Small-curvature Tunneling Research Articles

Cl Atom-Initiated Photo-Oxidation Reactions of Vinyl Trifluoroacetate and Allyl Trifluoroacetate in Tropospheric Conditions.

Relative rate (RR) technique coupled with gas chromatography (GC) was used to investigate the kinetics of the reactions of two hydrofluoroesters (allyl trifluoroacetate (CF3C(O)OCH2CH═CH2, ATFA) and vinyl trifluoroacetate (CF3C(O)OCH═CH2, VTFA)) with Cl atoms between 268 and 363 K and at 760 Torr of N2 (or air). The temperature-dependent Arrhenius expressions for the title reactions were obtained to be k268-363KVTFA+Cl = [(7.83 ± 2.26) × 10-12 exp((974 ± 89)/T)] cm3 molecule-1 s-1 and k268-363KATFA+Cl = [(9.03 ± 1.92) × 10-12 exp((883 ± 65)/T)] cm3 molecule-1 s-1, respectively. A negative temperature dependency was observed for both the reactions. In addition to this, the kinetics of the studied reactions was evaluated computationally at the CCSD(T)/cc-pVTZ//MP2/6-311++G(d,p) level of theory in the temperature range of 200-400 K using canonical variational transition (CVT) state theory in conjunction with small curvature tunneling (SCT) corrections and interpolated single point energy (ISPE) methods. The product analysis of the reactions of VTFA and ATFA with Cl atoms in the presence of O2 was also investigated using gas chromatography-mass spectrometry (GC-MS) and gas chromatography-infrared spectroscopy (GC-IR). The plausible oxidation mechanism of the title reactions was proposed based on the product analyses. Further, to comprehend the impact of these molecules on the troposphere, atmospheric lifetimes, lifetime-corrected radiative forcing (RF), and global warming potential (GWP) were estimated and are presented in this manuscript.

Effect of water and ammonia on the HO + NH3 → NH2 + H2O reaction in troposphere: Competition between single and double hydrogen atom transfer pathways

A comprehensive investigation of the roles of H2O and NH3 on the HO + NH3 → NH2 + H2O reaction in the troposphere has been carried out by CCSD(T)-F12a/cc-pVDZ-F12//M06-2X/6–311+G(2d,2p) method, and the canonical variational transition state theory with small curvature tunneling correction. The results show that both H2O and NH3 catalyzed reactions prefer the single hydrogen atom transfers (HAT) pathways than the double HAT routes. Meanwhile, the catalytic effect of H2O is more obvious as compared with NH3 with its effective rate constant (k'(WM)) larger by 2–5 orders of magnitude. However, within the temperature range of 213–320 K, the calculated value of k'(WM) is smaller by 5–9 orders of magnitude than the corresponding rate constant of the naked reaction. This result is in agreement with H2O catalyzed OH + CH2CH2, OH + CH2O and OH + CH2NH reactions, where water-assisted reaction cannot accelerate the reaction.

Hydrolysis of HNSO2: A potential route for atmospheric production of H2SO4 and NH3

AbstractThe hydrolysis of sulfonylamine (HNSO2) results in the formation of sulfuric acid along with ammonia, and is of significant interest due to their negative impact on environment and life on Earth. The formation of H2SO4 through the reaction of HNSO2 with (H2O)2‐4 has been studied using high level electronic structure calculations. This hydrolysis reaction is a step‐wise process, in the first step a H‐atom from H2O is transferred to the N‐atom of HNSO2 which results in the formation of NH2, and in the next step, H2SO4, NH3 and water molecule(s) are formed. The results show that the energy barrier associated with the formation of intermediates and product complexes is reduced by 7 to 10 kcal/mol when the number of water molecules is increased from 2 to 4. The rate constant was calculated using canonical variational transition state theory with small curvature tunneling correction over the temperature range of 200 to 1000 K. At 298 K, the calculated rate constant for the formation of intermediate in the first step is 2.24 × 10−16, 1.03 × 10−12, and 2.10 × 10−11 cm3 mol−1 s−1, respectively, for the reaction with water dimer, trimer and tetramer. The calculated enthalpy and free energy show that the reaction corresponding to the formation of H2SO4 is highly exothermic and exoergic in nature.

Mechanism and kinetics of diuron oxidation by hydroxyl radical addition reaction

Diuron is a phenyl urea herbicide used to control weeds in agricultural lands. The degradation of diuron in the atmosphere takes place dominantly via reaction with OH radicals. In this work, the OH addition reaction of diuron has been studied by using density functional theory methods M06-2X, ωB97X-D and MPWB1K with 6-31G(d,p) basis set. The calculated thermochemical parameters show that OH addition reaction occurs favourably at C2 position of diuron. The rate constant is calculated for the favourable initial reaction pathway by using canonical variational transition state theory with small curvature tunnelling (SCT) correction over the temperature range of 200-1000K. The reaction of initially formed diuron-OH adduct intermediate with O2 leads to the formation of peroxy radical intermediate. The reaction of peroxy radical intermediate with HO2 and NOx (x = 1, 2) radicals is studied in detail. The results obtained from time-dependent density functional theory (TDDFT) calculations show that the intermediates and products formed from oxidation of diuron can be easily photolyzed in the sunlight. This study provides thermodynamical and kinetic data for the atmospheric oxidation of diuron by OH radical addition reaction and demonstrates the atmospheric chemistry of diuron and its derivatives.

Gas-Phase Reaction Kinetics of Pyruvic Acid with OH Radicals: The Role of Tunneling, Complex Formation, and Conformational Structure.

The gas-phase reaction of pyruvic acid (PA) with the OH radical is studied theoretically using accurate quantum chemistry and transition state theory. Two chemically distinct H-atom abstraction reactions and two distinct OH addition reactions have been identified. The rate coefficients for these four processes were calculated. Quantum tunneling was included in each rate using the small curvature tunneling method. The influence of the conformational structure of PA was found to be particularly intriguing. While the trans-cis structure was found to dominantly react by H-atom abstraction from the methyl site, the trans-trans conformer was found to react mostly through H-atom abstraction from the acid site. A general formalism was developed to model the kinetics of the reactions that involve multiple conformers, interconverting prereactive complexes, and multiple transition states. Comparison of the results obtained with available experimental rate observations reveals agreement with the trans-trans conformer of PA but disagreement with the results obtained for a full statistical mixture of reagents. The role of these reactions in the atmospheric processing of PA is discussed.

Reaction mechanism, energetics, and kinetics of the water-assisted thioformaldehyde + ˙OH reaction and the fate of its product radical under tropospheric conditions.

The reactions of thioformaldehyde (H2CS) with OH radicals and assisted by a single water molecule have been investigated using high level ab initio quantum chemistry calculations. The H2CS + ˙OH reaction can in principle proceed through: (1) abstraction, and (2) addition pathways. The barrier height for the addition reaction in the absence of a catalyst was found to be -0.8 kcal mol-1, relative to the separated reactants, which has a ∼1.0 kcal mol-1 lower barrier than the abstraction channel. The H2CS + ˙OH reaction assisted by a single water molecule reduces the barrier heights significantly for both the addition and abstraction channels, to -5.5 and -6.7 kcal mol-1 respectively, compared to the un-catalyzed H2CS + ˙OH reaction. These values suggest that water lowers the barriers by ∼6.0 kcal mol-1 for both reaction paths. The rate constants for the H2CSH2O + ˙OH and OHH2O + H2CS bimolecular reaction channels were calculated using Canonical Variational Transition state theory (CVT) in conjunction with the Small Curvature Tunneling (SCT) method over the atmospherically relevant temperatures between 200 and 400 K. Rate constants for the H2CS + ˙OH reaction paths for comparison with the H2CS + ˙OH + H2O reaction in the same temperature range were also computed. The results suggest that the rate of the H2CS + ˙OH + H2O reaction is slower than that of the H2CS + ˙OH reaction by ∼1-4 orders of magnitude in the temperatures between 200 and 400 K. For example, at 300 K, the rates of the H2CS + ˙OH + H2O and H2CS + ˙OH reactions were found to be 2.2 × 10-8 s-1 and 6.4 × 10-6 s-1, respectively, calculated using [OH] = 1.0 × 106 molecules cm-3, and [H2O] = 8.2 × 1017 molecules cm-3 (300 K, RH 100%) atmospheric conditions. Electronic structure calculations on the H2C(OH)S˙ product in the presence of 3O2 were also performed. The results show that H2CS is removed from the atmosphere primarily by reacting with ˙OH and O2 to form thioformic acid, HO2, formaldehyde, and SO2 as the main end products.

Rate coefficients and product branching ratios for (E)-2-butenal + H reactions.

Thermal rate constants for the hydrogen abstraction reactions of (E)-2-butenal by hydrogen atoms were calculated, for the first time, using the multipath canonical variational theory with small-curvature tunneling (MP-CVT/SCT). After a torsional potential energy surface exploration, ten conformations of the transition states (including the mirror images) were found and separated into four conformational reaction channels (CRCs). Individual energy paths of each CRC were built, recrossing and quantum tunneling effects estimated, and the thermal rate constants obtained. Due to the hindered rotors, the torsional anharmonicity was incorporated in the rate coefficient through the calculations of the rovibrational partition functions using the extended two-dimensional torsional method (E2DT). For comparison, the one-well (1W-CVT/SCT) and harmonic multipath (MP-CVT/SCT) thermal rate constants were also estimated. In addition, kinetic Monte Carlo (KMC) simulations were performed to predict the product branching ratios. For all kinetic approaches, the formation of products of (R1) is predominant. Compared to the harmonic multipath estimation, the percentage of reaction (R4) increases by approximately 9% when the torsional anharmonicity is taken into account. For the reactions (R2) and (R3), the product branching ratio is slightly decreased when compared with the harmonic simulation.

A protocol to obtain multidimensional quantum tunneling corrections derived from QM(DFT)/MM calculations for an enzyme reaction.

The significance of tunneling contributions to the rate constants of enzymatic reactions has been described at length using experimental evidence as well as theoretical computations. Within the frame of variational transition state theory (VTST), tunneling corrections are included using the so-called ground-state tunneling transmission coefficient. For the calculation of those enzymatic rate constants using the ensemble-averaged extension of VTST on QM/MM potential energy surfaces, the transmission coefficient at a given temperature is averaged over a set of coefficient values, each one obtained from an individual minimum energy path (MEP). However, the calculation of accurate QM/MM MEPs for tunneling calculations, also using a reliable QM method like DFT, is highly costly in enzyme models. For this reason, more affordable methodologies have been used. In this paper, we validate a feasible computational strategy to compute multidimensional tunneling corrections that describes better than cheaper alternatives the physics of the hydrogen abstraction from linoleic acid catalyzed by the enzyme 15-rLOX-1. Our recommendations to obtain better values of kinetic isotope effects and, especially, of rate constants are based on multidimensional small-curvature tunneling (SCT) coefficients derived from electrostatic embedding QM(DFT)/MM MEPs. The MEPs used must be calculated with a small enough step-size. Also, the number of gradients and Hessians along the reaction path must be checked to cover the whole tunneling region and to obtain converged adiabatic potential energy profiles. Distinguished reaction coordinates (DCPs) that are commonly used to describe enzyme reaction mechanisms are not adequate for tunneling calculations in such biological systems.

Oxidative Degradation Kinetics and Mechanism of Two Biodiesel Constituents (Ethyl-2-Methyl Propionate (E2MP) and Ethyl-2,2-Dimethyl Propionate (E22DMP)) Initiated by Cl Atoms

The oxidative reaction kinetics and mechanism of Ethyl-2-methyl propionate (E2MP) and Ethyl-2, 2-dimethyl propionate (E22DMP) initiated by Cl atoms were investigated both experimentally and computationally. Temperature dependent kinetic studies on the reaction of E2MP and E22DMP with Cl atoms were measured using relative rate technique over the temperature range of 268-363 K and at 760 Torr. The rate coefficients of both the reactions at 298 K were measured (using IUPAC recommended reference rate coefficients) to be k_(E2MP+Cl)^(298 K)= (7.19±1.60) × 10-11 cm3 molecule-1 s-1 and k_(E22DMP+Cl)^(298 K)= (5.87±1.33) × 10-11 cm3 molecule-1 s-1 respectively. In addition to complement our experimental results, the kinetics were computed at CCSD(T)/MG3S//BHandHLYP/6-311+G(d,p) level of theory over the temperature range of 200-400 K using Canonical Variational Transition State theory (CVT) in conjunction with Small Curvature Tunneling (SCT) and Interpolated Single Point Energy (ISPE). Furthermore, the product deg...

Kinetics and Mechanistic Study for Gas Phase Tropospheric Photo-oxidation Reactions of 2,2,2-Trifluoroethyl Methacrylate with OH Radicals and Cl Atoms: An Experimental and Computational Approach.

The photo-oxidation reactions of 2,2,2-trifluoroethyl methacrylate (TFEMA) initiated by OH radicals and Cl atoms were investigated via experimental as well as computational methodologies. The rate coefficients for these reactions were investigated using the relative rate technique (RR) at temperatures between 268 and 363 K. The rate coefficients for the reaction of OH radicals with TFEMA were measured with reference to diethyl ether and propylene. Propane and propylene were used in the kinetic measurements as reference compounds. At 298 K, experimentally obtained rate coefficients for the reaction of TFEMA with OH radicals and Cl atoms are kTFEMA+OHexp-298K = (2.81 ± 0.54) × 10-11 and kTFEMA+Clexp-298K = (1.91 ± 0.44) × 10-10 cm3 molecule-1 s-1, respectively. The Arrhenius expression obtained for the respective reactions are kTFEMA+OHexp-(268-363K) = (7.32 ± 0.62) × 10-12 exp[(400 ± 53) and kTFEMA+Clexp-(268-363K) = (4.10 ± 0.78) × 10-12 exp[(1228 ± 115)/T]. To further complement our experimental findings, rate coefficients were also calculated computationally for the reactions of OH radicals and Cl atoms with TFEMA at CCSD(T)/cc-pVDZ//M062X/6-31+G(d,p) and CCSD(T)/cc-pVDZ//MP2/6-31+G(d,p) levels of theory using canonical variational transition state theory (CVT) with small curvature tunneling (SCT) over the temperature range 200-400 K. Moreover, to analyze the end products formed during the title reactions, qualitative analyses were performed using gas chromatography-mass spectrometry (GC-MS) and gas chromatography-infrared spectroscopy (GC-IR) as analytical tools and degradation mechanisms were proposed for the title reactions. Branching ratios, thermochemical parameters of these reactions, and their impact on the troposphere were discussed.

Computational investigation on the reaction of dimethyl ether with nitric dioxide. II. Detailed chemical kinetic modeling

Hydrogen abstraction from dimethyl ether (DME) by nitric dioxide (NO2) has been considered to be very sensitive to the DME ignition behavior. Thermal rate coefficients for the reaction of DME with NO2 are computed by employing the multi-structural canonical variational transition-state theory with multidimensional tunneling (MS-CVT/MT) at temperatures from 200 to 3000 K. The B2PLYP/TZVP theoretical level has been verified in the first part of this study against experiment for geometries of stationary points and against accurate CCSD(T)/CBS and CCSD(T)/6-311+G(2df,2p) computations for the energetics related to the title reaction, and then the compound CCSD(T)/6-311+G(2df,2p)//B2PLYP/TZVP computations are performed here to obtain the properties of all stationary points and the geometries, energies, gradients, and Hessians of non-stationary points along each reaction minimum-energy pathway. It has been proposed that the internal rotations in the transition structures are anharmonic and strongly coupled to each other to produce multiple conformations whose contributions are included in their partition function approximations. The previous evaluations for these rate coefficients based on transition-state theory with one-dimensional tunneling corrections and one-dimensional hindered rotor treatments of torsions are essentially distinct from our MS-CVT/SCT results where the critical effects of corner cutting due to small reaction-path curvature are seriously treated via multidimensional small-curvature tunneling (SCT) treatment and torsions are explicitly considered by multistructural treatments, and thus the MS-CVT/SCT rate coefficients are proposed particularly to refine the current kinetic model for DME combustion.

Reaction mechanism and kinetics of Criegee intermediate CH2OO with CH2 = C(CH3)CHO

The comprehensive mechanism and kinetics for CH2OO with methacrolein (CH2 = C(CH3)CHO, MACR) reaction were investigated at the CCSD(T)/6-311+G(2df,2p)//M06-2X/6-311+G(2df,2p) level of theory. The results show that three kinds of mechanisms including cycloaddition, oxidation and insertion have been identified. Among them, the cycloaddition of CH2OO adding to CO bond of MACR is more favorable, which occurs via a deeply submerged barrier leading to the formation of energized secondary ozonide (SOZ). The nascent SOZ mainly further decomposes into methacrylic acid and formaldehyde or formic acid and methacrolein, in which formic acid generation follows stepwise and concerted mechanism involving a catalysis for rearrangement of CH2OO by methacrolein. The calculated rate constant for the title reaction at 300 K using canonical variational transition state theory with small curvature tunneling correction is 1.00 × 10−12 cm3 molecule−1 s−1, which is close to the experimental value of (4.4 ± 1.0) × 10−13 cm3 molecule−1 s−1. The results reveal that the reaction provides a new pathway to produce organic acids. However, the contribution of acids from this reaction is small as its relative lower rate, compared to atmospheric CH2OO loss occurring through the hydrolysis of CH2OO + (H2O)n (n = 1–2).

Gas Phase Kinetics and Mechanistic Insights for the Reactions of Cl atoms with Isopropyl Formate and Isobutyl Formate.

Rate coefficients for the reactions of Cl atoms with isopropyl formate (IPF) and isobutyl formate (IBF) were measured experimentally over the temperature range of 268-363 K and at 760 Torr of nitrogen using relative rate method. Ethyl acetate and ethyl formate were used as reference compounds for the measurement of rate coefficients for the reaction of IPF with Cl atoms. Ethane and ethylene were used as reference compounds for the measurement of rate coefficients for the reaction of IBF with Cl atoms. The obtained rate coefficients for the reactions of IPF and IBF with Cl atoms at 298 K are (1.56 ± 0.47) × 10-11 and (7.60 ± 1.10) × 10-11 cm3 molecule-1 s-1 respectively. The derived temperature dependent Arrhenius expression for the reactions of IPF and IBF with Cl atoms are kR1(268-363K)Experimental = (3.87 ± 0.88) × 10-12 exp [(418 ± 70)/T] and kR2(268-363K)Experimental = (1.83 ± 0.45) × 10-11 exp [(421 ± 70)/T] cm3 molecule-1 s-1 respectively. A qualitative analysis of the products formed during the reactions of Cl atoms with IPF and IBF were carried out using gas chromatography-mass spectrometry (GC-MS) and gas chromatography-infrared spectroscopy (GC-IR) as analytical tools, and the degradation mechanisms were proposed on the basis of end products. To rationalize our experimentally obtained results, computational calculations were performed to calculate the temperature dependent rate coefficients for these reactions over the temperature range of 200-400 K at CCSD(T)/cc-pVDZ//MP2/6-31+G(d, p) level of theory using canonical variational transition state theory (CVT) with small curvature tunneling (SCT). Detailed discussions on the thermochemistry of the reactions, branching ratios, and atmospheric implications are discussed in the manuscript.

The Gas-Phase Formation Mechanism of Dibenzofuran (DBF), Dibenzothiophene (DBT), and Carbazole (CA) from Benzofuran (BF), Benzothiophene (BT), and Indole (IN) with Cyclopentadienyl Radical.

Benzofuran (BF), benzothiophene (BT), indole (IN), dibenzofuran (DBF), dibenzothiophene (DBT), and carbazole (CA) are typical heterocyclic aromatic compounds (NSO-HETs), which can coexist with polycyclic aromatic hydrocarbons (PAHs) in combustion and pyrolysis conditions. In this work, quantum chemical calculations were carried out to investigate the formation of DBF, DBT, and CA from the reactions of BF, BT, and IN with a cyclopentadienyl radical (CPDyl) by using the hybrid density functional theory (DFT) at the MPWB1K/6-311+G(3df,2p)//MPWB1K/6-31+G(d,p) level. The rate constants of crucial elementary steps were deduced over 600−1200 K, using canonical variational transition state theory with a small-curvature tunneling contribution (CVT/SCT). This paper showed that the production of DBF, DBT, and CA from the reactions of BF, BT, and IN with CPDyl involved six elementary steps: the addition reaction, ring closure, the first H shift, C–C cleavage, the second H shift, and elimination of CH3 or H. The cleavage of the C–C bond was regarded as the rate-determining step for each pathway due to the extremely high barrier. The 1-methyl substituted products were more easily formed than the 4-methyl substituted products. The main products were DBF and 1-methyl-DBF, DBT and 1-methyl-DBT, and CA and 1-methyl-CA for reactions of BF, BT, and IN with CPDyl, respectively. The ranking of DBF, DBT, and CA formation potential was as follows: DBT and methyl-DBT formation > DBF and methyl-DBF formation > CA, and methyl-CA formation. Comparison with the reaction of naphthalene with CPDyl indicated that the reactions of CPDyl attacking a benzene ring and a furan/thiophene/pyrrole ring could be inferred to be comparable under high temperature conditions.

Investigation of the Absorption Cross Section of Phenyl Radical and Its Kinetics with Methanol in the Gas Phase Using Cavity Ring-Down Spectroscopy and Theoretical Methodologies.

Cavity ring-down spectroscopy (CRDS) was used to measure the absorption cross section of phenyl radicals (C6H5•) at 504.8 nm (2B1 ← 2A1 transition) in the nitrogen atmosphere at 40 Torr total pressure and 298 K using nitrosobenzene (C6H5NO) as the radical precursor. At 504.8 nm, the absorption cross section was measured to be σphenyl504.8nm = (5.7 ± 1.4) × 10-19 cm2 molecule-1. The absorption cross section was independent of the total pressure range (40-200 Torr) over which it was studied with a precursor concentration of (4-5) × 1013 molecules cm-3. In addition to this, the absolute rate coefficients for the reaction of phenyl radicals with methanol were measured over the temperature range of 263-298 K and at 40 Torr pressure with N2 using CRDS. The temperature-dependent rate coefficient for the title reaction over the studied temperature range was obtained to be k263-298Kexperiment (T) = (1.38 ± 0.60) × 10-11 exp [-(1764 ± 321)/T] cm3 molecule-1 s-1 with a rate coefficient of k(T) = (3.50 ± 0.32) × 10-14 cm3 molecule-1 s-1 at 298 K. The effect of pressure and laser fluence was found to be negligible within the experimental uncertainties in the studied range. In addition, to complement our experimental findings, the T-dependent rate coefficients for the title reaction were investigated using computational methods. The B3LYP/6-311 + G(d,p) level of theory was used in combination with canonical variational transition-state theory with small-curvature tunneling to calculate the rate coefficients. The T-dependent rate coefficient in the range of 200-400 K was obtained as k200-400Ktheory (T) = 2.43 × 10-13 exp[-(478.38/T)] cm3 molecule-1 s-1 with a room-temperature (298 K) rate coefficient of 4.67 × 10-14 cm3 molecule-1 s-1.

Calculations Predict That Heavy-Atom Tunneling Dominates Möbius Bond Shifting in [12]- and [16]Annulene.

The contribution of heavy-atom tunneling to reactions of [12]- and [16]annulene was probed using small-curvature tunneling rate calculations. At the CCSD(T)/cc-pVDZ//M06-2X/cc-pVDZ level, tunneling is predicted to account for more than 50% of the rate for Möbius bond shifting and ca. 35% of the rate for electrocyclization in [12]annulene, and over 80% of the rate for Möbius bond shifting in [16]annulene, at temperatures at which these reactions have been observed experimentally.

The HO4H → O3 + H2O reaction catalysed by acidic, neutral and basic catalysts in the troposphere

A detailed effects of catalyst X (X = H2O, (H2O)2, NH3, NH3···H2O, H2O···NH3, HCOOH and H2SO4) on the HO4H → O3 + H2O reaction have been investigated by using quantum chemical calculations and canonical vibrational transition state theory with small curvature tunnelling. The calculated results show that (H2O)2-catalysed reactions much faster than H2O-catalysed one because of the former bimolecular rate constant larger by 2.6–25.9 times than that of the latter one. In addition, the basic H2O···NH3 catalyst was found to be a better than the neutral catalyst of (H2O)2. However it is marginally less efficient than the acidic catalysts of HCOOH, and H2SO4. The effective rate constant (k't) in the presence of catalyst X have been assessed. It was found from k't that H2O (at 100% RH) completely dominates over all other catalysts within the temperature range of 280–320 K at 0 km altitude. However, compared with the rate constant of HO4H → H2O + O3 reaction, the k eff values for H2O catalysed reaction are smaller by 1–2 orders of magnitude, indicating that the catalytic effect of H2O makes a negligible contribution to the gas phase reaction of HO4H → O3 + H2O. Highlights A detailed effects of catalyst of H2O, (H2O)2, NH3, NH3···H2O, H2O···NH3, HCOOH and H2SO4 on the HO4H → O3 + H2O reaction has been performed. From energetic viewpoint, H2SO4 exerts the strongest catalytic role in HO4H → O3 + H2O reaction as compared with the other catalysts. At 0 km altitude H2O (at 100% RH) completely dominates over all other catalysts within the temperature range of 280–320 K. HO4H → H2O + O3 reaction with H2O cannot be compete with the reaction without catalyst, due to the fact that the effective rate constants in the presence of H2O are smaller.

Understanding H2 Formation on Hydroxylated Pyroxene Nanoclusters: Ab Initio Study of the Reaction Energetics and Kinetics.

The rate constants of H2 formation on five models of silicate nanoclusters with varying degrees of hydroxylation, (Mg4Si4O12)(H2O)N, were computed over a wide temperature range [180-2000 K]. We tested nine combinations of density functional methods and basis sets for their suitability for calculating reaction energies and barrier heights, and we computed the minimum energy H + H → H2 reaction paths on each nanocluster. Subsequently, we computed the rate constants employing three semiclassical approaches that take into account tunneling and nonclassical reflection effects by means of the zero curvature tunneling (ZCT), the small curvature tunneling (SCT), and the one-dimensional semiclassical transition state theory (SCTST) methods, which all provided comparable results. Our investigations show that the H2 formation process following the Langmuir-Hinshelwood (LH) mechanism is more efficient on the hydroxylated (N = 1-4) nanoclusters than on the bare (N = 0) one due to relatively higher reaction barrier height on the latter. H2 formation is found to have the smallest barrier and the most exothermic reaction for the moderately hydroxylated (Mg4Si4O12)(H2O)2 nanocluster for all nine considered methods. Overall, we conclude that all the considered nanoclusters are very efficient catalyzing grains for H2 formation in the physical conditions of the interstellar medium (ISM) with pyroxene nanosilicates having moderate to high hydroxylation being more efficient than bare nanograins.

Theoretical study of H-atom abstraction reactions from CH3CH2OCH2CH3, CHF2CF2OCH2CF3 and CF3CH2OCH3 by NO3 radical & subsequent degradation

The nocturnal reactions of CH3CH2OCH2CH3, CHF2CF2OCH2CF3 and CF3CH2OCH3 initiated by NO3 radicals are important sources of alkyl radicals and nitric acids. In this paper, the thermodynamics and kinetics of CH3CH2OCH2CH3, CHF2CF2OCH2CF3 and CF3CH2OCH3 induced by NO3 radical in gas phase are studied in detail by BHandHLYP method combined with 6-311G(d,p) basis set, and the single point correction is calculated by relatively accurate CCSD(T) method. In the temperature range of 200–400 K, the rate constants of title reactions are fitted to the three-parameter Arrhenius formula:k1 = 1.13 × 10−40T9.24exp(1675.99/T)k2 = 2.23 × 10−23T2.81exp(-4476.24/T)k3 = 5.63 × 1043T−19.20exp(-9344.12/T)All the rate constants calculated by the canonical variational transition state theory and the small curvature tunneling are basically consistent with the limited experimental data. By comparing the reaction rate constants of ethyl ether and its isomer methyl propyl ether with NO3 radical at 293 ± 2 K, the higher the symmetry is, the faster the reaction rate of ether is.Thermodynamic calculations and kinetic data of the title reactions indicate that the H-abstraction reactions at the –OCH2- sites are the main reaction pathways. The thermodynamic and kinetic data of the reaction CH3CH2OCH2CH3, CHF2CF2OCH2CF3 with NO3 radical, showing that the reaction activity could be reduced due to the addition of fluorine atoms, which is further verified by the enthalpies, Gibbs free energies of the title reactions and C–H bond dissociation energies of the CH3CH2OCH2CH3, CHF2CF2OCH2CF3 and CF3CH2OCH3 molecules. The reaction thermodynamics and kinetics are determined, and the formation mechanisms of the products are proposed, which are crucial to determine the influence of CH3CH2OCH2CH3, CHF2CF2OCH2CF3 and CF3CH2OCH3 on air quality, as well as its atmospheric lifetime and durability.The atmospheric lifetimes of CH3CH2OCH2CH3, CHF2CF2OCH2CF3 and CF3CH2OCH3 are evaluated in the NO3-concentration range of 5 × 108-2 × 109 molecule cm−3 to fully consider the effects of different regions on their nocturnal migration. The radiation efficiency and global warming potentials (GWPs) have been reported. The products of title reaction CH3CH2OCHCH3, CF3CHOCF2CHF2 and CF3CHOCH3 are further oxidized into organic nitrates in the presence of O2 and NO. Organic nitrites can be isomerized into organic nitrates or degraded to form CH3C(O•)HOCH2CH3, CF3C(O•)HOCF2CHF2 and CH3C(O•)HOCF3 alkoxy radicals and NO2. This work provides deep insight into the night migration and transformation mechanism of the three ethers.

Investigation on the Thermal Dissociation of Vinyl Nitrite with a Saddle Point Involved.

Hybrid and double-hybrid density functionals are employed to explore the O–NO bond dissociation mechanism of vinyl nitrite (CH2=CHONO) into vinoxy (CH2=CHO) and nitric monoxide (NO). In contrast to previous investigations, which point out that the O–NO bond dissociation of vinyl nitrite is barrierless, our computational results clearly reveal that a kinetic barrier (first-order saddle point) in the O–NO bond dissociation is involved. Furthermore, a radical–radical adduct is recommended to be present on the dissociation path. The activation and reaction enthalpies at 298.15 K for the vinyl nitrite dissociation are calculated to be 91 and 75 kJ mol–1 at the M062X/MG3S level, respectively, and the calculated reaction enthalpy compares very well with the experimental result of 76.58 kJ mol–1. The M062X/MG3S reaction energetics, gradient, Hessian, and geometries are used to estimate vinyl nitrite dissociation rates based on the multistructural canonical variational transition-state theory including contributions from hindered rotations and multidimensional small-curvature tunneling at temperatures from 200 to 3000 K, and the rate constant results are fitted to the four-parameter Arrhenius expression of 4.2 × 109 (T/300)4.3 exp[−87.5(T – 32.6)/(T2 + 32.62)] s–1.