A quantum chemical study on •Cl-initiated atmospheric degradation of CH2BrO2•

The singlet and triplet potential energy surfaces for the CH₂BrO₂??+??Cl reaction have been researched theoretically. All of the probable reaction routes were investigated by using B3LYP and G3(MP2) models. Addition/elimination and S_N2 displacement exist on the singlet potential energy surfaces (PES), and the foremost approach process of CH₂BrO₂??+??Cl is generating IM1 (CH₂BrOOCl) with no barrier, followed by the O-O bond breaking accompanied by an H-migrate resulting in the most abundant product P1 (CHBrO?+?HClO). One direct H-abstraction and three S_N2 displacement reaction pathways exist on the triplet PES, and direct H-abstraction is the foremost pathway. RRKM-TST theory was employed to predict product distribution of the CH₂BrO₂??+??Cl reaction. At atmospheric pressure, the production of P1 (CHBrO?+?HClO) by addition/elimination dominants the reaction at T?≤?800?K, while the direct H-abstraction takes over the reaction at T?>?800?K. The total rate constants are insensitive to pressure, and the branching rate constants are just the opposite. The lifetime of CH₂BrO₂? in the presence of ?Cl was predicted to 3.2?d. Moreover, time-dependent density functional theory (TDDFT) calculations suggest that IM1 (CH₂BrOOCl), IM2 (CH₂BrOClO) and IM3 (CH₂(OBr)OCl) will photolyze under the sunlight.     

Computational study on mechanisms and pathways of the atmospheric NH2 + IO reaction

ABSTRACT

The potential-energy surfaces of the amino radical (NH₂) with IO reaction have been studied at the CCSD(T)/cc-pVTZ//MP2/6-311++G(d,p) level. Two kinds of pathways are revealed, namely H-abstraction and addition/elimination. Rice–Ramsperger–Kassel–Marcus theory and transition state theory are employed to calculate the overall and individual rate constants over a wide range of temperatures and pressures. It is predicted that, at atmospheric pressure with N2 as bath gas, the formation of P1 (HI?+?HNO) is the dominant pathways at 200–700?K, while the direct H-abstraction leading to P3 (³NH?+?HOI) takes over the reaction at a temperature above 700?K. At the high-pressure limit, IM1 [IONH₂] formed by collisional stabilisation is dominant at 200–700?K; the direct H-abstraction resulting in P3 (³NH?+?HOI) plays an important role at higher temperatures. However, the total rate constants are independence on the pressure; however, the individual rate constants are sensitive to pressure. The atmospheric lifetime of NH₂ in IO is around one week. TD-DFT computations imply that IM1 [IONH₂], IM1A [IONH₂′], IM2 [IN(H₂)O], IM3 [OINH₂], IM4 [HOINH], tra-IM5 [tra-HON(H)I] and cis-IM5 [cis-HON(H)I] will photolyze under the sunlight.     

The gas-phase pyrolysis of methyl azidoformate in the absence and presence of water: a theoretical study

The potential energy surface profiles for the gas-phase pyrolysis of methyl azidoformate (MA, CH₃OC(O)N₃) in the absence and presence of one water molecule have been investigated by ab initio methods at CCSD(T)/6-311++G(2df,2pd)//MP2(full)/6-311++G(d,p)+0.95×ZPE levels of theory. Three types of mechanisms are discussed for the gas-phase decomposition of CH₃OC(O)N₃. Ab initio calculations show that a four-membered-ring intermediate can be formed by the stepwise routes. The resulting intermediate can undergo two competitive decomposition channels to generate the major products CO₂?+?CH₂?=?NH and HNCO?+?HC(O)H. The calculated results are in qualitative agreement with the observed experimental data. However, CH₃ONCO can be produced from the Curtius-type rearrangement route. This is an intriguing finding in this study. Moreover, the effect of one water molecule on the gas-phase pyrolysis of MA has been also explored. We find that the relative energy of the hydrated transition states is effectively lowered when water is added to the reaction. However, the estimated rate constant at 625?K for the naked reaction is about 30 times faster than the reaction with water. Thus, a single water molecule cannot play an important role in the thermal decomposition of MA.     

Mechanistic study of the reaction of methyl peroxy radical (CH3O2) with formaldehyde (CH2O)

ABSTRACT

A direct dynamic study on the reactions of CH₃O₂?+?CH₂O was carried out over the temperature range of 300–1500?K. All stationary points were calculated with the M06-2X/6-311++G(d,p) level of theory and identified for local minimum. The energetic parameters were refined at QCISD (T)/cc-pVTZ and CCSD (T)/cc-pVTZ levels of theory. Three channels were explored and a reaction of hydrogen abstraction from CH₂O by CH₃O₂ was identified as dominant channel which involves the formation of a prereactive complex in the entrance channel. The rate coefficient of the dominant channel was calculated with TST and TST/Eck and the Eckart tunnelling effect is only important over the lower temperature region. The calculated rate coefficient of the dominant channel has positive temperature dependence and agrees reasonably with the available literature data.     

Theoretical study of mechanisms for NCO with CH3 reaction

A detailed computational study has been performed at the QCISD(T)/6-311++G(d,p)//B3LYP/6-311++G(d,p) level for the NCO with CH₃ reaction by constructing singlet and triplet potential energy surfaces (PES). The results show that the title reaction is more favorable for the singlet PES than the triplet PES. On the singlet PES, the dominant channel is the barrierless addition of the O or N atom to the C atom of the methyl group to form CH₃NCO (IM1) and CH₃OCN (IM2). On the triplet PES, the favorable channel is the barrierless addition of the N atom to the C atom of the methyl group to form an intermediate CH₃NCO (³IM2), which then undergoes a N–C bond scission process to give out CH₃N + CO.     

Quantum chemical studies of the OH-initiated oxidation reactions of propenols in the presence of O2

ABSTRACT

The atmospheric oxidation mechanisms of 1- and 2-propenol initiated by OH radical have been theoretically investigated at the CCSD(T)//BH&;HLYP/6-311?+?+G(d,p) level of theory. Conventional transition state theory was employed to predict the rate constants for the initial reaction channels. The calculations clearly indicate that OH-addition channels contribute maximum to the total reaction, both for 1- and 2-propenol, while H-abstraction channels can be neglected at the temperature range of 220–520?K. The calculated total rate constants at 298?K are 1.66?×?10^?11 and 7.69?×?10^?12 cm³?molecule^?1?s^?1 respectively for 1- and 2-propenol, which are in reasonable agreement with the experimental values of similar systems (vinyl ethers?+?OH reactions). The deduced Arrhenius expressions are k(OH?+?1-propenol)?=?1.43?×?10^?12 exp[(743.7?K)/T] and k(OH?+?2-propenol)?=?2.86?×?10^?12 exp[(310.5?K)/T] cm³?molecule^?1?s^?1. Under atmospheric condition, the OH-addition intermediates (CH₃C?HCH(OH)₂, CH₃CH(OH)C?H(OH), CH₃CH(OH)₂?CH₂, CH₃?C(OH)CH₂(OH)) are likely to react rapidly with O₂, the theoretically identified major products for 1-propenol are HCOOH, CH₃CHO and CH₃CH(OH)CHO, and the dominant products for 2-propenol are CH₃COOH, HCHO and CH₃COCH₂OH, both companied with the regeneration of OH and HO₂ radicals (crucial reactive radicals in the atmosphere).     

Reaction mechanism and rate constants of the CH+CH4 reaction: a theoretical study

Ab initio and density functional calculations have been performed to elucidate the mechanism of CH radical insertion into methane. The results show that the reaction can be viewed to occur via two stages. On the first stage, the CH radical approaches methane without large structural changes to acquire proper positioning for the subsequent stage, where H-migration occurs from CH₄ to CH, along with a C–C bond formation. Where the first stage ends and the second begins, a tight transition state was located using the B3LYP/6-311G(d,p) and MP4(SDQ)/6-311++G(d,p) methods. Using a rigid rotor – harmonic oscillator approach within transition state theory, we show that at the MP5/6-311++G(d,p)//MP4(SDQ)/6-311++G(d,p) level the calculated rate constants are in a reasonably good agreement with experiment in a broad temperature range of 145–581 K. Even at low temperatures, the insertion reaction bottleneck is found about the location of the tight transition state, rather than at long separations between the CH and CH₄ reactants. In addition, high level CCSD(T)-F12/CBS calculations of the remainder of the C₂H₅ potential energy surface predict the CH+CH₄ reaction to proceed via the initial insertion step to the ethyl radical which then can emit a hydrogen atom to form highly exothermic C₂H₄+H products.     

Ab initio Study of the Potential Energy Surface and Product Branching Ratios for Reaction of O(1D) with C2H5Cl

用QCISD(T)/6-311++G(d,p)//MP2/6-31G(d,p)方法研究了O(1D)与CH3CH2Cl的反应．计算表明此反应存在一个插入-消去机理．此反应先形成IM1和IM2两个中间体,两个中间体再分解成各种产物．用RRKM理论计算了碰撞能分别为0、20.9、41.8、62.7、83.6、104.5和125.4 kJ/mol时通过IM1、IM2分解的各个通道的分支比率．IM1的主要分解产物是HCl,IM2的主要分解产物是CH2OH．因为IM1比IM2稳定,HCl很可能是反应的主要产物．计算结果能够提供反应机理而且可以对以后的实验提供可能的解释．     

A computational perspective on the kinetics and thermochemistry of the gas phase reactions of 1, 1-dichlorodimethylether (DCDME) with OH radical at 298 K

Kinetics and thermochemistry of the gas phase reactions between CH₃OCHCl₂ (DCDME) and OH radical are investigated theoretically. The geometries and all the stationary points on the potential energy surface are calculated at BHandHLYP/6-311G(d,p) method. The energy information is further refined at CCSD(T)/6-311G(d,p) level of theory. Reaction profiles are modelled including the formation of two pre-reactive and post-complexes. The rate constants, which are evaluated by Canonical Transition State Theory (CTST) including tunnelling correction at 298 K, are in very good agreement with the available experimental data. The percentage contributions of both reaction channels are also reported at 298 K. The hydrogen abstraction reaction from the –CHCl₂ group is found to be dominant leading to the formation of CH₃OCCl₂ + H₂O. Using group-balanced isodesmic reactions, the standard enthalpies of formation for CH₃OCHCl₂, CH₃OCCl₂ and CH₂OCHCl₂ are also reported.     

Isomers of OCS and their reaction with H2O on singlet potential energy surface

The isomers of the carbonyl sulfide (OCS) molecule are investigated in detail at CCSD(T)/cc-pVTZ//MP2/6-311++G(2d,2p) level of theory. One cyclic isomer was identified along with three different linear minima of the OCS molecule. Three interconversion transition states were also located between cyclic and linear forms of OCS. Among these four isomers, the singlet potential energy surface (PES) for the molecule–molecule reaction between the three most energetically favoured isomers of OCS and H₂O has been explored theoretically at the CCSD(T)/cc-pVTZ//MP2/6-311++G(2d,2p) level. This singlet PES comprises of three paths. Path 1 is the reaction of linear OCS molecule with water producing the major product P1 (CO₂?+?H₂S), minor product P2 (S?+?HCOOH) and two isomers via 14 minima and 15 transition states. The Path 2 is an isomerization process in which cyclic isomer of OCS reacts with water molecule via another initial barrierless aduct producing five isomers of the OCS–H₂O system through five interconversion transition states. The reaction of linear COS isomer with water is shown in Path 3. This path produces the radicals SH and COOH from another COS–H₂O complex via a transition state. Among these three products, the product P1 is energetically most favoured. The overall exothermicity of the product channels for the formation of major product P1 on PES is calculated to be about 10.60?kcal/mol possessing initial high entrance barriers of 45.48 and 55.47?kcal/mol in two possible pathways. As the process is favoured thermodynamically but not kinetically, the reaction is expected to be very slow.     

Atmospheric oxidation chemistry of 1–methoxy 2–propyl acetate initiated by OH radicals: kinetics and mechanisms

Kinetics and mechanism of the gas-phase reaction of CH₃C(O)OCH(CH₃)CH₂OCH₃ (MPA) with OH radicals in the presence of O₂ and NO have been investigated theoretically by performing a high and reliable level of theory, viz., CCSD(T)/6-311?+?G(d,p)//BH&HLYP/6-311++G(d,p)?+?0.9335×ZPE. The calculations predict that the H-abstraction from the ?CH₂?O? position of MPA is the most facile channel, which leads to the formation of the corresponding alkoxy radicals CH₃C(O)OCH(CH₃)C(O ?)HOCH₃ under atmospheric conditions. This activated radicals CH₃C(O)OCH(CH₃)C(O ?)HOCH₃ will undergo further rearrangement, fragmentation and oxidative reactions and predominantly leads to the formation of various products (methyl formate HC(O)OCH₃ and acetic anhydride CH₃C(O)OC(O)CH₃). In the presence of water, acetic anhydride can convert into acetic acid CH₃C(O)OH via the hydrolysis reaction. The calculated total rate constants over the temperature range 263–372?K are used to derive a negative activation energy (E_a= ?5.88 kJ/mol) and an pre-exponential factor (A?=?1.78×10^?12 cm³ molecule^?1 s^?1). The obtained Arrhenius parameters presented here are in strong agreement with the experimental values. Moreover, the temperature dependence of the total rate constant over a temperature range of 263?1000?K can be described by k?=?5.60 × 10^?14×(T/298?K)^3.4×exp(1725.7?K/T) cm³ molecule^?1 s^?1.     

Calculation of accurate imaginary frequencies and tunnelling coefficients for hydrogen abstraction reactions using IRCmax

The effect of level of theory on the imaginary frequency and corresponding tunnelling coefficients has been studied for a test set of hydrogen abstraction reactions: ?CH₂X + CH₃Y → CH₃X + ?CH₂Y for (X,Y) = (H,H), (H,CN), (H,F), (H,Li) and (F,Li). It is found that the imaginary frequency is very sensitive to the level of theory used, with Hartree-Fock (HF) methods severely overestimating the imaginary frequency compared with high-level CCSD(T)/6-311G(d,p) calculations. The errors for the other methods are smaller but nonetheless significant, with MP2 methods overestimating the imaginary frequency and density functional theory (DFT) methods underestimating it. In the case of the HF methods, this leads to errors in the tunnelling coefficient of several orders of magnitude, while for the better DFT and MP2 methods errors of a factor of 2–3 are observed. To address this problem, an IRCmax procedure for estimating the imaginary frequency has been developed and it is found that IRCmax imaginary frequencies calculated with CCSD(T)/6-311G(d,p) single points along a low-level HF/6-31G(d) minimum energy path provide excellent approximations to the high-level values, at a fraction of the computational cost.     

Theoretical study on the reactions of CH3NHNH2 with ground state O(3P) atom and excited state O(1D) atom

The reaction mechanisms of methylhydrazine (CH₃NHNH₂) with O(³P) and O(¹D) atoms have been explored theoretically at the MPW1K/6-311+G(d,p), MP2/6-311+G(d,p), MCG3-MPWPW91 (single-point), and CCSD(T)/cc-pVTZ (single-point) levels. The triplet potential energy surface for the reaction of CH₃NHNH₂ with O(³P) includes seven stable isomers and eight transition states. When the O(³P) atom approaches CH₃NHNH₂, the heavy atoms, namely N and C atoms, are the favourable combining points. O(³P) atom attacking the middle-N atom in CH₃NHNH₂ results in the formation of an energy-rich isomer (CH₃NHONH₂) followed by migration of O(³P) atom from middle-N atom to middle-H atom leading to the product P6 (CH₃NNH₂+OH), which is one of the most favourable routes. The estimated major product CH₃NNH₂ is consistent with the experimental measurements. Reaction of O(¹D) + CH₃NHNH₂ presents different features as compared with O(³P) + CH₃NHNH₂. O(¹D) atom will first insert into C–H2, N1–H4, and N2–H5 bonds barrierlessly to form the three adducts, respectively. There are two most favourable paths for O(¹D) + CH₃NHNH₂. One is that the C–N bond cleavage accompanied by a concerted H shift from O atom to N atom (mid-N) leads to the product P_I (CH₂O + NH₂NH₂), and the other is that the N–N bond rupture along with a concerted H shift from O to N (end-N) forms P_IV (CH₃NH₂ + HNO). The similarities and discrepancies between two reactions are discussed.     

Quantum chemical study of the (Z)-2-penten-1-ol (HOCH2–CH = CHCH2CH3) + OH + O2 reactions

ABSTRACT

The mechanism and products of the reaction of (Z)-2-penten-1-ol [(Z)-PO21] with OH radical in the presence of O₂ have been elucidated by using high-level quantum chemical methods CCSD(T)/6-311+G(d,p)//BH&;HLYP/6-311++G(d,p). The calculations clearly indicate that addition channels contribute maximum to the total reaction and H-abstraction channels can be neglected at temperatures of 220–500 K. The rate constant for the reaction of OH radical with (Z)-PO21 at 298 K is computed to be 1.22 × 10^?10 cm³ molecule^?1 s^?1, which is in stronger agreement with the previously reported experimental values. The kinetic data obtained over the temperature range 220?500 K are used to derive an non-Arrhenius expression: k = 3.69 × 10^?13 × exp(1763.7/T) cm³ molecule^?1 s^?1. For the reaction of (Z)-PO21with OH radical in the presence of O₂, the major primary reaction products found in this study are propanal [CH₃CH₂C(O)H] and glycolaldehyde [HOCH₂C(O)H], whereas formaldehyde [HC(O)H], 2-hydroxybutanal [CH₃CH₂CH(OH)C(O)H] and the epoxide P18 are anticipated to be minor products. The calculated results are consistent with the recent experimental observations.     

Molecular structure and IR spectra of bromomethanes by DFT and post-Hartree-Fock MP2 and CCSD(T) calculations

The molecular parameters (geometries, rotational constants, dipole moments) and vibrational IR spectra (harmonic wavenumbers, absolute intensities) of bromomethanes (CH₃Br, CH₂Br₂, CHBr₃, CBr₄) are predicted by a density functional theory with the hybrid Becke3-LYP functional (DFT) and post-Hartree-Fock methods (MP2, CCSD(T)) using a 6-311G(2d,2p)-type basis set. The MP2 calculations are carried out with different numbers of frozen core orbitals to find how the number of bromine orbitals used for electron correlation influences the predicted molecular parameters and IR spectra of the species in question. Three options were used: (a) all electrons (full), with both the core and valence orbitals considered; (b) partial frozen core option (pfc), when the orbitals up to 3p of bromine were frozen; and (c) full frozen core option (ffc), when all core orbitals up to 3d were frozen. The CCSD(T) calculations for geometric parameters were carried out with both the pfc and ffc options, while for the prediction of the IR spectra only the ffc option was used. In addition, the calculations at the DFT and MP2(pfc) levels with inclusion of f functions on carbon and bromine atoms in bromomethanes (and also the CCSD(T)(pfc) calculations for CH₃Br) were carried out to predict the changes in the geometric parameters and/or vibrational IR spectra of the molecules upon inclusion of f functions The geometries of bromomethanes (particularly the CBr bond lengths) are predicted better by the DFT and CCSD(T) calculations when the f functions (in particular on bromine atom) are included, while the MP2 calculations without f functions are good enough for correct predictions of the molecular geometries. The molecular parameters and vibrational IR spectra of bromomethanes in question and their deuterated species predicted by the DFT, MP2(ffc) and CCSD(T)(ffc) with the 6-311G(2d,2p) basis set agree well with the available experimental data.     

Theoretical study of the CH3 + NS and related reactions: mechanism of HCN formation

More than thirty equilibrium and transition structures on the [CH₃NS] potential energy surface have been located using the B3LYP/6-311 + + G(d,p) method. Thioformaldoxime (3) turns out to be the most stable isomer followed by thionitrone (2), a three-membered ring, thionitrosyl methane (1) and thiazyl methane. These isomers are connected to each other by 1,2H and 1,3H shifts and ring—chain rearrangement, but the associated energy barriers are rather high, making most of them stable with respect to unimolecular transformations. Starting from CH₃ + NS, a possible initial atmospheric reaction, HCN formation appears to be the most favoured process through a cascade involvement of 1, 2 and 3. The standard heats of formation, ΔH ⁰ _f,298 are calculated to be: 3, 149kJmol^?1; and 1, 218kJmol^?1; using the CCSD(T)/6-311+ +G(3df,2p) method, with an error of ±10kJ mol^?1.     

Reaction of C2HCl2+O2: Combined TR-FTIR Spectroscopy and Electronic Structure

用时间分辨傅立叶变换红外发射光谱(TR-FTIR)和G3MP2//B3LYP/6-311G(d,p)水平的电子结构计算研究了环境化学中重要的二氯代乙烯自由基C₂HCl₂和O₂分子的基元反应通道和机理. 通过0.5 cm^-1高分辨的TR-FTIR发射光谱观察到三种振动激发态产物CO₂、CO和HCl,由光谱拟合得到CO和HCl的振动态分布,结合电子结构计算的反应势能曲线,提出反应机理和能量上最可能的反     

The catalytic effects of H2CO3, CH3COOH,HCOOH and H2O on the addition reaction of CH2OO + H2O → CH2(OH)OOH

The addition reaction of CH₂OO + H₂O → CH₂(OH)OOH without and with X (X = H₂CO₃, CH₃COOH and HCOOH) and H₂O was studied at CCSD(T)/6-311+ G(3df,2dp)//B3LYP/6-311+G(2d,2p) level of theory. Our results show that X can catalyse CH₂OO + H₂O → CH₂(OH)OOH reaction both by increasing the number of rings, and by adding the size of the ring in which ring enlargement by COOH moiety of X inserting into CH₂OO···H₂O is favourable one. Water-assisted CH₂OO + H₂O → CH₂(OH)OOH can occur by H₂O moiety of (H₂O)₂ or the whole (H₂O)₂ forming cyclic structure with CH₂OO, where the latter form is more favourable. Because the concentration of H₂CO₃ is unknown, the influence of CH₃COOH, HCOOH and H₂O were calculated within 0–30 km altitude of the Earth's atmosphere. The results calculated within 0–5 km altitude show that H₂O and HCOOH have obvious effect on enhancing the rate with the enhancement factors are, respectively, 62.47%–77.26% and 0.04%–1.76%. Within 5–30 km altitude, HCOOH has obvious effect on enhancing the title rate with the enhancement factor of 2.69%–98.28%. However, compared with the reaction of CH₂OO + HCOOH, the rate of CH₂OO···H₂O + HCOOH is much slower.     

CX2+CH2O (X=F, Cl, Br)环加成反应的密度泛函理论计算

用密度泛函理论,在B3LYP/6-311 G(d)水平上研究了CX2 CH2O(X=F,Cl,Br)环加成反应一条三过渡态三中间体路径的反应机理,全参数优化了反应势能面各驻点的几何构型,用内禀反应坐标(IRC)和频率分析方法,对过渡态进行了验证.用高级电子相关校正的耦合簇[CCSD(T)/6-311 G(d)]方法对优化构型进行了单点能计算.采用经Wigner校正的Eyring过渡态理论和热力学方法,研究了该反应通道的热力学及动力学性质.从热力学和动力角度综合分析,该途径CF2与GH2O的环加成反应难以发生,而CCl2及CBr2与CH2O反应的适宜温度范围均为400～1000K,如此,反应既具有较大的自发趋势和平衡常数,又具有较快的反应速率.     

The X2 Pi and A2 sigma states of FCN+ and ClCN+ : ab initio calculations and simulation of the HeI photoelectron spectra of FCN and ClCN

Geometry optimization and harmonic vibrational frequency calculations at the CASSCF, MP2 and CCSD(T) levels with basis sets up to 6-311G(2df) quality were carried out on the X¹Σ⁺states of FCN and ClCN and the X²Π and A²Σ⁺ states of their cations. Adiabatic ionization energies were calculated up to the CCSD(T)/6-311G(3df)//CCSD(T)/6-311G(2d) level. Some B3LYP calculations were performed also for the ground states of the neutral molecules and the cations. Franck-Condon simulations were performed for the first two bands in the He I photoelectron spectra of FCN and ClCN by employing the ab initio computed geometries and frequencies. By comparing the observed and the simulated spectra obtained from different CN and CX (X = F or Cl) ionic bond lengths chosen on the basis of the ab initio computed values, the following structural parameters are obtained for the two lowest-lying states of FCN⁺ and ClCN⁺ (the method of deriving the uncertainties is described):