Theoretical study of the oxidation mechanisms of thiophene initiated by hydroxyl radicals

Impact of a single water molecule on the atmospheric oxidation of thiophene by hydroperoxyl radical

Water as an important assistant can alter the reactivity of atmospheric species. This project is designed to investigate the impact of a single water molecule on the atmospheric reactions of aromatic compounds that have not been attended to comprehensively. In the first part, the atmospheric oxidation mechanisms of thiophene initiated by hydroperoxyl radical through a multiwell-multichannel potential energy surface were studied to have useful information about the chemistry of the considered reaction. It was verified that for the thiophene plus HO₂ reaction, the addition mechanism is dominant the same as other aromatic compounds. Due to the importance of the subject and the presence of water molecules in the atmosphere with a high concentration that we know as relative humidity, and also the lack of insight into the influence of water on the reactions of aromatic compounds with active atmospheric species, herein, the effect of a single water molecule on the addition pathways of the title reaction is evaluated. In another word, this research explores how water can change the occurrence of reactions of aromatic compounds in the atmosphere. For this, the presence of one water molecule is simulated by higher-level calculations (BD(T) method) through the main interactions with the stationary points of the most probable pathways. The results show that the mechanism of the reaction with water is more complicated than the bare reaction due to the formation of the ring-like structures. Also, water molecule decreases the relative energies of all addition pathways. Moreover, atoms in molecule theory (AIM) along with the kinetic study by the transition state (TST) and the Rice–Ramsperger–Kassel–Marcus (RRKM) theories demonstrate that the overall interactions of a path determine how the rate of that path changes. In this regard, our results establish that the interactions of water with HO₂ (thiophene) in the initial complex 1WHA (1WTA or 1WTB) are stronger (weaker) than the sum of its interactions in transition states. Also, for the water-assisted pathways, the ratio of the partition function of the transition state to the partition functions of the reactants is similar to the respective bare reaction. Therefore, the reaction rates of the bare pathways are more than the water-assisted paths that include the 1WHA complex and are less than the paths that involve the 1WTA and 1WTB complexes.

Computational studies on thermo-kinetics aspects of pyrolysis of isopropyl acetate and its methyl, bromide and hydroxyl derivatives

The gas-phase decomposition kinetics of isopropyl acetate (IPA) and its methyl, bromide and hydroxyl derivatives into the corresponding acid and propene were investigated using density functional theory (DFT) with the ωB97XD and M06–2x functionals, as well as the benchmark CBS-QB3 composite method. Transition state theory (TST) and RRKM theory calculations of rate constants under atmospheric pressure and in the fall-off regime were used to supplement the measured energy profiles. The results show that the formation of propene and bromoacetic acid is the most dominant pathway at the CBS-QB3 composite method, both kinetically and thermodynamically. There was a good agreement with experimental results. Pressures greater than 0.01 bar, corresponding to larger barrier heights are insufficient to ensure saturation of the measured rate coefficient when compared to the RRKM kinetic rates.

Natural bond orbitals (NBO) charges, bond orders, bond indices, and synchronicity parameters all point to the considered pathways taking place via a homogenous, first-order concerted, as well as an asynchronous mechanism involving a non-planar cyclic six-membered transition state. The calculated data exhibit that the elongation of the C_α−O bond length and subsequent polarization of the C_α^+δ…O^−δ bond is the rate-determining step of the considered reactions in the cyclic transition state, which appears to be involved in this type of reaction.

Phototransformation of an emerging cyanotoxin (Aerucyclamide A) in simulated natural waters

Aerucyclamide A (ACA) is an emerging cyanopeptide toxin produced by cyanobacteria, and its transformation pathway has rarely been reported. In the present study, ACA was purified from cyanobacterial extracts, and photodegradation processes were investigated in dissolved organic matter (DOM) solutions. Under simulated solar irradiation, the photodegradation of ACA was dominated by ^•OH oxidation, accounting for ~72% of the indirect photodegradation. The bimolecular reaction rate constant of ACA with ^•OH was (6.4 ± 0.2) × 10⁹M ^- 1s ^- 1. Our results indicated that the major reactive sites of ACA toward ^•OH are thiazoline and thiazole moieties. Product analysis via high-resolution mass spectrometry suggested that hydrogen abstraction and gradual hydroxylation are the main photodegradation pathways. The acute toxicity assessment indicate that the products generated in photolysis process did not show any measurable toxicity to Thamnocephalus platyurus. Photodegradation experiments with various DOM-phycocyanin mixtures demonstrated that the half-life of ACA is much longer than that of microcystin-LR.

Chemical insights into the atmospheric oxidation of thiophene by hydroperoxyl radical

The reaction mechanisms and kinetics of thiophene oxidation reactions initiated by hydroperoxyl radical, and decomposition of the related intermediates and complexes, have been considered herein by using high-level DFT and ab initio calculations. The main energetic parameters of all stationary points of the suggested potential energy surfaces have been computed at the BD(T) and CCSD(T) methods, based on the geometries optimized at the B3LYP/6-311 + g(d,p) level of theory. Rate constants of bimolecular reactions (high-pressure limit rate constants) at temperatures from 300 to 3000 K for the first steps of the title reaction have been obtained through the conventional transition state theory (TST), while the pressure dependent rate constants and the rate constants of the second and other steps have been calculated employing the Rice-Ramsperger-Kassel-Marcus/Master equation (RRKM/ME). The results show that the rate constants of addition to α and β carbons have positive temperature dependence and negative pressure dependence. It is found that the additions of HO₂ to the α and β carbons of thiophene in the initial steps of the title reaction are the most favored pathways. Also, the addition to the sulfur atom has a minor contribution. But, all efforts for simulating hydrogen abstraction reactions have been unsuccessful. In this complex oxidation reaction, about 12 different products are obtained, including important isomers such as thiophene-epoxide, thiophene-ol, thiophene-oxide, oxathiane, and thiophenone. The calculated total rate constants for generation of all minimum stationary points show that the addition reactions to the α and β carbons are the fastest among all at temperatures below 1000 K, while the proposed multi-step parallel reactions are more competitive at temperatures above 1200 K. Furthermore, important inter-and intra-molecular interactions for some species have been investigated by two well-known quantum chemistry method, the NBO and AIM analyses. Thermochemical properties such as free energy, enthalpy, internal energy, and entropy for thiophene and hydroperoxyl radical and related species in the simulated reactions have been predicted using a combination of the B3LYP and BD(T) methods.

Atmospheric chemistry of oxazole: the mechanism and kinetic studies of the oxidation reaction initiated by OH radicals

Oxidation of oxazole by OH˙ radicals studied by DFT methods coupled with reaction kinetics calculations using TST and RRKM theories.

Production of Amidinyl Radicals via UV-Vis-Light Promoted Reduction of N-Arylthiophene-2-carboxamidoximes and Application to the Preparation of Some New N-Arylthiophene-2-carboxamidines

A modern method for the preparation of some new N-arylthiophene-2-carboxamidines via amidinyl radicals generated using UV-vis-light promoting the reduction of N-arylthiophene-2-carboxamidoximes without any catalyst in a short amount of time, highly straight forward, and in an efficient manner is described. This method defeats the flaws of the conventional methods for the reduction of amidoxime derivatives to amidine derivatives, which require harsh conditions such as using a strong acid, high temperature, and expensive catalysts. Benzo[d]imidazoles, benzo[d]oxazoles, and amides can also be synthesized by applying this method. The photoproducts were analyzed by various spectroscopic and analytical techniques, including thin-layer chromatography, column chromatography, high-performance liquid chromatography, gas chromatography/mass spectrometry, IR, ¹H NMR, ¹³C NMR, and MS. Notably, the chromatographic analyses proved that the best time for the production of N-arylthiophene-2-carboxamidines is 20 min. The reaction mechanism comprising pathways and intermediates was also suggested via the homolysis of N-O and C-N bonds.

Mechanism and kinetics studies of the atmospheric oxidation of p,p'-Dicofol by OH and NO3 radicals

As an effective organochlorine pesticide, Dicofol has been extensively applied in more than 30 countries for protecting over 60 different crops. Considering its large consumption and potential adverse effect on human health (endocrine disrupting and carcinogenicity), the fate of Dicofol sprayed into the air is of public concern. In this study, we conducted a comprehensive study on the reaction mechanisms of p,p'-Dicofol with OH and NO₃ radicals using DFT method. Comparing the abstrations by OH and NO₃ radical, OH and NO₃ radical addition reactions are predominant due to the lower potential barriers and stronger heat release. The phenolic substances (P1P5), epoxides (P11 and P15), dialdehyde (P13) and other species (P8, P9, P10 and P14) are generated by OH additions and their subsequent reactions while OH abstraction reactions produce DCBP, P7 and chlorphenyl radical. Particularly, NO₃ additions and their subsequent reactions yield dialdehydes (P16 and P17) and 2,8-DCDD, which is the first report of the generation of dioxin from atmospheric oxidation of p,p'-Dicofol. Additionally, based on the structure optimization and energy calculation, rate constants and Arrhenius formulas of the elementary reactions of p,p'-Dicofol with OH and NO₃ radicals were obtained over the temperature range of 280-380 K and at 1 atm. The rate constants for the reactions of p,p'-Dicofol with OH and NO₃ radicals are 1.51 × 10^-12 and 8.88 × 10^-14 cm³ molecule^-1 s^-1, respectively. The lifetime (τ_Total) of p,p'-Dicofol determined by the reactions of OH and NO₃ radical is 5.86 h, indicating a potential long-range transport in the atmosphere.

Reaction Mechanisms and Kinetics of the O2 Addition Pathways to the Main Thiophene-OH Adduct: A Theoretical Study

Density functional theory, along with the ΩB97XD and UM06-2x exchange-correlation functional, has been used to study the reaction mechanisms and kinetics of the atmospheric oxidation of the main (kinetically dominant) thiophene-OH adduct [C4H4S-OH]• (R1) by molecular oxygen in its triplet electronic ground state. Kinetic rate constants and branching ratios under atmospheric pressure and in the fall-off regime have been calculated by means of transition state theory (TST), variational transition state theory (VTST) and statistical Rice–Ramsperger–Kassel–Marcus (RRKM) theory. In line with the computed energy profiles, the dominant process under both the thermodynamic and kinetic control of the reaction is O2 addition at the C5 position in syn mode. The computed branching ratios indicate that the regioselectivity of the reaction decreases with increasing temperature and decreasing pressure.