Correcting the Experimental Enthalpies of Formation of Some Members of the Biologically Significant Sulfenic Acids Family

Theoretical investigation of the OH-initiated atmospheric degradation mechanism of CX2CHX (X = H, F, Cl) by advanced quantum chemical and transition state theory methods

Halogenated olefins are anthropogenic compounds with many industrial applications but at the same time raising many environmental and health concerns. Gas-phase electrophilic addition of the OH radical to the olefinic CC bond represents the primary sink for these chemicals in the atmosphere, with the degree and type of halogenation playing a significant role in their overall reactivity. In this work, we present a theoretical investigation of the reaction mechanisms and kinetics for the reactions between the OH radical and CH2CH2 (ethylene, ETH), CF2CHF (trifluoroethylene, TFE) and CCl2CHCl (trichloroethylene, TCE), simulated by state-of-the-art protocols and methods, with the aim of providing a detailed interpretation of the available experimental results, as well as new data of relevance to tropospheric chemistry. Specifically, potential energy surfaces (PESs) are obtained using the jun-Cheap (jChS) composite scheme, whereas temperature and pressure dependent rate coefficients and product distributions in the 100-600 K temperature range are calculated within the Rice-Ramsperger-Kassel-Marcus/master equation (RRKM/ME) framework. The rates for barrierless channels are obtained from variable reaction coordinate-variational transition state theory (VRC-VTST) combined with the two transition state model. While the reactions with ETH and TFE proceed mainly via the formation of addition adducts at P = 1 atm and T = 298 K, the dominant channel for TCE is the Cl-elimination reaction. Global rate constants for the two halogenated olefins, TFE and TCE, are found to be pressure-independent, contrary to the case of ETH. The computed rate constants, as well as their temperature and pressure dependence, are in remarkable agreement with the available experimental data, and they are used to derive atmospheric lifetimes (τ) for both TFE and TCE as a function of altitude (h) in the atmosphere, by taking into account variations in the rate coefficients (k (T, P)) and [OH] concentration.

Spectroscopic Characterization and Photochemistry of the Atmospherically Relevant Methanesulfenic Acid

Methanesulfenic acid, CH3SOH, is a fleeting intermediate in the ·OH-initiated oxidation reactions of dimethyl sulfide (DMS) and dimethyl disulfide (DMDS) in the atmosphere. Herein, we report the characterization and photochemistry of CH3SOH in Ar- and N2-matrices at 10 K. The characterization of CH3SOH with matrix-isolation IR and UV-vis spectroscopy is supported by D and 13C isotope labeling experiments and quantum chemical calculations. In line with the observed absorption at 260 nm for CH3SOH, its photolysis at 254 nm leads to dissociation by yielding the novel water complex H2CS···H2O, which exhibits a five-membered ring structure with intermolecular S···HO and CH···O hydrogen bonding interactions. Upon further irradiation at 193 nm, the H2CS···H2O complex undergoes dehydrogenation to form CS···H2O, which can further convert to HC(O)SH under irradiation at 254 nm. When the photolysis of CH3SOH was performed in an O2-doped Ar-matrix, methanesulfonic acid (MSA, CH3SO3H) was obtained as the oxidation product.

A new chapter in the never ending story of cycloadditions: The puzzling case of SO2 and acetylene

A comprehensive study of the different classes of cycloaddition reactions ([3+2], [2+2], and [2+1]) of SO2 to acetylene and ethylene has been performed using density functional theory (DFT) and composite wavefunction methods. The [3+2] cycloaddition reaction, that was previously explored in the context of the cycloaddition of thioformaldehyde S-methylide (TSM) to ethylene and acetylene, proceeds in a concerted way to the formation of stable heterocycles. In this paper, we extend our study to the [2+2] and [2+1] cycloadditions of SO2 to acetylene, which would produce 1,1-oxathiete-2-oxide and thiirene-1,1-dioxide, respectively. One of the main conclusions is that cyclic 1,1-oxathiete-2-oxide can open through a relatively easy breaking of the SO single bond and rearrange toward sulfinyl acetaldehyde (SA). The SA molecule can easily undergo several internal rearrangements, which eventually lead to sulfenic acid and sulfoxide derivatives of ethenone, 1,2,3-dioxathiole, and CO plus sulfinylmethane. The most probable path, however, produces 2-thioxoacetic acid, whose derivatives (or those of the corresponding acetate) are usually obtained by Willgerodt-Kindler-type sulfuration of acetates. This product can in turn decompose, leading to the final products CO2 and H2CS. Comparison of this decomposition path with that of 2-amino-2-thioxoacetic acid shows that the process occurs through different H-transfer processes.

Ab Initio and Kinetic Modeling of β-d-Xylopyranose under Fast Pyrolysis Conditions

Lignocellulosic biomass is an abundant renewable resource that can be upgraded to chemical and fuel products through a range of thermal conversion processes. Fast pyrolysis is a promising technology that uses high temperatures and fast heating rates to convert lignocellulose into bio-oils in high yields in the absence of oxygen. Hemicellulose is one of the three major components of lignocellulosic biomass and is a highly branched heteropolymer structure made of pentose, hexose sugars, and sugar acids. In this study, β-d-xylopyranose is proposed as a model structural motif for the essential chemical structure of hemicellulose. The gas-phase pyrolytic reactivity of β-d-xylopyranose is thoroughly investigated using computational strategies rooted in quantum chemistry. In particular, its thermal degradation potential energy surfaces are computed employing Minnesota global hybrid functional M06-2X in conjunction with the 6-311++G(d,p) Pople basis set. Electronic energies are further refined by performing DLPNO-CCSD(T)-F12 single-point calculations on top of M06-2X geometries using the cc-pVTZ-F12 basis set. Conformational analysis for minima and transition states is performed with state-of-the-art semiempirical quantum chemical methods coupled with metadynamics simulations. Key thermodynamic quantities (free energies, barrier heights, enthalpies of formation, and heat capacities) are computed. Rate coefficients for the initial steps of thermal decomposition are computed by means of reaction rate theory. For the first time, a detailed elementary reaction kinetic model for β-d-xylopyranose is developed by utilizing the thermodynamic and kinetic information acquired from the aforementioned calculations. This model specifically targets the initial stages of β-d-xylopyranose pyrolysis in the high-pressure limit, aiming to gain a deeper understanding of its reaction kinetics. This approach establishes a systematic strategy for exploring reactive pathways, evaluating competing parallel reactions, and selectively accepting or discarding pathways based on the analysis. The findings suggest that acyclic d-xylose plays a significant role as an intermediary in the production of key pyrolytic compounds during the pyrolysis of xylose. These compounds include furfural, anhydro-d-xylopyranose, glycolaldehyde, and dihydrofuran-3(2H)-one.