A Computational Study of the Kinetics and Mechanism for the C2H3 + CH3OH Reaction

The mechanism for the C₂H₃ + CH₃OH reaction has been investigated by the Gaussian‐4 (G4) method based on the geometric parameters of the stationary points optimized at the B3LYP/6–31G(2df, p) level of theory. Four transition states have been identified for the production of C₂H₄ + CH₃O (TSR/P1), C₂H₄ + CH₂OH (TSR/P2), C₂H₃OH + CH₃ (TSR/P3), and C₂H₃OCH₃ + H (TSR/P4) with the corresponding barriers 8.48, 9.25, 37.62, and 34.95 kcal/mol at the G4 level of theory, respectively. The rate constants and branching ratios for the two lower energy H‐abstraction reactions were calculated using canonical variational transition state theory with the Eckart tunneling correction at the temperature range 300–2500 K. The predicted rate constants have been compared with existing literature data, and the uncertainty has been discussed. The branching ratio calculation suggests that the channel producing CH₃O is dominant up to about 1070 K, above which the channel producing CH₂OH becomes very competitive.     

Infrared chemiluminescence in the reactions of atomic fluorine with ethanol,propanol-(2) and some deuterated analogs

In the reactive systems F+C₂H₅OH, F+C₂D₅OD, F+C₂H₅OD, F+(CH₃)₂CHOH, F+(CD₃)₂CHOH, and F+(CD₃)₂CDOH the infrared emission spectra were recorded from HF and/or DF in the fundamental region. Hydrogen abstraction takes place from CH and OH bonds. Vibrational relaxation was suppressed and rotational relaxation took place only to a minor extent. HF(DF) excitation reaches the thermodynamic limit within error limits in all cases. The vibrational distributions of HF for the systems F+(CD₃)₂CHOH, F+(CD₃)₂CDOH show no populati inversion. The vibrational distribution of HF for all other systems and all the DF vibrational distributions obtained show population inversion. Inform theory was used to describe the results of those reaction channels that could be studied separately because of isotopic substitution. The results are c to the systems F + methanol and deuterated analogs investigated before in our laboratory, and to the F+CH₄, F+CD₄, and F+H₂O₂ reactio     

A comprehensive mechanism for methanol oxidation

A comprehensive detailed chemical kinetic mechanism for methanol oxidation has been developed and validated against multiple experimental data sets. The data are from static-reactor, flow-reactor, shock-tube, and laminar-flame experiments, and cover conditions of temperature from 633–2050 K, pressure from 0.26–20 atm, and equivalence ratio from 0.05–2.6. Methanol oxidation is found to be highly sensitive to the kinetics of the hydroperoxyl radical through a chain-branching reaction sequence involving hydrogen peroxide at low temperatures, and a chain-terminating path at high temperatures. The sensitivity persists at unusually high temperatures due to the fast reaction of CH₂OH+O₂=CH₂O+HO₂ compared to CH₂OH+M=CH₂O+H+M. The branching ratio of CH₃OH+OH=CH₂OH/CH₃O+H₂O was found to be a more important parameter under the higher temperature conditions, due to the rate-controlling nature of the branching reaction of the H-atom formed through CH₃O thermal decomposition. © 1998 John Wiley & Sons, Inc. Int J Chem Kinet 30: 805–830, 1998     

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     

Direct Ab Initio Dynamics Study on the Reaction of CH3CHF2 (HFC‐152a) with the Cl Atom

A direct ab initio dynamics method is used to investigate the hydrogen‐abstraction reaction CH₃CHF₂+Cl. One transition state is located for α‐H abstraction, and two are identified for β‐H abstraction. The potential‐energy surface (PES) is obtained at the G3(MP2)//MP2/6‐311G(d, p) level. Furthermore, the rate constants of the three channels are evaluated by using canonical variational transition‐state theory (CVT) with small‐curvature tunneling (SCT) contributions over a wide temperature range of 200–2500 K. The dynamic calculations show that the reaction proceeds mainly by α‐H abstraction over the whole temperature range. The calculated rate constants and branching ratios are both in good agreement with the available experimental values.     

Direct dynamics study on the mechanism and the kinetics of the reaction of CH3NH2 with OH

A theoretical study of the mechanism and the kinetics for the hydrogen abstraction reaction of methylamine by OH radical has been presented at the CCSD(T)/6‐311 ++G(2d,2p)//CCSD/6‐31G(d) level of theory. Our theoretical calculations suggest a stepwise mechanism involving the formation of a prereactant complex in the entrance channel and a preproduct complex in the exit channel, for the two hydrogen abstraction channels involving the methyl and amine groups. For clarity, the diagram of potential for the reaction is given. The calculated standard reaction enthalpies are ?98.48 and ?76.50 kJ mol^?1 and barrier heights are 0.36 and 25.25 kJ mol^?1, respectively. The rate constants are evaluated by means of the improved canonical variational transition state theory with small‐curvature tunneling correction (ICVT/SCT) in the temperature range of 299–3000 K. The calculated results show that the rate constants at experimentally measured temperatures are in good agreement with the experimental values. It is shown that the calculated rate constants exhibit a non‐Arrhenius behavior. Moreover, the variational effect is obvious in the calculated temperature range. The dominant product channel is to form CH₂NH₂ and H₂O via hydrogen abstraction from the CH₃ group of CH₃NH₂ by OH in the calculated temperature range. © 2009 Wiley Periodicals, Inc. Int J Quantum Chem, 2009     

Theoretical influence of third molecule on reaction channels of weakly bound complex CO2… HF systems

In this investigation, reaction channels of weakly bound complexes CO₂…HF, CO₂…HF…NH₃, CO₂…HF…H₂O and CO₂…HF…CH₃OH systems were established at the B3LYP/6‐311++G(3df,2pd) level, using the Gaussian 98 program. The conformers of syn‐fluoroformic acid or syn‐fluoroformic acid plus a third molecule (NH₃, H₂O, or CH₃OH) were found to be more stable than the conformers of the related anti‐fluoroformic acid or anti‐fluoroformic acid plus a third molecule (NH₃, H₂O, or CH₃OH). However, the weakly bound complexes were found to be more stable than either the related syn‐ and anti‐type fluoroformic acid or the acid plus third molecule (NH₃, H₂O, or CH₃OH) conformers. They decomposed into CO₂ + HF, CO₂ + NH₄F, CO₂ + H₃OF or CO₂ + (CH₃)OH₂F combined molecular systems. The weakly bound complexes have four reaction channels, each of which includes weakly bound complexes and related systems. Moreover, each reaction channel includes two transition state structures. The transition state between the weakly bound complex and anti‐fluoroformic acid type structure (T₁₃) is significantly larger than that of internal rotation (T₂₃) between the syn‐ and anti‐FCO₂H (or FCO₂H…NH₃, FCO₂H…H₂O, or FCO₂H…CH₃OH) structures. However, adding the third molecule NH₃, H₂O, or CH₃OH can significantly reduce the activation energy of T₁₃. The catalytic strengths of the third molecules are predicted to follow the order H₂O < NH₃ < CH₃OH. © 2006 Wiley Periodicals, Inc. Int J Quantum Chem, 2006     

Theoretical study on the gas‐phase reaction mechanism between rhodium monoxide and methane for methanol production

The gas‐phase reaction mechanism between methane and rhodium monoxide for the formation of methanol, syngas, formaldehyde, water, and methyl radical have been studied in detail on the doublet and quartet state potential energy surfaces at the CCSD(T)/6‐311+G(2d, 2p), SDD//B3LYP/6‐311+G(2d, 2p), SDD level. Over the 300–1100 K temperature range, the branching ratio for the Rh(⁴F) + CH₃OH channel is 97.5–100%, whereas the branching ratio for the D‐CH₂ORh + H₂ channel is 0.0–2.5%, and the branching ratio for the D‐CH₂ORh + H₂ channel is so small to be ruled out. The minimum energy reaction pathway for the main product methanol formation involving two spin inversions prefers to both start and terminate on the ground quartet state, where the ground doublet intermediate CH₃RhOH is energetically preferred, and its formation rate constant over the 300–1100 K temperature range is fitted by k_CH3RhOH = 7.03 × 10⁶ exp(?69.484/RT) dm³ mol^?1 s^?1. On the other hand, the main products shall be Rh + CH₃OH in the reactions of RhO + CH₄, CH₂ORh + H₂, Rh + CO +2H₂, and RhCH₂ + H₂O, whereas the main products shall be CH₂ORh + H₂ in the reaction of Rh + CH₃OH. Meanwhile, the doublet intermediates H₂RhOCH₂ and CH₃RhOH are predicted to be energetically favored in the reactions of Rh + CH₃OH and CH₂ORh + H₂ and in the reaction of RhCH₂ + H₂O, respectively. © 2009 Wiley Periodicals, Inc. J Comput Chem 2010     

Collaborative control of branching ratio in the O + HO2 → OH + O2 reaction via vibrational and rotational excitation

To understand the effect of different vibrational and rotational modes of reactant to enhance the reactivity of the O + HO₂ → OH + O₂ reaction, we revisited this important atmospheric reaction. We report here a quasi-classical trajectory (QCT) study of the reaction dynamics on a recently developed full-dimensional potential energy surface (PES). Our previous work has indicated that this reaction has two pathways, the H abstraction (HA) channel and the O abstraction (OA) channel, which lead to totally different product energy distribution. In this work, we identified that the vibrational excitation of the OH stretching (v₁) mode of HO₂ is the switch of the HA channel at low collision energy; meanwhile, the rotational excitation can also greatly change the branching ratio of the two pathways. With the excitation of v₁ mode, the original negligible HA channel controlled by the tight transition state becomes quite important. This work presents an approach to control the branching ratio via collaboration between vibrational and rotational excitation and will enrich the knowledge of the O + HO₂ reaction in atmospheric chemistry and physics.     

Quasi-classical Trajectory Study of F+H2O→HF+OH Reaction: Influence of Barrier Height,Reactant Rotational Excitation,and Isotopic Substitution (cited: 4)

The reaction dynamics of the F+H₂O/D₂O→HF/DF+OH/OD are investigated on an ac-curate potential energy surface (PES) using a quasi-classical trajectory method. For bothisotopomers, the hydrogen/deuterium abstraction reaction is dominated by a direct rebound mechanism over a very low “reactant-like” barrier, which leads to a vibrationally hot HF/DF product with an internally cold OH/OD companion. It is shown that the lowered reaction barrier on this PES, as suggested by high-level ab initio calculations, leads to a much better agreement with the experimental reaction cross section, but has little impact on the product state distributions and mode selectivity. Our results further indicate that rotational exci-tation of the H₂O reactant leads to significant enhancement of the reactivity, suggesting a strong coupling with the reaction coordinate.     

Atmospheric degradation mechanisms of a simulant organophosphorus pesticide isopropyl methyl methylphosphonate: A theoretical consideration

Calculations using density functional theory were performed to explore the mechanisms for atmospheric degradation of isopropyl methyl methylphosphonate (IMMP). The potential energy surface profiles for OH‐initiated reaction of IMMP were constructed, and all possible degradation channels were considered. Rate constants were further calculated using transition state theory. It was established from these calculations that H‐abstractions from alkyl groups have much lower energy barriers than substitutions of alkoxyl groups, and four possible H‐abstraction channels are competitive. Investigations into the secondary reactions under the presence of O₂/NO were also performed. It is shown that O₂ addition, reaction of peroxide radicals with NO to form RO radicals, and removal of ·RO are the major degradation pathways for alkyl radicals. Four selected products, CH₃OP(O)(CH₃)OC(O)CH₃, CH₃OP(O)(O)CH₃, (CH₃)₂CHOP(O)(CH₃)OH, and (CH₃)₂CHOP(O)(CH₃)OCH?O, are predicted to be the major products in this study. © 2013 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.     

Mechanistic and kinetic study of the O + CH2OH reaction

The mechanism for the O + CH2OH reaction was investigated by various ab initio quantum chemistry methods. For the chemical activation mechanism, that is, the addition/elimination path, the couple-cluster methods including CCSD and CCSD(T) were employed with the cc-pVXZ (X = D, T, Q, 5) basis sets. For the abstraction channels, multireference methods including CASSCF, CASPT2, and MRCISD were used with the cc-pVDZ and cc-pVTZ basis sets. It has been shown that the production of H + HCOOH is the major channel in the chemical activation mechanism. The minor channels include HCO + H2O and OH + CH2O. The hydrogen abstraction by an O atom from the CH2OH radical produces either OH + CH2O or OH + HCOH. Moreover, the two abstraction reactions are essentially barrierless processes. The rate constants for the association of O with CH2OH have been calculated using the flexible transition state theory. A weak negative temperature dependence of the rate constants is found in the range 250-1000 K. Furthermore, it is estimated that the abstraction processes also play an important role in the O + CH2OH reaction. Additionally, the falloff behavior for the OCH2OH --> H + HCOOH reaction has been investigated. The present theoretical results are compared to the experimental measurements to understand the mechanism and kinetic behavior of the O + CH2OH reaction and the unimolecular reaction of the OCH2OH radical.     

Theoretical Study of the Reactions M++CH3F (M=Ge,As, Se,Sb)

CASSCF–MRMP2 calculations have been carried out to analyze the reactions of the methyl fluoride molecule with the atomic ions Ge⁺, As⁺, Se⁺ and Sb⁺. For these interactions, potential energy curves for the low‐lying electronic states were calculated for different approaching modes of the fragments. Particularly, those channels leading to C? H and C? F oxidative addition products, H₂FC? M? H⁺ and H₃C? M? F⁺, respectively were explored, as well as the paths which evolve to the abstraction (M? F⁺+CH₃) and the elimination (CH₂M⁺+HF) asymptotes. For the reaction Ge⁺+CH₃F the only favorable channel leads to fluorine abstraction by the ion. As⁺ and Sb⁺ can react with CH₃F along pathways yielding stable addition products. However, a viable path joining the oxidative addition product H₃C? M? F⁺ with the elimination asymptote CH₂M⁺+HF was found for the reaction of the fluorocarbon compound with As⁺. No favorable channels were detected for the interaction of fluoromethane with Se⁺. The results discussed herein allow rationalizing some of the experimental data found for these interactions through gas‐phase mass spectrometry.     

Atomic radical—molecule reactions F + CH<Subscript>3</Subscript>C≡CH: mechanistic study

The reaction of atomic radical F with propyne has been studied theoretically using ab initio quantum chemistry methods and transition state theory. The potential energy surface was calculated at the CCSD(T)/aug-cc-pVDZ (single-point) level using the UMP2/6-311++G(d,p) optimized structures. Two reaction mechanisms including the addition–isomerization–elimination reaction mechanism and the directed hydrogen abstraction reaction mechanism are considered. For the hydrogen abstraction reactions, i.e., the most probable evolution pathway in the title reaction, the HF formation occurs via direct abstraction mechanism dominantly and the H atom picked up by the atomic radical F should come mostly from the methyl group of normal propyne. On the other hand, for the addition–isomerization–elimination mechanism, the most feasible pathway should be the atomic radical F attacking on the C≡C triple bond in propyne (CH₃C≡CH) to form a weakly-bound adduct A1 with no barrier, followed by F addition to the C≡C triple bond to form the low-lying intermediate isomer 5. Subsequently, isomer 5 directly dissociates to P3 H₂CCCHF + H via transition state TS₅/P3. The other reaction pathways on the doublet PES are less competitive due to thermodynamical or kinetic factors. Furthermore, based on the analysis of the kinetics of all channels through which the addition and abstraction reaction proceed, we expect that the competitive power of reaction channels may vary with experimental conditions for the title reaction. The present work will provide useful information for understanding the processes of atomic radical F reaction with other unsaturated hydrocarbons. This material is available from author via E-mail.     

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     

On the kinetic mechanism of the hydrogen abstraction reactions of the hydroxyl radical with CH3CF2Cl and CH3CFCl2: A dual level direct dynamics study

By means of the dual‐level direct dynamics method, the mechanisms of the reactions, CH₃CF₂Cl + OH → products (R1) and CH₃CFCl₂ + OH → products (R2), are studied over a wide temperature range 200–2000 K. The optimized geometries and frequencies of the stationary points are calculated at the MP2/6‐311G(d,p) level, and then the energy profiles of the reactions are refined with the interpolated single‐point energy method at the G3(MP2) level. The canonical variational transition‐state theory with the small‐curvature tunneling (SCT) correction method is used to calculate the rate constants. For the title reactions, three reaction channels are identified and the H‐abstraction channel is the major pathway. The results indicate that F substitution has a significant (reductive) effect on hydrochlorofluorocarbon reactivity. Also, for all H‐abstraction reaction channels the variational effect is small and the SCT effect is only important in the lower temperature range on the rate constants calculation. © 2009 Wiley Periodicals, Inc. J Comput Chem, 2010     

Gas-phase kinetics of hydroxyl radical reactions with C3H6 and C4H8: product branching ratios and OH addition site-specificity

Products of the reaction of OH radicals with propene, trans-2-butene, and 1-butene have been investigated in a fast flow reactor, coupled with time-of-flight mass spectrometry, at pressures between 0.8 and 3.0 Torr. The product determination includes H atom abstraction channels as well as site-specific OH addition. The OH radicals are produced by the H + NO(2) → OH + NO reaction or by the F + H(2)O → OH + HF reaction, the H or F atoms being produced in a microwave discharge. The gas mixture is ionized using single photon ionization (SPI at 10.54 eV), and products are detected using time-of-flight mass spectrometry (TOF-MS). The H atom abstraction channels are measured to be <2% for OH + propene, 8 ± 3% for OH + 1-butene, and 3 ± 1% for OH + trans-2-butene. Analysis of ion fragmentation patterns leads to 72 ± 16% OH addition to the propene terminal C atom and 71 ± 16% OH addition to the 1-butene terminal C atom. The errors bars represent 95% confidence intervals and include estimated uncertainties in photoionization cross sections.     

Theoretical Study on the Reaction Mechanism of Ketene CH2CO with Isocyanate NCO Radical

The bimolecular single collision reaction potential energy surface of an isocyanate NCO radical with a ketene CH2CO molecule was investigated by means of B3LYP and QCISD（T） methods. The computed results indicate that two possible reaction channels exist on the surface. One is an addition-elimination reaction process, in which the CH2CO molecule is attacked by the nitrogen atom at its methylene carbon atom to lead to the formation of the intermediate OCNCH2CO followed by a C-C rupture channel to the products CH2NCO＋CO. The other is a direct hydrogen abstraction channel from CHzCO by the NCO radical to afford the products HCCO＋HNCO. Because of a higher barrier in the hydrogen abstraction reaction than in the addition-elimination reaction, the direct hydrogen abstraction pathway can only be considered as a secondary reaction channel in the reaction kinetics of NCO＋ CH2CO. The predicted results are in good agreement with previous experimental and theoretical investigations.     

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