Rate constants calculation of hydrogen abstraction reactions CH3CHBr + HBr and CH3CBr2 + HBr

Dual‐level direct dynamics method is used to study the kinetic properties of the hydrogen abstraction reactions of CH₃CHBr + HBr → CH₃CH₂Br + Br (R1) and CH₃CBr₂ + HBr → CH₃CHBr₂ + Br (R2). Optimized geometries and frequencies of all the stationary points and extra points along the minimum‐energy path are obtained at the MPW1K/6‐311+G(d,p), MPW1K/ma‐TZVP, and BMK/6‐311+G(d,p) levels. Two complexes with energies less than that of the reactants are located in the entrance of each reaction at the MPW1K/6‐311+G(d,p) and MPW1K/ma‐TZVP levels, respectively. The energy profiles are further refined with the interpolated single‐point energies method at the G2M(RCC5)//MPW1K/6‐311+G(d,p) level of theory. By the improved canonical variational transition‐state theory with the small‐curvature tunneling correction (SCT), the rate constants are evaluated over a wide temperature range of 200–2000 K. Our calculations have shown that the radical reactivity decreases from CH₃CHBr to CH₃CBr₂. Finally, the total rate constants are fitted by two modified Arrhenius expression. © 2012 Wiley Periodicals, Inc.     

Ab initio study of the potential energy surface and product branching ratios for the reaction of O(D) with CH3CH2F

The potential energy surface of O(¹D) + CH₃CH₂F 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) atom inserting into C–F bond of CH₃CH₂F produces one energy-rich intermediate CH₃CH₂OF and another is the O(¹D) atom inserting into one of the C–H bonds of CH₃CH₂F produces 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 is the main decomposition product of CH₃CH₂OF. HF and CH₃ are the main decomposition products for IM1; CH₂OH is the main decomposition product for IM2. Since IM1 is more stable and more likely to form than CH₃CH₂OF and IM2, HF and CH₃ are probably the main products of the O(¹D) + CH₃CH₂F reaction. Our computational results can give insight to reaction mechanism and provide probable explanations for future experiments.     

Mechanism for the gas‐phase reaction between OH and 3‐methylfuran: A theoretical study

The mechanism for the OH + 3‐methylfuran reaction has been studied via ab initio calculations to investigate various reaction pathways on the doublet potential energy surface. Optimizations of the reactants, products, intermediates, and transition structures are conducted using the MP2 level of theory with the 6‐311G(d,p) basis set. The single‐point electronic energy of each optimized geometry is refined with G3MP2 and G3MP2B3 calculations. The theoretical study suggests that the OH + 3‐methylfuran reaction is dominated by the formation of HC(O)CH?C(CH₃)CHOH (P7) and CH(OH)CH?C(CH₃)C(O)H (P9), formed from two low‐lying adducts, IM1 and IM2. The direct hydrogen abstraction pathways and the S_N2 reaction may play a minor or negligible role in the overall reaction of OH with 3‐methylfuran. © 2008 Wiley Periodicals, Inc. Int J Quantum Chem, 2008     

Theoretical investigation on mechanism for OH-initiated oxidation of CH2=C(CH3)CH2OH

The mechanism of the gas-phase reaction OH with CH₂=C(CH₃)CH₂OH (2-methyl-2-propen-1-ol) has been elucidated using high-level ab initio method, i.e., CCSD(T)/6-311++g(d,p)//MP2(full)/6-311++g(d,p). Various possible H-abstraction and addition–elimination pathways are identified. The calculations indicate that the addition–elimination mechanism dominates the OH+MPO221 reaction. The addition reactions between OH radicals and CH₂=C(CH₃)CH₂OH begin with the barrierless formation of a pre-reactive complex in the entrance channel, and subsequently the CH₂(OH)C(CH₃)CH₂OH (IM1) and the CH₂C(OH)(CH₃)CH₂OH (IM2) are formed by OH radicals’ electrophilic additions to the double bond. IM1 can easily rearrange to IM2 via a 1,2-OH migration. Subsequently, rearrangement of IM2 to form (CH₃)₂C(OH)CH₂O (IM11) followed by dissociation to HCHO + (CH₃)₂COH (P21) is the most favorable pathway. The decomposition of IM2 to CH₂OH + CH₂=C(OH)CH₃ (P16) is the secondary pathway. The other pathways are not expected to play any important role in forming final products.     

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     

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     

Computational study on the mechanism for the gas‐phase reaction of dimethyl disulfide with OH

The mechanisms for the reaction of CH₃SSCH₃ with OH radical are investigated at the QCISD(T)/6‐311++G(d,p)//B3LYP/6‐311++G(d,p) level of theory. Five channels have been obtained and six transition state structures have been located for the title reaction. The initial association between CH₃SSCH₃ and OH, which forms two low‐energy adducts named as CH₃S(OH)SCH₃ (IM1 and IM2), is confirmed to be a barrierless process, The S? S bond rupture and H? S bond formation of IM1 lead to the products P1(CH₃SH + CH₃SO) with a barrier height of 40.00 kJ mol^?1. The reaction energy of Path 1 is ?74.04 kJ mol^?1. P1 is the most abundant in view of both thermodynamics and dynamics. In addition, IMs can lead to the products P2 (CH₃S + CH₃SOH), P3 (H₂O + CH₂S + CH₃S), P4 (CH₃ + CH₃SSOH), and P5 (CH₄ + CH₃SSO) by addition‐elimination or hydrogen abstraction mechanism. All products are thermodynamically favorable except for P4 (CH₃ + CH₃SSOH). The reaction energies of Path 2, Path 3, Path 4, and Path 5 are ?28.42, ?46.90, 28.03, and ?89.47 kJ mol^?1, respectively. Path 5 is the least favorable channel despite its largest exothermicity (?89.47 kJ mol^?1) because this process must undergo two barriers of TS5 (109.0 kJ mol^?1) and TS6 (25.49 kJ mol^?1). Hopefully, the results presented in this study may provide helpful information on deep insight into the reaction mechanism. © 2009 Wiley Periodicals, Inc. Int J Quantum Chem, 2011     

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.     

Reaction of CH3O2 and HO2: Ab initio characterization of dimer structure and vibrational mode analysis for reaction mechanisms

All species involved in the multi‐channel reaction of CH₃O₂ with HO₂ have been investigated using density functional theory (DFT). The molecular geometries for various species are optimized employing the B3LYP method implementing the 6‐311++G** basis set. The relative energies of all species are calculated at the same level theory. The results show that there are two kinds of channels: singlet and triplet. The singlet channel involves four intermediates and six transition states. The triplet channel includes two intermediates and two transition states. There are four kinds of reaction products: CH₃OOH + ¹O₂, CH₃OH + O₃, CH₄ + 2O₂, and CH₃OOH + ³O₂. The vibrational mode analysis is used to elucidate the relationships of the intermediates, the transition states, and the products. The extensive investigation shows that the reaction mechanism is reliable. © 2005 Wiley Periodicals, Inc. Int J Quantum Chem, 2006     

Theoretical study on the ion–molecule reaction of NH+ with CH2O

An in‐depth theoretical study is carried out at the B3LYP/6‐311G(d,p), M062X/aug‐cc‐pVDZ and CCSD(T)/6‐311++G(3df,2dp) (single‐point) levels as an attempt to explore the mechanism of the little‐understood ion–molecule reaction between NH⁺ and CH₂O. Various possible reaction pathways are taken into account. It is shown that six dissociation products, including P ₁ (²N + CH₂OH⁺), P ₂ (⁴N + CH₂OH⁺), P ₃ (³NH + CH₂O⁺), P ₄ (NH₂ + HCO⁺), P ₅ (NH + CO), and P ₉ (H + CONH) are all accessible both kinetically and thermodynamically. Among these products, P ₄ is the most competitive product with predominant abundance, and the second most feasible product is P ₃ , followed by P ₂ and P ₁ . The remaining products, P ₅ and P ₉ , may have negligible yield under room temperature condition. As the intermediates and transition states involved in the NH⁺ + CH ₂ O reaction all stay below the reactant, the title reaction is expected to be rapid, which is consistent with the measured large rate constant in experiment. The present study will enrich our knowledge of the chemistry of NH⁺. Furthermore, our calculated result is compared with the previous experimental research, and, meanwhile, it provides a useful guide for understanding analogous reaction, NH⁺ with CH₂NH. © 2011 Wiley Periodicals, Inc. Int J Quantum Chem, 2012     

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     

Rate constant for the reaction of CH3C(O)CH2 radical with HBr and its thermochemical implication

The fast flow method with laser induced fluorescence detection of CH₃C(O)CH₂ was employed to obtain the rate constant of k₁ (298 K) = (1.83 ± 0.12 (1σ)) × 10¹⁰ cm³ mol^?1 s^?1 for the reaction CH₃C(O)CH₂ + HBr ? CH₃C(O)CH₃ + Br (1, ?1). The observed reduced reactivity compared with n‐alkyl or alkoxyl radicals can be attributed to the partial resonance stabilization of the acetonyl radical. An application of k₁ in a third law estimation provides Δ_fH(CH₃C(O)CH₂) values of ?24 kJ mol^?1 and ?28 kJ mol^?1 depending on the rate constants available for reaction ( ‐1 ) from the literature. © 2005 Wiley Periodicals, Inc. Int J Chem Kinet 38: 32–37, 2006     

Computational study of the reaction of the methylsulfonyl radical,CH3S(O)2, with NO2

The mechanism of the reaction between the methylsulfonyl radical, CH₃S(O)₂, and NO₂ is examined using density functional theory and ab initio calculations. Two stable association intermediates, CH₃SNO₂ and CH₃S(O)ONO, may be formed through the attack of the nitrogen or the oxygen atom of NO₂ radical to the S atom. Interisomerization and decomposition of these intermediates are investigated using high level energy methods and specifically, CCSD(T), CBS‐QB3, and G3//B3LYP. The computational investigation indicates that the lowest energy reaction pathway leads to the products CH₃S(O)₃ + NO, through the decomposition of the most stable association adduct CH₃S(O)ONO. This result fully supports the relevant assumption of Ray et al. (Ray et al., J. Phys. Chem. 1996, 100, 8895], on which the experimental evaluation of the rate constant was based, namely that CH₃S(O)₃ + NO are the most probable products of the reaction CH₃S(O)₂ + NO₂. © 2014 Wiley Periodicals, Inc.     

Master Equation Modeling of the Unimolecular Decompositions of α‐Hydroxyethyl (CH3CHOH) and Ethoxy (CH3CH2O) Radicals

The unimolecular decomposition of two radical isomers of C₂H₅O (CH₃CH₂O/ethoxy, CH₃CHOH/α‐hydroxyethyl) are investigated by means of Rice–Ramsperger–Kassel–Marcus/master equation simulations in helium and nitrogen bath gases on an accurate one‐dimensional potential energy surface. For ethoxy, simulations are carried out between temperatures of 406 and 1200 K and pressures of 0.001 and 100 atm. For CH₃CHOH, simulations are carried out between temperatures of 800 and 1500 K and pressures of 0.001 and 100 atm. Results are compared with available experimental data, with good agreement. The dominant product of α‐hydroxyethyl decomposition is CH₃CHO + H, with C₂H₃OH + H and CH₃ + CH₂O, being minor channels. Rate coefficients are strongly dependent on temperature and pressure and are recommended with attendant uncertainty factor estimates. The relative roles of vinyl alcohol and acetaldehyde in the context of combustion chemistry are also discussed.     

An FTIR spectroscopic study of the reactions Br + CH3CHO → HBr + CH3CO and CH3C(O)OO + NO2 ⇌ CH3C(O)OONO2 (PAN)

Based on an FTIR-product study of the photolysis of mixtures containing Br₂? CH₃CHO and Br₂? CH₃CHO? HCHO in 700 torr of N₂, the rate constant for the reaction Br + CH₃CHO → HBr + CH₃CO was determined to be 3.7 × 10^?12 cm³ molecule^?1 s^?1. In addition, the selective photochemical generation of Br at λ > 400 nm in mixtures containing Br₂? CH₃CHO? ¹⁴NO₂ (or ¹⁵NO₂)? O₂ was shown to serve as a quantitative preparation method for the corresponding nitrogen-isotope labeled CH₃C(O)OONO₂ (PAN). From the dark-decay rates of ¹⁵N-labeled PAN in large excess ¹⁴NO₂, the rate constant for the unimolecular reaction CH₃C(O)OO¹⁵NO₂ → CH₃C(O)OO + ¹⁵NO₂ was measured to be 3.3 (±0.2) × 10^?4 s^?1 at 297 ± 0.5 K.     

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     

Theoretical studies on the reaction mechanism of CH2CH radical with HNCO

The reaction mechanism of CH₂CH radical with HNCO has been investigated systematically by density functional theory (DFT). The geometries and harmonic frequencies of reactants, intermediates, transition states, and products have been optimized with the B3LYP at different levels. At the same time, AIM is performed to calculate the charge density of some bonding critical points and the charges of some atoms. Nine feasible reaction pathways have been investigated. The results indicated that the main pathway is CH₂CH + HNCO → IMA1 → TSA1 → CH₂CH₂ + NCO, which is characterized by hydrogen atom transferring. © 2006 Wiley Periodicals, Inc. Int J Quantum Chem, 2006     

Theoretical mechanistic study on the radical-molecule reaction of CH2Br/CHBrCl with NO2

The radical-molecule reaction mechanisms of CH₂Br and CHBrCl with NO₂ have been explored theoretically at the UB3LYP/6-311G(d, p) level. The single-point energies were calculated using UCCSD(T) and UQCISD(T) methods. The results show that the title reactions are more favorable on the singlet potential energy surface than on the triplet one. For the singlet potential energy surface of CH₂Br + NO₂ reaction, the association of CH₂Br with NO₂ is found to be a barrierless carbon-to-oxygen attack forming the adduct IM1 (H₂BrCONO-trans), which can isomerize to IM2 (H₂BrCNO₂), and IM3 (H₂BrCONO-cis), respectively. The most feasible pathway is the 1, 3-Br shift with C–Br and O–N bonds cleavage along with the N–Br bond formation of IM1 lead to the product P1 (CH₂O + BrNO) which can further dissociate to give P4 (CH₂O + Br + NO). The competitive pathway is the 1, 3-H-shift associated with O–N bond rupture of IM1 to form P2 (CHBrO + HNO). For the singlet potential energy surface of CHBrCl + NO₂ reaction, there are three important reaction pathways, all of which may have comparable contribution to the reaction of CHBrCl with NO₂. The theoretically obtained major products CH₂O and CHClO for CH₂Br + NO₂ and CHBrCl + NO₂ reactions, respectively, are in good agreement with the kinetic detection in experiment.     

Polar effect in the reaction of CH3O with HBr

The discharge flow method with laser induced fluorescence detection of CH₃O was applied to determine the rate constant, k ₁, for the reaction CH₃O + HBr → products (1) k ₁ (298 K) = (8.41 ± 0.80(1σ)) 10¹¹ cm³ mol^-1 s^-1. The unusually large k ₁ value was explained by the polar nature of the transition state of the reaction. This revised version was published online in June 2006 with corrections to the Cover Date.     

Reaction Mechanism Investigation Using Vibrational Mode Anal—ysis for the Multichannel Reacition of CH3O＋CO

On the basis of the computed results got by the Gaussian 94 package at B3LYP/6-311 G** level,the reaction mechanism of CH3O radical with CO has been investiagted thoroughly via the vibrational model analysis ,And the relationships among the reactants,eight transition states,four intermediates and various products involved this multichannel reation are eluci-dated,The vibrational mode anaysis shows that the reaction mechanism is relialbe.