Canonical Variational Transition State Theory Research Articles

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.

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...

Atmospheric oxidation of HFE-7300 [n-C2F5CF(OCH3)CF(CF3)2] initiated by •OH/Cl oxidants and subsequent degradation of its product radical: a DFT approach.

To understand the atmospheric chemistry of hydrofluoroethers, we have studied the oxidation of a highly fluorinated compound n-C2F5CF(OCH3)CF(CF3)2 (HFE-7300) by OH/Cl oxidants. Here, we have employed M06-2X functional along with a 6-31 + G(d,p) basis set to obtain the optimized structures, various forms of energies, and different modes of frequencies for all species. We have characterized energies of all species on the potential energy surface, and it indicates that H-abstraction from n-C2F5CF(OCH3)CF(CF3)2 by Cl atom is kinetically more dominant than the H-abstraction reaction initiated by OH radical. In contrast, the calculated energy change (ΔrH°298 and ΔrG°298) results govern that OH-initiated H-abstraction reaction is highly exothermic and spontaneous compared to the Cl-initiated H-abstraction reaction. Rate constants are estimated using transition state theory as well as canonical variation transition state theory at the temperature range 200-1000 K and 1 atm pressure. The calculated rate constants of the H-abstraction channels are found to be in good agreement with the reported experimental rate constant at 298 K. Moreover, we have estimated the atmospheric lifetimes of HFE-7300 for the reaction with OH radical and Cl atom and are found to be 1.75 and 153.93 years, respectively. Additionally, the global warming potentials for HFE-7300 molecule are also estimated for 20-, 100-, and 500-year time horizons. Further, subsequent aerial oxidation of product radical (n-C2F5CF(OCH2)CF(CF3)2) in the presence of NO radical is performed, and it produced alkoxy radical via formation of peroxy radical. This alkoxy radical undergoes unimolecular decompositions via two different ways and formed n-C2F5CF(OCHO)CF(CF3)2 and n-C2F5CF(OH) CF(CF3)2 products.

Kinetics of the hydrogen abstraction alkane + O2 → alkyl + HO2 reaction class: an application of the reaction class transition state theory

In this work, the kinetics of reaction class of hydrogen abstraction from saturated hydrocarbons by O2 molecules has been studied. The high-pressure reaction rate constants were determined using reaction class transition state theory/structure-activity relationship (RC-TST/SAR) methodology, augmented by linear energy relationship (LER) and/or barrier height grouping (BHG) approximations for evaluation of the reaction barrier heights. The parameters needed have been derived from DFT calculations at M06-2X/aug-cc-pVTZ level for a training set of 23 reactions, involving hydrogen abstraction by O2 molecule at primary, secondary, and tertiary carbon sites. The reference reaction rate constant C2H6 + O2 → C2H5 + HO2 was obtained by extrapolation of the simplest reaction within the title family CH4 + O2 → CH3 + HO2. Kinetic parameters of the later one, calculated from canonical variational transition state theory (CVT), were taken from literature. The influence of low-frequency internal rotations has been investigated in details. The error analysis shows that the average systematic error of RC-TST/SAR-derived rate constants at low temperatures is within 25% compared to the explicit RC-TST results and diminishes at higher temperatures. This suggests that the proposed methodology can be effectively implemented in the automated mechanism generation codes to create the fuel combustion mechanisms.

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.

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.

Mechanism and Kinetics of Diuron Oxidation Initiated by Hydroxyl Radical: Hydrogen and Chlorine Atom Abstraction Reactions.

Diuron is a herbicide that has been classified as an environmental pollutant because of its harmful effects on living beings and environment. In the present work, the OH-initiated oxidation reaction of diuron is investigated by performing electronic structure calculations based on density functional theory (DFT) methods, M06-2X, ωB97X-D, and MPWB1K using the 6-31G(d,p) basis set. The CBS-QB3 method is used to validate the results obtained from the DFT methods. All possible initial hydrogen and chlorine atom abstraction reaction pathways involved in the oxidation of diuron were studied, and the favorable reaction pathways were found by analyzing the potential energy surface and thermochemistry of the reactions. The results obtained from the present work show that hydrogen atom abstraction from methyl and amine groups of diuron are energetically favorable, which leads to the formation of diuron radical intermediate and water molecule. The rate constant is calculated for most favorable reaction pathways by using canonical variation transition state theory (CVT) with small curvature tunnelling (SCT) correction over the temperature range 298-1000 K. The atmospheric lifetime of diuron is found to be around 15 days. The subsequent reaction of most favorable diuron radical intermediate with other atmospheric reactive species, such as O2, H2O, HO2, and NOx (x = 1, 2) radicals are studied. The time-dependent DFT calculation is performed to study the photolysis of diuron and favorable diuron radical intermediates. This study provides thermochemical and kinetic data for the oxidation of diuron initiated by OH radical through H atom abstraction reaction.

Cl-Atom-Initiated Atmospheric Degradation of Saturated Cyclic Hydrocarbons: Kinetic and Mechanistic Investigation.

The temperature-dependent reaction kinetics of methyl cyclohexane (MCH) and methyl cyclopentane (MCP) with Cl atoms were experimentally explored via the relative rate technique. Gas chromatography coupled with a flame ionization detector was employed to follow the reactant as well as the reference compound concentrations during the course of reaction. Gas chromatography coupled with mass spectrometry and gas chromatography coupled with infrared spectroscopy were used as the diagnostic tools for the detection and identification of the products in the title reaction. The rate coefficients for the reaction of Cl atoms with methyl cyclohexane and methyl cyclopentane were measured in the temperature range of 283-363 K at 760 Torr using isoprene and propylene as reference compounds. To support the experimental results, computational calculations were performed over the temperature range of 200-400 K using canonical variational transition state theory coupled with small curvature tunneling (CVT/SCT), conventional transition state theory (CTST) coupled with Wigner tunneling corrections, and CVT coupled with Wigner tunneling methods using the MP2 level of theory with 6-31G(d,p) as the basis set. The temperature-dependent rate coefficients were measured for the reaction of methyl cyclohexane and methyl cyclopentane with Cl atoms to be k(MCH) = [(4.48 ± 0.75) × 10-11]exp[(604 ± 25)/T] cm3 molecule-1 s-1 and k(MCP) = [(5.71 ± 0.66) × 10-12]exp[(1819 ± 669)/T] cm3 molecule-1 s-1, respectively. The measured rate coefficients at 298 K for the reactions of Cl atoms with methyl cyclohexane and methyl cyclopentane are k298KMCH = (3.36 ± 0.34) × 10-10 cm3 molecule-1 s-1 and k298KMCP = (2.25 ± 0.24) × 10-10 cm3 molecule-1 s-1, respectively. The conceivable atmospheric degradation mechanism for the reaction of methyl cyclohexane as well as methyl cyclopentane with Cl atoms was projected based on the products obtained during the reaction. Atmospheric implications, cumulative atmospheric lifetimes, thermochemistry, ozone formation potentials, and branching ratios of these molecules were also calculated and have been reported in this article.

Theoretical studies of $${{\mathrm{{CN} + {H}}_{2}({\mathrm{D}}_{2})}}$$ reactions: competition between H(D)-abstractions in $${\mathrm{H(D) + HCN(DCN)/HNC(DNC)}} $$ channels

The $$\hbox {CN} + \hbox {H}_{2}$$ reaction was investigated by considering the two possible channels, $$\hbox {H} + \hbox {HCN}$$ and $$\hbox {H} + \hbox {HNC}$$ , taking into account the isotopic effects and with the vibrationally excited states. The frequencies and structures for all species of the $$\hbox {CN} + \hbox {H}_{2}/\hbox {D}_{2}$$ reaction were calculated using G3 method for further kinetics calculation. The thermal rate constants were calculated using the conventional transition-state theory (TST) and canonical variational transition-state theory (CVT) by APUAMA code, over the temperature range from 200 to 4000 K. In addition, rate coefficients for vibrationally excited reactants CN (v = 1) or $$\hbox {H}_{2}$$ (v = 1) or $$\hbox {D}_{2}$$ (v = 1) are presented. The branching ratio for the partitioning into H/D + HCN/DCN or H/D + HNC/DNC has, also, been determined. The results showed that the $$\hbox {CN} (v=0) + \hbox {H}_{2} (v=0) \rightarrow \hbox {H} + \hbox {HCN} $$ channel is dominant at all range of temperature, while $$\hbox {CN } (v=1) + \hbox {H}_{2} (v=0) \rightarrow \hbox {H} + \hbox {HNC}$$ channel is dominant at T $$\ge $$ 1900 K. The isotopic effects are the same behavior that $$\hbox {CN}(v=0,1) + \hbox {H}_{2}(v=0,1) \rightarrow \hbox {H} + \hbox {HCN/HNC}$$ reactions. Reasonable agreement was found between the experimental results and the rate constants predicted by conventional transition-state theory, with tunneling correction, using the theoretical transition-state properties.

Mechanistic and Kinetic Study on Self-/Cross- Condensation of PCTA/DT Formation Mechanisms from Three Types of Radicals of 2,4-Dichlorothiophenol.

Chlorothiophenols (CTPs) are known to be key and direct precursors of polychlorinated thianthrene/dibenzothiophenes (PCTA/DTs). Self/cross-coupling of the chlorothiophenoxy radicals (CTPRs), sulfydryl-substituted phenyl radicals and thiophenoxyl diradicals evolving from CTPs are initial and important steps for PCTA/DT formation. In this study, quantum chemical calculations were carried out to investigate the homogenous gas-phase formation of PCTA/DTs from self/cross-coupling of 2,4-dichlorothiophenoxy radical (R1), 2-sulfydryl-3,5-dichlorophenyl radical (R2) and 3,5-dichlorothiophenoxyl diradical (DR) 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. For the formation of PCTAs, the S•/σ-C• condensation with both thiophenolic sulfur in one radical and ortho carbon in the other radical bonded to single electron is the most efficient sulfur-carbon coupling mode, and the ranking of the PCTA formation potential is DR + DR > R2 + DR > R1 + DR > R1 + R2 > R1 + R1. For the formation of PCDTs, the σ-C•/σ-C• coupling with both ortho carbon in the two radicals bonded to single electron is the energetically favored carbon-carbon coupling mode, and the ranking of the PCDT formation potential is: R2 + DR > R2 + R2 > R1 + DR > R1 + R2 > R1 + R1. The PCTA/DTs could be produced from R1, R2 and DR much more readily than PCDD/DFs from corresponding oxygen substituted radicals.

Theoretical study of the formation of dinitro-pyrenes from mononitro-pyrenes initiated by OH radicals

Nitrated polycyclic aromatic hydrocarbons are more mutagenic and carcinogenic than parent PAHs. The formation mechanism of dinitro-pyrenes initiated by OH from mononitro-pyrenes was conducted by quantum chemical calculations. The products are 1,2-, 1,7-, 2,4-, 2,5- and 2,7- dinitropyrenes. Water molecules can decrease the Gibbs activation barriers of water loss step of NO2-OH-nitropyrene. By canonical variational transition state theory with small curvature tunneling correction, the calculated overall rate constants for 1-, 2- and 4-nitropyrene are 8.13 × 10−13 cm3 molecule−1 s−1, 8.90 × 10−13 cm3 molecule−1 s−1 and 1.50 × 10−12 cm3 molecule−1 s−1 at 298 K and 1 atm, respectively.

Photo-Oxidation Reaction Kinetics and Mechanistics of 4-Hydroxy-2-butanone with Cl Atoms and OH Radicals in the Gas Phase.

The temperature-dependent rate coefficients for the gas-phase reaction of 4-hydroxy-2-butanone (4H2BN) with Cl atoms and OH radicals were explored experimentally using relative rate technique and computational methods. The concentrations of the reactants as well as products were followed using gas chromatography (GC) with the flame ionization detector, GC/mass spectrometry, and GC/infrared spectroscopy as analytical techniques. Formaldehyde was obtained as the major product during the title reaction. The kinetics of 4H2BN with Cl atoms and OH radicals were measured over the temperature range of 298-363 K at 760 Torr in the N2 atmosphere using C3H8, C2H4, isopropanol, and n-propanol as reference compounds. The temperature-dependent rate coefficients for the reaction of 4H2BN with Cl atoms and OH radicals were obtained as kExpt( T) = [(1.52 ± 0.86) × 10-26] T5 exp[(2474 ± 450)/ T] cm3 molecule-1 s-1 and kexpt( T) = [(2.09 ± 0.24) × 10-12] exp[-(409 ± 15)/ T] cm3 molecule-1 s-1, respectively. Theoretical calculations were carried out at the M062X/6-31G(d,p) and M06-2X/6-31+G(d,p) level of theories, and the rate coefficients for H abstraction reactions were evaluated using the canonical variational transition state theory with the inclusion of small-curvature tunneling correction over the temperature range of 200-400 K. The rate coefficients obtained over the studied temperature range were used to fit the data, and the Arrhenius expression was obtained to be kCl(Theory) (200-400 K) = (6.10 × 10-25) T4.42 exp(2397/ T) cm3 molecule-1 s-1, kOH(Theory) (200-400 K) = (1.13 × 10-19) T2.27 exp(1505/ T) cm3 molecule-1 s-1, respectively, for the reactions of Cl atoms and OH radicals with 4H2BN. The possible reaction mechanism proposed based on the obtained products for the title reaction, thermochemistry, branching ratios, and atmospheric implications and cumulative lifetime of 4H2BN were also explored in this study.

A theoretical insight on the kinetics for the reaction of (E)-/(Z)-CHF=CF(CF2)x=1,2CF3 with OH radicals under tropospheric conditions

Hydrofluoroolefins (HFOs) are the potential replacements to chlorofluorocarbons (CFCs) and hydrochlorofluorocarbons (HCFCs). HFOs are well-known alternates to regular refrigerants and foam expansion agents. The aim of this work is to provide comprehensive description on the environmental acceptability and atmospheric impact of some of these HFOs. In this study, OH-radical initiated photo-degradation of (E)-/(Z)-CHF=CF(CF2)x=1,2CF3 was investigated over the temperature range of 200–400 K using CCSD(T)/aug-cc-pVTZ//M062x/6-31+G(d,p) level of theory. The reaction kinetics was computed using the canonical variational transition state theory (CVT) in conjunction with the small curvature tunneling (SCT) and interpolated single-point energy (ISPE) corrections. A negative temperature-dependent Arrhenius behavior was observed for all the HFO + OH reactions. Various fluorinated aldehydes and acids formed via the interaction of the products with O2 and NOx are also reported as part of this manuscript. Short lifespans and low global warming potentials (GWPs) furthermore characterized the importance of these HFOs as propitious replacements to high GWP CFCs.

Can a single water molecule catalyze the OH+CH2CH2 and OH+CH2O reactions?

The reaction of OH radicals with ethylene (CH2CH2) and formaldehyde (CH2O) molecules with and without water have been investigated using ab initio/DFT potential energy surfaces (PESs) at CCSD(T)/aug-cc-pVTZ//BHandHLYP//aug-cc-pVTZ levels of theory. The rate coefficients for the bimolecular reaction pathways OH + CH2X···H2O (X = CH2, O) and CH2X + H2O⋯HO were calculated using canonical variational transition state theory (CVT) with small curvature tunneling (SCT) correction. The kinetic results show that OH radical adds to C=C bond in CH2CH2, and abstract the hydrogen atoms from CH2O, similar to its isoelectronic analogous OH + CH2NH (+H2O) reaction. The catalytic effect of a single water molecule on OH + CH2X (X = CH2, O) reaction system shows that the initial water complexation step is essential in the rate coefficients calculation. The calculated rate coefficient for the OH + CH2CH2(+H2O) reaction at 300K is 6 × 10−16 cm3 molecule−1 s−1 and for OH + CH2O(+H2O) reaction at 300K is 8.1 × 10−14 cm3 molecule−1 s−1. The rate coefficient for OH + CH2CH2(+H2O) reaction is at least two orders of magnitude smaller than OH + CH2NH(+H2O) reaction (at 300K is 5.1 × 10−14 cm3 molecule−1 s−1) and rate coefficient for OH + CH2O reaction is in good agreement with OH + CH2NH reaction. In general, the rate coefficients for OH + CH2X (X = CH2, O)⋯H2O and for CH2X + H2O⋯HO reactions are ∼3–4 orders of magnitude smaller than reaction without water molecule. Our results predict that catalytic effect of single water molecule on OH + CH2CH2 and OH + CH2O reactions can make a negligible contribution to the gas phase removal of CH2CH2 and CH2O by OH radicals because the dominated water-assisted process depends parametrically on water concentration. As a result, the overall reaction rate coefficients are smaller. The present results provide a better understanding of gas phase catalytic effect of a water molecule on the most important atmospheric and combustion reaction prototypes.