1,720,971 research outputs found
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
Full text linkHalogenated 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
Thermochemistry and Kinetics of the OH- and Cl-Initiated Degradation Pathways of the HCFC-132b Atmospheric Pollutant
Full text linkThe gas-phase hydrogen abstraction reaction of 1,2-dichloro-1,1-difluoroethane (CH2ClCClF2) with OH and Cl radicals is theoretically investigated by employing density functional theory methods combined with coupled-cluster-based composite schemes. The mechanism and kinetics of the degradation entrance channels of CH2ClCClF2, recently detected with increasing concentration in the atmosphere, is elucidated by using cost-effective ab initio triple-slash dual-level direct dynamics, jChS// B2PLYP-D3(BJ)/jun-cc-pV(T+d)Z///M06-2X-D3/jun-cc-pV(T +d)Z. Thermal rate constants are calculated over the temperature range of 200-1000 K by adopting both canonical-(CVT) and microcanonical (mu VT)-variational transition state theory also including the microcanonically optimized multidimensional tunnel-ing transmission coefficient (mu OMT). The theoretical rate coefficient for the H-abstraction reaction initiated by the OH radical is computed to be kOH CVT/mu OMT = 1.72 x 10-14 cm3 molecule-1 s-1 and kOH mu VT/mu OMT = 1.57 x 10-14 cm3 molecule-1 s-1 at 298 K, in excellent agreement with the available experimental data. The rate constant for the H-abstraction reaction by the Cl atom, here obtained for the first time, is predicted to be kCl CVT/mu OMT = 5.96 x 10-14 cm3 molecule-1 s-1 and kCl mu VT/mu OMT = 5.65 x 10-14 cm3 molecule-1 s-1 at 298.15 K, thus showing increased efficiency with respect to the OH-prompted reaction
Mechanistic Insights into the Silica-Mediated Synthesis of Glyceraldehyde from Glycolaldehyde and Hydroxymethylene
No full textMinerals are crucial ingredients in prebiotic chemistry as they could have promoted
the evolution of simple organic molecules toward proto-biomolecules that are on the route of the
emergence of self-replicating information-rich macromolecules. In this respect, the formose
reaction, involving the sequential autocatalytic condensation of formaldehyde, is the generally
accepted pathway to sugar synthesis. Although obtained under controlled laboratory conditions,
with enhanced sugar yields promoted by the presence of silicate in the reaction medium, it presents
a number of limitations, and the underlying reaction mechanism remains an unsolved riddle. In
this work, the focus is on the second step of the formose reaction, namely the synthesis of
glyceraldehyde, which is accomplished by considering the reaction between glycolaldehyde and
hydroxymethylene taking place on the edingtonite mineral. The reaction mechanism is explored
by quantum chemical simulations performed at various degrees of sophistication to shed light on
the thermochemical and kinetic feasibility of the reaction. The same pathway is also investigated
in the gas-phase, in order to disentangle the role played by the zeolitic mineral. The obtained results
show that the endothermic reaction between glycolaldehyde and hydroxymethylene yields
glyceraldehyde by a submerged reaction path, both in the gas-phase and on the edingtonite surface.
The mineral substrate provides further stabilization, by about 20 kcal mol-1, of all the species
involved in the reaction pathway and acts as a scaffold favoring the interaction of the two reactants
Reliable gas phase reaction rates at affordable cost by means of the parameter-free junChS-F12 model chemistry
Full text linkA recently developed strategy for the computation at affordable cost of reliable barrier heights ruling reactions in the gas phase (junChS, [Barone, V.; et al. J. Chem. Theory Comput. 2021, 17, 4913−4928]) has been extended to the employment of explicitly correlated (F12) methods. A thorough benchmark based on a wide range of prototypical reactions shows that the new model (referred to as junChS-F12), which employs cost-effective revDSD-PBEP86-D3(BJ) reference geometries, has an improved performance with respect to its conventional counterpart and outperforms the most well-known model chemistries without the need of any empirical parameter and at an affordable computa- tional cost. Several benchmarks show that revDSD-PBEP86- D3(BJ) structures and force fields provide zero point energies
and thermal contributions, which can be confidently used, together with junChS-F12 electronic energies, for obtaining accurate reaction rates in the framework of the master equation approach based on the ab initio transition-state theory
Paradigms and paradoxes : systematics in the study of the simplest sulfenic acids and sulfoxides, and a comparison between sulfur–oxygen and nitrogen–oxygen bonds
No full textSome particularities in the bonding of simple sulfenic acids and their sulfoxide isomers
are explored using accurate theoretical methods. Some unexpected results are
described using thermochemical results on diverse nitrogen– and/or oxygen–
containing functionalities such as amino, nitro, nitroso and nitrite derivatives
Enthalpy of Formation of Carbocycles: A Precise Theoretical Determination of Experimentally Imprecise Measurements.
Full text linkDespite being one of the best-known families of organic compounds, the fact that hydrocarbons exhibit a rich
variety of structures owing to branching, cyclization, and the presence of multiple bonds, means that many of
their properties are yet to be determined accurately, or even at all. Among cyclic hydrocarbons, those with three-
membered rings are particularly interesting because of their strain energy. In this paper, we report accurate
calculations of the enthalpy of formation of three-membered carbocycles, whose experimental values have not
been obtained by direct measurement of the heat of combustion. For this purpose, we used several accurate
composite methods to obtain the gas-phase enthalpies of atomization and derived from them the gas-phase
enthalpies of formation, using experimentally determined accurate values for the atoms. Moreover, to mini-
mize the inaccuracy that can possibly arise in this procedure, we also used homodesmotic reactions designed to
balance systematic errors in the geometric and electronic structure of some of the species. A careful analysis of
the results shows that some of the indirectly derived values reported in the literature are far from the most
accurate theoretical outcomes, and we suggest that these new ones should be adopted
A new chapter in the never ending story of cycloadditions : The puzzling case of SO2 and acetylene
Full text linkA 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 SO 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 H2CS. Comparison of this decomposition path with that of 2-amino-2-thioxoacetic acid shows that the process occurs through different H-transfer processes
Exploring the Maze of C2N2H5 Radicals and Their Fragments in the Interstellar Medium with the Help of Quantum-Chemical Computations
Full text linkAmong the species discovered in the interstellar medium and planetary atmospheres, a crucial role is played by the so-called “interstellar” complex organic molecules (iCOMs) because they are the signature of the increasing molecular complexity in space. Indeed, they may represent the connection between simple molecules and biochemical species like amino acids and nucleobases. In particular, HCN and the related CN radical are the starting points of rich nitrile chemistry. In this framework, we have undertaken a computational investigation of the gas-phase reaction mechanisms involving different C2N2H5 radicals and their fragments, stemming from the addition of the cyano radical to the nitrogen atom of methylamine. Aiming at exploiting an accurate yet cost-effective protocol, a combination of CCSD(T)-based composite schemes and density functional theory has been employed. The exploration of the plausible chemical reaction channels has led to the identification of 12 different products, as well as 28 transition states connecting reactants, intermediates, and products. Aminoacetonitrile (H2NCH2CN), proposed as an intermediate in the formation of the smallest amino acid glycine, and the CH2NH2 radical appear as products energetically accessible under astrophysical conditions
Unraveling the role of additional OH-radicals in the H–Abstraction from Dimethyl sulfide using quantum chemical computations
No full textDimethyl sulfide (DMS) is the main organosulfur compound in the atmosphere. Oxidation by OH radicals to form the methyl thiomethyl species (MTMr) has been studied under normal atmospheric conditions and in reaction chambers at different O2 partial pressure, including complete absence of oxygen. Scarce attention has been devoted however to the possibility of further reaction of OH with MTMr. We present here a computational study using DFT, CCSD(T) and composite methods, on the properties of two stable intermediates never fully investigated before, methanesulfenyl methanol (MSMOH) and S-methyl-methanesulfenic acid (SMMSA), arising from addition of a second OH radical to MTMr
Energetics of the OH radical H-abstraction reactions from simple aldehydes and their geminal diol forms
Full text linkContextCarbonyl compounds, especially aldehydes, emitted to the atmosphere, may suffer hydration in aerosols or water droplets in clouds. At the same time, they can react with hydroxyl radicals which may add or abstract hydrogen atoms from these species. The interplay between hydration and hydrogen abstraction is studied using density functional and quantum composite theoretical methods, both in the gas phase and in simulated bulk water. The H-abstraction from the aldehydic and geminal diol forms of formaldehyde, acetaldehyde, glycolaldehyde, glyoxal, methylglyoxal, and acrolein is studied to determine whether the substituent has any noticeable effect in the preference for the abstraction of one form or another. It is found that abstraction of the H-atom adjacent to the carbonyl group gives a more stable radical than same abstraction from the geminal diol in the case of formaldehyde, acetaldehyde, and glycolaldehyde. The presence of a delocalizing group in the C alpha (a carbonyl group in glyoxal and methylglyoxal, and a vinyl group in acrolein), reverts this trend, and now the abstraction of the H-atom from the geminal diol gives more stable radicals. A further study was conducted abstracting hydrogen atoms from the other different positions in the species considered, both in the aldehydic and geminal diol forms. Only in the case of glycolaldehyde, the radical formed by H-abstraction from the -CH2OH group is more stable than any of the other radical species. Abstraction of the hydrogen atom in one of the hydroxyl groups in the geminal diol is equivalent to the addition of the center dot OH radical to the aldehyde. It leads, in some cases, to decomposition into a smaller radical and a neutral molecule. In these cases, some interesting theoretical differences are observed between the results in gas phase and (simulated) bulk solvent, as well as with respect to the method of calculation chosen.MethodsDFT (M06-2X, B2PLYP, PW6B95), CCSD(T), and composite (CBS-QB3, jun-ChS, SCVECV-f12) methods using Dunning basis sets and extrapolation to the CBS limit were used to study the energetics of closed shell aldehydes in their keto and geminal-diol forms, as well as the radical derived from them by hydrogen abstraction. Both gas phase and simulated bulk solvent calculations were performed, in the last case using the Polarizable Continuum Model
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