Theoretical studies on energetics and mechanisms of the decomposition of CF3OH

Mechanistic Insights into Criegee Intermediate-Hydroperoxyl Radical Chemistry

The reaction between a Criegee intermediate and the hydroperoxyl radical (HO₂) is believed to play a role in the formation of new particles in the troposphere. Although the reaction has been previously studied in the gas phase, there are several knowledge gaps that still need to be filled. We simulated the reaction of anti-CH₃CHOO with HO₂ and HO₂-H₂O radical complexes in the gas phase at 0 K, which exhibited a low-barrier profile for water-containing systems and a barrierless profile for water-free systems. Moreover, the reaction was found to follow a proton-transfer mechanism, which challenges previous assumptions that the gas-phase reaction involves a hydrogen atom transfer. The HO₂ radical was found to mediate the Criegee hydration reaction in the gas phase. Metadynamics simulations further confirmed that the expected radical adduct formation between anti-CH₃CHOO and the HO₂ radical, as well as the HO₂- and H₂O-mediated reactions in the gas phase, followed a concerted mechanism. By combining constrained ab initio molecular dynamics simulations with thermodynamic integration, we quantitively evaluated the free-energy barriers at high temperatures. The barriers obtained for all three Criegee-HO₂ reaction systems were found to be temperature-dependent. We also compared the free-energy barriers of water-free and water-containing systems; the results revealed that water could hinder the reaction between the Criegee and HO₂ radical. These results suggest that HO₂ radicals may be involved in the formation of tropospheric radical adducts, and water molecules may also play important roles in the reactions of Criegee intermediates.

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.

Penetrating the Elusive Mechanism of Copper-Mediated Fluoromethylation in the Presence of Oxygen through the Gas-Phase Reactivity of Well-Defined [LCuO](+) Complexes with Fluoromethanes (CH(4-n)Fn, n = 1-3)

Traveling wave ion mobility spectrometry (TWIMS) isomer separation was exploited to react the particularly well-defined ionic species [LCuO](+) (L = 1,10-phenanthroline) with the neutral fluoromethane substrates CH(4-n)Fn (n = 1-3) in the gas phase. Experimentally, the monofluoromethane substrate (n = 1) undergoes both hydrogen-atom transfer, forming the copper hydroxide complex [LCuOH](•+) and concomitantly a CH2F(•) radical, and oxygen-atom transfer, yielding the observable ionic product [LCu](+) plus the neutral oxidized substrate [C,H3,O,F]. DFT calculations reveal that the mechanism for both product channels relies on the initial C-H bond activation of the substrate. Compared to nonfluorinated methane, the addition of fluorine to the substrate assists the reactivity through a lowering of the C-H bond energy and reaction preorganization (through noncovalent interaction in the encounter complex). A two-state reactivity scenario is mandatory for the oxidation, which competitively results in the unusual fluoromethanol product, CH2FOH, or the decomposed products, CH2O and HF, with the latter channel being kinetically disfavored. Difluoromethane (n = 2) is predicted to undergo the analogous reactions at room temperature, although the reactions are less favored than those of monofluoromethane. The reaction of trifluoromethane (n = 3, fluoroform) through C-H activation is kinetically hindered under ambient conditions but might be expected to occur in the condensed phase upon heating or with further lowering of reaction barriers through templation with counterions, such as potassium. Overall, formation of CH(3-n)Fn(•) and CH(3-n)FnOH occurs under relatively gentle energetic conditions, which sheds light on their potential as reactive intermediates in fluoromethylation reactions mediated by copper in the presence of oxygen.

Theoretical study of the OH-initiated atmospheric oxidation mechanism of perfluoro methyl vinyl ether, CF3OCFCF2

The fluorinated product glycolaldehyde can be catalytically converted to glyoxal + HF by H₂O.