Reaction Class Transition State Research Articles

Reaction Rate Rules of Intramolecular H-Migration Reaction Class for RIORIIOO·Radicals in Ether Combustion.

The intramolecular H-migration reaction of RIORIIOO· radicals constitute a key class of reactions in the low-temperature combustion mechanism of ethers. Despite this, there is a dearth of direct computations regarding the potential energy surface and rate constants specific to ethers, especially when considering large molecular systems and intricate branched-chain structures. Furthermore, combustion kinetic models for large molecular ethers generally utilize rate constants derived from those of structurally similar alcohols or alkane fuels. Consequently, chemical kinetic studies involve the calculation of energy barriers and rate rules for the intramolecular H-migration reaction class of RIORIIOO· radicals, which are systematically conducted using the isodesmic reaction method (IRM). The geometries of the species participating in these reactions are optimized, and frequency calculations are executed using the M06-X method in tandem with the 6-31+G(d,p) basis set by the Gaussian 16 program. Moreover, the M06-2X/6-31+G(d,p) method acts as the low-level ab initio method, while the CBS-QB3 method is utilized as the high-level ab initio method for calculating single-point energies. Rate constants at the high-pressure-limit are computed based on the reaction class transition state theory (RC-TST) by ChemRate program, incorporating asymmetric Eckart tunneling corrections for intramolecular H-migration reactions across a temperature range of 500 to 2000 K. It was found that the isodesmic reaction method gives accurate energy barriers and rate constants, and the rate constants of the H-migration reaction for RIORIIOO· radicals diverge from those of comparable reactions in alkanes and alcohol fuels. There are significant disparities in energy barriers and rate constants across the entire reaction classes of the H-migration reaction for RIORIIOO· radicals, necessitating the subdivision of the H-migration reaction into subclasses. Rate rules are established by averaging the rate constants of representative reactions for each subclass, which is pivotal for the advancement of accurate low-temperature combustion reaction mechanisms for ethers.

High-Pressure-Limit Rate Coefficients for HO2 Elimination Reactions of Hydroperoxyalkenylperoxy Radicals based on the Reaction Class Transition State Theory.

Thermokinetic parameters and transport parameters are of great importance to the combustion model and the reaction rate rules are of great importance to construct the combustion reaction mechanism for hydrocarbon fuels. The HO2 elimination reaction class for hydroperoxyalkenylperoxy radicals is one of the key reaction classes for olefin, for which the rate coefficients are lacking. Therefore, the rate coefficients and rate rules of the HO2 elimination reaction class for hydroperoxyalkenylperoxy radicals are studied in this work. The reaction class transition state theory (RC-TST) is used to calculate the rate coefficients. In addition, the HO2 elimination reaction class of hydroperoxyalkenylperoxy radicals is divided into four subclasses depending upon the type of H–Cβ bond that is broken in the reactant molecules, and the rate rules are calculated by taking the average of rate coefficients from a representative set of reactions in a subclass. The calculated kinetics data would be valuable for the construction of the combustion reaction mechanism for olefin.

On-The-Fly Kinetics of the Hydrogen Abstraction by Hydroperoxyl Radical: An Application of the Reaction Class Transition State Theory.

A Reaction Class Transition State Theory (RC-TST) is applied to calculate thermal rate constants for hydrogen abstraction by OOH radical from alkanes in the temperature range of 300–2500 K. The rate constants for the reference reaction C2H6 + ∙OOH → ∙C2H5 + H2O2, is obtained with the Canonical Variational Transition State Theory (CVT) augmented with the Small Curvature Tunneling (SCT) correction. The necessary parameters were obtained from M06-2X/aug-cc-pVTZ data for a training set of 24 reactions. Depending on the approximation employed, only the reaction energy or no additional parameters are needed to predict the RC-TST rates for other class representatives. Although each of the reactions can in principle be investigated at higher levels of theory, the approach provides a nearly equally reliable rate constant at a fraction of the cost needed for larger and higher level calculations. The systematic error is smaller than 50% in comparison with high level computations. Satisfactory agreement with literature data, augmented by the lack of necessity of tedious and time consuming transition state calculations, facilitated the seamless application of the proposed methodology to the Automated Reaction Mechanism Generators (ARMGs) programs.

Kinetics of the hydrogen abstraction alkane + O2 → alkyl + HO2 reaction class: an application of the reaction class transition state theory

In this work, the kinetics of reaction class of hydrogen abstraction from saturated hydrocarbons by O2 molecules has been studied. The high-pressure reaction rate constants were determined using reaction class transition state theory/structure-activity relationship (RC-TST/SAR) methodology, augmented by linear energy relationship (LER) and/or barrier height grouping (BHG) approximations for evaluation of the reaction barrier heights. The parameters needed have been derived from DFT calculations at M06-2X/aug-cc-pVTZ level for a training set of 23 reactions, involving hydrogen abstraction by O2 molecule at primary, secondary, and tertiary carbon sites. The reference reaction rate constant C2H6 + O2 → C2H5 + HO2 was obtained by extrapolation of the simplest reaction within the title family CH4 + O2 → CH3 + HO2. Kinetic parameters of the later one, calculated from canonical variational transition state theory (CVT), were taken from literature. The influence of low-frequency internal rotations has been investigated in details. The error analysis shows that the average systematic error of RC-TST/SAR-derived rate constants at low temperatures is within 25% compared to the explicit RC-TST results and diminishes at higher temperatures. This suggests that the proposed methodology can be effectively implemented in the automated mechanism generation codes to create the fuel combustion mechanisms.

Kinetics of the Hydrogen Abstraction PAH + •OH → PAH Radical + H2O Reaction Class: An Application of the Reaction Class Transition State Theory (RC-TST) and Structure-Activity Relationship (SAR).

A reaction class transition state theory (RC-TST) augmented with structure-activity relationship (SAR) methodology is applied to predict high-pressure limit thermal rate constants for hydrogen abstraction by •OH radical from polycyclic aromatic hydrocarbons (PAHs) reaction class in the temperature range of 300-3000 K. The rate constants for the reference reaction of C6H6 + •OH → C6H5 + H2O is calculated by the canonical variational transition state theory (CVT) with small curvature tunneling (SCT). Only the reaction energy is needed to predict RC-TST rates for other processes within the family, the parameters needed were obtained from M06-2X/cc-pVTZ data for a training set of 34 reactions. The systematic error of the resulting RC-TST rates is smaller than 50% in comparison with explicit rate calculations, which facilitates application of the proposed methodology to the automated reaction mechanism generators (ARMGs) schemes.

Advances in modeling hydrocarbon cracking kinetic predictions by quantum chemical theory: A review

In the modeling of hydrocarbon thermal cracking processes, complicated reaction networks of multicomponent reactants make kinetic parameter predictions very challenging. To solve this problem, the additivity method and Evans-Polanyi equation were proposed for the prediction of kinetic factors. As the calculation of kinetic parameters using quantum chemical theory was used for higher accuracy, new additivity methods at the quantum theory level were developed for the prediction of thermodynamic kinetic parameters. At the same time, a reaction class transition state theory RC-TST, analogous to the Evans-Polanyi equation, was proposed by considering quantum factors for kinetic constant evaluation. Quantum calculation methods promoted mechanistic studies of electronic effects to accurately reveal the kinetic constant rules with different structural factors. Under these circumstances, kinetic parameter predictions by different quantum chemical computation methods were firstly introduced and reviewed. Meanwhile, development of the new additivity method and RC-TST theory at the quantum chemistry level were summarized, and the new outlook on electronic effects with an appropriate quantum chemical computational method into the new additivity method was conducted to develop more perspicuous and accuracy in a simplified method for the calculation of complicated thermal cracking reaction.

On-the-fly kinetics of hydrogen abstraction from polycyclic aromatic hydrocarbons by methyl/ethyl radicals.

This work provides a rigorous procedure, within the framework of the Reaction Class Transition State Theory (RC-TST) and the Structure-Activity Relationship (SAR), for predicting reliable thermal rate constants on-the-fly for hydrogen abstraction reactions by methyl/ethyl radicals from Polycyclic Aromatic Hydrocarbons (PAHs) in a temperature range of 300-3000 K. All necessary RC-TST parameters were derived from ab initio calculations for a representative set of 36 reactions on which different error analyses and comparisons with available literature data were carried out. In addition to the good agreement between the RC-TST rate constants and the literature data, the detailed error analyses show that RC-TST/SAR, utilizing either the Linear Energy Relationship (LER) where only the reaction energy is needed or Barrier Height Grouping (BHG) where no additional data is needed, can predict the thermal rate constants for any reaction in the title reaction class with an average systematic error of less than 50% when compared to the explicit rate calculations. Therefore, the constructed RC-TST procedure can be confidently used to obtain reliable rate constants on the fly in an attempt to effectively construct detailed kinetic mechanisms for PAH-related fuels.

Calculation of the rate constants for concerted elimination reaction class of hydroperoxyl-alkyl-peroxyl radicals

Hydroperoxyl-alkyl-peroxyl radicals (O2QOOH) are an important radical intermediate in low-temperature combustion, and its concerted elimination reaction to produce Q″OOH and HO2 is an important reaction class in low-temperature combustion modeling of hydrocarbons. In this paper, isodesmic reaction method is used for the accurate calculation of rate constants for a representative set of 45 reactions in the class. All geometries of the reactants, the transition states and the products are optimized under the B3LYP/6-311 + G(d, p) level. It is shown that the geometries at the reaction center of the transition states in this reaction class are conserved, and hence, the minus of any two reactions for the formation of transition states can be considered as an isodesmic reaction. One reaction is called reference reaction and as is usually done in reaction class-transition state theory, the reaction in the class with the minimum size is chosen as the reference reaction, and other reactions are called target reactions. According to isodesmic reaction theory, it can be derived that the energy difference between the high-level ab initio result and the low-level result for the reference reaction can be used to correct the low-level energy barriers for the target reactions. In this study, B3LYP/6-311 + G(d, p) method is used as the low-level ab initio method and the composite Gaussian-4 (G4) method is used as the high-level ab initio method. CCSD(T)/cc-PVTZ method is chosen as the benchmarking method to validate the accuracy of G4 method through a comparison of the energy barriers for three selected small molecular reactions calculated by G4 method with the results by CCSD(T)/cc-PVTZ method, and it is found that the difference of the energy barriers by these two methods are between 0.35 and 2.11 kJ/mol, indicating that the accuracy of G4 method is close to that of the benchmarking method. Meanwhile, ten representative target reactions in this class from the O2QOOH to Q″OOH + HO2 are selected to validate the correction scheme through a comparison of the energy barriers before and after correction with the energy barriers by G4 method. It is found that the deviations of the energy barriers calculated by direct DFT method are between 16.58 and 24.95 kJ/mol, and after correction, the deviations of the energy barriers are reduced to −3.28 to 5.09 kJ/mol, indicating that the results of the energy barriers corrected by the isodesmic reaction method are close to that of the G4 method. Finally, using this correction scheme, the energy barriers, reaction enthalpies and rate constants of all reactions are calculated and kinetic parameters in the form of (A, n, E) are provided for this important elimination reaction class of hydroperoxyl-alkyl-peroxyl radicals (O2QOOH), which is useful for the simulation of low-temperature combustion for hydrocarbon.

DFT Studies of SN2 Dechlorination of Polychlorinated Biphenyls.

Nucleophilic dechlorination of all 209 PCBs congeners by ethylene glycol anion has been studied theoretically at the DFT level. The obtained Gibbs free energies of activation are in the range 7-22 kcal/mol. The reaction Gibbs free energies indicate that all reactions are virtually irreversible. Due to geometric constrains these reactions undergo rather untypical attack with attacking oxygen atom being nearly perpendicular to the attacked C-Cl bond. The most prone to substitution are chlorine atoms that occupy ortho- (2, 2', 6, 6') positions. These results provide extensive information on the PEG/KOH dependent PCBs degradation. They can also be used in further developments of reaction class transition state theory (RC-TST) for description of complex reactive systems encountered for example in combustion processes.

ChemInform Abstract: Performance of First‐Principles‐Based Reaction Class Transition State Theory

AbstractReview: 96 refs.

Performance of First-Principles-Based Reaction Class Transition State Theory.

Performance of the Reaction Class Transition State Theory (RC-TST) for prediction of rates constants of elementary reactions is examined using data from its previous applications to a number of different reaction classes. The RC-TST theory is taking advantage of the common structure denominator of all reactions in a given family combined with structure activity relationships to provide a rigorous theoretical framework to obtain rate expression of any reaction within a reaction class in a simple and cost-effective manner. This opens the possibility for integrating this methodology with an automated mechanism generator for "on-the-fly" generation of accurate kinetic models of complex reacting systems.

First-principles kinetics of n-octyl radicals

Kinetics of the isomerisation and unimolecular degradation of n-octyl radicals have been studied with the reaction class transition state theory (RC-TST) method. To explore the kinetics of the 1,7-H migration reactions family, the accurate high-pressure limits of the rate constants for the reference reaction of this class (1-heptyl → 1-heptyl) have been calculated. Finally, both the achievements reported in this paper and previous developments are employed to obtain theoretical branching ratios of intramolecular H-transfers and unimolecular degradations of all possible n-octyl radicals; the results are in satisfactory agreement when compared to experiment. The application of the rates obtained to the simulation of a simple reactor is also reported.

An application of the reaction class transition state theory to the kinetics of hydrogen abstraction reactions of hydrogen with methyl esters at the methoxy group

The hydrogen abstraction reactions of methyl esters from methyl formate (MF) to methyl decanoate (MD) by hydrogen (H) atom at the methoxy group have been investigated on the basis of isodesmic reactions within the framework of reaction class transition state theory (RC-TST). The rate constants for the four reactions of H with MF, methyl acetate (MA), methyl propanoate (MP), and methyl butanoate (MB) are used as the benchmarking values, and are calculated via the transition state theory (TST) with the Eckart method to account for tunneling effect by employing potential energy surface information computed at the CCSD(T)/CBS//BH&HLYP/6-31G(d,p) level. The calculated rate constants of reactions H with MF and MA are in excellent agreement with experimental results. For the target reactions of H with MA, MP and MB, it is demonstrated that the calculated energy barriers and rate constants at the BH&HLYP/6-31G(d,p) level with a simple correction scheme based on isodesmic reactions within the framework of RC-TST can approach the accuracy of the CCSD(T)/CBS level. Based on the correction scheme, reaction enthalpies, energy barriers and rate constants for large reaction systems including the reactions of H with methyl valerate (MV), methyl hexanoate (MHex), methyl heptanoate (MHep), methyl octanoate (MO), methyl nonanoate (MN) and MD at the methoxy group are computed and fitted into the modified Arrhenius expression in the temperature range 300–2500K for chemical kinetic modeling studies. Further, it is found that the reaction rate constants of H with large methyl ester molecules (from MA to MD) is nearly identical and is slightly larger than that of H with MF at the methoxy group.

Interpretation and Application of Reaction Class Transition State Theory for Accurate Calculation of Thermokinetic Parameters Using Isodesmic Reaction Method

We present a further interpretation of reaction class transition state theory (RC-TST) proposed by Truong et al. for the accurate calculation of rate coefficients for reactions in a class. It is found that the RC-TST can be interpreted through the isodesmic reaction method, which is usually used to calculate reaction enthalpy or enthalpy of formation for a species, and the theory can also be used for the calculation of the reaction barriers and reaction enthalpies for reactions in a class. A correction scheme based on this theory is proposed for the calculation of the reaction barriers and reaction enthalpies for reactions in a class. To validate the scheme, 16 combinations of various ab initio levels with various basis sets are used as the approximate methods and CCSD(T)/CBS method is used as the benchmarking method in this study to calculate the reaction energies and energy barriers for a representative set of five reactions from the reaction class: R(c)CH(R(b))CR(a)CH2 + OH(•) → R(c)C(•)(R(b))CR(a)CH2 + H2O (R(a), R(b), and R(c) in the reaction formula represent the alkyl or hydrogen). Then the results of the approximate methods are corrected by the theory. The maximum values of the average deviations of the energy barrier and the reaction enthalpy are 99.97 kJ/mol and 70.35 kJ/mol, respectively, before correction and are reduced to 4.02 kJ/mol and 8.19 kJ/mol, respectively, after correction, indicating that after correction the results are not sensitive to the level of the ab initio method and the size of the basis set, as they are in the case before correction. Therefore, reaction energies and energy barriers for reactions in a class can be calculated accurately at a relatively low level of ab initio method using our scheme. It is also shown that the rate coefficients for the five representative reactions calculated at the BHandHLYP/6-31G(d,p) level of theory via our scheme are very close to the values calculated at CCSD(T)/CBS level. Finally, reaction barriers and reaction enthalpies and rate coefficients of all the target reactions calculated at the BHandHLYP/6-31G(d,p) level of theory via the same scheme are provided.

Application of the reaction class transition state theory to the kinetics of hydrogen abstraction reactions of alkanes by atomic chlorine

Kinetics of the hydrogen abstraction reaction of the class alkane+Cl→alkyl+HCl was studied using reaction class transition state theory (RC-TST) combined with linear energy relationships (LERs). The thermal rate coefficients for the reference reaction of ethane+Cl→ethyl+HCl, calculated by the microcanonical variational transition state theory (μVT) incorporating small curvature tunneling (SCT), were taken from the literature. All necessary parameters were derived from density functional theory (DFT) calculations for a representative set of 29 reactions involving a range of hydrocarbons. Direct comparison to available experimental data reveals that the RC-TST/LER can predict rate coefficients for any reaction in the reaction class with acceptable accuracy. For the two test reactions outside of the representative set, our derived rate coefficients were in reasonable agreement with available data. Furthermore, our analyses indicate that RC-TST/LER gave systematic errors of less than 25% when compared to TST with one-dimensional Eckart tunneling approximation rate coefficients.

Kinetics of the hydrogen abstraction ·C2H5 + alkane → C2H6 + alkyl reaction class: an application of the reaction class transition state theory

This paper presents an application of the reaction class transition state theory (RC-TST) to predict thermal rate constants for hydrogen abstraction reactions at alkane by the ·C2H5 radical on-the-fly. The linear energy relationship (LER), developed for acyclic alkanes, was also proven to hold for cyclic alkanes. We have derived all RC-TST parameters from rate constants of 19 representative reactions, coupling with LER and the barrier height grouping (BHG) approach. Both the RC-TST/LER, where only reaction energy is needed, and the RC-TST/BHG, where no other information is needed, can predict rate constants for any reaction in this reaction class with satisfactory accuracy for combustion modeling. Our analysis indicates that less than 50 % systematic errors on the average exist in the predicted rate constants using either the RC-TST/LER or RC-TST/BHG method, while in comparison with explicit rate calculations, the differences are within a factor of 2 on the average. The results also show that the RC-TST method is not sensitive to the choice of density functional theory used.

Kinetics of 1,6-hydrogen migration in alkyl radical reaction class

The kinetics of the 1,6-intramolecular hydrogen migration in the alkyl radical reaction class has been studied using the reaction class transition state theory (RC-TST) combined with the linear energy relationship (LER) and the barrier height grouping (BHG) approach. The RC-TST/LER, where only reaction energy is needed, and RC-TST/BHG, where no other information is needed, are found to be promising methods for predicting rate constants for any reaction in the 1,6-intramolecular H migration in alkyl radicals reaction class. Direct comparison with available experimental data indicates that the RC-TST/LER, where only reaction energy is needed, can predict rate constants for any reaction in this reaction class with satisfactory accuracy.

Kinetics of the C–C Bond Beta Scission Reactions in Alkyl Radical Reaction Class

Kinetics of the β-scission in alkyl radical reaction class was studied using the reaction class transition state theory (RC-TST) combined with the linear energy relationship (LER) and the barrier height grouping (BHG) approach. All necessary parameters were derived from first-principle density functional calculations for a representative set of 21 reactions. Different error analyses and comparisons with available literature data were made. Direct comparison with available experimental data indicates that the RC-TST/LER, where only reaction energy is needed, can predict rate constants for any reaction in this reaction class with excellent accuracy. Specifically for this reaction class, the RC-TST/LER method has less than 60% systematic errors on average in the predicted rate constants when compared to explicit rate calculations.

Investigations on the Kinetics for the Reaction Class of Hydrogen Abstractions from Substituted Cyclohexane by Hydrogen Atom

This work presents an application of the reaction class transition state theory(RC-TST) to predict kinetics parameters for hydrogen abstraction reactions of the substituted cyclohexane.All parameters for the RC-TST method of this reaction class were derived from the 14 representative reactions in this class.And the linear energy relationship(LER) was also established.The rate constants for selected reactions predicted by the RC-TST/LER are in good agreement with those calculated with TST/Eckart method.Compared with results from the TST/Eckart calculations,the RC-TST/LER method is found to be quite efficient and time-saving.

Kinetics of 1,5-Hydrogen Migration in Alkyl Radical Reaction Class

Kinetics of the 1,5-intramolecular hydrogen migration in the alkyl radicals reaction class has been studied using the reaction class transition state theory combined with the linear energy relationship (LER) and the barrier height grouping (BHG) approach. The high pressure limits of the rate constants for the reference reaction of 1-pentyl → 1-pentyl, calculated by the Canonical Variational Transition State Theory (CVT) with the Small Curvature Tunneling (SCT), are taken from the literature. Direct comparison with available experimental data indicates that the RC-TST/LER, where only reaction energy is needed, can predict rate constants for any reaction in this reaction class with excellent accuracy. Specifically for this reaction class, the RC-TST/LER method has less than 65% systematic errors in the predicted rate constants when compared to explicit rate calculations.