一种确定反应中间态几何特征和能量的综合性方法

通过综合使用传统的过渡态优化算法、数学统计工具以及人工神经网络算法(ANN)找到一种不依赖于反应物起始构象而得到化学反应中过渡态结构和能量的方法. 在两个反应物互相接近的过程中, 每一步的几何构象都对应着一个系统能量值. 本研究的目的是尽可能地收集处在反应能量面上的这种能量点值. 通过采用几何参数作为自变量对势能面进行模拟研究, 得到了势能面上对应过渡态结构的一阶鞍点. 采用乙醛负离子和甲醛作为反应物, 对经典的醛醇缩合反应中的亲核进攻步骤进行了研究. 对内禀反应坐标(IRC)路径的计算是从反应物的三组不同起始构象出发, 最终获得了反应势能面上的96个点. 本研究中的势能面采用人工神经网络算法进行模拟研究, 并利用交叉验证方法评估得到的结果, 避免了采用人工神经网络算法时过度拟合情况的发生.     

氧杂环丁烷热解机理的量子化学研究

本文利用半经验分子轨道理论研究了氧杂环丁烷热解为甲醛和乙烯的反应机理计算是采用半经验方法AM1进行的, 各种驻点全部运用Berny梯度方法优化. 同时, 对过渡态的结构进行了振动分析的确证. 计算表明: 1)不存在协同的同面-同面反应途径的过渡态, 其驻点只是一个二级鞍点; 2) 协同的同面-异面反应途径需要经过一个能量很高的过渡态; 3)有利的反应途径是包含了双自由基中间体的分步过程。     

Lewis碱稳定的硼代苯与亲二烯体的Diels-Alder反应的理论研究

采用密度泛函理论方法在B3LYP/6-31G(d)水平上研究了Lewis碱稳定的硼代苯与一些亲二烯体的两种可能的Diels-Alder反应的微观机理和势能剖面, 并研究了反应的溶剂效应和取代基效应. 计算结果表明, 一部分反应以直接的近同步的协同方式进行, 而在另一部分反应中, 两个反应物分子先形成分子间复合物, 然后再经过协同的过渡态生成产物. 与气相中相比, 二氯甲烷溶剂使所研究的大部分反应的活化能垒有所增加. 在乙炔或乙烯分子中分别引入吸电子基团CO2Me或CN能显著降低反应的活化能垒. 形成一个C—B键的杂Diels-Alder反应都比相应的Diels-Alder反应在热力学和动力学上容易进行, 这与实验结果一致.     

烯酮及取代烯酮与环戊二烯环加成反应机理的理论研究

利用半经验分子轨道理论AM1方法,研究了烯酮及取代烯酮与环戊二烯环加成反应机理。采用Berny梯度法优化得到各反应的过渡态和中间体,并进行了振动分析确认。计算结果表明,该环加成反应是按照协同的非同步途径进行的,经过一个四元环发生扭曲的过渡态,并有部分电荷从环戊二烯迁移到烯酮或取代烯酮上,前线轨道分析表明反应机理为“2×[1+1]”机理;而氯甲基取代的烯酮与环戊二烯的环加成反应是按照分步途径发生的。计算结果可以很好地说明实验所观察到的立体选择性,并根据烯酮上取代基的电子效应和位阻效应对反应机理的影响进行了分析。     

N-亚苯基氨基酰胺气相高温分解反应机理的密度泛函研究

利用密度泛函理论研究了N-亚苯基氨基酰胺气相高温分解生成苯腈和苯甲酰胺的反应机理。首先用B3LYP／6-31G(d,p)方法优化反应中反应物、过渡态、中间体及产物的几何构型,通过振动分析确认了过渡态的结构,并通过内稟反应坐标方法(IRC)确认能量最低的反应途径。本文报道了三条可能的反应通道,包括一条直接协同高温分解反应和两条先成环后协同高温分解反应途径,其中直接协同高温分解反应由于能垒低,因此发生的几率较大     

N-亚苯基氨基酰胺气相高温分解反应机理的密度泛函研究

利用密度泛函理论研究了N-亚苯基氨基酰胺气相高温分解生成苯腈和苯甲酰胺的反应机理。首先用B3LYP／6-31G(d,p)方法优化反应中反应物、过渡态、中间体及产物的几何构型,通过振动分析确认了过渡态的结构,并通过内稟反应坐标方法(IRC)确认能量最低的反应途径。本文报道了三条可能的反应通道,包括一条直接协同高温分解反应和两条先成环后协同高温分解反应途径,其中直接协同高温分解反应由于能垒低,因此发生的几率较大     

乙炔与氢化锂单体和二聚体加成反应中的HOMO-HOMO相互作用

用前线分子轨道分析乙炔与氢化锂单体和二聚体加成反应的过渡态.结果表明,在其反应过渡态形成过程中,反应物间的HOMO-LUMO和HOMO-HOMO相互作用均起重要作用.较强烈的HOMO-HOMO相互作用与过渡态附近反应由亲电性加成向亲核性加成转变相关,该反应特性的转变又与LiH中的氢原子在反应中的电荷转移相关.     

CH2 CHF与臭氧反应机理的量子化学研究

用量子化学MP2方法,在6-311+ +G(d,p)基组水平上研究了烯烃CH2CHF与臭氧反应的机理,对氟代乙烯臭氧化反应Criegee机理进行了系统的计算,全参数优化了反应过程中反应物、中间体、过渡态和产物的几何构型,在QCISD(T)/6-311+ +G(d,p)水平上计算了它们的能量.并对它们进行了频率分析,以确定中间体和过渡态的真实性.研究结果表明,氟代乙烯与臭氧反应沿Criegee机理是可信的、合理的.同时研究还发现,就氟代乙烯与臭氧反应活性而言,其控制步骤的位垒较低,可以说氟代乙烯与臭氧反应活性较强,也就是说氟代乙烯对臭氧的损耗较大.     

β-羟基丙醛基态与激发态氢迁移反应的从头算研究

本文用从头算RHF和UHF方法在3-21G基组上研究了β-羟基丙醛基态和激发态分解为甲醛和乙烯醇的反应机理。优化得到了各反应途径的过渡态和中间体, 其结果为: 基态β-羟基丙醛经过一个六元环过渡态和一个氢键中间体形成产物, 反应属于氢迁移和断键的协同过程; 激发三态β-羟基丙醛的分解途径首先经过一个氢迁移六元环过渡态形成双自由基中间体, 然后该中间体的分解包括两条相互竞争的途径, 它们各自经过一个断碳碳键的过渡态和一个氢键激-基态配合物中间体而形成两类产物, 一类为甲醛的基态和乙烯醇的激发态, 另一类为甲醛的激发态和乙烯醇的基态。激发态反应的两条通道均属于先氢迁移后断键分解的分步过程, 且反应的第二步为速控步骤。计算结果表明, 激发态反应活化位垒都比基态的低。     

烯烃光敏氧化反应机理——Ⅰ.富电子烯烃与单线态氧1,2-加成反应机理的量子化学研究

本文用量子化学(简称量化)的半经验及从头算法对氨基乙烯及乙烯与单线态氧1,2-加成反应机理进行了研究。首先应用MINDO/3辅以Powell法对两个体系各基元过程过渡态进行了优化。得到了与前人不同的结构。而后利用Fukui提出的IRC理论计算了这两个体系整个反应途径。计算结果不但证实我们所得过渡态结构的可靠性,也证实对于富电子烯烃,反应确实经由两性离子中间体。同时基于反应途径对反应全过程进行了详细分析并结合从头算相互作用能的分解对取代基电子效应提出了较为深入合理的理论解释。     

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.     

Oxidation of CO by SO2: a theoretical study

The elementary reaction SO(2) + CO --> CO(2) + SO((3)Sigma) (1) and the subsequent reaction SO((3)Sigma) + CO --> CO(2) + S((3)P) (2) have been studied by the application of the Gaussian-3//B3LYP quantum chemical approach to characterize the potential energy surfaces and transition state kinetic analysis to derive rate coefficients. Reaction 1 is found to take place via two transition states (TS), a cis-OSOCO TS and a trans-OSOCO TS. Reaction via the cis-TS is concerted and takes place on a singlet surface. Intersystem crossing to the final products occurs after passage through the barrier on the singlet surface. The trans-TS leads to a very weakly bound singlet OSOCO intermediate that then passes through a second TS (on the triplet surface) to form the products. Reaction 2 takes place on triplet surfaces. There is a concerted reaction through a cis-SOCO TS and a weakly bound trans-SOCO has also been identified. Reaction 2 is analogous to the reaction CO + O(2)((3)Sigma) --> CO(2) + O((3)P) (3), and this reaction has been reinvestigated at a similar level of theory and the rate coefficient derived by quantum chemistry is compared with experiment. The sensitive effects of trace impurities such as H(2), H(2)O, and hydrocarbons on the accurate experimental determination of the rate coefficient of reaction 3 is discussed. Using rate coefficients for reactions 1 and 2 obtained via quantum chemical calculations, we have been unable to model the extent of decomposition of SO(2) measured in a shock tube study of reaction between SO(2) and CO [Bauer, S. H.; Jeffers, P.; Lifshitz, A.; Yadava, B. P. Proc. Combust. Inst. 1971, 13, 417]. In light of the known sensitivity of reaction 3 to trace impurities, we have incorporated trace amounts of H(2), CH(4), or H(2)O, together with our rate coefficients for (1) and (2), in a kinetic model of Alzueta et al. [Combust. Flame 2001, 127, 2234], which is then shown to be able to substantially model the SO(2) data of Bauer et al. In the course of this modeling study we also computed heats of formation for a number of sulfur-containing small molecules: HS, HSO, HSOH, HOSO, HS(2), HSO(2), HOSO(2), HOSOH, and HOSHO.     

Energy-flow dynamics in the molecular channel of propanal photodissociation, C2H5CHO --> C2H6 + CO: direct ab initio molecular dynamics study

Direct ab initio molecular dynamics calculations have been carried out for the molecular channel of the photodissociation of propanal, C2H5CHO --> C2H6 + CO, at the RMP2(full)/cc-pVDZ level of ab initio molecular orbital theory. The initial conditions were generated using the microcanonical sampling to put the excess energy randomly into all vibrational modes of the TS. Starting from the TS, a total of approximately 700 trajectories were numerically integrated for 100 fs. The obtained final energy distributions for the C2H6 and CO fragments and their relative translational motion were found to be quite similar to those obtained for the acetaldehyde reaction, CH3CHO --> CH4 + CO, in our previous study (Chem. Phys. Lett. 2006, 421, 549) despite the fact that the number of degree of freedom for C2H6 is larger than that for CH4. The coupling between the intrinsic reaction coordinate and one of the generalized normal modes orthogonal to it was predicted substantially strong around s = 1.4 amu(1/2) bohr, and it is expected that the energy flow out of C2H6 proceeds through this coupling. However, the obtained energy distributions strongly suggest that the coupling among the modes in C2H6 is quite small and the intramolecular energy redistribution does not occur efficiently in this molecule.     

Infrared chemiluminescence study of CO2 formation in CO + NO reaction on Pd(110) and Pd(111) surfaces

Infrared (IR) chemiluminescence studies of CO2 formed during steady-state CO + NO reaction over Pd(110) and Pd(111) surfaces were carried out. Kinetics of the CO + NO reaction were studied over Pd(110) using a molecular-beam reaction system in the pressure range of 10-2-10-1 Torr. The activity of the CO + NO reaction on Pd(110) was much higher than that of Pd(111), which was quite different from the result of other experiments under a higher pressure range. On the basis of the experimental data on the dependence of the reaction rate on CO and NO pressures and the reaction rate constants obtained by using a reaction model, the coverage of NO, CO, N, and O was calculated under various flux conditions. From the analysis of IR emission spectra in the CO + O2 reaction on Pd(110) and Pd(111), the antisymmetric vibrational temperature (TVAS) was seen to be higher than the bending vibrational temperature (TVB) on Pd(110). In contrast, TVB was higher than TVAS on Pd(111). These behaviors suggest that the activated complex for CO2 formation is more bent on Pd(111) than that on Pd(110), which is reflected by the surface structure. Both TVB and TVAS for the CO + O2 reaction on Pd(110) and Pd(111) increased gradually with increasing surface temperature (TS). On the other hand, in the case of the CO + NO reaction on Pd(110) and Pd(111), TVAS decreased and TVB increased significantly with increasing TS. TVB was lower than TVAS at lower TS, while TVB was higher than TVAS at higher TS. Comparison of the data obtained for the two reactions indicates that TVB in the CO + NO reaction on Pd(110) at TS = 800 and 850 K is much higher than that in the CO + O2 reaction on Pd(110).     

Ab initio potential energy surfaces of the ion-molecule reaction: C2H2 + O+

High level ab initio calculations using complete active space self-consistent field and multi reference single and double excitation configuration interaction methods with cc-pVDZ (correlation consistent polarized valence double zeta) and cc-pVTZ (triple zeta) basis sets have been performed to elucidate the reaction mechanism of the ion-molecule reaction, C2H2(1Sigmag+) + O+(4S), for which collision experiment has been performed by Chiu et al. [J. Chem. Phys. 109, 5300 (1998)]. The minor low-energy process leading to the weak spin-forbidden product C2H2+ (2Piu) + O(1D) has been studied previously and will not be discussed here. The major pathways to form charge-transfer (CT) products, C2H2+ (2Piu) + O(3P) (CT1) and C2H2+ (4A2) + O(3P) (CT2), and the covalently bound intermediates are investigated. The approach of the oxygen atom cation to acetylene goes over an energy barrier TS1 of 29 kcal/mol (relative to the reactant) and adiabatically leads the CT2 product or a weakly bound intermediate Int1 between CT2 products. This transition state TS1 is caused by the avoided crossing between the reactant and CT2 electronic states. As the C-O distance becomes shorter beyond the above intermediate, the C1 reaction pathway is energetically more favorable than the Cs pathway and goes over the second transition state TS2 of a relative energy of 39 kcal/mol. Although this TS connects diabatically to the covalent intermediate Int2, there are many states that interact adiabatically with this diabatic state and these lead to the other charge-transfer product CT1 via either of several nonadiabatic transitions. These findings are consistent with the experiment, in which charge transfer and chemical reaction products are detected above 35 and 39 kcal/mol collision energies, respectively.     

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     

Optimizing the structures of minimum and transition state on the free energy surface

Presented here is the application of a scheme for optimizing the structures of minima and transition states on the free energy surface (FES) for a path along a fixed reaction coordinate with the aid of ab initio molecular dynamics (AIMD) simulation. In the direction of the reaction coordinate, the values corresponding to the stationary points were optimized using the quasi-Newton method, in which the gradient of the free energy along the reaction coordinate was obtained by a constraint AIMD method, and the Bofill Hessian update scheme was used. The equilibrium values for the other directions were taken as the corresponding averages in the dynamic simulation. This scheme was applied to several elementary bimolecular addition reactions: (A) BH(3) + H(2)O --> H(2)O.BH(3); (B) BF(3) + NH(3) --> FB(3).NH(3); (C) SO(3) + NH(3) --> O(3)S.NH(3); (D) C(2)H(4) + CCl(2) --> H(4)C(2).CCl(2); (E) Ni(NH(2))(2) + PH(3) --> (NH(2))(2)Ni.PH(3); (F) W(CO)(5) + CO --> W(CO)(6). For reactions A, B, C, and F, no transition state (TS) exists on the potential energy surface (PES). However there is a TS on the FES. This stems from the curvature difference of the PES and -TDeltaS as a function of the reaction coordinate. For all reactions, it is found that the TS shifts toward the complexation product with increasing temperature because of the curvature increase of -TDeltaS. The equilibrium bond distances for the inactive coordinates perpendicular to the reaction coordinate always increase with temperature, which is due to the thermal excitation and anharmonicity of the PES.     

C(3P)与H2S反应的反应机理

The mechanisms of the C(3P)+H 2S→HCS+H and C(3P)+H 2S → HSC+H reactions have been studied at the UMP2/6-31G(d,p),UMP2/6-311G(d,p),and G2 levels, and six transition states and three intermediates have been located along the reaction paths. The predicted path for the C(3P)+H2S→HCS+H reaction is: C(3P)+H2S→IM1→TS1→IM2→TS4→HCS+H, in line with the reaction process suggested by Lee et al. [1] in which only the intermediates were given. Our energetic results indicate that the C(3P)+H2S→HCS+H reaction is more favorable than the C(3P)+H 2S→HSC+H reaction, in agreement with experiment.     

The reaction of phenyl radical with molecular oxygen: a G2M study of the potential energy surface

Ab initio G2M calculations have been performed to investigate the potential energy surface for the reaction of C6H5 with O2. The reaction is shown to start with an exothermic barrierless addition of O2 to the radical site of C6H5 to produce phenylperoxy (1) and, possibly, 1,2-dioxaspiro[2.5]octadienyl (dioxiranyl, 8) radicals. Next, 1 loses the terminal oxygen atom to yield the phenoxy + O products (3) or rearranges to 8. The dioxiranyl can further isomerize to a seven-member ring 2-oxepinyloxy radical (10), which can give rise to various products including C5H5 + CO2, pyranyl + CO, o-benzoquinone + H, and 2-oxo-2,3-dihydrofuran-4-yl + C2H2. Once 10 is produced, it is unlikely to go back to 8 and 1, because the barriers separating 10 from the products are much lower than the reverse barrier from 10 to 8. Thus, the branching ratio of C6H5O + O against the other products is mostly controlled by the critical transition states between 1 and 3, 1 and 8, and 8 and 10. According to the calculated barriers, the most favorable product channel for the decomposition of 10 is C5H5 + CO2, followed by pyranyl + CO and o-benzoquinone + H. Since C6H5O + O and C5H5 + CO2 are expected to be the major primary products of the C6H5 + O2 reaction and thermal decomposition of C6H5O leads to C5H5 + CO, cyclopentadienyl radicals are likely to be the major product of phenyl radical oxidation, and so it results in degradation of the six-member aromatic ring to the five-member cyclopentadienyl ring. Future multichannel RRKM calculations of reaction rate constants are required to support these conclusions and to quantify the product branching ratios at various combustion conditions.     

First principle and ReaxFF molecular dynamics investigations of formaldehyde dissociation on Fe(100) surface

Detailed formaldehyde adsorption and dissociation reactions on Fe(100) surface were studied using first principle calculations and molecular dynamics (MD) simulations, and results were compared with available experimental data. The study includes formaldehyde, formyl radical (HCO), and CO adsorption and dissociation energy calculations on the surface, adsorbate vibrational frequency calculations, density of states analysis of clean and adsorbed surfaces, complete potential energy diagram construction from formaldehyde to atomic carbon (C), hydrogen (H), and oxygen (O), simulation of formaldehyde adsorption and dissociation reaction on the surface using reactive force field, ReaxFF MD, and reaction rate calculations of adsorbates using transition state theory (TST). Formaldehyde and HCO were adsorbed most strongly at the hollow (fourfold) site. Adsorption energies ranged from ?22.9 to ?33.9 kcal/mol for formaldehyde, and from ?44.3 to ?66.3 kcal/mol for HCO, depending on adsorption sites and molecular direction. The dissociation energies were investigated for the dissociation paths: formaldehyde → HCO + H, HCO → H + CO, and CO → C + O, and the calculated energies were 11.0, 4.1, and 26.3 kcal/mol, respectively. ReaxFF MD simulation results were compared with experimental surface analysis using high resolution electron energy loss spectrometry (HREELS) and TST based reaction rates. ReaxFF simulation showed less reactivity than HREELS observation at 310 and 523 K. ReaxFF simulation showed more reactivity than the TST based rate for formaldehyde dissociation and less reactivity than TST based rate for HCO dissociation at 523 K. TST‐based rates are consistent with HREELS observation. © 2013 Wiley Periodicals, Inc.