Showing papers on "Elementary reaction published in 2022"

Quantum Chemical Calculations to Trace Back Reaction Paths for the Prediction of Reactants

Abstract: The long-due development of a computational method for the ab initio prediction of chemical reactants that provide a target compound has been hampered by the combinatorial explosion that occurs when reactions consist of multiple elementary reaction processes. To address this challenge, we have developed a quantum chemical calculation method that can enumerate the reactant candidates from a given target compound by combining an exhaustive automated reaction path search method with a kinetics method for narrowing down the possibilities. Two conventional name reactions were then assessed by tracing back the reaction paths using this new method to determine whether the known reactants could be identified. Our method is expected to be a powerful tool for the prediction of reactants and the discovery of new reactions.

Theoretical chemical reaction database construction based on quantum chemistry-aided retrosynthetic analysis

Abstract: A theoretical database comprising experimentally accessible and inaccessible chemical reactions could complement the existing experimental databases and contribute significantly to data-driven chemical reaction discovery. Quantum chemistry-aided retrosynthetic analysis (QCaRA) can generate a network of elementary steps called a reaction-path network and predict hundreds or more of chemical reactions along with their theoretical yields. In contrast to ordinary simulations, QCaRA traces back the reaction paths from the target product to various reactant candidates while solving the kinetic equations. In this study, we propose theoretical reaction database construction based on QCaRA. Seven reaction-path networks containing 13,190 reactions, 108,754 reaction paths, and 2,552,652 geometries have been identified and discussed as examples. In addition to well-known reactions (i.e., synthesis of fluoroglycine, Wöhler’s urea synthesis, base-catalysed aldol reaction, Lewis-acid-catalysed ene reaction, cobalt-catalysed hydroformylation, Strecker reaction, and Passerini reaction), numerous unexplored reactions with high, medium, low, near-zero, or zero yields have been identified. We anticipate that such a QCaRA-based theoretical reaction database will provide information on hitherto unexplored reactivities, especially those that are experimentally inaccessible.

New Insights into the (A)Synchronicity of Diels–Alder Reactions: A Theoretical Study Based on the Reaction Force Analysis and Atomic Resolution of Energy Derivatives

Abstract: In the present manuscript, we report new insights into the concept of (a)synchronicity in Diels–Alder (DA) reactions in the framework of the reaction force analysis in conjunction with natural population calculations and the atomic resolution of energy derivatives along the intrinsic reaction coordinate (IRC) path. Our findings suggest that the DA reaction transitions from a preferentially concerted mechanism to a stepwise one in a 0.10 Å window of synchronicity indices ranging from 0.90 to 1.00 Å. We have also shown that the relative position of the global minimum of the reaction force constant with respect to the TS is an alternative and quantifiable indicator of the (a)synchronicity in DA reactions. Moreover, the atomic resolution of energy derivatives reveals that the mechanism of the DA reaction involves two inner elementary processes associated with the formation of each of the two C-C bonds. This resolution goes on to indicate that, in asynchronous reactions, the driving and retarding components of the reaction force are mostly due to the fast and slow-forming C-C bonds (elementary processes) respectively, while in synchronous reactions, both elementary processes retard and drive the process concomitantly and equivalently.

Product molecule numbers and reaction rate fluctuations in elementary reactions

Abstract: In many chemical reactions, reaction rate fluctuations are inevitable. Whenever chemical reactions occur, reaction rates vary due to their dependence on the number of reaction events or products. Accordingly, understanding the impact of rate fluctuations on the product number counting statistics is of the utmost importance when developing a quantitative explanation of chemical reactions. In this work, we examine the relationship between the reaction rate and product number fluctuations. Product number counting statistics uncover stochastic properties of the product number; the latter directly manipulates the reaction rate. Specifically, we find that the product number displays super-Poisson characteristics as the reaction rate increases with the product number. On the other hand, when the product number shows sub-Poisson behavior, a decrease in the reaction rate follows. Furthermore, our analysis, dealing with reaction rate fluctuations, allows for quantifying the deviations of an elementary reaction process from a renewal process.

A Hierarchical Theoretical Study of the Hydrogen Abstraction Reactions of H2/C1–C4 Molecules by the Methyl Peroxy Radical and Implications for Kinetic Modeling

Abstract: The hydrogen atom abstraction by the methyl peroxy radical (CH3O2) is an important reaction class in detailed chemical kinetic modeling of the autoignition properties of hydrocarbon fuels. Systematic theoretical studies are performed on this reaction class for H2/C1–C4 fuels, which is critical in the development of a base model for large fuels. The molecules include hydrogen, alkanes, alkenes, and alkynes with a carbon number from 1 to 4. The B2PLYP-D3/cc-pVTZ level of theory is employed to optimize the geometries of all of the reactants, transition states, and products and also the treatments of hindered rotation for lower frequency modes. Accurate benchmark calculations for abstraction reactions of hydrogen, methane, and ethylene with CH3O2 are performed by using the coupled cluster method with explicit inclusion of single and double electron excitations and perturbative inclusion of triple electron excitations (CCSD(T)), the domain-based local pair-natural orbital coupled cluster method (DLPNO-CCSD(T)), and the explicitly correlated CCSD(T)-F12 method with large basis sets. Reaction rate constants are computed via conventional transition state theory with quantum tunneling corrections. The computed rate constants are compared with literature values and those employed in detailed chemical kinetic mechanisms. The calculated rate constants are implemented into the recently developed NUIGMECH1.1 base model for kinetic modeling of ignition properties.

Effect of Ammonia Addition on the Ignition Delay Mechanism of Methyl Decanoate

Abstract: In this study, the effect of mixing a small amount of ammonia on the ignition delay time of methyl decanoate under different conditions was studied from the perspective of the combustion mechanism. The effect of adding ammonia on the ignition delay time of methyl decanoate at different pressures and temperatures was studied by means of simulation calculations and numerical comparison. Integrating the detailed mechanism and reaction path of methyl decanoate, the sensitivity of the ignition delay time was investigated. Analyses of the ignition delay time and rate of production were conducted to explore the transformation and influence of ammonia on the oxidation/decomposition process of the main elementary reaction during the ignition of methyl decanoate. The research illustrated that the ignition delay time of methyl decanoate increased with the number of moles of mixed ammonia at a certain temperature range, and in the negative temperature coefficient region, the effect of ammonia on the ignition delay time was the greatest. In addition, the susceptibility and yield analysis of methyl decanoate showed that the addition of ammonia had a weakening effect on the elementary reactions that originally promoted and inhibited methyl decanoate, and its consumption and production rates were reduced.

Combustion Characteristic of 2,3,3,3-tetrafluroropropene (R1234yf)

Abstract: To describe the combustion process of hydrofluoroolefins (HFOs) 2,3,3,3-tetrafluroropropene (R1234yf), a kinetics mechanism was developed. On the M06–2X/6–311++G(d,p) level, dynamic calculations have been performed using the density function theory (DFT) method. Proposed R1234yf kinetic reactions included unimolecular decomposition and intermolecular reactions, such as collisions with O2, OH, and H radicals. The results indicated that R1234yf combustion is a step-by-step chain reaction involving chain initiation (unimolecular lysis and collision reaction with oxygen), chain transmission (collision reactions with free radicals), and chain termination. In conjunction with the priority rule of kinetics reactions, a schematic diagram of the primary microscopic combustion pathways for R1234yf was created at 298.15 K and 1 atm. All the simulation results could provide a data basis to establish the kinetic model of hydrofluoroolefin pyrolysis and combustion. In addition, to verify the product direction of the reaction path, experiments were conducted to determine the properties of the R1234yf combustion products.

Operating parametric analysis and kinetic modeling of methanol gasification in supercritical water

Abstract: Experimental analysis and quantitative kinetic modeling were performed on methanol gasification in supercritical water using continuous reactor at different reaction temperatures (550 ℃ to 650 ℃), pressures (21 MPa to 25 MPa), residence times (6 s to 60 s), and initial methanol concentrations (0.05 mol·L-1 to 0.20 mol·L-1) to investigate the mechanism underlying methanol gasification and conversion. The kinetic model consisted of eight separate reaction pathways. Simultaneously, reaction rate and sensitivity analyses were conducted. Higher temperatures, longer residence times and lower feedstock concentrations were beneficial for the yields of H2 and CO2. Kinetic analysis showed that H2 and CO2 generation mainly occurred through the hydrolysis of methanol. CO was mainly consumed during the forward water-gas shift reaction, and the production of CH4 depended primarily on methanol methanation. Additionally, methanol hydrolysis was identified as an important elementary reaction for CH4 production by sensitivity analysis.

Theoretical Study on the Kinetics of the Rubisco Carboxylase Reaction by a Model Based on Quantum Chemistry and Absolute Reaction Rate Theory

Abstract: The rate of the Rubisco carboxylase reaction is evaluated by statistical mechanics and hybrid density functional theory (DFT). The Rubisco molecular model given by Kannappan et al. was modified and used in the present calculation. The activation energies of CO2 addition reaction, H2O addition reaction, C2–C3 bond scission, and C2 protonation are estimated. We calculated the turnover number (TON) for each of the four reaction steps based on a revised absolute reaction rate theory, which became applicable to soft matter reactions. The molecular parameters used in TON calculations were obtained by DFT calculations. The TON of the total Rubisco reaction was finally evaluated using rate equations. The calculation in a vacuum gave the total TON to be around 5 × 10–5, which was much lower than the experimental value. The DFT calculation in water solvent gave the total TON to be around 0.1, which agreed reasonably well with experimentally reported values (∼2.71). The rate-limiting process was the scission reaction. The present calculation showed that both the phosphate groups in the substrate accelerate each reaction step. The present calculation showed that a more comprehensive molecular model including enolization and quantum chemical methods is necessary to make a more precise reaction model including the irreversibility of some reactions.

Describing Chemical Kinetics in the Time Dimension with Mean Reaction Time

Abstract: The chemical kinetics is such a fundamental topic in chemistry. By analyzing how the experimental conditions and parameters influence the reaction rate, chemists have deciphered the molecular details on how the chemical reactions occur. The quantitative analysis of the reaction rate requires the formulation of the rate equation, which describes the dependence of the reaction rate on the concentrations of the reactants and also the rate constants of the elementary steps. Though the methods to derive the rate equation from the kinetic model have been known for a century, it is still mathematically challenging to derive the rate equation for complex reactions with multiple steps, which requires a solution for simultaneous differential equations. Therefore, chemists frequently resort to the steady-state approximation or the numerical simulation. One way to avoid the mathematical difficulty is to describe the chemical kinetics in the time dimension. Describing the mean reaction time, the average of the time required for the completion of the chemical reaction, using the elementary rate constants of the kinetic model is much simpler mathematically than deriving the rate equation for the kinetic model. Here, we describe the basic rules for the derivation of the formula of the mean reaction time, derive the generalized equation for the mean reaction time, and analyze the components of the formula to determine how the individual steps in the complex reaction contribute to the mean reaction time. Being the ensemble-averaged value, the mean reaction time does not provide the information on the actual distribution of the reaction time of individual chemical entity. However, the formula of the mean reaction time reveals invaluable insights on how the energy levels of the ground state and the transition states affect the kinetics of the complex reaction and offers a way to identify the most time-consuming process of the complex reaction in a straightforward manner. We also apply the mean reaction time to enzyme kinetics and demonstrate that one can derive the expressions of the kinetic parameters (kcat/KM and kcat) in a surprisingly simple way even without resorting to the steady-state application.