High-Pressure Rate Rules for Alkyl + O₂ Reactions. 1. The Dissociation, Concerted Elimination, and Isomerization Channels of the Alkyl Peroxy Radical

The reactions of alkyl peroxy radicals (RO₂) play a central role in the low-temperature oxidation of hydrocarbons. In this work, we present high-pressure rate estimation rules for the dissociation, concerted elimination, and isomerization reactions of RO₂. These rate rules are derived from a systematic investigation of sets of reactions within a given reaction class using electronic structure calculations performed at the CBS-QB3 level of theory. The rate constants for the dissociation reactions are obtained from calculated equilibrium constants and a literature review of experimental rate constants for the reverse association reactions. For the concerted elimination and isomerization channels, rate constants are calculated using canonical transition state theory. To determine if the high-pressure rate expressions from this work can directly be used in ignition models, we use the QRRK/MSC method to calculate apparent pressure and temperature dependent rate constants for representative reactions of small, medium, and large alkyl radicals with O₂. A comparison of concentration versus time profiles obtained using either the pressure dependent rate constants or the corresponding high-pressure values reveals that under most conditions relevant to combustion/ignition problems, the high-pressure rate rules can be used directly to describe the reactions of RO₂

Mechanism and Kinetics of Low-Temperature Oxidation of a Biodiesel Surrogate: Methyl Propanoate Radicals with Oxygen Molecule

This paper presents a computational study on the low-temperature mechanism and kinetics of the reaction between molecular oxygen and alkyl radicals of methyl propanoate (MP), which plays an important role in low-temperature oxidation and/or autoignition processes of the title fuel. Their multiple reaction pathways either accelerate the oxidation process via chain branching or inhibit it by forming relatively stable products. The potential energy surfaces of the reactions between three primary MP radicals and molecular oxygen, namely, C^•H₂CH₂COOCH₃ + O₂, CH₃C^•HCOOCH₃ + O₂, and CH₃CH₂COOC^•H₂ + O₂, were constructed using the accurate composite CBS-QB3 method. Thermodynamic properties of all species as well as high-pressure rate constants of all reaction channels were derived with explicit corrections for tunneling and hindered internal rotations. Our calculation results are in good agreement with a limited number of scattered data in the literature. Furthermore, pressure- and temperature-dependent rate constants for all reaction channels on the multiwell-multichannel potential energy surfaces were computed with the quantum Rice–Ramsperger–Kassel (QRRK) and the modified strong collision (MSC) theories. This procedure resulted in a thermodynamically consistent detailed kinetic submechanism for low-temperature oxidation governed by the title process. A simplified mechanism, which consists of important reactions, is also suggested for low-temperature combustion at engine-like conditions

High-Pressure Rate Rules for Alkyl + O₂ Reactions. 2. The Isomerization, Cyclic Ether Formation, and β-Scission Reactions of Hydroperoxy Alkyl Radicals

The unimolecular reactions of hydroperoxy alkyl radicals (QOOH) play a central role in the low-temperature oxidation of hydrocarbons as they compete with the addition of a second O₂ molecule, which is known to provide chain-branching. In this work we present high-pressure rate estimation rules for the most important unimolecular reactions of the β-, γ-, and δ-QOOH radicals: isomerization to RO₂, cyclic ether formation, and selected β-scission reactions. These rate rules are derived from high-pressure rate constants for a series of reactions of a given reaction class. The individual rate expressions are determined from CBS-QB3 electronic structure calculations combined with canonical transition state theory calculations. Next we use the rate rules, along with previously published rate estimation rules for the reactions of alkyl peroxy radicals (RO₂), to investigate the potential impact of falloff effects in combustion/ignition kinetic modeling. Pressure effects are examined for the reaction of <i>n</i>-butyl radical with O₂ by comparison of concentration versus time profiles that were obtained using two mechanisms at 10 atm: one that contains pressure-dependent rate constants that are obtained from a QRRK/MSC analysis and another that only contains high-pressure rate expressions. These simulations reveal that under most conditions relevant to combustion/ignition problems, the high-pressure rate rules can be used directly to describe the reactions of RO₂ and QOOH. For the same conditions, we also address whether the various isomers equilibrate during reaction. These results indicate that equilibrium is established between the alkyl, RO₂, and γ- and δ-QOOH radicals

Ab Initio Chemical Kinetics for the HCCO + H Reaction

<p>Ketenyl radical (HCCO) is an important hydrocarbon combustion intermediate. The mechanisms and kinetics for the reaction of HCCO (X²A″) with H(²S) occurring on both singlet and triplet surfaces have been studied by a combination of ab initio calculations and rate constant predictions at the CCSD(T)/6-311++G(3df,2p)//CCSD/6-311++G(d,p) level of theory. The kinetics and product branching ratios have been investigated in the temperature range of 297–3000 K by variational transition state and Rice–Ramsperger–Kassel–Marcus (RRKM) theories for the production of CH₂(a¹A₁) + CO(X¹Σ⁺) and CH₂(X³B₁) + CO(X¹Σ⁺). Our prediction for the primary product CH₂(a¹A₁) + CO(X¹Σ⁺) formation is in good agreement with earlier experimental results. The pressure independent rate constant for this channel can be expressed by <i>k</i>₁(T) = 8.62 × 10^–11T⁰^.16exp(–20/T) cm³ molecule^–1 s^–1. For the production of CH₂(X³B₁) + CO(X¹Σ⁺), the rate constant <i>k</i>₂ can be represented as <i>k</i>₂(T) = 7.63 × 10^–16T^1.56exp(–386/T) cm³ molecule^–1 s^–1. The predicted product branching ratios for the reaction are in close agreement with experimental data as well. We also predicted the heat of formation at 0 K for ²HCCO, ³CCO, and ¹CCO by CCSD(T)/6-311++G(3df,2p), CBS-QB3, and G2M; the values are in good agreement among one another. Specifically, the CCSD(T) values are: Δ<i>_f</i>H°(HCCO, X²A″) = 42.52 ± 0.70; Δ<i>_f</i>H°(CCO, X³Σ_g) = 91.50 ± 0.54; and Δ<i>_f</i>H°(CCO, a¹Δ) = 110.22 ± 0.54 kcal/mol.</p

Kinetics of Thermal Unimolecular Decomposition of Acetic Anhydride: An Integrated Deterministic and Stochastic Model

An integrated deterministic and stochastic model within the master equation/­Rice–Ramsperger–­Kassel–Marcus (ME/RRKM) framework was first used to characterize temperature- and pressure-dependent behaviors of thermal decomposition of acetic anhydride in a wide range of conditions (i.e., 300–1500 K and 0.001–100 atm). Particularly, using potential energy surface and molecular properties obtained from high-level electronic structure calculations at CCSD­(T)/CBS, macroscopic thermodynamic properties and rate coefficients of the title reaction were derived with corrections for hindered internal rotation and tunneling treatments. Being in excellent agreement with the scattered experimental data, the results from deterministic and stochastic frameworks confirmed and complemented each other to reveal that the main decomposition pathway proceeds via a 6-membered-ring transition state with the 0 K barrier of 35.2 kcal·mol^–1. This observation was further understood and confirmed by the sensitivity analysis on the time-resolved species profiles and the derived rate coefficients with respect to the ab initio barriers. Such an agreement suggests the integrated model can be confidently used for a wide range of conditions as a powerful postfacto and predictive tool in detailed chemical kinetic modeling and simulation for the title reaction and thus can be extended to complex chemical reactions

Identification of potential anti-hyperglycemic compounds in <i>Cordyceps militaris</i> ethyl acetate extract: <i>in vitro</i> and <i>in silico</i> studies

Cordyceps militaris has been long known for valuable health benefits by folk experience and was recently reported with diabetes-tackling evidences, thus deserving extending efforts on screening for component-activity relationship. In this study, experiments were carried out to find the evidence, justification, and input for computations on the potential against diabetes-related protein structures: PDB-4W93, PDB-3W37, and PDB-4A3A. Liquid chromatography identified 14 bioactive compounds in the ethyl acetate extract (1–14) and quantified the contents of cordycepin (0.11%) and adenosine (0.01%). Bioassays revealed the overall potential of the extract against α-amylase (IC50 = 6.443 ± 0.364 mg.mL−1) and α-glucosidase (IC50 = 2.580 ± 0.194 mg.mL−1). A combination of different computational platforms was used to select the most promising candidates for applications as anti-diabetic bio-inhibitors, i.e. 1 (ground state: −888.49715 a.u.; dipole moment 3.779 Debye; DS¯ −12.3 kcal.mol−1; polarizability 34.7 Å3; logP − 1.30), 10 (ground state: −688.52406 a.u.; dipole moment 5.487 Debye; DS¯ −12.6 kcal.mol−1; polarizability 24.9 Å3; logP − 3.39), and 12 (ground state: −1460.07276 a.u.; dipole moment 3.976 Debye; DS¯ −12.5 kcal.mol−1; polarizability 52.4 Å3; logP − 4.39). The results encourage further experimental tests on cordycepin (1), mannitol (10), and adenosylribose (12) to validate their in-practice diabetes-related activities, thus conducive to hypoglycemic applications. Communicated by Ramaswamy H. Sarma</p