Quantum Chemical Study of Autoignition of Methyl Butanoate

Investigating the kinetics of the intramolecular H-migration reaction class of methyl-ester peroxy radicals in low-temperature oxidation mechanisms of biodiesel

Biodiesel is a promising, sustainable, and carbon-neutral fuel. However, studying its combustion mechanisms comprehensively, both theoretically and experimentally, presents challenges due to the complexity and size of its molecules. One significant obstacle in determining low-temperature oxidation mechanisms for biodiesel is the lack of kinetic parameters for the reaction class of intramolecular H-migration reactions of alkyl-ester peroxy radicals, labeled as R(CO)OR'-OO˙ (where the 'dot' represents the radical). Current biodiesel combustion mechanisms often estimate these parameters from the analogous reaction class of intramolecular H-migration reactions of alkyl peroxy radicals in alkane combustion mechanisms. However, such estimations are imprecise and neglect the unique characteristics of the ester group. This research aims to explore the kinetics of the reaction class of H-migration reactions of methyl-ester peroxy radicals. The reaction class is divided into 20 subclasses based on the newly formed cycle size of the transition state, the positions of the peroxy radical and the transferred H atom, and the types of carbons from which the H atom is transferred. Energy barriers for each subclass are calculated by using the CBS-QB3//M06-2X/6-311++G(d,p) method. High-pressure-limit and pressure-dependent rate constants ranging from 0.01 to 100 atm are determined using the transition state theory and Rice-Ramsberger-Kassel-Marcus/master-equation method, respectively. It is noted that the pressure-dependent rate constants calculated for each individual isomerization channel could bring some uncertainties while neglecting the interconnected pathways. A comprehensive comparison is made between our values of selected reactions and high-level calculated values of the corresponding reactions reported in the literature. The small deviation observed between these values indicates the accuracy and reliability of the energy barriers and rate constants calculated in this study. Additionally, our calculated high-pressure-limit rate constants are compared with the corresponding values in combustion mechanisms of esters, which were estimated based on analogous reactions of alkyl peroxy radicals. These comparative analyses shed light on the significant impact of the ester group on the kinetics, particularly when the ester group is involved in the reaction center. Finally, the high-pressure-limit rate rule and pressure-dependent rate rule for each subclass are derived by averaging the rate constants of reactions in each subclass. The accurate and reasonable rate rules for methyl-ester peroxy radicals developed in this study play a crucial role in enhancing our understanding of the low-temperature oxidation mechanisms of biodiesel.

Mechanistic and Kinetic Approach on the Propargyl Radical (C3H3) with the Criegee Intermediate (CH2OO)

The detailed reaction mechanism and kinetics of the C₃H₃ + CH₂OO system have been thoroughly investigated. The CBS-QB3 method in conjunction with the ME/vRRKM theory has been applied to figure out the potential energy surface and rate constants for the C₃H₃ + CH₂OO system. The C₃H₃ + CH₂OO reaction leading to the CH₂-[cyc-CCHCHOO] + H product dominates compared to the others. Rate constants of the reaction are dependent on temperatures (300-2000 K) and pressures (1-76,000 Torr), for which the rate constant of the channel C₃H₃ + CH₂OO → CH₂-[cyc-CCHCHOO] + H decreases at low pressures (1-76 Torr), but it increases with rising temperature if the pressure P ≥ 760 Torr. Rate constants of the three reaction channels C₃H₃ + CH₂OO → CHCCH₂CHO + OH, C₃H₃ + CH₂OO → OCHCHCHCHO + H, and C₃H₃ + CH₂OO → CHCHCHO + CH₂O fluctuate with temperatures. The branching ratio of the C₃H₃ + CH₂OO → CH₂-[cyc-CCHCHOO] + H channel is the highest, accounting for 51-98.7% in the temperature range of 300-2000 K and 760 Torr pressure, while those of the channels forming the products PR10 (OCHCHCHCHO + H) and PR11 (CHCHCHO + CH₂O) are the lowest, less than 0.1%, indicating that the contribution of these two reaction paths to the title reaction is insignificant. The proposed temperature- and pressure-dependent rate constants, together with the thermodynamic data of the species involved, can be confidently used for modeling CH₂OO-related systems under atmospheric and combustion conditions.

Structure determination of fatty acid ester biofuels via in situ cryocrystallisation and single crystal X-ray diffraction

In situ cryocrystallisation enabled the crystal structure determination of a homologous series of low-melting n-alkyl methyl esters C_n−1H_2n+1CO₂CH₃.

Benchmarking dual-level MS-Tor and DLPNO-CCSD(T) methods for H-abstraction from methyl pentanoate by an OH radical

Methyl pentanoate (MP) was recently proposed as a potential biodiesel surrogate due to its negative temperature coefficient region.

Thermochemistry and Kinetic Studies on the Autoignition of 2-Butanone: A Computational Study

Unimolecular reactions of alkylperoxy(ROO^•), hydroperoxyalkyl(^•QOOH), and hydroperoxyalkylperoxy(^•OOQOOH) radicals of 2-butanone, which is a potential biofuel molecule, have been studied computationally. These radicals are responsible for the chain branching at low temperature oxidation and play a significant role in modeling the autoignition. The composite CBS-QB3 method was used to study the thermochemistry and energetics of all the species involved. Intrinsic reaction coordinate (IRC) calculations were carried out for all the transition states along various reaction pathways. All the possible reactions like H-migration, ^•OH elimination, and HO^•₂ elimination reactions were studied for these radicals. It was found that, the isomerization of ^•OOQOOH to HOOQOO^• is the most favorable channel, which involves 8- and 9-membered cyclic transition states. However, the decomposition pathway involves the H-migration from carbon to oxygen. The mechanism for the decomposition of all ^•OOQOOH radicals with their potential energy level diagrams are reported. The temperature dependent rate coefficients were also studied using Canonical Variational Transition state theory (CVT) with small curvature tunneling (SCT) in the temperature range of 400-1500 K, which is relevant to the combustion. Thermodynamic parameters for all the reactions involved were calculated. The high barrier (1,3 H-migration) reactions were found to be exothermic and spontaneous, which is unexpected.

Thermodynamic DFT analysis of natural gas

Density functional theory was performed for thermodynamic predictions on natural gas, whose B3LYP/6-311++G(d,p), B3LYP/6-31+G(d), CBS-QB3, G3, and G4 methods were applied. Additionally, we carried out thermodynamic predictions using G3/G4 averaged. The calculations were performed for each major component of seven kinds of natural gas and to their respective air + natural gas mixtures at a thermal equilibrium between room temperature and the initial temperature of a combustion chamber during the injection stage. The following thermodynamic properties were obtained: internal energy, enthalpy, Gibbs free energy and entropy, which enabled us to investigate the thermal resistance of fuels. Also, we estimated an important parameter, namely, the specific heat ratio of each natural gas; this allowed us to compare the results with the empirical functions of these parameters, where the B3LYP/6-311++G(d,p) and G3/G4 methods showed better agreements. In addition, relevant information on the thermal and mechanic resistance of natural gases were investigated, as well as the standard thermodynamic properties for the combustion of natural gas. Thus, we show that density functional theory can be useful for predicting the thermodynamic properties of natural gas, enabling the production of more efficient compositions for the investigated fuels. Graphical abstract Investigation of the thermodynamic properties of natural gas through the canonical ensemble model and the density functional theory.

Theoretical Analysis of the Effect of C═C Double Bonds on the Low-Temperature Reactivity of Alkenylperoxy Radicals

Biodiesel contains a large proportion of unsaturated fatty acid methyl esters. Its combustion characteristics, especially its ignition behavior at low temperatures, have been greatly affected by these C═C double bonds. In this work, we performed a theoretical analysis of the effect of C═C double bonds on the low-temperature reactivity of alkenylperoxy radicals, the key intermediates from the low-temperature combustion of biodiesel. To understand how double bonds affect the fate of peroxy radicals, we selected three representative peroxy radicals from heptane, heptene, and heptadiene having zero, one, and two double C═C bonds, respectively, for study. The potential energy surfaces were explored at the CBS-QB3 level, and the reaction rate constants were computed using canonical/variational transition state theories. We have found that the double bond is responsible for the very different bond dissociation energies of the various types of C-H bonds, which in turn affect significantly the reaction kinetics of alkenylperoxy radicals.

Biotransformation of neuro-inflammation inhibitor kellerin using Angelica sinensis (Oliv.) Diels callus

This paper mainly focused on biotransformation of coumarins using Angelica sinensis (Oliv.) Diels callus.