Theoretical study of the photodissociation reaction of methanol

Direct Observation of Gas-Phase Hydroxymethylene: Photoionization and Kinetics Resulting from Methanol Photodissociation

Carbene species play an integral role in high-energy chemistry, transition-metal-carbene chemistry, catalysis, photolytic formation of carbohydrates, and possibly even the formation of interstellar sugars. In 1921, "reactive formaldehyde"─now known as hydroxymethylene (HCOH)─was first implicated as an intermediate in photocatalytic processes. However, due to its transient nature, direct observation of HCOH has predominantly been attained using cryogenic isolation methods. As a result, HCOH gas-phase reactivity measurements have been limited. We directly observed HCOH using photoionization spectroscopy following UV photodissociation of methanol. Our measurements show it reacts slowly with O2 at room temperature. This work provides evidence for the formation mechanism of HCOH from CH3OH and its subsequent reactivity under gas-phase reaction conditions.

Stability Evaluation of Aflatoxin B1 Solution Certified Reference Material via Ultra-High Performance Liquid Chromatography Coupled with High-Resolution Mass Spectrometry

Aflatoxin B₁ (AFB₁) solution certified reference materials (CRMs) have been widely utilized in the measurements of AFB₁ contaminations in foods and agricultural products. It is of great importance to evaluate the stability of AFB₁ solution CRMs in different matrices for their practical applications. In this study, the stability of AFB₁ solution CRM was investigated and its degradation products under various conditions were elucidated using ultra-high performance liquid chromatography coupled with high-resolution mass spectrometry for the first time. Exposure to high temperatures and UV light irradiation accelerated the degradation of AFB₁ solution significantly, and the degradation products were largely dependent on the solvents. Two degradation pathways were proposed based on the degradation products. The addition reaction, oxidation reaction, and modification of the methoxy group are the major processes involved in the degradation of the AFB₁ solution. The results of this study indicate that the property value of the acetonitrile solution of AFB₁ can be well retained when it is stored at temperatures lower than 60 °C, and the exposure to UV light irradiation is avoided.

Mechanism, Kinetics, and Ecotoxicity Assessment of ·OH-Initiated Oxidation Reactions of Sulfoxaflor

The ·OH-initiated reaction mechanism and kinetics of sulfoxaflor were investigated by using electronic structure calculations. The possible hydrogen atom and cyano group abstraction reaction pathways were studied, and the calculated thermochemical parameters show that the hydrogen atom abstraction from the C7 carbon atom is the more favorable reaction pathway. The subsequent reactions for the favorable intermediate (I4) with other atmospheric reactive species, such as O₂, H₂O, HO₂·, and NO_x· (x = 1, 2), were studied in detail. The products identified from the subsequent reactions could contribute to secondary organic aerosol (SOA) formation in the atmosphere. The intermediates and products formed from the initial and subsequent reactions are equally as toxic as the parent sulfoxaflor. At 298 K, the rate constant calculated for the formation of the favorable intermediate I4 is 2.54 × 10^-12 cm³ molecule^-1 s^-1, which shows that the lifetime of sulfoxaflor is 54 h. The excited-state calculation performed through time-dependent density functional theory shows that the photolysis of the title molecule is unlikely in the atmosphere. The global warming potentials (GWPs) for different time horizons, photochemical ozone creation potential (POCP), and ecotoxicity analysis were also studied for the insecticide sulfoxaflor.

Aqueous picloram degradation by hydroxyl radicals: Unveiling mechanism, kinetics, and ecotoxicity through experimental and theoretical approaches

Pesticides are chemical compounds widely used to combat pests in crops, and they thus play a key role in agricultural production. However, due to their persistence in aquatic environments, even at low concentrations, their use has been considered an environmental problem and caused concern regarding the adverse effects on human health. This paper reports, for the first time, the mechanisms, kinetics, and an evaluation of the toxicity of picloram degradation initiated by OH radicals in the aqueous environment using quantum chemistry and computational toxicology calculations. The rate constants are calculated using a combination of formulations derived from the Transition State Theory in a realistic temperature range (250-310 K). The results indicate that the two favorable pathways (R1 and R5) of OH -based reactions occur by addition to the pyridine ring. The calculated rate constant at 298 K is compared with the overall second-order reaction rate constant, quantified herein experimentally via the competition kinetics method and data available in the literature showing an excellent agreement. The toxicity assessment and a photolysis study provide important information: i) picloram and the majority of degradation products are estimated as harmful; however, ii) these compounds can suffer photolysis in sunlight. The results of the present study can help understand the mechanism of picloram, also providing important clues regarding risk assessment in aquatic environments as well as novel experimental information.

First-Principles Study of the Reaction Mechanism of CHO + H on Graphene Surface

Many organic molecules observed in the interstellar medium are considered to be formed on dust grains and populated into the gas phase. We analyzed the reaction of HCO + H on a graphene surface using ab initio molecular dynamics simulations as a case study of the formation and desorption of organic molecules on interstellar dust particles. During the reactions of chemisorbed CHO (chemisorbed at the C atom) with free H, CO was generated and efficiently desorbed from the surface. These results suggest that the reactions, of which the reactant forms a covalent bond with the surface while the product does not, cause efficient desorption of the product upon reaction. In such reactions a repulsive force between the product and the surface would be generated and accelerate translation of the product in a specific direction. In addition, it was also shown that the branching ratio of the reactions between radical species on the surface would be affected by the form of the adsorption on the surface, e.g., when a free H reacted with the CHO chemisorbed at the C atom, CH₂O was not generated.