Atmospheric Chemistry of the Z and E Isomers of CF3CFCHF; Kinetics, Mechanisms, and Products of Gas-Phase Reactions with Cl Atoms, OH Radicals, and O3

Oxidation pathways and kinetics of the 1,1,2,3-tetrafluoropropene (CF2CF-CH2F) reaction with Cl-atoms and subsequent aerial degradation of its product radicals in the presence of NO

To give a comprehensive account of the environmental acceptability of 1,1,2,3-tetrafluoropropene (CF2CF-CH2F) in the troposphere, we have examined the oxidation reaction pathways and kinetics of CF2CF-CH2F initiated by Cl-atoms using the second-order Møller-Plesset perturbation (MP2) theory along with the 6-31+G(d,p) basis set. We also performed single-point energy calculations to further refine the energies at the CCSD(T) level along with the basis sets 6-31+G(d,p) and 6-311++G(d,p). The estimation of the relative energies and thermodynamic parameters of the CF2CF-CH2F + Cl reaction clearly shows that Cl-atom addition reaction pathways are more dominant compared to H-abstraction reaction pathways. The value of the rate coefficient for each reaction channel is calculated using the conventional transition state theory (TST) over the temperature range of 200-1000 K at 1 atm. The estimated overall rate coefficients for the title reaction are found to be 1.10 × 10-12, 1.21 × 10-10, and 1.13 × 10-8 cm3 per molecule per s via the respective calculation methods viz. MP2/6-31+G(d,p), CCSD(T)//MP2/6-31+G(d,p), and CCSD(T)/6-311++G(d,p)//MP2/6-31+G(d,p), at 298.15 K. Moreover, the calculated rate coefficients and percentage branching ratio values suggest that the Cl-atom addition reaction at the β-carbon atom is more preferable to that of the α-carbon addition to CF2CF-CH2F. Based on the rate coefficient values calculated by the three different methods, the atmospheric lifetime for the title reaction at 298.15 K is estimated. The radiative efficiency (RE) and Global Warming Potential (GWP) results of the title molecule show that its GWP would be negligible. Further, we have explored the degradation of its product radicals in the presence of O2 and NO. From the degradation results, we have found that CF2(Cl)COF, FCOCH2F, FCFO and FCOCl are formed as stable end products along with various radicals such as ˙CF2Cl and ˙CH2F. Therefore, these findings of kinetic and mechanistic data can be applied to the development and implementation of a novel CFC replacement.

Atmospheric chemistry of (Z)- and (E)-1,2-dichloroethene: kinetics and mechanisms of the reactions with Cl atoms, OH radicals, and O3

Smog chambers interfaced with in situ FT-IR detection were used to investigate the kinetics and mechanisms of the Cl atom, OH radical, and O₃ initiated oxidation of (Z)- and (E)-1,2-dichloroethene (CHClCHCl) under atmospheric conditions. Relative and absolute rate methods were used to measure k(Cl + (Z)-CHClCHCl) = (8.80 ± 1.75) × 10^-11, k(Cl + (E)-CHClCHCl) = (8.51 ± 1.69) × 10^-11, k(OH + (Z)-CHClCHCl) = (2.02 ± 0.43) × 10^-12, k(OH + (E)-CHClCHCl) = (1.94 ± 0.43) × 10^-12, k(O₃ + (Z)-CHClCHCl) = (4.50 ± 0.45) × 10^-21, and k(O₃ + (E)-CHClCHCl) = (1.02 ± 0.10) × 10^-19 cm³ molecule^-1 s^-1 in 700 Torr of N₂/air diluent at 298 ± 2 K. Pressure dependencies for the Cl atom reaction kinetics were observed for both isomers, consistent with isomerization occurring via Cl atom elimination from the chemically activated CHCl-CHCl-Cl adduct. The observed products from Cl initiated oxidation were HC(O)Cl (117-133%), CHCl₂CHO (29-30%), and the corresponding CHClCHCl isomer (11-20%). OH radical initiated oxidation gives HC(O)Cl as a major product. For reaction of OH with (E)-CHClCHCl, (Z)-CHClCHCl was also observed as a product. A significant chlorine atom elimination channel was observed experimentally (HCl yield) and supported by computational results. Photochemical ozone creation potentials of 12 and 11 were estimated for (Z)- and (E)-CHClCHCl, respectively. Finally, an empirical kinetic relationship is explored for the addition of OH radicals or Cl atoms to small alkenes. The results are discussed in the context of the atmospheric chemistry of (Z)- and (E)-CHClCHCl.

Kinetic Analysis of Unimolecular Reactions Following the Addition of the Hydroxyl Radical to 1,1,2-Trifluoroethene

Fluorinated olefins are valued chemicals in industry, especially as heat transfer fluids in refrigeration applications. As these volatile compounds are widely used, they may be released into the atmosphere, and investigation of their reactions in the atmosphere are therefore of importance. The kinetic analysis of the reaction mechanisms of trifluoroethene (CF₂═CHF) with hydroxyl radicals is studied using computational chemistry at the M06-2X level with the 6-311++G(2d,d,p) and aug-cc-pVDZ basis sets as well as the composite CBS-QB3 method. Rate coefficients for the proposed mechanisms are calculated using transition state theory (TST) with tunneling corrections. The calculated rate constants for OH addition to CF₂═CHF are in excellent agreement with experimental values. Kinetic parameters as a function of temperature and pressure are evaluated for the chemically activated formation and unimolecular dissociation of hydroxylfluoroalkyl intermediates. Important forward reactions result in adduct stabilization, H atoms, hydrogen fluoride (HF) via molecular elimination, and formation of fluorinated carbonyl radicals with CF₂(═O) and CHF(═O) product channels. Stabilization of initial adducts along with HF elimination are important reaction pathways under high pressure and low temperatures. Important HF eliminations and H atom transfers primarily involve H atoms from the hydroxyl group, as the C-H bonds on or adjacent to carbons with F atoms are stronger and show high barriers to H atom transfer.

FTIR product study of the Cl-initiated oxidation products of CFC replacements: (E/Z)-1,2,3,3,3-pentafluoropropene and hexafluoroisobutylene

A product study of the reactions of (E/Z)-1,2,3,3,3-pentafluoropropene ((E/Z)-CF3CF[double bond, length as m-dash]CHF) and hexafluoroisobutylene ((CF3)2C[double bond, length as m-dash]CH2) initiated by Cl atoms were developed at 298 ± 2 K and atmospheric pressure. The experiments were carried out in a 1080 L quartz-glass environmental chamber coupled via in situ FTIR spectroscopy to monitor the reactants and products. The main products observed and their yields were as follows: CF3C(O)F (106 ± 9)% with HC(O)F (100 ± 8)% as a co-product for (E/Z)-CF3CF[double bond, length as m-dash]CHF, and CF3C(O)CF3 (94 ± 5)% with HC(O)Cl (90 ± 7)% as a co-product for (CF3)2C[double bond, length as m-dash]CH2. Atmospheric implications of the end-product degradation are assessed in terms of their impact on ecosystems to help environmental policymakers consider HFOs as acceptable replacements.

Kinetic and Mechanistic Investigation for the Gas-Phase Tropospheric Photo-oxidation Reactions of 2,2,2-Trifluoroethyl Acrylate with OH Radicals and Cl Atoms

The photo-oxidation of 2,2,2-trifluoroethyl acrylate (TFEA) (CH₂CHC(O)OCH₂CF₃) initiated by OH radicals and Cl atoms was investigated in tropospheric conditions using both experimental and computational methods. The kinetic measurements were carried out in the temperature range of 268-363 K using the relative rate method. The rate coefficients for the reaction of OH radicals with TFEA were measured relative to diethyl ether, ethylene, and acetaldehyde. The rate coefficients for the reaction of Cl atoms with TFEA were measured relative to propylene and ethylene. The rate coefficients for the reaction of TFEA with OH radicals and Cl atoms at 298 K were experimentally measured to be k_R1^{exp - 298 K} = (1.41 ± 0.31) × 10^-11 cm³ molecule^-1 s^-1 and k_R2^{exp - 298 K} = (2.37 ± 0.50) × 10^-10 cm³ molecule^-1 s^-1, respectively. The deduced temperature-dependent Arrhenius expressions for the reactions of OH radicals and Cl atoms with TFEA are k_R1^{exp - (268 - 363 K)} = (9.82 ± 1.37) × 10^-12 exp. [(812 ± 152)/T] cm³ molecule^-1 s^-1 and k_R2^{exp - (268 - 363 K)} = (1.25 ± 0.17) × 10^-11 exp. [(862 ± 85)/T] cm³ molecule^-1 s^-1, respectively. To complement our experimental results, computational calculations were performed at CCSD(T)/cc-pVDZ//M062X/6-31+G(d,p) and CCSD(T)/cc-pVDZ//MP2/6-311+G(d,p) levels of theory, respectively, in combination with canonical variational transition-state theory (CVT) with small curvature tunneling (SCT) over the temperature range of 200-400 K. Furthermore, the degradation mechanisms initiated by OH radicals and Cl atoms were proposed for the titled reactions based on the qualitative analysis of the products in gas chromatography-mass spectrometry (GC-MS) and gas chromatography-infrared spectroscopy (GC-IR). Atmospheric implications, thermochemistry, and branching ratios for the titled reactions are discussed in detail in the paper.

Atmospheric oxidation of 2-fluoropropene (CH3CFCH2) with Cl atom and aerial degradation of its product radicals by computational study

Primary degradation of 2-fluoropropene initiated by Cl atom and subsequent degradation of its product radials.

Tropospheric degradation of 2-fluoropropene (CH3CFCH2) initiated by hydroxyl radical: Reaction mechanisms, kinetics and atmospheric implications from DFT study

Degradation of hydrofluoro-olefins (HFOs) with oxidants plays a significant role in the troposphere. Thus, we have investigated detail theoretical calculations of hydroxyl radical (^•OH) initiated oxidation of 2-fluoropropene (CH₃CFCH₂) using M06-2X/6-311++G(d,p) level of theory. Here, we have considered different possible H-abstraction and OH addition for the degradation of CH₃CFCH₂ molecule. The potential energy analysis shows that OH-addition channels are more dominant than H-abstraction channels. The calculated reaction enthalpies (Δ_rH°) and Gibbs free energies (Δ_rG°) also suggest that OH-addition reaction channels are more favourable than H-abstraction channels. The overall rate coefficients for CH₃CFCH₂ + ^•OH reaction is calculated within the temperature range of 250-450 K. The observed overall rate coefficient (2.01 × 10^-11 cm³ molecule^-1 s^-1) at 298 K for the titled reaction is found to be in good agreement with the earlier reported experimental rate coefficient. The calculated percentage branching ratio shows that the contribution of OH-addition to α-carbon and β-carbon of CH₃CFCH₂ molecule are 85.10% and 14.20% to the overall rate coefficient while H-abstractions have a negligible contribution. Based on the kinetics calculations, the atmospheric lifetime of the titled molecule is found to be 0.6 days. Further, we have also explored the degradation pathways of OH-addition product radicals and found acetyl fluoride (CH₃CFO) and formaldehyde (HCHO) are the end degradation products.

Rate Coefficients for the Gas-Phase Reaction of ( E)- and ( Z)-CF3CF═CFCF3 with the OH Radical and Cl-Atom

The rate coefficients, k, for the gas-phase reaction of the OH radical and Cl-atom with ( E)- and ( Z)-CF₃CF═CFCF₃ were measured using a relative rate technique over a range of temperature (240-375 K) and bath gas pressure (50-630 Torr, He). The obtained rate coefficients were found to be independent of pressure under these conditions. The obtained rate coefficients for the reaction of Cl-atom with ( E)- and ( Z)-CF₃CF═CFCF₃ at 296 K were k₁(296 K) = (7.23 ± 0.3) × 10^-12 cm³ molecule^-1 s^-1 and k₂(296 K) = (6.70 ± 0.3) × 10^-12 cm³ molecule^-1 s^-1, respectively, with the temperature dependence described by the Arrhenius expressions: k₁( T) = (3.47 ± 0.35) × 10^-12 exp[(210 ± 25)/ T] cm³ molecule^-1 s^-1 and k₂( T) = (3.37 ± 0.35) × 10^-12 exp[(199 ± 25)/ T] cm³ molecule^-1 s^-1. The rate coefficients for the OH radical reaction with ( E)- and ( Z)-CF₃CF═CFCF₃ were found to be k₃(296-375 K) = (4.34 ± 0.45) × 10^-13 cm³ molecule^-1 s^-1 and k₄(296-375 K) = (3.30 ± 0.35) × 10^-13 cm³ molecule^-1 s^-1, respectively. The quoted rate coefficient uncertainties are 2σ (95% confidence level) and include estimated systematic errors. The rate coefficients for the reaction of OH with a mixture of the two stereoisomers were determined using a pulsed laser photolysis-laser-induced fluorescence (PLP-LIF) technique for comparison with previous kinetic measurements using stereoisomer mixtures. The effective rate coefficient for the 0.7/0.3 ( E)/( Z) stereoisomer sample was found to be nearly independent of temperature over the range 222-375 K with a value of (4.47 ± 0.36) × 10^-13 cm³ molecule^-1 s^-1. The atmospheric lifetimes for ( E)- and ( Z)-CF₃CF═CFCF₃ due to OH-reactive loss are estimated to be 25 and 35 days, respectively. The lifetime-corrected radiative efficiencies (W m^-2 ppb^-1) and 100 year time horizon global warming potentials derived in this work are 0.05 and 1.2 for ( E)-CF₃CF═CFCF₃ and 0.13 and 4.1 for ( Z)-CF₃CF═CFCF₃. The photochemical ozone creation potentials for ( E)- and ( Z)-CF₃CF═CFCF₃ are estimated to be 2.5 and 2.1, respectively.

Rate Constants for the Reactions of OH Radicals with the ( E)/( Z) Isomers of CFCl=CFCl and ( E)-CHF=CHF

The rate constants for the OH radical reactions with halogenated ethenes were investigated experimentally and computationally. The rate constants for the reactions of OH radicals with ( E)-CFCl=CFCl ( k1), ( Z)-CFCl=CFCl ( k2), and ( E)-CHF=CHF ( k3) were measured using flash and laser photolysis methods. The temporal profile of the OH radical was monitored by a laser-induced fluorescence technique. Kinetic measurements were carried out over the temperature range of 250-430 K. Arrhenius rate constants were determined to be k1 = (1.67 ± 0.06) × 10-12·exp[(140 ± 10) K/ T], k2 = (1.75 ± 0.04) × 10-12·exp[(140 ± 10) K/ T], and k3 = (3.99 ± 0.15) × 10-12·exp[(260 ± 10) K/ T] cm3 molecule-1 s-1. The quoted uncertainties are 95% confidence levels and do not include systematic errors. Infrared absorption spectra were measured at room temperature. The atmospheric lifetimes and the global warming potentials of ( E)-CFCl=CFCl, ( Z)-CFCl=CFCl, and ( E)-CHF=CHF were estimated to be 4.3, 4.2, and 1.2 days and 0.035, 0.036, and 0.0056, respectively. The ozone depletion potentials of ( E)-CFCl=CFCl and ( Z)-CFCl=CFCl were determined to be 0.00011 and 0.00010, respectively. The photochemical ozone creation potentials of the halogenated ethenes were less than 1/4 that of ethene. In addition, the ( E)/( Z) differences in the energy and IR spectra of the CFCl=CFCl and CHF=CHF molecules were computationally examined. The reactivities of these halogenated ethenes toward OH radicals were investigated through the combination of DFT and ab initio computations. The rate constants calculated for the OH radical reactions of these halogenated ethenes showed reasonable agreement with the experimentally determined values. Our computational results for the CFCl=CFCl and CHF=CHF ( E)/( Z) isomeric pairs indicated that the rate constants toward OH radicals are larger for the higher-energy geometrical isomers than for the lower-energy counterparts.

Product distribution and mechanism of the OH− initiated tropospheric degradation of three CFC replacement candidates: CH3CFCH2, (CF3)2CCH2 and (E/Z)-CF3CFCHF

The OH initiated degradation of 2-fluoropropene, 3,3,3-trifluoro-2-(tri-fluoromethyl)propene and (E/Z)-1,2,3,3,3-pentafluoropropene has been investigated using a 1080 L chamber at 298 K and 1000 mbar of air coupled with in situ FTIR spectroscopy.

Atmospheric chemistry of (Z)-CF3CH[double bond, length as m-dash]CHCl: products and mechanisms of the Cl atom, OH radical and O3 reactions, and role of (E)-(Z) isomerization

The chemical mechanisms of the OH radical, Cl-atom and O3 initiated oxidation of (Z)-CF3CH[double bond, length as m-dash]CHCl were studied at 296 ± 1 K in 10-700 Torr air of N2/O2 diluent. Cl atoms add to the [double bond splayed left]C[double bond, length as m-dash]C[double bond splayed right] double bond: 12 ± 5% to the terminal carbon and 85 ± 5% to the central carbon. In 700 Torr of air the products are CF3CHClCHO, HCOCl, CF3COCl, CF3CHO, (E)-CF3CH[double bond, length as m-dash]CHCl, CF3C(O)CHCl2, and CF3CHClCOCl. The yield of (E) isomer was dependent on total pressure, but independent of O2 partial pressure; consistent with isomerization occurring via Cl atom elimination from the chemically activated rather than the thermalized CF3CHCHCl-Cl adduct. The rate constant for (Z)-CF3CH[double bond, length as m-dash]CHCl + Cl was measured at low pressure (10-15 Torr) and found to be indistinguishable from that determined at 700 Torr total pressure, whereas the low pressure rate constant for (E)-CF3CH[double bond, length as m-dash]CHCl was 36% smaller. G4MP2 ab initio calculations showed that the (E) isomer is 1.2 kcal mol-1 more stable than the (Z) isomer. Cl atom elimination from the adduct will preferentially form the (E) isomer and hence the rate of CF3CH[double bond, length as m-dash]CHCl loss will be more sensitive to pressure for the (Z) than the (E) isomer. Reaction of (Z)-CF3CH[double bond, length as m-dash]CHCl with OH radicals gives CF3CHO, HCOCl, (E)-CF3CH[double bond, length as m-dash]CHCl, and HCl. A significant chlorine atom elimination channel was observed experimentally, and supported by computational results. The oxidation products of the reaction of O3 with (Z)- and (E)-CF3CH[double bond, length as m-dash]CHCl were determined with no evidence of isomerization. The results are discussed with respect to the atmospheric chemistry and environmental impact of (Z)- and (E)-CF3CH[double bond, length as m-dash]CHCl.

Kinetic investigations of Cl atom initiated photo-oxidation reactions of cyclic unsaturated hydrocarbons in the gas phase: an experimental and theoretical study

Cl atom initiated photo oxidation kinetics of cyclohexene and cycloheptene.

Atmospheric fate of hydrofluoroolefins, CxF2x+1CHCH2 (x = 1,2,3,4 and 6): Kinetics with Cl atoms and products

Rate coefficients for the gas-phase reactions of C_xF_2x+1CHCH₂ (x = 1, 2, 3, 4 and 6) with Cl atoms were determined at (298 ± 2) K and (710 ± 5) Torr of air using a relative rate technique. Two experimental setups with simulation chambers were employed with Fourier Transform Infrared (FTIR) spectroscopy and Gas Chromatography coupled to Mass Spectrometry (GC-MS) as detection techniques. The Cl-rate coefficients obtained were (in 10^-10 cm³ molecule^-1 s^-1): (0.85 ± 0.11) for CF₃CHCH₂, (1.11 ± 0.08) for C₂F₅CHCH₂, (1.12 ± 0.18) for C₃F₇CHCH₂, (0.97 ± 0.09) for C₄F₉CHCH₂, and (0.99 ± 0.08) for C₆F₁₃CHCH₂. Additionally, the gas-phase products were identified and quantified, when possible, by FTIR spectroscopy or GC-MS. The main reaction product was reported to be C_xF_2x+1C(O)CH₂Cl. The fluorinated species, C_xF_2x+1CHO and C_xF_2x+1C(O)CH₂Cl, were identified. CF₃C(O)CH₂Cl and CF₃CHO were found to be formed with molar yield of (69 ± 5)% and (9 ± 1)%, respectively. The global lifetime of the investigated C_xF_2x+1CHCH₂ due to their Cl-reaction is more than 100 days so this route does not compete with the removal by OH radicals. This lifetime is long enough for C_xF_2x+1CHCH₂ to be transported to remote areas where they can be degraded. However, at a local scale, in marine regions at dawn the removal of C_xF_2x+1CHCH₂ is expected to occur in ca. 1 day. The atmospheric degradation of these hydrofluoroolefins by Cl atoms is not expected to be a source of bioaccumulative perfluorinated carboxylic acids, C_xF_2x+1C(O)OH. Additionally, the UV absorption cross sections of CF₃C(O)CH₂Cl were determined together with the rate coefficient of the OH reaction by an absolute kinetic method at room temperature.

Atmospheric chemistry of Z- and E-CF3CHCHCF3

The first determination of the atmospheric chemistry of E-CF₃CHCHCF₃ and of the Cl and O₃ initiated atmospheric chemistry of Z-CF₃CHCHCF₃.

Kinetic study of the OH and Cl-initiated oxidation, lifetimes and atmospheric acceptability indices of three halogenated ethenes

First relative kinetic study for the OH/Cl reactions with three chloro-fluoro-ethenes at room-temperature and atmospheric pressure and environmental acceptability.

Atmospheric chemistry of (CF3)2CCH2: OH radicals, Cl atoms and O3 rate coefficients, oxidation end-products and IR spectra

OH, Cl and O₃ kinetics and IR spectra of (CF₃)₂CCH₂ utilized to estimate tropospheric lifetimes, radiative efficiencies, global warming potentials, estimated photochemical ozone creation potentials and tropospheric oxidation end-products.

Assessment of hydrofluoropropenes as insulating blowing agents for extruded polystyrene foams

Thermally insulating extruded polystyrene foams are currently produced with hydrofluorocarbon blowing agents. Hydrofluorocarbons have zero ozone depletion potential but rather high greenhouse warming potential. Various unsaturated fluoropropenes, with greenhouse warming potential values <15, have been assessed as HFC-134a replacements for styrenic extrusion foaming. The screening is first based on the modeling of solubility and diffusivity properties, followed by foaming experiments with a conventional extrusion process. Some fluoropropenes appear to be excellent blowing agents for extruded polystyrene foams and can be used alone for making very low-density foams with regular and large cell sizes, while some others require the use of a co-blowing agent for processing good quality foam. A few others are not suitable as a blowing agent for extruded polystyrene foams due to their toxicity or their very poor transport properties.

Atmospheric perfluorinated acid precursors: chemistry, occurrence, and impacts

Perfluorocarboxylic acids (PFCAs) can be found from the hydrolysis of perfluoroacyl fluorides and chlorides, which can be produced in three separate ways in the atmosphere. Alternatively, PFCAs can be formed directly in the gas phase through reaction of perfluoroacyl peroxy radicals or perfluorinated aldehyde hydrates. All five mechanisms have been elucidated using smog chamber techniques. Yields of the PFCAs from this process vary from less than 10% to greater than 100%, depending on the mechanism. The formation of perfluorosulfonic acids in the atmosphere can also occur, though the mechanism has not been entirely elucidated. A large number of compounds have been confirmed as perfluorinated acid precursors, including CFC-replacement compounds, anesthetics, fluorotelomer compounds, and perfluorosulfonamides. Levels of some of these compounds have been measured in the atmosphere, but concentration for the majority have yet to be detected. It is clear that atmospheric oxidation of volatile precursors contributes to the overall burden of PFAs, though the extent to which this occurs is compound and environment dependent and is difficult to assess accurately.