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     

Theoretical studies of the gas‐phase pyrolysis kinetics of ω‐bromonitriles,ZCH2CH2Br [Z = NC,NCCH2, NCCH2CH2]

The gas‐phase elimination of several ω‐bromonitriles (ZCH₂CH₂Br, Z = NC, NCCH₂, NCCH₂CH₂) has been examined at the MP2/6‐31G(d,p), MP2/6‐31G(2d,2p), B3PW91/6‐31G(2d,2p), and MPW91PW91/6‐31(2p,2d) levels of theory. The bromonitriles yield the corresponding cyano‐olefin and HBr gas in a rate‐determining step. The MPW91PW91/6‐31G(2p,2d) results suggest a concerted mechanism, with a polar, four‐membered transition state. The calculated kinetic and thermodynamic parameters were found to be within reasonable agreement with the experimental determinations. Substituent effects are discussed in terms of electronic structure. © 2008 Wiley Periodicals, Inc. Int J Chem Kinet 41: 168–175, 2009     

DFT investigation of the mechanism of CH2CO + O(3P) reaction

A detailed theoretical survey of the potential energy surface (PES) for the CH₂CO + O(³P) reaction is carried out at the QCISD(T)/6‐311+G(3df,2p)//B3LYP/6‐311+G(d,p) level. The geometries, vibrational frequencies, and energies of all stationary points involved in the reaction are calculated at the B3LYP/6‐311+G(d,p) level. More accurate energy information is provided by single‐point calculations at the QCISD(T)/6‐311+G(3df,2p) level. Relationships of the reactants, transition states, intermediates, and products are confirmed by the intrinsic reaction coordinate (IRC) calculations. The results suggest that P1(CH₂+CO₂) is the most important product. This study presents highlights of the mechanism of the title reaction. © 2005 Wiley Periodicals, Inc. Int J Quantum Chem, 2005     

DFT studies on the multi‐channel reaction of CH3S+NO2

The mechanisms for the reaction of CH₃S with NO₂ are investigated at the QCISD(T)/6‐311++G(d,p)//B3LYP/6‐311++G(d,p) on both single and triple potential energy surfaces (PESs). The geometries, vibrational frequencies, and zero‐point energy (ZPE) correction of all stationary points involved in the title reaction are calculated at the B3LYP/6‐311++G(d,p) level. More accurate energies are obtained at the QCISD(T)/6‐311++G(d,p). The results show that 5 intermediates and 14 transition states are found. The reaction is more predominant on the single PES, while it is negligible on the triple PES. Without any barrier height for the whole process, the main channel of the reaction is to form CH₃SONO and then dissociate to CH₃SO+NO. © 2006 Wiley Periodicals, Inc. Int J Quantum Chem, 2007     

Structures,Vibrational Spectra,and Relative Energetics of CH3COIO3 Isomers

The structures, vibrational spectra, relative energetics, and enthalpies of formation of CH₃COIO₃ isomers have been investigated with B3LYP, B3P86 and B3PW91 methods in conjugation with the 6‐31+G(d), 6‐311+G(d,p) and 6‐311++G(3df,3pd) basis sets. The CH₃COOIO₂ structure was found to be the most stable form among the isomers with an estimated enthalpy of formation of ?314.6 kJ·mol^?1. The enthalpies of formation for CH₃COOOOI, CH₃COOOIO and CH₃COIO₃ are ?180.7, ?184.9 and ?50.6 kJ·mol^?1, respectively. The implication of the formation of CH₃COIO₃ isomers from the atmospheric cross‐reactions of the acetylperoxy (CH₃COO₂) and iodine monoxide (IO) radicals was examined and the possible dissociation products of the most likely CH₃COIO₃ isomers were determined.     

Kinetics of the R + HBr RH + Br (CH3CHBr, CHBr2 or CDBr2) equilibrium. Thermochemistry of the CH3CHBr and CHBr2 radicals

The kinetics of the reaction of the CH₃CHBr, CHBr₂ or CDBr₂ radicals, R, with HBr have been investigated in a temperature-controlled tubular reactor coupled to a photoionization mass spectrometer. The CH₃CHBr (or CHBr₂ or CDBr₂) radical was produced homogeneously in the reactor by a pulsed 248 nm exciplex laser photolysis of CH₃CHBr₂ (or CHBr₃ or CDBr₃). The decay of R was monitored as a function of HBr concentration under pseudo-first-order conditions to determine the rate constants as a function of temperature. The reactions were studied separately from 253 to 344 K (CH₃CHBr + HBr) and from 288 to 477 K (CHBr₂ + HBr) and in these temperature ranges the rate constants determined were fitted to an Arrhenius expression (error limits stated are 1σ + Student’s t values, units in cm³ molecule⁻¹ s⁻¹, no error limits for the third reaction): k(CH₃CHBr + HBr) = (1.7 ± 1.2) × 10⁻¹³ exp[+ (5.1 ± 1.9) kJ mol⁻¹/RT], k(CHBr₂ + HBr) = (2.5 ± 1.2) × 10⁻¹³ exp[−(4.04 ± 1.14) kJ mol⁻¹/RT] and k(CDBr₂ + HBr) = 1.6 × 10⁻¹³ exp(−2.1 kJ mol⁻¹/RT). The energy barriers of the reverse reactions were taken from the literature. The enthalpy of formation values of the CH₃CHBr and CHBr₂ radicals and an experimental entropy value at 298 K for the CH₃CHBr radical were obtained using a second-law method. The result for the entropy value for the CH₃CHBr radical is 305 ± 9 J K⁻¹ mol⁻¹. The results for the enthalpy of formation values at 298 K are (in kJ mol⁻¹): 133.4 ± 3.4 (CH₃CHBr) and 199.1 ± 2.7 (CHBr₂), and for α-C–H bond dissociation energies of analogous compounds are (in kJ mol⁻¹): 415.0 ± 2.7 (CH₃CH₂Br) and 412.6 ± 2.7 (CH₂Br₂), respectively.     

Reaction‐path dynamics and theoretical rate constants for the reaction CH4 + O3 → HOOO + CH3

We present a direct ab initio dynamics study of thermal rate constants of the hydrogen abstraction reaction of CH₄ + O₃ → HOOO +CH₃. The geometries of all the stationary points are optimized at MPW1K/6‐31+G(d,p), MPWB1K/6‐31+G(d,p), and BHandHLYP/6‐31+G(d,p) levels of theory. The energies are refined at a multi‐high‐level method. The extended Arrhenius expression fitted from the CVT/SCT and μVT/Eckart rate constants of ozonolysis of methane in the temperature range 200–2500 K are k^CVT/SCT(T) = 5.96 × 10^?29T^4.49e^(?17321.3/T) and k^μVT/Eckart(T) = 7.92 × 10^?29T^4.46e^(?17301.7/T), respectively. © 2007 Wiley Periodicals, Inc. Int J Quantum Chem, 2007     

Theoretical study on the rate constants for the C2H5 + HBr --> C2H6 + Br reaction

The reaction C(2)H(5) + HBr --> C(2)H(6) + Br has been theoretically studied over the temperature range from 200 to 1400 K. The electronic structure information is calculated at the BHLYP/6-311+G(d,p) and QCISD/6-31+G(d) levels. With the aid of intrinsic reaction coordinate theory, the minimum energy paths (MEPs) are obtained at the both levels, and the energies along the MEP are further refined by performing the single-point calculations at the PMP4(SDTQ)/6-311+G(3df,2p)//BHLYP and QCISD(T)/6-311++G(2df,2pd)//QCISD levels. The calculated ICVT/SCT rate constants are in good agreement with available experimental values, and the calculate results further indicate that the C(2)H(5) + HBr reaction has negative temperature dependence at T < 850 K, but clearly shows positive temperature dependence at T > 850 K. The current work predicts that the kinetic isotope effect for the title reaction is inverse in the temperature range from 200 to 482 K, i.e., k(HBr)/k(DBr) < 1.     

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     

Competitive bromination kinetics of CH3Br and CH2ClBr

The relative-rate method with gas-chromatographic product analysis was applied to study the kinetics of the reactions Br + CH₃Br → CH₂Br + HBr (1) and Br + CH₂ClBr → CHClBr + HBr (2) The rate coefficient ratio of k ₁/ k ₂ = (1.6 ± 0.2) exp[(-15.2 ± 0.3) kJ mol^-1/ RT] was determined in the temperature range of 353 - 410 K. This revised version was published online in August 2006 with corrections to the Cover Date.     

Kinetics and mechanism for the CH2O + NO2 reaction: A computational study

The reactants, products, and transition states of the CH₂O + NO₂ reaction on the ground electronic potential energy surface have been searched at both B3LYP/6?311+G(d,p) and MPW1PW91/6?311+G(3df,2p) levels of theory. The forward and reverse barriers are further improved by a modified Gaussian‐2 method. The theoretical rate constants for the two most favorable reaction channels 1 and 2 producing CHO + cis‐HONO and CHO + HNO₂, respectively, have been calculated over the temperature range from 200 to 3000 K using the conventional and variational transition‐state theory with quantum‐mechanical tunneling corrections. The former product channel was found to be dominant below 1500 K, above which the latter becomes competitive. The predicted total rate constants for these two product channels can be presented by k_t (T) = 8.35 × 10^?11 T^6.68 exp(?4182/T) cm³/(mol s). The predicted values, which include the significant effect of small curvature tunneling corrections, are in quantitative agreement with the available experimental data throughout the temperature range studied (390–1650 K). © 2003 Wiley Periodicals, Inc. Int J Chem Kinet 35: 184–190, 2003     

Ab initio and RRKM calculations for the reaction channels of O(1D) + CH3CHF2

Ab initio and Rice–Ramsperger–Kassel–Marcus theories are carried out to study the potential energy surface and the energy‐dependent rate constants and branching ratios of the products for O(¹D) + CH₃CHF₂ reaction. Optimized geometries and vibrational frequencies have been obtained by MP2/6‐311G(d,p) method. The main products of the title reaction are CH₃CFO + HF, CH₂CFOH + HF, and CH₃ + CF₂OH at lower collision energy; and CH₃ + CF₂OH, CH₃CF₂ + OH are the main products at higher collision energy. CHF₂ + CH₂OH are the main products in the whole range of collision energy. © 2011 Wiley Periodicals, Inc. Int J Quantum Chem, 2012     

A dual‐level direct dynamics study on the hydrogen abstraction reaction of oxygen atom with methylhydrazine

The mechanism of the multichannel reaction CH₃NHNH₂ (SC₁ and SC₂) + O → products is investigated theoretically using ab initio and density functional theory, and dynamics properties are explored by a dual‐level direct dynamics method. The calculation of the potential energy surface is carried out at the BMC‐CCSD//MPW1K/6‐311G(d,p) level. Using canonical variational transition state theory with a small‐curvature tunneling correction, the rate constants of each channel are evaluated over a wide temperature range of 200–2000 K on the basis of obtained electronic structures and energy information. The total rate constants are calculated from the sum of the individual rate constants taking into account the Boltzmann distribution of two conformers. The reactivity of the H atom located in different groups is compared. © 2013 Wiley Periodicals, Inc.     

Isomerizations of CH3SO2: Quantum Chemistry and Topological Study

The structures, stabilities and the isomerization reactions of CH₃SO₂ isomers in a doublet electronic state have been studied at B3LYP/6‐311+ +G (d,p), MP2/6‐311++G (d,p) and CCSD(T)/6‐311++G (d,p) levels. The three different levels of calculation give the similar results: thirteen minimum isomers were located and they were connected by eleven transition states. Among the thirteen isomers, cis‐CH₃OSO, trans‐CH₃OSO and CH₃SO₂ are the most stable species, and they should be detected easily in experiment. This is well consistent with the experimental result. These isomers could isomerize to each other by chemical bond vibration, chemical bond rotation and atom migration. The non‐planar ring structure transition state (STS), which was found in this paper, extended the concept of ring STS to the non‐planar systems.     

Theoretical Studies on Reaction Mechanism of CH3SO with NO

The potential energy surface （PES） of CH3SO radical with NO reaction has been studied at MP2/6-311G（2df, p） and QCISD/6-311G（2df, p） levels. Geometries of the reactants, transition states （TS） and products were optimized at B3LYP/6-311G （d,p） level. The geometries of the transition states were found for the first time. The calculated results show that the reaction can proceed via singlet-state or triplet-state PES. Because of the high energy barrier of triplet surface, the singlet surface reactions are dominant. The topological analysis of electron density shows that there are two kinds of structaral transition states （the bifurcation-type ring structure transition state and the T-shaped conflict structure transition state） in the titled reaction. The total electronic density of the reactants, TS and products and the spin electronic density on the triplet surface were also discussed in this paper.     

Branching ratios for BrO production from reactions of O(1D) with HBr,CF3Br,CH3Br,CF2ClBr,and CF2HBr

Laser-flash photolysis of RBr/O₃/SF₆/He mixtures at 248 nm has been coupled with BrO detection by time-resolved UV absorption spectroscopy to measure BrO product yields from O(¹D) reactions with HBr, CF₃Br, CH₃Br, CF₂ClBr, and CF₂HBr at 298±3 K. The measured yields are: HBr, 0.20±0.04; CF₃Br, 0.49±0.07; CH₃Br, 0.44±0.05; CF₂ClBr, 0.31±0.06; and CF₂HBr, 0.39±0.07 (uncertainties are 2σ and include estimates of both random and systematic errors). The results are discussed in light of other available information or O(¹D)+RBr reactions. © 1998 John Wiley & Sons, Inc. Int J Chem Kinet 30: 555–563, 1998     

Hydrogen abstraction reactions of OH radicals with CH₃CH₂CH₂Cl and CH₃CHClCH₃: a mechanistic and kinetic study

The hydrogen abstraction reactions of OH radicals with CH₃CH₂CH₂Cl (R1) and CH₃CHClCH₃ (R2) have been investigated theoretically by a dual‐level direct dynamics method. The optimized geometries and frequencies of the stationary points are calculated at the B3LYP/6‐311G(d,p) level. To improve the reaction enthalpy and potential barrier of each reaction channel, the single point energy calculation is performed by the BMC‐CCSD method. Using canonical variational transition‐state theory (CVT) with the small‐curvature tunneling correction, the rate constants are evaluated over a wide temperature range of 200–2000 K at the BMC‐CCSD//B3LYP/6‐311G(d,p) level. For the reaction channels with the negative barrier heights, the rate constants are calculated by using the CVT. The calculated total rate constants are consistent with available experimental data. The results show that at lower temperatures, the tunneling correction has an important contribution in the calculation of rate constants for all the reaction channels with the positive barrier heights, while the variational effect is found negligible for some reaction channels. For reactions OH radicals with CH₃CH₂CH₂Cl (R1) and CH₃CHClCH₃ (R2), the channels of H‐abstraction from –CH₂– and –CHCl groups are the major reaction channels, respectively, at lower temperatures. With temperature increasing, contributions from other channels should be taken into account. Finally, the total rate constants are fitted by two models, i.e., three‐parameter and four‐parameter expressions. The enthalpies of formation of the species CH₃CHClCH₂, CH₃CHCH₂Cl, and CH₂CH₂CH₂Cl are evaluated by isodesmic reactions. © 2011 Wiley Periodicals, Inc. J Comput Chem, 2011     

Inverse Halogen Bonds Interactions Involving Br Atom in the Electronic Deficiency Systems of CH3⊕ … Br-Y (Y=H,CCH, CN,NC)

Inverse halogen bonds interactions involving Br in the electronic deficiency sys-tems of CH₃^⊕…Br-Y (Y=H, CCH, CN, NC) have been investigated by B3LYP/6-311++G(d, p) and MP2/6-311++G(d, p) methods. The calculated interaction energies with basis set super-position error correction of the four IXBs com-plexes are 218.87, 219.48, 159.18, and 143.05 kJ/mol (MP2/6-311++G(d, p)), re-spectively. The relative stabilities of the four complexes increased in the order:CH₃^⊕…BrCN3^⊕…BrNC3^⊕…BrH≈CH₃^⊕…BrCCH. Natural bond orbital theory analysis and the chemical shifts calculation of the related atoms revealed that the charges flow from Br-Y to CH₃^⊕. Here, the Br of Br-Y acts as both a halogen bond donor and an electron donor. Therefore, compared with conventional halogen bonds, the IXBs complexes formed between Br-Y and CH₃^⊕. Atoms-in-molecules theory has been used to investigate the topological properties of the critical points of the four IXBs structures which have more covalent content.     

CH_3SO+HO_2气相反应机理与主通道速率常数

在MPW1PW91/6-311G(d,p)水平上优化了标题反应各驻点物种的几何构型,并在相同水平上通过频率计算和内禀反应坐标(IRC)分析对过渡态结构及连接性进行了验证.采用QCISD(T)/6-311G(d,p)方法对所有驻点及反应路径的部分选择点进行单点能校正,分别构建了CH3SO+HO2反应体系的单、三重态反应势能剖面.研究结果表明,CH3SO+HO2反应体系存在6条反应通道7条路径,优势通道(1)R→3IM→P1(CH3SOH+3O2)发生在三重态势能面上,此通道包含两条路径,其表观活化能分别为12.01和-30.04kJ?mol-1,主路径(2)R→3IM→3TS2→P1(CH3SOH+3O2)是一个无势垒氢迁移过程.利用经典过渡态理论(TST)与变分过渡态理论(CVT)并结合小曲率隧道效应模型(SCT),分别计算了主路径(2)在200～2500K温度范围内的速率常数kTST,kCVT和kCVT/SCT,在此温度区间内的表观反应速率常数三参数表达式为kCVT/SCT=4.08×10-24T3.13exp(8012.2/T)cm3imolecule-1is-1,具有负温度系数效应.速率常数计算结果显示,变分效应在计算温度段内影响较小,而量子力学隧道效应在低温段有显著影响.     

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.