Theoretical insights into the HO●-induced oxidation of chlorpyrifos pesticide: Mechanism, kinetics, ecotoxicity, and cholinesterase inhibition of degradants.

The oxidation of the common pesticide chlorpyrifos (CPF) initiated by HO● radical and the risks of its degradation products were studied in the gaseous and aqueous phases via computational approaches. Oxidation mechanisms were investigated, including H-, Cl-, CH3- abstraction, HO●-addition, and single electron transfer. In both phases, HO●-addition at the C of the pyridyl ring is the most energetically favorable and spontaneous reaction, followed by H-abstraction reactions at methylene groups (i.e., at H19/H21 in the gas phase and H22/H28 in water). In contrast, other abstractions and electron transfer reactions are unfavorable. However, regarding the kinetics, the significant contribution to the oxidation of CPF is made from H-abstraction channels, mostly at the hydrogens of the methylene groups. CPF can be decomposed in a short time (5-8 h) in the gas phase, and it is more persistent in natural water with a lifetime between 24 days and 66 years, depending on the temperature and HO● concentration. Subsequent oxidation of the essential radical products with other oxidizing reagents, i.e., HO●, NO2●, NO●, and 3O2, gave primary neutral products P1-P15. Acute and chronic toxicity calculations estimate very toxic levels for CPF and two degradation products, P7w and P12w, in aquatic systems. The neurotoxicity of these products was investigated by docking and molecular dynamics. P7w and P12w show the most significant binding scores with acetylcholinesterases, while P8w and P13w are with butyrylcholinesterase enzyme. Finally, molecular dynamics illustrate stable interactions between CPF degradants and cholinesterase enzyme over a 100 ns time frame and determine P7w as the riskiest degradant to the neural developmental system.

New insight into environmental oxidation of phosmet insecticide initiated by HO˙ radicals in gas and water - a theoretical study.

Phosmet is an organophosphorus insecticide widely used in agriculture to control a range of insects; recently, it was banned by the European Union in 2022 due to its harmful effects. However, its environmental degradation and fate have not yet been evident. Thus, phosmet oxidation by HO˙ radicals was theoretically studied in this work using the DFT approach at the M06-2X/6-311++G(3df,3pd)//M06-2X/6-31+G(d,p) level of theory. Three different mechanisms were considered, including formal hydrogen transfer (FHT), radical adduct formation (RAF), and single electron transfer (SET). The mechanisms, kinetics, and lifetime were studied in the gas and aqueous phases, in addition to its ecotoxicity evaluation. The results show that FHT reactions were dominant in the gas phase, while RAF was more favourable in the aqueous phase at 298 K, while SET was negligible. The branching ratio indicated that H-abstractions at the methyl and the methylene groups were the most predominant, while the most favourable HO˙-addition was observed at the phosphorus atom of the dithiophosphate group. The overall rate constant values varied from 1.2 × 109 (at 283 K) to 1.40 × 109 M-1 s-1 (at 323 K) in the aqueous phase and from 6.29 × 1010 (at 253 K) to 1.32 × 1010 M-1 s-1 (at 323 K) in the gas phase. The atmospheric lifetime of phosmet is about 6 hours at 287 K, while it can persist from a few seconds to several years depending on the temperature and [HO˙] concentration in the aqueous environment. The QSAR-based ecotoxicity evaluation indicates that phosmet and its degradation products are all dangerous to aquatic organisms, although the products are less toxic than phosmet. However, they are generally developmental toxicants and mutagenicity-negative compounds.

Hydroxyl radical-initiated decomposition of metazachlor herbicide in the gaseous and aqueous phases: Mechanism, kinetics, and toxicity evaluation.

The oxidation of widely-used herbicide metazachlor (MTZ) by hydroxyl radical (HO•) in the gas and the aqueous phases was investigated in terms of mechanistic and kinetic behaviors using the M06-2X/6-311++G (3df, 3pd)//M06-2X/6-31 + G (d,p) level of theory over the temperature range 250-400 K. The formal hydrogen transfer, HO•-addition, and single electron transfer mechanisms were considered. The overall rate constants in the gas phase range from 8.40 × 1010 to 8.31 × 109 M-1 s-1 at the temperature from 250 to 400 K, respectively, while the ones in the aqueous phase are close to diffusion-controlled rates, with diffusion-corrected rate constants being 1.31 × 109 to 1.27 × 109 M-1 s-1. The formal hydrogen transfer mechanism is the most dominant in the gas phase, whereas the HO•-addition is the most favorable in the aqueous phase. The H-abstraction at two methyl groups and the HO•-addition to C11 and C12 atoms (pyrazole ring), C16 and C18 atoms (benzyl ring) are significant. The short lifetime in the environment, equal to only 4.16 h, requires more attention to this herbicide compound, whereas its lifetime in the aqueous condition varies sharply from half second to several thousand days depending on the HO• concentration. The ecotoxicity estimation of MTZ and its principal transformation products to aquatic organisms suggests that they are harmful or toxic substances. Moreover, the MTZ is a developmental toxicant and mutagenicity-positive, while its decomposed products are developmental toxicants with no mutagenic toxicity. Their bioaccumulation in aquatic organisms is negligible.

Theoretical study of H-abstraction reactions from CH3Cl and CH3Br molecules by ClO and BrO radicals.

The rate constants of the H-abstraction reactions from CH(3)Cl and CH(3)Br molecules by ClO and BrO radicals have been estimated over the temperature range of 300-2500 K using four different levels of theory. Calculations of optimized geometrical parameters and vibrational frequencies are performed using B3LYP and MP2 methods combined with the cc-pVTZ basis set. Single-point energy calculations have been carried out with the highly correlated ab initio coupled cluster method in the space of single, double, and triple (perturbatively) electron excitations CCSD(T) using the cc-pVTZ and cc-pVQZ basis sets. Canonical transition-state theory combined with an Eckart tunneling correction has been used to predict the rate constants as a function of temperature. In order to choose the appropriate levels of theory with chlorine- and bromine-containing species, the reference reaction Cl ((2)P(3/2)) + CH(3)Cl → HCl + CH(2)Cl (R(ref)) was first theoretically studied because its kinetic parameters are well-established from numerous experiments, evaluation data, and theoretical studies. The kinetic parameters of the reaction R(ref) have been determined accurately using the CCSD(T)/cc-pVQZ//MP2/cc-pVTZ level of theory. This level of theory has been used for the rate constant estimation of the reactions ClO + CH(3)Cl (R(1)), ClO + CH(3)Br (R(2)), BrO + CH(3)Cl (R(3)), and BrO + CH(3)Br (R(4)). Six-parameter Arrhenius expressions have been obtained by fitting to the computed rate constants of these four reactions (including cis and trans pathways) over the temperature range of 300-2500 K.

Thermochemical data and additivity group values for ten species of o-xylene low-temperature oxidation mechanism.

o-Xylene could be a good candidate to represent the family of aromatic hydrocarbons in a surrogate fuel. This study uses computational chemistry to calculate standard enthalpies of formation at 298 K, Δ(f)H°(298 K), standard entropies at 298 K, S°(298 K), and standard heat capacities C(p)°(T) over the temperature range 300 K to 1500 K for ten target species present in the low-temperature oxidation mechanism of o-xylene: o-xylene (1), 2-methylbenzyl radical (2), 2-methylbenzylperoxy radical (3), 2-methylbenzyl hydroperoxide (4), 2-(hydroperoxymethyl)benzyl radical (5), 2-(hydroperoxymethyl)benzaldehyde (6), 1-ethyl-2-methylbenzene (7), 2,3-dimethylphenol (8), 2-hydroxybenzaldehyde (9), and 3-hydroxybenzaldehyde (10). Δ(f)H°(298 K) values are weighted averages across the values calculated using five isodesmic reactions and five composite calculation methods: CBS-QB3, G3B3, G3MP2, G3, and G4. The uncertainty in Δ(f)H°(298 K) is also evaluated. S°(298 K) and C(p)°(T) values are calculated at B3LYP/6-311G(d,p) level of theory from molecular properties and statistical thermodynamics through evaluation of translational, rotational, vibrational, and electronic partition functions. S°(298 K) and C(p)°(300 K) values are evaluated using the rigid-rotor-harmonic-oscillator model. C(p)°(T) values at T ≥ 400 K are calculated by treating separately internal rotation contributions and translational, external rotational, vibrational, and electronic contributions. The thermochemical properties of six target species are used to develop six new additivity groups taking into account the interaction between two substituents in ortho (ORT/CH2OOH/ME, ORT/ET/ME, ORT/CHO/OH, ORT/CHO/CH2OOH) or meta (MET/CHO/OH) positions, and the interaction between three substituents (ME/ME/OH123) located one beside the other (positions numbered 1, 2, 3) for two- or three-substituted benzenic species. Two other additivity groups are also developed using the thermochemical properties of benzenic species taken from the literature: the C/CB/H2/OO and the CB/CO groups. These groups extend the capacities of the group additivity method to deal with substituted benzenic species.

A CASPT2 theoretical study of the kinetics of the 2-, 3-, and 4-methylbenzylperoxy radical isomerization.

The rate constants of the 2-, 3-, and 4-methylbenzylperoxy isomerization reactions have been computed using the elaborated CASPT2 method. Geometry optimizations and vibrational frequency calculations are performed with two methods (B3LYP and MPW1K) combined with the cc-pVDZ and 6-31+G(d,p) basis sets, respectively. Single-point energy calculations are performed at the CASPT2/ANO-L-VDZP//B3LYP/cc-pVDZ level of theory as recommended by Canneaux et al. (J. Phys. Chem. A 2008, 112, 6045). Canonical transition-state theory with a simple Wigner tunneling correction is used to predict the high-pressure limit rate constants as a function of temperature. They are given by the following relations for the 2-, 3-, and 4-methylbenzylperoxy (MBP) (1,3s) isomerizations and for the 2-methylbenzylperoxy (1,6p) isomerization, respectively: k(2-MBP(1,3s))(600-2000 K) (in s(-1)) = (3.33 x 10(10))T(0.79) exp((-142.6 in kJ mol(-1))/RT); k(3-MBP(1,3s))(600-2000 K) (in s(-1)) = (0.74 x 10(10))T(0.79) exp((-130.7 in kJ mol(-1))/RT); k(4-MBP(1,3s))(600-2000 K) (in s(-1)) = (1.12 x 10(10))T(0.79) exp((-133.6 in kJ mol(-1))/RT); k(2-MBP(1,6p))(600-2000 K) (in s(-1)) = (5.10 x 10(8))T(0.85) exp((-87.1 in kJ mol(-1))/RT). These parameters can be used in the thermokinetic models involving aromatic compounds at high pressure. In the case of the 2-methylbenzylperoxy radical, the (1,6p) H-atom transfer reaction is consistently the most important channel over the studied temperature range, and the (1,3s) H-atom transfer reaction is not energetically favored.

A theoretical study of the kinetics of the benzylperoxy radical isomerization.

The rate constant of the benzylperoxy isomerization reaction has been computed using 54 different levels of theory and has been compared to the experimental value reported at 773 K. The aim of this methodology work is to demonstrate that standard theoretical methods are not adequate to obtain quantitative rate constants for the reaction under study. The use of the elaborated CASPT2 method is essential to estimate a quantitative rate constant. Geometry optimizations and vibrational frequency calculations are performed using three different methods (B3LYP, MPW1K, and MP2) and six different basis sets (6-31G(d,p), 6-31+G(d,p), 6-31++G(d,p), 6-311G(d,p), 6-311+G(d,p), and cc-pVDZ). Single-point energy calculations are performed with the highly correlated ab initio coupled cluster method in the space of single, double, and triple (pertubatively) electron excitations CCSD(T) using the 6-31G(d,p) basis set, and with the CASPT2 level of theory with the ANO-L-VDZP basis set. Canonical transition-state theory with a simple Wigner tunneling correction is used to predict the high-pressure limit rate constants as a function of temperature. We recommend the use of the CASPT2/ANO-L-VDZP//B3LYP/cc-pVDZ level of theory to compute the temperature dependence of the rate constant of the four-center isomerization of the benzylperoxy radical. It is given by the following relation: k(600-2000 K) (in s (-1)) = (1.29 x 10 (10)) T (0.79) exp[(-133.1 in kJ mol (-1))/ RT]. These parameters can be used in the thermokinetic models involving aromatic compounds at high pressure. This computational procedure can be extended to predict rate constants for other similar reactions where no available experimental data exist.

Detailed chemical kinetic modeling of cyclohexane oxidation.

A detailed chemical kinetic mechanism has been developed and used to study the oxidation of cyclohexane at both low and high temperatures. Rules for reaction rate constants are developed for the low-temperature combustion of cyclohexane. These rules can be used for in chemical kinetic mechanisms for other cycloalkanes. Because cyclohexane produces only one type of cyclohexyl radical, much of the low-temperature chemistry of cyclohexane is described in terms of one potential energy diagram showing the reaction of cyclohexyl radical with O2 through five-, six-, and seven-membered-ring transition states. The direct elimination of cyclohexene and HO2 from RO2 is included in the treatment using a modified rate constant of Cavallotti et al. (Proc. Combust. Inst. 2007, 31, 201). Published and unpublished data from the Lille rapid compression machine, as well as jet-stirred reactor data, are used to validate the mechanism. The effect of heat loss is included in the simulations, an improvement on previous studies on cyclohexane. Calculations indicated that the production of 1,2-epoxycyclohexane observed in the experiments cannot be simulated according to the current understanding of low-temperature chemistry. Possible "alternative" H-atom isomerizations leading to different products from the parent O2QOOH radical were included in the low-temperature chemical kinetic mechanism and were found to play a significant role.