Kinetics of the gas-phase reactions of CHXCFX (X = H, F) with OH (253-328 K) and NO3 (298 K) radicals and O3 (236-308 K)

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.

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.

Laser-induced fluorescence of the CHFCHO radical and reaction of OH radicals with halogenated ethylenes

A new laser-induced fluorescence spectrum of the 2-fluorovinoxy (CHFCHO) radical was first observed around 335 nm. The radical was produced in the reaction of an OH radical with 1,2-difluoroethylene (CHF=CHF). A single weak band was observed, which was assigned to the 0₀ ⁰ band of the B̃-X̃ transition of the trans-CHFCHO radical. The B̃←X̃ electronic transition energy (T₀) for trans-CHFCHO was 29 871 cm^-1, which was just 3 cm^-1 lower than that of its isomer, the 1-fluorovinoxy (CH₂CFO) radical. The fluorescence lifetime at 29 871 cm^-1 was shorter than 20 ns. This means that strong predissociation is probable at v' = 0 in the excited B̃ state of trans-CHFCHO. From an analysis of the dispersed fluorescence spectrum, some of the vibrational frequencies can be assigned for the ground electronic state: ν₃ = 1557 cm^-1 (C-O stretch), ν₇ = 1162 cm^-1 (C-C stretch), and ν₈ = 541 cm^-1 (CCO bend). These vibrational assignments were supported by ab initio calculations. The structure of the C-C-O skeleton and the spectroscopic character of trans-CHFCHO were close to those of CHClCHO and CH₂CHO than those of CH₂CFO. For the reaction of CH₂=CHF with O(³P), the formation of both the regioisomeric radicals, i.e., 1- and 2-fluorovinoxy radicals, was confirmed. The regioselectivity of the oxygen atom added to the double bond of monofluoroethylene is discussed.

Rate Constants for the Reactions of OH Radicals with Fluorinated Ethenes: Kinetic Measurements and Correlation between Structure and Reactivity

The rate constants for the reaction of OH radicals with four fluorinated ethenes (CF₂═CHF, ( E)-CHF═CHF, CF₂═CH₂, and CHF═CH₂) have been measured over the temperature range of 250-430 K. Kinetic measurements have been carried out using flash photolysis and laser photolysis methods combined with a laser-induced fluorescence technique. The Arrhenius expressions for the rate constant have been determined as k(CF₂═CHF) = (3.12 ± 0.11) × 10^-12 exp[(270 ± 10)/ T], k(( E)-CHF═CHF) = (3.75 ± 0.08) × 10^-12 exp[(230 ± 10)/ T], k(CF₂═CH₂) = (1.15 ± 0.07) × 10^-12 exp[(230 ± 20)/ T], and k(CHF═CH₂) = (1.16 ± 0.09) × 10^-12 exp[(390 ± 20)/ T] cm³ molecule^-1 s^-1. Infrared absorption spectra of the fluorinated ethenes have been measured at room temperature. The atmospheric lifetimes and global warming potentials of the fluorinated ethenes have been estimated. The correlation between the reactivity and the structure of the halogenated ethenes has been investigated by considering the structure containing the atoms attached to the carbons on both sides of the double bond. The calculated rate constants of 14 halogenated ethenes showed agreement with the measured rate constants within a factor of 2, except for that of one compound.

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.