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.

Molecular electrostatic potential as a general and versatile indicator for electronic substituent effects: statistical analysis and applications

It is necessary to quantitatively determine substituent effects to accurately elucidate reaction mechanisms in the field of organic chemistry. This paper reports that the molecular electrostatic potential (MESP) can be used as a general and versatile measure for the substituent effects in various chemical reactions by performing extensive density functional theory (DFT) calculations for more than 400 molecules, followed by statistical analyses. We observed a robust and linear correlation between the electrostatic potential and the substituent parameters for various cases of reactive systems, regardless of the DFT functionals, basis sets, and solvation models used. In addition, we statistically analysed the normality of the residuals from the linear regression to demonstrate that strong linear relationships hold universally, which indicates that the electrostatic potential can serve as a physically meaningful quantity for the predictive estimation of substituent effects. In contrast, conventionally used methods based on the charge deviation in the aromatic carbons, as computed using various charge analysis methods, (e.g., Hirshfeld charge analysis) do not demonstrate the statistical normality. Furthermore, we illustrate that MESP can be extensively adopted to strengthen the validity of the linear free energy relationships (LFERs) under various chemical conditions. The results revealed that the MESP shift derived by a functional group on a mono-substituted benzene ring is a strong predictor for the substituent effects on the electronic behaviours in chemical reactions; thus, it can serve as an alternative to other empirical parameters such as the Hammett or Swain-Lupton parameters, or the charge shift.

Self-Reaction of Acetonyl Peroxy Radicals and Their Reaction with Cl Atoms

The rate constant for the self-reaction of the acetonyl peroxy radicals, CH₃C(O)CH₂O₂, has been determined using laser photolysis/continuous wave cavity ring down spectroscopy (cw-CRDS). CH₃C(O)CH₂O₂ radicals have been generated from the reaction of Cl atoms with CH₃C(O)CH₃, and the concentration time profiles of four radicals (HO₂, CH₃O₂, CH₃C(O)O₂, and CH₃C(O)CH₂O₂) have been determined by cw-CRDS in the near-infrared. The rate constant for the self-reaction was found to be k = (5.4 ± 1.4) × 10^-12 cm³ s^-1, in good agreement with a recently published value (Zuraski, K., et al. J. Phys. Chem. A 2020, 124, 8128); however, the branching ratio for the radical path was found to be ϕ_1b = (0.6 ± 0.1), which is well above the recently published value (0.33 ± 0.13). The influence of a fast reaction of Cl atoms with the CH₃C(O)CH₂O₂ radical became evident under some conditions; therefore, this reaction has been investigated in separate experiments. Through the simultaneous fitting of all four radical profiles to a complex mechanism, a very fast rate constant of k = (1.35 ± 0.8) × 10^-10 cm³ s^-1 was found, and experimental results could be reproduced only if Cl atoms would partially react through H-atom abstraction to form the Criegee intermediate with a branching fraction of ϕ_Criegee = (0.55 ± 0.1). Modeling the HO₂ concentration-time profiles was possible only if a subsequent reaction of the Criegee intermediate with CH₃C(O)CH₃ was included in the mechanism leading to HO₂ formation with a rate constant of k = (4.5 ± 2.0) × 10^-14 cm³ s^-1.

An Extended Computational Study of Criegee Intermediate-Alcohol Reactions

High-level ab initio calculations (DF-LCCSD(T)-F12a//B3LYP/aug-cc-pVTZ) are performed on a range of stabilized Criegee intermediate (sCI)-alcohol reactions, computing reaction coordinate energies, leading to the formation of α-alkoxyalkyl hydroperoxides (AAAHs). These potential energy surfaces are used to model bimolecular reaction kinetics over a range of temperatures. The calculations performed in this work reproduce the complicated temperature-dependent reaction rates of CH₂OO and (CH₃)₂COO with methanol, which have previously been experimentally determined. This methodology is then extended to compute reaction rates of 22 different Criegee intermediates with methanol, including several intermediates derived from isoprene ozonolysis. In some cases, sCI-alcohol reaction rates approach those of sCI-(H₂O)₂. This suggests that in regions with elevated alcohol concentrations, such as urban Brazil, these reactions may generate significant quantities of AAAHs and may begin to compete with sCI reactions with other trace tropospheric pollutants such as SO₂. This work also demonstrates the ability of alcohols to catalyze the 1,4-H transfer unimolecular decomposition of α-methyl substituted sCIs.

Reactions of hydroxyl radicals with benzoic acid and benzoate

Density functional theory was used to study the mechanism and kinetics of benzoic acid with hydroxyl radicals in both gas and aqueous phases as well as benzoate with hydroxyl radicals in the aqueous phase at the M06-2X/6-311+G(d,p) level of theory.