Ab initio chemical kinetics for reactions of H atoms with SiHx (x = 1–3) radicals and related unimolecular decomposition processes

Hydrogen atoms and SiH_x (x = 1–3) radicals coexist during the chemical vapor deposition (CVD) of hydrogenated amorphous silicon (a‐Si:H) thin films for Si‐solar cell fabrication, a technology necessitated recently by the need for energy and material conservation. The kinetics and mechanisms for H‐atom reactions with SiH_x radicals and the thermal decomposition of their intermediates have been investigated by using a high high‐level ab initio molecular‐orbital CCSD (Coupled Cluster with Single and Double)(T)/CBS (complete basis set extrapolation) method. These reactions occurring primarily by association producing excited intermediates, ¹SiH₂, ³SiH₂, SiH₃, and SiH₄, with no intrinsic barriers were computed to have 75.6, 55.0, 68.5, and 90.2 kcal/mol association energies for x = 1–3, respectively, based on the computed heats of formation of these radicals. The excited intermediates can further fragment by H₂ elimination with 62.5, 44.3, 47.5, and 56.7 kcal/mol barriers giving ¹Si, ³Si, SiH, and ¹SiH₂ from the above respective intermediates. The predicted heats of reaction and enthalpies of formation of the radicals at 0 K, including the latter evaluated by the isodesmic reactions, SiH_x + CH₄ = SiH₄ + CH_x, are in good agreement with available experimental data within reported errors. Furthermore, the rate constants for the forward and unimolecular reactions have been predicted with tunneling corrections using transition state theory (for direct abstraction) and variational Rice–Ramsperger–Kassel–Marcus theory (for association/decomposition) by solving the master equation covering the P,T‐conditions commonly employed used in industrial CVD processes. The predicted results compare well experimental and/or computational data available in the literature. © 2013 Wiley Periodicals, Inc.     

Identification of decomposition reactions for HMDSO organosilicon using quantum chemical calculations

Hexamethyldisiloxane [HMDSO, (CH₃)₃-SiOSi-(CH₃)₃] is an important precursor for SiO₂ formation during flame-based silica material synthesis. As a result, HMDSO reactions in flame have been widely investigated experimentally, and many results have indicated that HMDSO decomposition reactions occur very early in this process. In this paper, quantum chemical calculations are performed to identify the initial decomposition of HMDSO and its subsequent reactions using the density functional theory at the level of B3LYP/6-311+G (d, p). Four reaction pathways—(a) Si O bond dissociation of HMDSO, (b) Si C bond dissociation of HMDSO, (c) dissociation and recombination of Si O and Si C bonds, and (d) elimination of a methane molecule from HMDSO—have been examined and identified. From the results, it is found that the barrier of 84.38 kcal/mol and Si O bond dissociation energy of 21.55 kcal/mol are required for the initial decomposition reaction of HMDSO in the first pathway, but the highest free energy barrier (100.69 kcal/mol) is found in the third reaction pathway. By comparing the free energy barriers and reaction rate constants, it is concluded that the most possible initial decomposition reaction of HMDSO is to eliminate the CH₃ radical by Si C bond dissociation.     

Tautomeric preferences and electron delocalization in biurets,thiobiurets, and dithiobiurets: An ab initio study

In several literature reports biuret and its sulfur analogs are reported to exist in their diketo form with general formula H₂N? CX? NH? CY? NH₂ (X = O, Y = O, biuret; X = Y = S, dithiobiuret; and X = O, Y = S, thiobiuret). On the other hand, recently reported results on the electronic structure of biguanide analogs (X = Y = NH)demonstrated that a form equivalent to diketo is not the preferred structure. Thus, a systematic ab initio study on the tautomeric preferences of biuret and its sulfur analogs (dithiobiuret and thiobiuret) has been carried out. The results indicate that an interplay of conjugative stabilization and intramolecular hydrogen bonding to play a role in tautomeric preferences. Energy and geometric parameters, natural bond orbital analyses have been employed to understand the chemistry of the title compounds. The results indicate that unlike biguanides, these compounds prefer diketo forms containing hydrogen on the bridging nitrogen (N4) and in a trans‐arrangement (1a–4a). However, tautomerization of these keto forms to the corresponding enol isomers was also found to be highly probable. © 2008 Wiley Periodicals, Inc. Int J Quantum Chem, 2008     

An ab initio potential energy surface for the υ4 vibrational mode of hydrogen peroxide

In this paper, the levels and the torsional microstates of hydrogen peroxide are determined from fully optimized ab initio calculations using a nuclear model in one dimension. Calculations have been performed at the MP2 level with the 6-311 G(2df,2pd), 6-31 1+G(2df,2pd), cc-pVTZ and AUG-cc-pVTZ basis sets including polarization orbitals and diffuse functions. The most stable conformation, calculated with the MP2/AUG-cc-pVTZ approach, is a trans–gauche conformer lying at 67.5° from the trans structure. By using the same level of calculations, the heights of the trans and cis barriers have been determined to be 386.5 and 2643.8 cm⁻¹ in a good agreement with the experimental data. The variational torsional levels split into four components by the tunnelling effect of the barriers. The splitting of the fundamental level caused by the trans barrier has been found to be 11.8683 cm⁻¹, whereas the splitting caused by the cis barrier is insignificant under n=2. Current ab initio energies confirm the experimental assignments and verify the separability of the torsion from the rest of the vibrations. However, the experimental relation of dependence on the torsion of the rotational constants cannot be reproduced in one-dimension and depends on several additional vibrational effects.     

Ab initio study of the potential energy surface and product branching ratios for the reaction of O(1D) with CH3CH2Br

The potential energy surface of O(¹D) + CH₃CH₂Br reaction has been studied using QCISD(T)/6‐311++G(d,p)//MP2/6‐311G(d,p) method. The calculations reveal an insertion‐elimination reaction mechanism of the title reaction. The insertion process has two possibilities: one is the O(¹D) inserting into C? Br bond of CH₃CH₂Br producing one energy‐rich intermediate CH₃CH₂OBr and another is the O(¹D) inserting into one of the C? H bonds of CH₃CH₂Br producing two energy‐rich intermediates, IM1 and IM2. The three intermediates subsequently decompose to various products. The calculations of the branching ratios of various products formed though the three intermediates have been carried out using RRKM theory at the collision energies of 0, 5, 10, 15, 20, 25, and 30 kcal/mol. CH₃CH₂O + Br are the main decomposition products of CH₃CH₂OBr. CH₃COH + HBr and CH₂CHOH + HBr are the main decomposition products for IM1; CH₂CHOH + HBr are the main decomposition products for IM2. As IM1 is more stable and more likely to form than CH₃CH₂OBr and IM2, CH₃COH + HBr and CH₂CHOH + HBr are probably the main products of the O(¹D) + CH₃CH₂Br reaction. Our computational results can give insight into reaction mechanism and provide probable explanations for future experiments. © 2009 Wiley Periodicals, Inc. Int J Quantum Chem, 2011     

CH自由基和NO~2反应研究: II.反应的动力学计算

采用MP2(FULL)/6-31G(d)方法从动力学计算上探讨了CH自由基与NO~2反应的可能途径,找到了反应物,中间体及产物之间的能量通道和过渡态,报道了它们的构型、电子态及能量。并通过频率分析和IRC方法对所有的过渡态进行了验证。在此基础上求出了各步反应的活化能。在以前热力学研究的基础上,对于可能的反应通道进一步作了动力学分析,找到了反应主产物通道的分支比,与实验得到的分支比基本吻合。     

Au_n团簇催化水煤气变换反应机理的密度泛函理论研究

利用密度泛函理论(DFT)研究了Au10、Au13和Au20三类团簇的稳定性和对水煤气变换(WGSR)反应的催化活性,考察了各物质在Aun团簇上的吸附行为和微观反应机理。结果表明,三类Aun团簇的稳定性顺序为Au10Au13Au20,而Aun团簇中电子离域性及吸附能力大小趋势为Au13Au10Au20。在三类Aun团簇上,水煤气变换反应的控速步骤均为H2O的解离,但其反应机理路径有所不同。Au10团簇上为羧基机理,COOH*中间体直接解离;Au13团簇上为氧化还原机理,两个OH*发生歧化反应;Au20团簇上为羧基机理,COOH*和OH*发生歧化反应。通过对三类团簇上的最佳反应路径进行比较发现,Au13团簇在低温下具有较好的催化活性。     

Automated reaction path searches for spin‐forbidden reactions

Many catalytic and biomolecular reactions containing transition metals involve changes in the electronic spin state. These processes are referred to as “spin‐forbidden” reactions within nonrelativistic quantum mechanics framework. To understand detailed reaction mechanisms of spin‐forbidden reactions, one must characterize reaction pathways on potential energy surfaces with different spin states and then identify crossing points. Here we propose a practical computational scheme, where only the lowest mixed‐spin eigenstate obtained from the diagonalization of the spin‐coupled Hamiltonian matrix is used in reaction path search calculations. We applied this method to the ^6,4FeO⁺ + H₂ → ^6,4Fe⁺ + H₂O, ^6,4FeO⁺ + CH₄ → ^6,4Fe⁺ + CH₃OH, and ⁷Mn⁺ + OCS → ⁵MnS⁺ + CO reactions, for which crossings between the different spin states are known to play essential roles in the overall reaction kinetics. © 2018 Wiley Periodicals, Inc.     

Mechanism and rate constants of the CH2 + CH2CO reactions in triplet and singlet states: A theoretical study

Ab initio and density functional CCSD(T)-F12/cc-pVQZ-f12//B2PLYPD3/6-311G** calculations have been performed to unravel the reaction mechanism of triplet and singlet methylene CH₂ with ketene CH₂CO. The computed potential energy diagrams and molecular properties have been then utilized in Rice–Ramsperger–Kassel–Marcus-Master Equation (RRKM-ME) calculations of the reaction rate constants and product branching ratios combined with the use of nonadiabatic transition state theory for spin-forbidden triplet-singlet isomerization. The results indicate that the most important channels of the reaction of ketene with triplet methylene lead to the formation of the HCCO + CH₃ and C₂H₄ + CO products, where the former channel is preferable at higher temperatures from 1000 K and above. In the C₂H₄ + CO product pair, the ethylene molecule can be formed either adiabatically in the triplet electronic state or via triplet-singlet intersystem crossing in the singlet electronic state occurring in the vicinity of the CH₂COCH₂ intermediate or along the pathway of CO elimination from the initial CH₂CH₂CO complex. The predominant products of the reaction of ketene with singlet methylene have been shown to be C₂H₄ + CO. The formation of these products mostly proceeds via a well-skipping mechanism but at high pressures may to some extent involve collisional stabilization of the CH₃CHCO and cyclic CH₂COCH₂ intermediates followed by their thermal unimolecular decomposition. The calculated rate constants at different pressures from 0.01 to 100 atm have been fitted by the modified Arrhenius expressions in the temperature range of 300–3000 K, which are proposed for kinetic modeling of ketene reactions in combustion. © 2018 Wiley Periodicals, Inc.     

Theoretical study and rate constants calculation for the reactions of SiH3 radical with SiH3CH3 and SiH2(CH3)2

The multiple‐channel reactions SiH₃ + SiH₃CH₃ → products and SiH₃ + SiH₂(CH₃)₂ → products are investigated by direct dynamics method. The minimum energy path (MEP) is calculated at the MP2/6‐31+G(d,p) level, and energetic information is further refined by the MC‐QCISD method. The rate constants for individual reaction channels are calculated by the improved canonical variational transition state theory (ICVT) with small‐curvature tunneling (SCT) correction over the temperature range of 200–2400 K. The theoretical three‐parameter expression k₁(T) = 2.39 × 10⁻²³T^4.01exp(−2768.72/T) and k₂(T) = 9.67 × 10⁻²⁷T^4.92exp(−2165.15/T) (in unit of cm³ molecule⁻¹ s⁻¹) are given. Our calculations indicate that hydrogen abstraction channel from SiH group is the major channel because of the smaller barrier height among eight channels considered. © 2009 Wiley Periodicals, Inc. J Comput Chem 2010     

Theoretical study and rate constants calculation for the reactions X + CF3CH2OCF3 (X = F,Cl, Br)

The multiple‐channel reactions X + CF₃CH₂OCF₃ (X = F, Cl, Br) are theoretically investigated. The minimum energy paths (MEP) are calculated at the MP2/6‐31+G(d,p) level, and energetic information is further refined by the MC‐QCISD (single‐point) method. The rate constants for major reaction channels are calculated by canonical variational transition state theory (CVT) with small‐curvature tunneling (SCT) correction over the temperature range 200–2000 K. The theoretical three‐parameter expressions for the three channels k_1a(T) = 1.24 × 10^?15T^1.24exp(?304.81/T), k_2a(T) = 7.27 × 10^?15T^0.37exp(?630.69/T), and k_3a(T) = 2.84 × 10^?19T^2.51 exp(?2725.17/T) cm³ molecule^?1 s^?1 are given. Our calculations indicate that hydrogen abstraction channel is only feasible channel due to the smaller barrier height among five channels considered. © 2011 Wiley Periodicals, Inc. J Comput Chem, 2012     

An ab initio study of water clusters in gas phase and bulk aqueous media: (H2O)n,n=1–12

Geometries of several clusters of water molecules including single minimum energy structures of n‐mers (n=1–5), several hexamers and two structures of each of heptamer to decamer derived from hexamer cage and hexamer prism were optimized. One structural form of each of 11‐mer and 12‐mer were also studied. The geometry optimization calculations were performed at the RHF/6‐311G* level for all the cases and at the MP2/6‐311++G** level for some selected cases. The optimized cluster geometries were used to calculate total energies of the clusters in gas phase employing the B3LYP density functional method and the 6‐311G* basis set. Frequency analysis was carried out in all the cases to ensure that the optimized geometries corresponded to total energy minima. Zero‐point and thermal free energy corrections were applied for comparison of energies of certain hexamers. The optimized cluster geometries were used to solvate the clusters in bulk water using the polarized continuum model (PCM) of the self‐consistent reaction field (SCRF) theory, the 6‐311G* basis set, and the B3LYP density functional method. For the cases for which MP2/6‐311++G** geometry optimization was performed, solvation calculations in water were also carried out using the B3LYP density functional method, the 6‐311++G** basis set, and the PCM model of SCRF theory, besides the corresponding gas‐phase calculations. It is found that the cage form of water hexamer cluster is most stable in gas phase among the different hexamers, which is in agreement with the earlier theoretical and experimental results. Further, use of a newly defined relative population index (RPI) in terms of successive total energy differences per water molecule for different cluster sizes suggests that stabilities of trimers, hexamers, and nonamers in gas phase and those of hexamers and nonamers in bulk water would be favored while those of pentamer and decamer in both the phases would be relatively disfavored. © 2001 John Wiley & Sons, Inc. Int J Quant Chem 81: 90–104, 2001     

CH3CH2,CH3CHCl,CH3CCl2与NO2反应微观动力学理论研究

运用量子化学密度泛函理论UB3LYP/6-311+G*和高级电子相关校正的偶合簇(CCSD(T)/6-311+G*)方法,对CH3CH2,CH3CHCl和CH3CCl2自由基与NO2反应的机理和动力学进行了理论研究,得到了体系的势能面信息和可能的反应机理.根据计算得到的各反应热力学参数及反应能垒,采用传统过渡态理论计算了各反应在温度T=298 K和T=700 K时的速率常数.研究结果表明,该类反应均通过1个中间体和1个过渡态生成产物,产物分别为CH3CHO+HNO,CH3CHO+ClNO和CH3CClO+ClNO.     

Direct ab initio dynamics study on the rate constants and kinetic isotope effect for the reactions of H atoms with GeDn(CH3)4-n (n = 1-4)

Direct ab initio dynamic calculations are performed on the reactions of atomic hydrogen with GeD(n)(CH(3))(4-n) (n = 1-4) over the temperature range 200-2000 K at the PMP4SDTQ/6-311 +G(3df,2p)//MP2/6-31 +G(d) (for n = 2-4) and G2//MP2/6-31 +G(d) (for n = 1) levels. The corresponding k(H)/k(D) ratios are then calculated in order to determine the kinetic isotope effect for the four reactions. For the simplest GeD(4) +H reaction, the only one that has available experimental data, the calculated canonical variational transition state theory incorporates small-curvature tunneling correction (CVT/SCT) thermal rate constants, and the k(H)/k(D) values are in good agreement with the experimental values within the experimental temperature range 293-550 K. For the four GeD(n)(CH(3))(4-4) (n = 1-4) reactions, the variational effect is small over the whole temperature range, whereas the small-curvature effect is important in the lower temperature range. Finally, the overall rate constants are fitted to the three-parameter expression over the whole temperature range 200-2000 K as 5.8 x 10(8)T(1.68)exp(-929/T), 1.7 x 10(8)T(1.80)exp(-691/T), 2.58 x 10(8)T(1.71)exp(-706/T), and 1.0 x 10(7)T(2.08)exp(-544/T) cm(3) mol(-1) s(-1) for the n = 4, 3, 2, and 1 reactions. Our work may represent the first theoretical study of the kinetic isotope effect for the H-attack on the G-H bonding.     

The form of the rate constant for elementary reactions at equilibrium from MD: framework and proposals for thermokinetics

The rates of formation and concentration distributions of a dimer reaction showing hysteresis behavior are examined in an ab initio chemical reaction designed as elementary and where the hysteresis structure precludes the formation of transition states (TS) with pre-equilibrium and internal sub-reactions. It was discovered that the the reactivity coefficients, defined as a measure of departure from the zero density rate constant for the forward and backward steps had a ratio that was equal to the activity coefficient ratio for the product and reactant species. This surprising result, never formally incorporated in elementary rate expressions over approximately one and a half centuries of quantitative chemical kinetics measurement and calculation is accepted axiomatically and leads to an outline of a theory for the form of the rate constant, in any one given substrate—here the vacuum state. A major deduction is that the long-standing definition of the rate constant for elementary reactions is not complete and is nonlinear, where previous works almost always implicitly refer to the zero density limit for strictly irreducible elementary reactions without any attending concatenation of side-reactions. This is shown directly from MD simulation, where for specially designed elementary reactions without any transition states, density dependence of reactants and products always feature, in contrast to current practice of writing rate equations. It is argued that the rate constant expression without reactant and product dependence is due to historical conventions used for strictly elementary reactions. From the above observations, a theory is developed with the aid of some proven elementary theorems in thermodynamics, and expressions under different state conditions are derived whereby a feasible experimental and computational method for determining the activity coefficients from the rate constants may be obtained under various approximations and conditions. Elementary relations for subspecies equilibria and its relation to the bulk activity coefficient are discussed. From one choice of reaction conditions, estimates of activity coefficients are given which are in at least semi-quantitative agreement with the data for non-reacting Lennard-Jones (LJ) particles for the atomic component. The theory developed is applied to ionic reactions where the standard Brönsted-Bjerrum rate equation and exceptions to this are rationalized.     

An in situ temperature‐programmed XPS study of the surface chemical reactions of thiophene with Ag/titania

The Ag/titania sorbent for the ultradeep desulfurization of liquid fuels was characterized by electron spin resonance and was found to contain nearly the stoichiometric titania, without significant concentration of Ti³⁺ or the reactive oxygen species. The surface chemical reactions of thiophene adsorbed on the Ag/titania were studied by temperature‐programmed XPS from 25 to 525 °C upon in situ thermal annealing in high vacuum and in situ oxidation by oxygen gas. The titania support is not chemically reactive under those conditions. Silver oxide in the Ag/titania sorbent is converted to Ag₂ S without formation of the transient surface sulfates or sulfites and is further oxidized by molecular oxygen. Copyright © 2010 John Wiley & Sons, Ltd.     

Automated exploration of stable isomers of H(+)(H(2)O)(n) (n = 5-7) via ab initio calculations: An application of the anharmonic downward distortion following algorithm

Isomers of protonated water clusters H(+)(H(2)O)(n) (n = 5-7) have been explored on ab initio potential energy surfaces by means of the anharmonic downward distortion following algorithm. Totally, 9, 24, and 131 isomers for n = 5, 6, and 7, respectively, were located by the automatic exploration, and all of known important isomers previously reported by conventional geometry optimization approaches have been included in the present results. Moreover, structure transitions depending on n and temperature, which were observed by experimental studies, could be reproduced via thermodynamic simulation on the basis of the superposition approach and the present isomer sets.