The reactions of alkyl peroxy radicals (RO(2)) 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(2). 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(2). 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(2).

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(2) 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(2), 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(2)), to investigate the potential impact of falloff effects in combustion/ignition kinetic modeling. Pressure effects are examined for the reaction of n-butyl radical with O(2) 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(2) 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(2), and γ- and δ-QOOH radicals.

Eighteen active substances, including 17 organosulfur compounds found in garlic essential oil (T), were identified by GC−MS analysis. For the first time, using the molecular docking technique, we report the inhibitory effect of the considered compounds on the host receptor angiotensin-converting enzyme 2 (ACE2) protein in the human body that leads to a crucial foundation about coronavirus resistance of individual compounds on the main protease (PDB6LU7) protein of SARS-CoV-2. The results show that the 17 organosulfur compounds, accounting for 99.4% contents of the garlic essential oil, have strong interactions with the amino acids of the ACE2 protein and the main protease PDB6LU7 of SARS-CoV-2. The strongest anticoronavirus activity is expressed in allyl disulfide and allyl trisulfide, which account for the highest content in the garlic essential oil (51.3%). Interestingly, docking results indicate the synergistic interactions of the 17 substances, which exhibit good inhibition of the ACE2 and PDB6LU7 proteins. The results suggest that the garlic essential oil is a valuable natural antivirus source, which contributes to preventing the invasion of coronavirus into the human body. Figure 1. Picture of garlic (A. sativum L.).Article

Accurate description of reactions between propyl radicals and molecular oxygen is an essential prerequisite for modeling of low-temperature propane oxidation because their multiple reaction pathways either accelerate the oxidation process via chain branching or inhibit it by forming relatively stable products. The CBS-QB3 level of theory was used to construct potential energy surfaces for n-C(3)H(7) + O(2) and i-C(3)H(7) + O(2). High-pressure rate constants were calculated using transition state theory with corrections for tunneling and hindered rotations. These results were used to derive pressure- and temperature-dependent rate constants for the various channels of these reactions under the framework of the Quantum Rice-Ramsperger-Kassel (QRRK) and the modified strong collision (MSC) theories. This procedure resulted in a thermodynamically consistent C(3)H(7) + O(2) submechanism, which was either used directly or as part of a larger extended detailed kinetic mechanism to predict the loss of propyl and the product yields of propylene and HO(2) over a wide range of temperatures, pressures, and residence times. The overall good agreement between predicted and experimental data suggests that this reaction subset is reliable and should be able to properly account for the reactions of propyl radicals with O(2) in propane oxidation. It is also demonstrated that for most conditions of practical interest only a small subset of reactions (e.g., isomerization, concerted elimination of HO(2), and stabilization) controls the oxidation kinetics, which makes it possible to considerably simplify the mechanism. Moreover, we observed strong similarities in the rate coefficients within each reaction class, suggesting the potential for development of relatively simple rate constant estimation rules that could be applied to analogous reactions involving hydrocarbon radicals that are too large to allow accurate detailed electronic structure calculations.

Fundamental understanding of mechanisms of the epoxy-amine curing reaction is crucial for developing new polymer materials. Nearly all experimental studies, to date for elucidating its mechanisms are based on thermometric measurements and thus cannot provide the molecular level details. This study used density functional theory (DFT) methods to examine the mechanism of epoxy-amine poly addition reactions at the molecular level. Different reaction pathways involving both acyclic and cyclic transition state structures were examined for different reaction conditions, namely isolated, self-promoted by amine, catalyzed by alcohol, and in different solvents. The results indicate that the reactions catalyzed by an alcohol dominate the rate over the self-promoted reaction by other amine species and the isolated one in early stages of the conversion. The concerted pathways involving cyclic transition-state complexes are not significant due to their high activation energies. Calculated activation energies are within the experimental uncertainty. In addition, solvent, not steric and electronic effects as suggested earlier, are shown to be responsible for secondary amines to react slower than primary amines.

Carboxymethyl cellulose (CMC), in solution and adsorbed on the surface of talc, has been studied with ATR FTIR spectroscopy as a function of the solution pH. The solution spectra enable the calculation of the extent of ionization of the polymer (due to protonation and deprotonation of the carboxyl group) at various pH values, yielding a value of 3.50 for the pK(app)(1/2) (pH at which half of all carboxyl groups are ionized) in a simple electrolyte solution and a value of 3.37 for the pK(app)(1/2) in solutions containing magnesium ions (3.33 x 10(-4) M). The spectra of the adsorbed layer reveal that CMC interacts with the talc surface through a chemical complexation mechanism, via the carboxyl groups substituted on the polymer backbone. The binding mechanism is active at all pH values down to pH 2 and up to pH 11. The adsorbed layer spectra reveal that protonation and deprotonation of the polymer are affected by adsorption, with an increase in the pK(app)(1/2) to a value of 4.80. Spectra of the adsorbed polymer were also acquired as a function of the adsorption time. Adsorption kinetic data reveal that the polymer most likely has two different interactions with the talc surface, with a stronger interaction with the talc edge through chemical complexation and a weaker interaction with the talc basal plane presumably through the hydrophobic interaction.

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Kinetics of enol generation from propene has been predicted in an effort to understand the presence of enols in flames. A potential energy surface for reaction of OH with propene was computed by CCSD(T)/cc-pVDZ//B3LYP/cc-pVTZ calculations. Rate constants of different product channels and branching ratios were then calculated using the Master Equation formulation (J. Phys. Chem. A 2006, 110, 10528). Of the two enol products, ethenol is dominant over propenol, and its pathway is also the dominant pathway for the OH + propene addition reactions to form bimolecular products. In the temperature range considered, hydrogen abstraction dominated propene + OH consumption by a branching ratio of more than 90%. Calculated rate constants of enol formation were included in the Utah Surrogate Mechanism to model the enol profile in a cyclohexane premixed flame. The extended model shows consistency with experimental data and gives 5% contribution of ethenol formation from OH + propene reaction, the rest coming from ethene + OH.