Theoretical studies on atmospheric chemistry of (CF3)2C(OH)CH3: Kinetics, mechanism and thermochemistry of gas phase reactions with OH radicals

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.

Atmospheric oxidation of fluoroalcohols initiated by ˙OH radicals in the presence of water and mineral dusts: mechanism, kinetics, and risk assessment

The transport and formation of fluorinated compounds are greatly significant due to their possible environmental risks. In this work, the ˙OH-mediated degradation of CF3CF2CF2CH2OH and CF3CHFCF2CH2OH in the presence of O2/NO/NO2 was studied by using density functional theory and the direct kinetic method. The formation mechanisms of perfluorocarboxylic/hydroperfluorocarboxylic acids (PFCAs/H-PFCAs), which were produced from the reactions of α-hydroxyperoxy radicals with NO/NO2 and the ensuing oxidation of α-hydroxyalkoxy radicals, were clarified and discussed. The roles of water and silica particles in the rate constants and ˙OH reaction mechanism with fluoroalcohols were investigated theoretically. The results showed that water and silica particles do not alter the reaction mechanism but obviously change the kinetic properties. Water could retard fluoroalcohol degradation by decreasing the rate constants by 3-5 orders of magnitude. However, the heterogeneous ˙OH-rate coefficients on the silica particle surfaces, including H4SiO4, H6Si2O7, and H12Si6O18, are larger than that of the naked reaction by 1.20-24.50 times. This finding suggested that these heterogeneous reactions may be responsible for the atmospheric loss of fluoroalcohols and the burden of PFCAs. In addition, fluoroalcohols could be exothermically trapped by H12Si6O18, H6Si2O7, and H4SiO4, in which the chemisorption on H12Si6O18 is stronger than that on H6Si2O7 or H4SiO4. The global warming potentials and radiative forcing of CF3CF2CF2CH2OH/CF3CHFCF2CH2OH were calculated to assess their contributions to the greenhouse effect. The toxicities of individual species were also estimated via the ECOSAR program and experimental measurements. This work enhances the understanding of the environmental formation of PFCAs and the transformation of fluoroalcohols.

Experimental and Computational Investigations of the Tropospheric Photooxidation Reactions of 1,1,1,3,3,3-Hexafluoro-2-Methyl-2-Propanol Initiated by OH Radicals and Cl Atoms

The gas-phase kinetics for the reactions of OH radicals and Cl atoms with 1,1,1,3,3,3-hexafluoro-2-methyl-2-propanol (HF2M2P) were measured at temperatures between 268 and 363 K using the relative rate experimental technique. Methane and acetonitrile were used as reference compounds to measure the rate coefficients of the title reactions. For the reactions of HF2M2P with OH radicals and Cl atoms, the rate coefficients were measured to be (7.07 ± 1.21) × 10^-15 and (2.85 ± 0.54) × 10^-14 cm³ molecule^-1 s^-1, respectively, at 298 K. The obtained Arrhenius expressions for the reactions of HF2M2P with OH radicals and Cl atoms are k_{HF2M2P + OH}^{Exp - (268 - 363 K)} = (7.84 ± 0.75) × 10^-14 exp [-(717 ± 59)/T] and k_{HF2M2P + Cl}^{Exp - (268 - 363 K)} = (3.21 ± 0.45) × 10^-12 exp [-(1395 ± 83)/T] cm³ molecule^-1 s^-1. In addition to the experimental measurements, computational kinetic calculations were also performed for the title reactions at the M06-2X/MG3S//M06-2X/6-31 + G(d,p) level of theory using advanced methods such as the canonical variational transition-state theory coupled with small curvature tunneling corrections at temperatures between 200 and 400 K. Theoretical calculations reveal that the H-abstraction from the CH₃ group is a more favorable reaction channel than that from the OH group. Thermochemistry, branching ratios, cumulative atmospheric lifetime, global warming potential, acidification potential, and photochemical ozone creation potential of HF2M2P were calculated in the present investigation.