Eckart Tunneling Correction Research Articles

Rate rules for hydrogen abstraction reaction kinetics of polycyclic aromatic hydrocarbons and vinyl radical

Hydrogen abstraction reaction from polycyclic aromatic hydrocarbons (PAHs) is an important reaction class for PAHs consumption and soot formation. In this work, the reaction kinetics for a series of PAHs containing up to seven aromatic rings by vinyl radical are systematically investigated using the M06-2X/cc-pVTZ method. Based on the electronic structure calculations, the rate constants of title reactions are calculated by using transition state theory coupled with Eckart tunneling correction at the temperature range of 500–2500 K. The effects of PAH sizes, structures, and reaction sites on the rate constants are examined. The results show that the PAH sizes and reaction sites have little effect on the rate constants, while the structures of PAHs influence the rate constant significantly. Hence, the hydrogen abstraction reactions are simplified into C5 and C6 reaction classes depending on the abstraction site on the five- or six-membered ring. The simple two classes are conducive to construct the combustion model of PAHs. The reactivity with C6 class possesses the higher activity than the C5 class. Moreover, the difference in rate constants between the two classes is large at low temperatures while the two reaction classes are competitive above 1000 K. The rate rules are summarized by taking the average values of rate constants of a representative set of reactions in each class, which are applicable for the chemical model construction of PAHs.

Theoretical Kinetics Study Of The HO2 + C2H5OH Hydrogen Abstraction Reaction

C2H5OH has been using as an alternative fuel for decades; HO2 also plays a pivotal role in the combustion. The kinetics and mechanism for the reaction between C2H5OH and HO2 radical has been investigated using the molecular parameters for the reactants, transition states and products predicted at the CCSD(T)//B3LYP/6-311++G(3df,2p) level of theory. There are ten pair products have been found including C2H5O + H2O2 (PR1), CH3CHOH + H2O2 (PR2), CH2CH2OH + H2O2 (PR3), CH3CH2OOOH + H (PR4), C2H5 + HOOOH (PR5), CH3CH2OOH + OH (PR6), CH3CH(OH)OOH + H (PR7), HOCH2CH2OH + H (PR8), HOOCH3 + CH2OH (PR9), and CH3 + HOOCH2OH (PR10) in which the second and third ones are the major channels. The rate constants and branching ratios for all H-abstraction reactions have been calculated using the conventional transition state theory with asymmetric Eckart tunneling corrections for the temperature ranging from 298 to 2000 K.

Insight into the reaction mechanism of CH2SH with HO2: A density functional theory study

AbstractDensity functional theory was adopted in this work to reveal the reaction mechanism of CH2SH with HO2. Reaction rate constants were computed from 200 to 2000 K using the transition state theory combined with Wigner and Eckart tunneling correction. Moreover, localized orbital locator, atoms in molecules and Mayer bond order analyses were used to study the essence of chemical bonding evolution. Eleven singlet paths and three triplet ones are located on the potential surface (PES). The results show that the main products on the singlet PES are 1CH2S and H2O2, whereas on the triplet PES they are CH3SH + 3O2, which are coincident with the similar reaction of CH3S and HO2. This conclusion is also supported by rate constant calculation results. Interestingly, all the possible paths are involved in the hydrogen transfer. The results have provided underlying insights to the analogous reactions and further experimental studies.

Kinetic Study and Rate Coefficient Calculations of the Reaction of 1-Hydroxyethyl Radical with Nitric Oxide.

The kinetic study of the reaction of 1-hydroxyethyl radicals (CH3CHOH) with nitric oxide (NO) was performed over the temperature range of 200-1100 K and the pressure range of 1.0 × 10-5 to 10.0 bar. The geometries of all of the stationary points were optimized at the B3LYP/6-311++G(df,pd) level of theory, and the energetics were refined at the CCSD(T)/cc-pVTZ level of theory. Eight reaction pathways were explored, and they all consisted of a common first step involving the formation of a deep potential well. Three favorable pathways were confirmed, and they were the channels producing the adducts CH3CO(NHOH) and CH3NOHCHO and the products H2O and CH3CNO. The Rice-Ramsperger-Kassel -Marcus-canonical variational transition state theory method with Eckart tunneling correction was used to calculate the rate coefficients of the system. The predicted total rate coefficients agree well with the available literature data and show negative temperature dependence and positive pressure dependence. The reaction producing the adduct CH3CHOHNO in the entrance channel is dominant at 1.0 bar, and its branching ratio is almost 100% at a temperature less than 670 K. At 3.0 Torr, it is only dominant at a temperature less than 600 K.

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 Study on the Reaction of Nitric Oxide with Propargyl Radical.

The reaction of nitric oxide (NO) with propargyl radical (C3H3) was investigated at the CCSD(T)/cc-pVTZ//B3LYP/6-311++G(df, pd) level of theory. The rate coefficients of the system were determined by using the RRKM-CVT method with Eckart tunneling correction over a temperature range of 200-800 K and a pressure range of 1.0 × 10-4 to 10.0 bar. Eight channels proceeding via the barrierless formation of excited intermediate ONCH2CCH or CH2CCHNO at the first step were explored. Three favorable channels (i.e., channels producing adduct of ONCH2CCH and CH2CCHNO and products of HCN and H2CCO) were confirmed. The rate coefficients of channels producing adduct of ONCH2CCH and CH2CCHNO are comparable and have weak negative temperature dependence and positive pressure dependence. Channel producing products of HCN and H2CCO is more important at low pressure and high temperature and less important after pressure greater than 1.0 × 10-2 bar (with a branching ratio less than 6% at 0.1 bar).

Eyringpy: A program for computing rate constants in the gas phase and in solution

AbstractEyringpy is a modular program for calculating thermochemical properties and rate constants for reactions in the gas phase and in solution. The code is written in Python and it has a user‐friendly interface and a simple input format. Unimolecular and bimolecular reactions with one and two products are supported. Thermochemical properties are estimated through canonical ensemble and rate constants are computed according to the transition state theory. One‐dimensional Wigner and Eckart tunneling corrections are also available. Rate constants of bimolecular reactions involving the formation of pre‐reactive complexes are also estimated. To compute rate constants in solution, Eyringpy uses the Collins–Kimball theory to include the diffusion‐limit, the Marcus theory for electron transfer processes, and the molar fractions to account for the solvent pH effect.

Theoretical Investigation on Hydrogen Abstraction by NO2 from Symmetric Ethers (CH3)2 xO ( x = 1-4).

Hydrogen abstractions by NO2 from symmetric ethers are investigated to determine the rate constants and explore the effect of the functional group on rate constants at different reaction sites. The involved ethers are dimethyl ether (DME), diethyl ether (DEE), dipropyl ether (DPE), and dibutyl ether (DBE). The B3LYP method with a 6-31G(2df,p) basis set is employed to optimize the ground-state geometries and for frequency and intrinsic reaction coordinate calculations. The G4 method is used to calculate the electronic energies for the small ethers (DME and DEE). Given the heavy computational cost of the G4 method, the modified G4MP2 method is applied for larger ethers (DPE and DBE) and also for DME to verify the accuracy of the G4MP2 method by benchmarking with the G4 method. The high-pressure limit rate constants are calculated within the temperature range of 500-2000 K, with the asymmetrical Eckart tunneling correction as well as one-dimensional hindered rotor treatment. The calculated rate constants agree well with the literature data, and the branch ratio analysis suggests that the cis-HONO channel basically dominates the hydrogen abstraction reactions and shows a decrease at high temperatures, followed by HNO2 and trans-HONO channels; in addition, the hydrogen abstraction at the C site adjacent to the ether bond (α reaction site) accounts for most of the reactions. Furthermore, the total rate constants of the ethers are compared to those of their half-structurally alkanes, and linear Bell-Evans-Polanyi correlations are observed.

Theoretical Insight into the Kinetics of H‐Abstraction Reaction of CHF2CH2OH with OH Radical, Atmospheric Lifetime and Global Warming Potential

AbstractThe mechanistic, kinetic and thermochemical analysis of the reaction of CHF2CH2OH with OH radical have been carried out at the CCSD(T)/aug‐cc‐pVTZ//M06‐2X/6‐311++G(d,p) level. The rate coefficients for all the reaction channels are evaluated using conventional transition state theory with Eckart's tunneling correction over a temperature range of 250–500 K. Our calculated rate coefficients agree reasonably well with those reported from the experiments. Heats of formation of CHF2CH2OH molecule and C•F2CH2OH, CHF2C•HOH and CHF2CH2O• radicals have been reported for the first time. Atmospheric lifetime, Global Warming Potential (GWP) and atmospheric degradation pathway of CHF2CH2OH have also been determined in the present study.

Reaction Kinetics of Hydrogen Atom Abstraction from C4-C6 Alkenes by the Hydrogen Atom and Methyl Radical.

Alkenes are important ingredients of realistic fuels and are also critical intermediates during the combustion of a series of other fuels including alkanes, cycloalkanes, and biofuels. To provide insights into the combustion behavior of alkenes, detailed quantum chemical studies for crucial reactions are desired. Hydrogen abstractions of alkenes play a very important role in determining the reactivity of fuel molecules. This work is motivated by previous experimental and modeling evidence that current literature rate coefficients for the abstraction reactions of alkenes are still in need of refinement and/or redetermination. In light of this, this work reports a theoretical and kinetic study of hydrogen atom abstraction reactions from C4-C6 alkenes by the hydrogen (H) atom and methyl (CH3) radical. A series of C4-C6 alkene molecules with enough structural diversity are taken into consideration. Geometry and vibrational properties are determined at the B3LYP/6-31G(2df,p) level implemented in the Gaussian-4 (G4) composite method. The G4 level of theory is used to calculate the electronic single point energies for all species to determine the energy barriers. Conventional transition state theory with Eckart tunneling corrections is used to determine the high-pressure-limit rate constants for 47 elementary reaction rate coefficients. To faciliate their applications in kinetic modeling, the obtained rate constants are given in the Arrhenius expression and rate coefficients for typical reaction classes are recommended. The overall rate coefficients for the reaction of H atom and CH3 radical with all the studied alkenes are also compared. Branching ratios of these reaction channels for certain alkenes have also been analyzed.

Mechanistic Study of the Reactions of Methyl Peroxy Radical with Methanol or Hydroxyl Methyl Radical.

An ab initio and direct dynamic study of the reactions of CH3O2 + CH3OH and CH3O2 + CH2OH has been carried out over the temperature range of 300-1500 K. All stationary points were calculated at the MP2/aug-cc-pVTZ level of theory for CH3O2 + CH3OH or at the M06-2X/MG3S level of theory for CH3O2 + CH2OH and identified for the local minimum. The energetic parameters were refined at the QCISD(T)/cc-pVTZ and CCSD(T)/aug-cc-pVTZ levels of theory. For the reaction of CH3OO + CH3OH, two hydrogen abstraction channels producing CH3OOH + CH2OH (R1) and CH3OOH + CH3O (R2) were confirmed. These two channels consist of the same reversible first step involving the formation of a prereactive complex in the entrance channel. The rate constants of these two channels have been calculated by canonical transition station theory (TST) and canonical variational transition station theory (VTST) with Eckart tunneling correction and compared with the available literature data. The positive temperature dependence of the rate constants was observed. The tunneling effect is important at low temperature and decreases with an increase of the temperature. The contribution of R1 to the total rate constant is dominant, with branching ratios of 0.93 at 500 K and 0.67 at 1000 K, although the branching ratio for R2 increases dramatically with the increase of the temperature from 500 K. For the reaction of CH3OO + CH2OH, eight channels were explored on the lowest singlet and triplet surfaces, and an excited intermediate was found to be formed on the singlet surface. A channel proceeding through the formation of an excited intermediate followed by its impulsive dissociation was confirmed as the dominant channels with a branching ratio more than 0.99 in the temperature range of 300-1500 K, where products of CH3O and OCH2OH were given. The rate constant of the dominant channel calculated by multichannel RRKM-VTST is comparable with the available literature data.

Hydrogen abstraction from CH3NH2, (CH3)[formula omitted]NH, and (CH3)[formula omitted]N by H[formula omitted] radicals: A theoretical study

Hydrogen abstraction from CH3NH2, (CH3)2NH, and (CH3)3N by HȮ2 radicals, is investigated via quantum chemistry calculations.The B3LYP/6-31G(2df, p) method is employed in geometry optimizations, frequencies, and intrinsic reaction coordinate calculations. The G4 method is applied in energy and thermodynamical property calculations. Rate constants are obtained from variation transition state theory and conventional transition state theory calculations with one-dimensional hindered rotor treatment and Eckart tunneling correction. A branching ratio analysis for each reaction system is further performed.

Ab Initio Chemical Kinetics for SiHxReactions with Si2Hy(x= 1,2,3,4;y= 6,5,4,3;x+y= 7) under a-Si:H CVD Condition

Gas-phase reactions of SiHx with Si2Hy (x = 1,2,3,4; y = 6,5,4,3) species, respectively, which may coexist under chemical vapor deposition (CVD) conditions, have been investigated by means of ab initio molecular orbital and statistical theory calculations. Potential energy surface (PES) predicted at the CCSD(T)/CBS//B3LYP/6–311++G(3df,2p) level shows that these reactions take place primarily via trisilany radicals, n-Si3H7 and i-Si3H7. For example, SiH2 can associate with Si2H5 producing n-H2SiSiH2SiH3 exothermically by 55.8 kcal/mol; SiH3 can undergo addition to H2SiSiH2 to produce n-Si3H7 or associate with H3SiSiH barrierlessly forming i-Si3H7; whereas SiH can insert into one of the Si─H bonds of Si2H6 to give excited n-Si3H7. Similarly, H2SiSiH and SiSiH3 can undergo insertion reactions with SiH4 producing n/i-Si3H7 intermediates, respectively, to be followed by fragmentation to various smaller species. These processes are fully depicted in the complete PES. The predicted heats of formation of various species agree well with available thermochemical data. The rate constants and product branching ratios for the low-energy channel products have been calculated for the temperature range 300–1000 K by variational RRKM (Rice–Ramsperger–Kassel–Macus) theory with Eckart tunneling corrections. The results may be employed for realistic kinetic modeling of the plasma-enhanced chemical vapor deposition growth of a-Si:H thin films under practical conditions.

The Effect of Solvation on the Radiation Damage Rate Constants for Adenine.

It is well known that water plays an important part in almost all biological systems and that inclusion of solvation effects might therefore be of utmost importance in studies of radiation damage to DNA. In the present investigation, we have studied the effect of different solvation models in calculations of Gibbs free energies and reaction rates for the reaction between the OH radical and the DNA nucleobase adenine by conducting density functional theory calculations at the ωB97X-D/6-311++G(2df,2pd) level with the Eckart tunnelling correction. The solvent, water, was included through either the implicit polarizable continuum model (PCM) or through explicit modelling of micro-solvation by a single water molecule at the site of reaction as well as by the combination of both. Scrutiny of the thermodynamics and kinetics of the individual sub-reactions suggests that the qualitative differences introduced by the solvation models do not significantly alter the conclusions made based solely on simple gas-phase calculations. Abstraction of the amine hydrogen atoms H61 and H62 and addition onto C8 remain the most likely reaction pathways.

Theoretical Study on Gas Phase Reactions of OH Hydrogen-Abstraction from Formyl Fluoride with Different Catalysts

The mechanisms and kinetics of the gas phase reactions that the hydrogen atom in formyl fluoride (FCHO) abstracted by OH in the presence of water, formic acid (FA), or sulfuric acid (SA) are theoretically investigated at the CCSD(T)/6-311++G(3df, 3pd)//M06-2X/6-311++G(3df, 3pd) level of theory. The calculated results show that the barriers of the transition states involving catalysts are lowered to −2.89, −6.25, and −7.76 kcal/mol from 3.64 kcal/mol with respect to the separate reactants, respectively, which reflects that those catalysts play an important role in reducing the barrier of the hydrogen abstraction reaction of FCHO with OH. Additionally, using conventional transition state theory with Eckart tunneling correction, the kinetic data demonstrate that the entrance channel X⋯FCHO+OH (X=H2O, FA, or SA) is significantly more favorable than the pathway X⋯OH+FCHO. Moreover, the rate constants of the reactions of FCHO with OH radical with H2O, FA, or SA introduced are computed to be smaller than that of the naked OH+FCHO reaction because the concentration of the formed X⋯FCHO or X⋯OH complex is quite low in the atmosphere.

Ab initio and direct dynamics study on the hydrogen abstraction reaction C2H3 + CH3CHO

The mechanism for the hydrogen abstraction reaction C2H3+CH3CHO has been investigated by the CCSD(T)/cc-pVTZ method based on the geometries of stationary points optimized at the B3LYP/6-311++G(d,p) level of theory. Two abstraction channels have been identified for the production of C2H4+CH2CHO and C2H4+CH3CO. The potential barrier heights of the corresponding transition states TSR/P1 and TSR/P2 were predicted to be 9.47 and 5.95kcal/mol at the CCSD(T)//B3LYP level of theory, respectively. The rate constants and branching ratios for the two H-abstraction channels were calculated using conventional transition state theory with Eckart tunneling correction at the temperature range 300–2500K. The predicted rate constants have been compared with available literature data. Both the forward and reverse rate constants have positive temperature dependence and the tunneling effect is only important at low temperatures. The branching ratio calculation shows that the channel producing C2H4+CH3CO remains predominant throughout the entire studied temperature range.

Ab initio and direct dynamics study on the C2H3 + CH3CH2OH reaction

ABSTRACTThe mechanism for the C2H3 + CH3CH2OH reaction has been investigated by the CCSD(T)/cc-pVTZ method based on the geometries of stationary points optimised at the B3LYP/6-311++G(d, p) level of theory. Five reaction channels have been identified for the production of C2H4 + CH2CH2OH, C2H4 + CH3CH2O, C2H4 + CH3CHOH, C2H3OH + CH3CH2, and C2H3OCH2CH3 + H (denoted as channels (1)–(5), respectively). The potential barrier heights of the corresponding transition states TSR/P1, TSR/P2, TSR/P3, TSR/P4 and TSR/P5 were predicted to be 12.40, 8.50, 8.11, 38.38 and 42.21 kcal/mol at the CCSD(T)//B3LYP level of theory, respectively. The rate constants and branching ratios for the three lower energy H-abstraction channels were calculated using conventional transition state theory with Eckart tunnelling correction at the temperature range 300–2500 K. The predicted rate constants have been compared with the available literature data. Both the forward and reverse rate constants have positive temperature dependence, and the tunnelling effect is only important at low temperatures. The branching ratio calculation shows that channel (2) is predominant and channel (3) is less competitive, while channel (1) is almost neglectable at low temperatures. However, all three channels are important in high-temperature region.

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.

Insight into the Mechanism of the Initial Reaction of an OH Radical with DNA/RNA Nucleobases: A Computational Investigation of Radiation Damage.

Earlier theoretical investigations of the mechanism of radiation damage to DNA/RNA nucleobases have claimed OH radical addition as the dominating pathway based solely on energetics. In this study we supplement calculations of energies with the kinetics of all possible reactions with the OH radical through hydrogen abstraction and OH radical addition onto carbon sites, using DFT at the ωB97X-D/6-311++G(2df,2pd) level with the Eckart tunneling correction. The overall rate constants for the reaction with adenine, guanine, thymine, and uracil are found to be 2.17×10(-12) , 5.64×10(-11) , 2.01×10(-11) , and 5.03×10(-12) cm(3) molecules(-1) s(-1) , respectively, which agree exceptionally well with experimental values. We conclude that abstraction of the amine group hydrogen atoms competes with addition onto C8 as the most important reaction pathway for the purine nucleobases, while for the pyrimidine nucleobases addition onto C5 and C6 competes with the abstraction of H1 . Thymine shows favourability against abstraction of methyl hydrogens as the dominating pathway based on rate constants. These mechanistic conclusions are partly explained by an analysis of the electrostatic potential together with HOMO and LUMO orbitals of the nucleobases.

Kinetic mechanisms of hydrogen abstraction reactions from methanol by methyl, triplet methylene and formyl radicals

Although H-abstraction reactions from methanol by methyl radical (CH3), triplet methylene radical (3CH2) and formyl radical (HCO) are important for methanol pyrolysis and combustion, the kinetic mechanisms of those reactions are indistinct and need to be better understood. Ab initio calculations were performed to optimize geometries of transition states using Gaussian 09W package with the B3LYP density functional at the 6-311++G (3df, 3pd) basis set. Energy barrier and minimum energy path (MEP) on the reaction potential energy surface were calculated with MP2 method at the same basis set. The canonical variational transition state theory (CVT) with Eckart tunneling correction were employed to compute rate constants over the temperature range 298–2000K. Results show that, for reaction CH3OH+CH3, barrier height for hydroxymethyl channel (R1a) is 12.59kcal/mol and for methoxyl channel (R1b) is 13.82kcal/mol. Rate constants for channels R1a and R1b are k1a=3.635×10−31T5.58exp(−1962/T)cm3molecule−1s−1, k1b=4.342×10−28T4.61exp(−4284/T)cm3molecule−1s−1, respectively. For reaction CH3OH+3CH2, barrier heights for hydroxymethyl channel (R2a) and methoxyl channel (R2b) are 7.68kcal/mol and 9.19kcal/mol, respectively. Rate constant for channel R2a is k2a=5.885×10−28T4.19exp(−1813/T)cm3molecule−1s−1 and for channel R2b is k2b=2.266×10−28T4.26exp(−2833/T)cm3molecule−1s−1. For reaction CH3OH+HCO, barrier heights for hydroxymethyl channel (R3a) and methoxyl channel (R3b) are 21.88kcal/mol and 28.83kcal/mol, respectively. Rate constant for channel R3a is k3a=8.156×10−38T7.37exp(−4900/T)cm3molecule−1s−1 and for channel R3b is k3b=2.007×10−27T4.34exp(−11,605/T)cm3molecule−1s−1. This study is beneficial for improving the kinetic mechanism of methanol pyrolysis and combustion.