Canonical Transition State Theory Research Articles

Chemical Kinetics of Hydrogen Atom Abstraction from Propargyl Sites by Hydrogen and Hydroxy Radicals.

Hydrogen atom abstraction from propargyl C-H sites of alkynes plays a critical role in determining the reactivity of alkyne molecules and understanding the formation of soot precursors. This work reports a systematic theoretical study on the reaction mechanisms and rate constants for hydrogen abstraction reactions by hydrogen and hydroxy radicals from a series of alkyne molecules with different structural propargyl C-H atoms. Geometry optimizations and frequency calculations for all species are performed at M06-2X/cc-pVTZ level of theory and the hindered internal rotations are also treated at this level. The high-level W1BD and CCSD(T)/CBS theoretical calculations are used as a benchmark for a series of DFT calculations toward the selection of accurate DFT functionals for large reaction systems in this work. Based on the quantum chemistry calculations, rate constants are computed using the canonical transition state theory with tunneling correction and the treatment of internal rotations. The effects of the structure and reaction site on the energy barriers and rate constants are examined systematically. To the best of our knowledge, this work provides the first systematic study for one of the key initiation abstraction reactions for compounds containing propargyl hydrogen atoms.

Kinetics of hydrogen abstraction from desflurane by OH and Cl radicals – A theoretical study

Based on the constructed CCSD(T)/cc-pVTZ//M06-2X/aug-cc-pVTZ potential energy surface, the detailed kinetics of the desflurane (CF3CHFOCHF2) + Cl/OH reactions was investigated using canonical transition state theory (TST) with corrections of the hindered internal rotation and tunneling treatments. The suggested rate constants, namely kCl (T) = 8.76 × 10−21 × T3.2 × exp(−1961 K/T) and kOH (T) = 1.19 × 10−23 × T3.7 × exp(−481 K/T) (cm3∙molecule−1∙s−1) in the temperature range of 200–2000 K, are in good agreement with experimental data. Moreover, the subsequent unimolecular reactions of the desflurane radicals were also investigated under the atmospheric conditions.

High-level theoretical study of the hydrogen abstraction reaction H2S + O2 = SH + HO2 and prediction of the rate constants

Hydrogen abstraction reaction of hydrogen sulfide (H2S) by molecule oxygen (O2) forming hydroperoxyl radical (HO2) and hydrosulfide radical (SH) is very important for both atmospheric and combustion chemistry. This work reports a systematic theoretical study on this reaction using a combination of high-level quantum chemistry methods and chemical kinetic theory. Geometry optimizations for all species are performed at QCISD/cc-pVQZ level of theory. The energies for minima and transition state are computed using both single- and multi-reference ab initio methods. It is found that the transition state of this reaction exhibits significant multi-reference nature based on 〈S2〉 and T1 diagnostic during CCSD(T) calculations. The predicted reaction barrier from high-level multi-reference CASPT2(20e,13o)/aug-cc-pVTZ method is higher than those from CCSD(T) and W1BD methods by 6–7 kcal mol−1. Reaction energy barriers from a series of DFT methods also show large deviations compared with both single- and multi-reference methods. Thermal rate constants are calculated over a temperature range from 300 to 2500 K by employing canonical transition state theory with Eckart tunneling correction and the treatment of torsional mode. This work provides accurate rate constants for this reaction and fundamental benchmark results for H/O/S reaction systems, which is valuable for theoretical chemistry and kinetic modeling of sulfur chemistry.

Theoretical ab-initio kinetics of the reactions between isobutene plus hydroxyl

The kinetic chemistry of H-abstraction reactions of isobutene by the OH radical and the channels of the C4H9O complex formed by addition of the hydroxyl to isobutene were investigated in current work. Their rate constants at high pressure limits were computed based on the canonical transition state theory with tunneling effect considered, following the construction of the potential energy surface (PES) based on a duel-level quantum chemistry method at CCSD(T)/CBS//BHandHLYP/6-311G(d,p) level. Comparison of reaction rate constants of H-abstractions by the OH radical for isobutene with previous experimental and calculated data showed acceptable agreements. Isomerization pathways of the hydroxybutyl radicals to each other and to the isobutoxy radical were discussed in details. It was observed that the rate constants for both H-abstraction and isomerization via H-transfer were improved by considering tunneling effect. Subsequently, the pressure-dependent rate constants of multiple channels for each hydroxybutyl radical and of the dominant channel for isobutoxy radical were calculated based on RRKM/ME theory. The effect of chemical activation and thermal initial distribution was discussed with the case of 2-hydroxymethyl-prop-2-yl (W1) radical.

Mechanistic investigation of the atmospheric oxidation of bis(2-chloroethyl) ether (ClCH2CH2OCH2CH2Cl) by OH and NO3 radicals and Cl atoms: a DFT approach.

The oxidation of bis(2-chloroethyl) ether (ClCH2CH2OCH2CH2Cl) in the atmosphere, as initiated by various oxidants (OH and NO3 radicals and Cl atoms), was examined using the functional M06-2X in conjunction with the basis set 6-31 + G(d,p). We explored the oxidation pathways and reaction energies (enthalpies and Gibb's free energies) for the abstraction of H from the -CH2Cl and -OCH2 sites of ClCH2CH2OCH2CH2Cl by oxidants. The energy profile shows that H abstraction from the -OCH2 site of the title molecule by each atmospheric oxidant is more likely than H abstraction from the other site. The resulting radical ClCH2C•HOCH2CH2Cl was found to more stable than any other product, as shown by their reaction energies. The rate constants of the oxidation reactions were also calculated using canonical transition state theory in the temperature range 298-400K. The calculated total rate constant at 298K is consistent with the reported experimental rate constant. The branching ratio percentages and global atmospheric lifetime of the title molecule are also reported herein.

Pressure-dependent rate rules for cycloaddition, intramolecular H-shift, and concerted elimination reactions of alkenyl peroxy radicals at low temperature.

The reactions of cycloaddition, intramolecular H-shift and concerted elimination of alkenyl peroxy radicals are three kinds of important reactions in the low temperature combustion of alkenes. In this study, the cycloaddition reactions are divided into classes considering endo-cycloaddition, exo-cycloaddition and the size of the transition states; the intramolecular H-shift reactions are divided into classes depending upon the ring size of the transition states and the type of C-H bonds from which the hydrogen atom is transferred; the concerted elimination reactions are divided into classes according to the type of H-CβCαOO bond that is broken. All geometry optimizations are performed at the B3LYP/6-31G(2df,p) level. With the electronic structure calculations being performed using the composite Gaussian-4 (G4) method, high pressure limit rate constants and pressure-dependent rate constants at pressures varying from 0.01 to 100 atm are calculated by using canonical transition state theory and the Rice-Ramsberger-Kassel-Marcus/master equation method, respectively. All rate constants are given in the form of the modified Arrhenius expression. The high pressure limit rate rules and the pressure-dependent rate rules are derived by averaging the rate constants of a representative set of reactions in each class. The results show that the rate rules for these three classes of reactions have a large uncertainty and the impact of the pressure on the rate constants increases as temperature increases.

Oxidation pathways, kinetics and branching ratios of chloromethyl ethyl ether (CMEE) initiated by OH radicals and the fate of its product radical: an insight from a computational study.

The OH-initiated oxidation reactions of chloromethyl ethyl ether (CH2ClOCH2CH3) have been presented by using quantum calculation methods. The Minnesota functional (M06-2X) of the density functional theory method along with a polarization and diffuse 6-311++G(d,p) basis set is chosen for optimization and frequency calculations for H-abstractions from CH2ClOCH2CH3 molecules by OH radicals. Furthermore, the CCSD(T) method along with the same basis set is used for energy refinement of all optimized structures to obtain more accurate energies of the species. Our thermo-chemical calculation results show that the C˙HClOCH2CH3 product radical is more stable, corresponding to hydrogen atom abstraction from the -CH2Cl site, than others while the energy profile results indicate that the H-atom abstracted from the -OCH2 site follows the minimum energy path compared to other channels. The rate constants are computed using canonical transition state theory (CTST) within the temperature range of 250-450 K at 1 atm. The overall rate constant (at 298 K) for the abstraction reactions is found to be consistent with the earlier reported rate constant. The percentage branching ratios of different abstraction channels and the lifetime of chloromethyl ethyl ether are also given herein. We further investigated the unimolecular decomposition pathways of the CH2ClOCH(O˙)CH3 radical and found that unimolecular C-C bond scission is the kinetically and thermodynamically more feasible pathway compared to other unimolecular decomposition reactions.

Formation of Criegee intermediates and peroxy acids: a computational study of gas-phase 1,3-cycloaddition of ozone with catechol.

A detailed theoretical investigation of gas-phase 1,3-cycloaddition of ozone with catechol is presented to explore the discrepancies in previous theoretical and experimental rate constants. DFT based PBE, TPSS, B3LYP, B3PW91, M06-2X, wB97XD, MN15 and high-level CCSD(T) methods are used for the calculation. Canonical transition state theory has been used to calculate the rate coefficients of individual steps. The calculated rate coefficients are compared with the experimental and previously calculated rate constant. The possible pathways for primary ozonide (POZ) formation and subsequent reactions to yield the Criegee Intermediates (CI) and peroxy acids (POA) are investigated. The endo-POZ may undergo conversion to exo-POZ or form the Creigee Intermediates. This work shows a novel pathway by which the exo-POZ can form more stable and chemically different species, peroxy acids, by abstracting an H atom from the OH group.

Kinetics and mechanism of 3-chloro-2-methyl-1-propene(3-ClMP) initiated by OH radical: an insight from DFT calculations[1

ABSTRACTIn this manuscript, OH-initiated gas phase reactions of 3-chloro-2-methyl-1-propene [CH2=C(CH3)CH2Cl] has been carried out by using density functional theory (DFT) method. We have chosen M06-2X hybrid density functional method along with 6-31 + G(d,p) basis set for optimisation and vibrational frequency calculations of all the relevant molecular species of the titled reaction. Energetic calculations have been further refined at CCSD(T) method along with cc-pVTZ basis set in order to obtain higher degree of precision of thermo-chemical and kinetic results. Energy profile is presented for all stationary points involved in the titled reaction and thermochemistry for all possible reaction channels (H-abstraction and OH-addition reactions) has been analysed. Canonical Transition State Theory (CTST) has been employed to determine the rate constants within the temperature range of 250–450 K and Our reported rate constant at 298 K are found to be close to the reported experimental rate constant. The atmospheric lifetime of the titled molecule has also been reported in this work. We have also reported standard enthalpies of formation of the titled molecule and generated product radicals by using group-balanced isodesmic reaction schemes.

Theoretical Study of the Reactions of H Atoms with CH3I and CH2I2.

High level ab initio methods have been used to provide reliable kinetic data for the H + CH3I and H + CH2I2 gas-phase reactions. The (H, I)-abstraction and I-substitution reaction pathways were identified. The structures were determined on the potential energy surface at the MP2/aug-cc-pVTZ level of theory. The energetics was then refined using the coupled cluster theory. For the iodinated species, the spin-orbit coupling was calculated using the MRCI approach. The core valence and the scalar relativistic corrections were considered. Thermal rate constants were reported using the canonical transition-state theory (TST) and compared to computed values with the canonical variational transition-state theory (CVT) using the zero curvature tunneling (ZCT) and the small curvature tunneling (SCT) corrections over a wide temperature range (250-2500 K) to show the importance of quantum tunneling effects at low temperatures. They are given by the following expressions for the overall reactions using the CVT/SCT method: kH+CH3I( T) = 1.07 × 10-17 × T2.13 exp(2.68 (kJ mol-1)/ RT) and kH+CH2I2( T) = 5.73 × 10-21 × T2.97 exp(3.15 (kJ mol-1)/ RT). The I-abstraction is predicted to be the major pathway for both H + CH3I and H + CH2I2 reactions. The obtained kinetic parameters for the H + CH3I reaction are in excellent agreement with their experimental counterparts over the temperature range 300-750 K. On the basis of our calculated reaction enthalpies, a new evaluation of the standard enthalpy of formation at 298 K of CH2I and CHI2 has been provided. Obtained values are Δf H°298K (CH2I) = 219.5 kJ mol-1 and Δf H°298K(CHI2) = 296.3 kJ mol-1.

Quantum-chemical and kinetic study of the reactions of the ClSO2 radical with H, O, Cl, S, SCl and ClSO2 in the atmosphere of Venus

Rate constants for the reactions between the ClSO2 radical and H, O, Cl, S, SCl and ClSO2 were studied over the 150–700 K temperature range employing the statistical adiabatic channel model/classical trajectory approach and the canonical transition state theory on potential energy surfaces based on G4//B3LYP/6-311++G(3df,3pd) calculations. For these processes the following rate constants (in cm3 molecule−1 s−1) are predicted: 2.71 x 10- 11(T/250)0.47 (ClSO2 + H → HCl + SO2); 7.69 x 10- 11(T/250)0.093 (ClSO2 + O → Cl + SO3); 1.44 x 10- 11(T/250)0.47 (ClSO2 + Cl → Cl2SO2); 6.73 x 10- 11(T/250)0.18 (ClSO2 + S → SCl + SO2); 9.38 x 10- 13(T/250)0.75 (ClSO2 + SCl → ClS(O2)SCl); 2.64 x 10- 14(T/250)0.61 (2 ClSO2 → (O2)ClSSCl(O2)). These data are in marked contrast with those normally used for the modeling of the lower and middle atmosphere of Venus. Therefore, in the absence of experimental and theoretical investigations, the above rate constants are proposed for these studies.

Kinetics, mechanism, and global warming potentials of HFO-1234yf initiated by O3 molecules and NO3 radicals: insights from quantum study.

In the present investigation, the oxidation of HFO-1234yf (2,3,3,3-tetrafluoropropene) with O3 molecule and NO3 radical is studied by quantum chemical methods. The possible reaction pathways of the titled molecule with O3 molecule and NO3 radical are analyzed using M06-2X meta-hybrid density functional with the 6-311++G(d,p) basis set. We have further employed a series of single-point energy calculations by using a potentially high-level couple cluster method with single and double excitations, including perturbative corrections ((CCSD(T)) at the same basis set. The addition reaction of HFO-1234yf with O3 molecule is initiated by the formation of primary ozonide complex, which leads to the formation of various carbonyl compounds and Criegee intermediates. The calculated energy barriers and thermochemical parameters inferred that decomposition of C˙H2OO˙ and CF3CFO is slightly more preferred over the formation of CF3C˙FOO˙ and CH2O. Further, the NO3 radical addition at α- and β-sits of CF3CF〓CH2 molecule is analyzed in details. The individual and overall rate constants for each reaction pathways are calculated by using canonical transition state theory over the temperature range of 250-450K. We have observed that the computed rate constants are in good agreement with the available experimental data. Atmospheric lifetimes and global warming potentials of the HFO-1234yf are also reported in this manuscript.

A thermochemical study on the primary oxidation of sulfur

ABSTRACTSeveral chemical reactions related to the oxidation and combustion of sulfur are investigated using a number of computational chemistry methods with the objective of determining appropriate methods for use in developing an elementary reaction mechanism for oxidation of sulfur. Calculations are focused on thermochemical properties and reaction energetics for reactive species and transition state structures for reactions in the oxidation/combustion of sulfur. Reactions involving several intermediates resulting from the reactions of S2 with oxygen were investigated with the density functional theory B3LYP (with several basis sets) and BB1K/GTLarge. The composite ab-initio methods G2, G3, G3MP2, G3B3, G3MP2B3 and CBS-QB3 were also used. Enthalpies of a series of sulfur compounds and transition state structures are calculated using the ab-initio and DFT calculations. The calculations were combined with isodesmic reaction analysis, whenever possible, in order to cancel error and improve the accuracy of the calculations. Results show that all B3LYP DFT calculations including the 6–311++G(3df,2p) basis set show poor outcome in estimating the enthalpy of reactions involving S2. The six composite methods have all shown consistency with each other and their calculated reaction energies/bond energies are in good agreement with the available literature. Kinetic parameters for calculation of the kinetic parameters on SO3 dissociation to SO2 and O using the canonical transition state theory are reported.

Quantum chemical study on the reaction mechanism and kinetics of Cl-initiated oxidation of methyl n-propyl ether

Oxidation of methyl n-propyl ether (CH3CH2CH2OCH3) molecule initiated by Cl atoms has been carried out using a dual level of quantum chemical investigation to understand the mechanistic pathways and kinetics of the H-atom abstraction reaction. Firstly, geometry optimization and frequency calculations for all the species are performed using BHandHLYP/6-311++G(d,p) level of theory at 298 K, and energetic calculations are further refined using CCSD(T) method with the same basis set to explore all stationary points on potential energy profile. We have observed from the energy profile that H-atom abstraction from –OCH2 group of CH3CH2CH2OCH3 is the kinetically predominant pathway. The reported bond dissociation energy for the dominant path is found to be in good agreement with the experimentally determined value. Further, standard Gibbs free energies (ΔG°298) and standard enthalpies (ΔH°298) analyses also indicate that the H-atom abstraction from –OCH2 group of CH3CH2CH2OCH3 is thermodynamically more favourable than other abstraction channels. The rate coefficients are also reported using canonical transition state theory, which is found to be in good agreement with the experimental data. The atmospheric lifetime of title molecule is also calculated.

Exploring unimolecular dissociation kinetics of ethyl dibromide through electronic structure calculations

Pathway for dissociation of multihalogenated alkyls is observed to be competitive between molecular and atomic elimination products. Factors such as molecular structure, temperature and pressure are known to influence the same. Hence present work is focussed to explore mechanism and kinetics of atomic (Br) and molecular (HBr and Br2) elimination upon pyrolysis of 1,1- and 1,2-ethyl dibromide (EDB). For this purpose, electronic structure calculations were performed at DFT and CCSD(T) level of theory. In addition to concerted mechanism, an alternate energetically efficient isomerisation pathway has been exploited for molecular elimination. Energy calculations are further complimented by detailed kinetic investigation, over wide range of temperature and pressure, using suitable models like Canonical Transition State Theory, Statistical Adiabatic Channel Model and Troe’s formalism. Our calculations suggest high branching ratio for dehydrohalogentation reaction, from both isomers of EDB. Fall off curve depicts good agreement between theoretically estimated and experimentally reported values.

Theoretical prediction of the mechanistic pathways and kinetics of methylcyclohexane initiated by OH radicals

ABSTRACTIn this manuscript, we have systematically depicted the theoretical prediction of H-absorption from methylcyclohexane initiated by OH radical. For this we have performed dual-level of quantum chemical calculations on the gas-phase reactions between methylcyclohexane (MCH) and OH radical. Geometry optimisation and vibrational frequency calculations have been performed at BHandHLYP/6-311G(d,p) level of theory along with energetic calculations at coupled cluster CCSD(T) method using the same basis set. All the stationary points of titled reaction have been located on the potential energy surface. It has also been found that the H-abstraction takes place from –CH site of MCH, which is the minimum energy pathway than others. The rate constant was calculated using canonical transition state theory for MCH with OH radical and is found to be 3.27 × 10−12 cm3 molecule−1 s−1, which is in sound agreement with reported experimental data. The atmospheric lifetime of MCH and branching ratios of the reaction channels are also reported in the manuscript.

Theoretical kinetics study of the reactions CHClBr + HBr ⇄ CH2ClBr + Br, CCl2Br + HBr ⇄ CHCl2Br + Br and CClBr2 + HBr ⇄ CHClBr2 + Br

The kinetics of CHClBr + HBr ⇄ CH2ClBr + Br (1, -1), CCl2Br + HBr ⇄ CHCl2Br + Br (2, -2) and CClBr2 + HBr ⇄ CHClBr2 + Br (3, -3) reactions at 293–787 K has been studied by using the canonical transition state theory with molecular information provided by different quantum chemical methods. The obtained rate constants (in cm3 molecule−1 s−1) are k1 = 5.24 × 10−13 exp[−1.47 kcal mol−1/RT], k−1 = 2.70 × 10−11 exp[−10.21 kcal mol−1/RT], k2 = 4.18 × 10−13 exp[−2.49 kcal mol−1/RT], k−2 = 6.96 × 10−12 exp[−7.36 kcal mol−1/RT], k3 = 3.29 × 10−13 exp[−2.20 kcal mol−1/RT], and k−3 = 8.45 × 10−13 exp[−7.10 kcal mol−1/RT]. Rate constants for (2, -2) and (3, -3) are here reported for the first time.

Quantum calculation on night-time degradation of 2-chloroethyl ethyl ether (CH3CH2OCH2CH2Cl) initiated by NO3 radical

A dual-level quantum chemical calculations have been carried out on the initiation of night-time degradation of 2-chloroethyl ethyl ether (CH3CH2OCH2CH2Cl) via H-abstraction by NO3 radical. Within the scope of density functional theory, the electronic structure of all the species involved in the titled reaction has been optimized at M06-2X functional along with 6-31[Formula: see text]G(d,p) basis set. A higher level of couple cluster CCSD(T) method in conjunction with 6-311[Formula: see text]G(d,p) basis set has been used for the refined energy of the species. All minima and saddle states involved in the reaction channel have been characterized on the potential energy surface (PES). From PES, it is confirmed that H-abstraction from methylene (–CH2–) of ethyl (CH3CH2–) part of CH3CH2OCH2CH2Cl follows the minimum energy path. The rate constants (individual and overall) of the titled reaction are obtained using Canonical Transition State Theory (CTST) over the temperature range of 250–350[Formula: see text]K. The atmospheric lifetime and radiative efficiency of the titled molecule have also been estimated, amounting to 0.23 years and 0.024 years, respectively. The Global Warming Potentials of the 2-chloroethyl ethyl ether in 20 years, 100 years and 500 years time horizon were found to be 0.13, 0.04 and 0.01, respectively.

H-X (X = H, CH3, CH2F, CHF2, CF3, and SiH3) Bond Activation by Criegee Intermediates: A Theoretical Perspective.

Using theoretical calculations and Born-Oppenheimer molecular dynamics simulations, it is shown here that Criegee intermediate, which is principally produced in the olefin ozonolysis, can activate H-X (X = H, CH3, CH2F, CHF2, CF3, and SiH3) under mild conditions, a reaction that has long been known for transition metals. The zwitterionic electronic structure of Criegee intermediate makes it an interesting metal-free system for activating enthalpically strong small molecules such as H2, methane, silanes, and boranes. The calculated barriers for the H2 or SiH4 reactions of CH2OO are significantly lower than those for the CH4 or its fluorinated analogue reactions. The distortion-interaction energy model is found to be successful in explaining the differential reactivity of the Criegee intermediate toward activating the various H-X bonds. The canonical transition state theory calculations suggest that the CH2OO-H2 reaction is 9-11 orders of magnitude faster than the CH2OO-CH4 reaction over the 200-300 K temperature range. Considering that the laboratory synthesis of Criegee intermediate is now feasible, these findings may open up new vistas in the metal-free activation of small molecules.

OH-initiated mechanistic pathways and kinetics of camphene and fate of product radical: a DFT approach.

Present manuscript represents the DFT studies on the oxidation reaction of camphene initiated by OH radical and fate of product radicals using M06-2X functional along with 6-31+G(d,p) basis set. Intrinsic reaction calculation is done for transition states involving OH-addition reactions which proceed via reaction complexes proceeding to the formation of transition states. The rate constant calculated by using canonical transition state theory at 298K and 1atm is found to be 5.67×10-11cm3molecule-1s-1 which is in good agreement with the experimental rate constant. The atmospheric lifetime of the titled molecule has also been reported in our work.