6 research outputs found
High-Pressure Rate Rules for Alkyl + O<sub>2</sub> Reactions. 1. The Dissociation, Concerted Elimination, and Isomerization Channels of the Alkyl Peroxy Radical
The reactions of alkyl peroxy radicals (RO<sub>2</sub>) 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 RO<sub>2</sub>. 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 O<sub>2</sub>. 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 RO<sub>2</sub>
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<sup>•</sup>H<sub>2</sub>CH<sub>2</sub>COOCH<sub>3</sub> + O<sub>2</sub>, CH<sub>3</sub>C<sup>•</sup>HCOOCH<sub>3</sub> + O<sub>2</sub>, and CH<sub>3</sub>CH<sub>2</sub>COOC<sup>•</sup>H<sub>2</sub> + O<sub>2</sub>, 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 + O<sub>2</sub> 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 O<sub>2</sub> 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 RO<sub>2</sub>, 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 (RO<sub>2</sub>),
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 O<sub>2</sub> 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 RO<sub>2</sub> 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, RO<sub>2</sub>, 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 (X<sup>2</sup>A″) with H(<sup>2</sup>S) 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 CH<sub>2</sub>(a<sup>1</sup>A<sub>1</sub>) + CO(X<sup>1</sup>Σ<sup>+</sup>) and CH<sub>2</sub>(X<sup>3</sup>B<sub>1</sub>) + CO(X<sup>1</sup>Σ<sup>+</sup>). Our prediction for the primary product CH<sub>2</sub>(a<sup>1</sup>A<sub>1</sub>) + CO(X<sup>1</sup>Σ<sup>+</sup>) formation is in good agreement with earlier experimental results. The pressure independent rate constant for this channel can be expressed by <i>k</i><sub>1</sub>(T) = 8.62 × 10<sup>–11</sup>T<sup>0</sup><sup>.16</sup>exp(–20/T) cm<sup>3</sup> molecule<sup>–1</sup> s<sup>–1</sup>. For the production of CH<sub>2</sub>(X<sup>3</sup>B<sub>1</sub>) + CO(X<sup>1</sup>Σ<sup>+</sup>), the rate constant <i>k</i><sub>2</sub> can be represented as <i>k</i><sub>2</sub>(T) = 7.63 × 10<sup>–16</sup>T<sup>1.56</sup>exp(–386/T) cm<sup>3</sup> molecule<sup>–1</sup> s<sup>–1</sup>. 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 <sup>2</sup>HCCO, <sup>3</sup>CCO, and <sup>1</sup>CCO 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><sub>f</sub></i>H°(HCCO, X<sup>2</sup>A″) = 42.52 ± 0.70; Δ<i><sub>f</sub></i>H°(CCO, X<sup>3</sup>Σ<sub>g</sub>) = 91.50 ± 0.54; and Δ<i><sub>f</sub></i>H°(CCO, a<sup>1</sup>Δ) = 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/ÂRice–Ramsperger–ÂKassel–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 CCSDÂ(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<sup>–1</sup>. 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