction was nearly independent of pressure as the pressure was much higher than the phase separation pressure of the mixed solvents, while it increased considerably as pressure approached the bubble points, critical point, and dew points from high pressure. The clustering of the solvent molecules with the reactants and the activated complex in the reaction systems near the phase boundary in the critical region may be the main reason for the interesting phenomena observed. This work also shows that, using pure CO2 as the solvent, the reaction cannot be carried out in the critical region of the solvent due to the limitations of the reactants, while it can be conducted in the critical region of mixed solvents of suitable compositions, where the solvents are highly compressible and the reaction rate can be tuned effectively by pressure.     

CH2与HNCO反应机理的量子化学研究

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所研究的领域,由于该反应在燃烧化学中讨论氮氧化物NOx的消除过程十分重要,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,本文采用量子化学方法,研究了CH2与HNCO体系的反应机理,力求从理论角度给出合理的解释。