Understanding the kinetics and atmospheric degradation mechanism of chlorotrifluoroethylene (CF2CFCl) initiated by OH radicals

The atmospheric degradation of chlorotrifluoroethylene (CTFE) by OH˙ was investigated using density functional theory (DFT). The potential energy surfaces were also defined in terms of single-point energies derived from the linked cluster CCSD(T) theory. With an energy barrier of -2.62 to -0.99 kcal mol^-1 using the M06-2x method, the negative temperature dependence was determined. The OH˙ attack on C_α and C_β atoms (labeled pathways R1 and R2, respectively) shows that reaction R2 is 4.22 and 4.42 kcal mol^-1, respectively, more exothermic and exergonic than reaction R1. The main pathway should be the addition of OH˙ to the β-carbon, resulting in ˙CClF-CF₂OH species. At 298 K, the calculated rate constant was 9.87 × 10^-13 cm³ molecule^-1 s^-1. The TST and RRKM calculations of rate constants and branching ratios were performed at P = 1 bar and in the fall-off pressure regime over the temperature range of 250-400 K. The formation of HF and ˙CClF-CFO species via the 1,2-HF loss process is the most predominant pathway both kinetically and thermodynamically. With increasing temperature and decreasing pressure, the regioselectivity of unimolecular processes of energized adducts [CTFE-OH]˙ gradually decreases. Pressures greater than 10^-4 bar are often adequate for assuring saturation of the estimated unimolecular rates when compared to the RRKM rates (in high-pressure limit). Subsequent reactions involve the addition of O₂ to the [CTFE-OH]˙ adducts at the α-position of the OH group. The [CTFE-OH-O₂]˙ peroxy radical primarily reacts with NO and then directly decomposes into NO₂ and oxy radicals. "Carbonic chloride fluoride", "carbonyl fluoride", and "2,2-difluoro-2-hydroxyacetyl fluoride" are predicted to be stable products in an oxidative atmosphere.

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.

Kinetics, mechanism, and global warming potentials of HFO-1234yf initiated by O3 molecules and NO3 radicals: insights from quantum study

In the present investigation, the oxidation of HFO-1234yf (2,3,3,3-tetrafluoropropene) with O₃ molecule and NO₃ radical is studied by quantum chemical methods. The possible reaction pathways of the titled molecule with O₃ molecule and NO₃ radical are analyzed using M06-2X meta-hybrid density functional with the 6-311++G(d,p) basis set. We have further employed a series of single-point energy calculations by using a potentially high-level couple cluster method with single and double excitations, including perturbative corrections ((CCSD(T)) at the same basis set. The addition reaction of HFO-1234yf with O₃ molecule is initiated by the formation of primary ozonide complex, which leads to the formation of various carbonyl compounds and Criegee intermediates. The calculated energy barriers and thermochemical parameters inferred that decomposition of C˙H₂OO˙ and CF₃CFO is slightly more preferred over the formation of CF₃C˙FOO˙ and CH₂O. Further, the NO₃ radical addition at α- and β-sits of CF₃CF〓CH₂ molecule is analyzed in details. The individual and overall rate constants for each reaction pathways are calculated by using canonical transition state theory over the temperature range of 250-450 K. We have observed that the computed rate constants are in good agreement with the available experimental data. Atmospheric lifetimes and global warming potentials of the HFO-1234yf are also reported in this manuscript.

Products and mechanism of the OH-initiated photo-oxidation of perfluoro ethyl vinyl ether, C2F5OCF[double bond, length as m-dash]CF2

The OH-initiated photo-oxidation of perfluoro ethyl vinyl ether (C2F5OCF[double bond, length as m-dash]CF2, PEVE) in air (298 K, 50 and 750 Torr total pressure) was studied in a photochemical reactor using in situ detection of PEVE and its products by Fourier transform IR absorption spectroscopy. The relative rate technique was used to derive the rate coefficient, k1, for the reaction of PEVE with OH as k1 = (2.8 ± 0.3) × 10-12 cm3 molecule-1 s-1. The photo-oxidation of PEVE in the presence of NOx at 1 bar results in formation of C2F5OCFO, FC(O)C(O)F and CF2O in molar yields of 0.50 ± 0.07, 0.46 ± 0.07 and 1.50 ± 0.22, respectively. FC(O)C(O)F and CF2O are formed partially in secondary, most likely heterogeneous processes. At a reduced pressure of 50 Torr, the product distribution is shifted towards formation of FC(O)C(O)F, indicating the important role of collisional quenching of initially formed association complexes, and enabling details of the reaction mechanism to be elucidated. An atmospheric photo-oxidation mechanism for PEVE is presented and the environmental implications of PEVE release and degradation are discussed.

Absolute and relative-rate measurement of the rate coefficient for reaction of perfluoro ethyl vinyl ether (C2F5OCF[double bond, length as m-dash]CF2) with OH

The rate coefficient (k₁) for the reaction of OH radicals with perfluoro ethyl vinyl ether (PEVE, C₂F₅OCF[double bond, length as m-dash]CF₂) has been measured as a function of temperature (T = 207-300 K) using the technique of pulsed laser photolysis with detection of OH by laser-induced fluorescence (PLP-LIF) at pressures of 50 or 100 Torr N₂ bath gas. In addition, the rate coefficient was measured at 298 K and in one atmosphere of air by the relative-rate technique with loss of PEVE and reference reactant monitored in situ by IR absorption spectroscopy. The rate coefficient has a negative temperature dependence which can be parameterized as: k₁(T) = 6.0 × 10^-13 exp[(480 ± 38/T)] cm³ molecule^-1 s^-1 and a room temperature value of k₁ (298 K) = (3.0 ± 0.3) × 10^-12 cm³ molecule^-1 s^-1. Highly accurate rate coefficients from the PLP-LIF experiments were achieved by optical on-line measurements of PEVE and by performing the measurements at two different apparatuses. The large rate coefficient and the temperature dependence indicate that the reaction proceeds via OH addition to the C[double bond, length as m-dash]C double bond, the high pressure limit already being reached at 50 Torr N₂. Based on the rate coefficient and average OH levels, the atmospheric lifetime of PEVE was estimated to be a few days.

Atmospheric oxidation mechanism of OH-initiated reactions of diethyl ether – the fate of the 1-ethoxy ethoxy radical

The 1-ethoxy ethoxy radical resulting from the secondary peroxy chemistry in the oxidation of diethyl ether (DEE) by hydroxyl radical leads to the formation of ethyl formate in major quantities and ethyl acetate in minor quantities.