Temperature and Pressure Dependence of the Reaction 2CF3 (+ M) ⇔ C2F6 (+ M)

Mechanistic and Kinetic Investigations on Decomposition of Trifluoromethanesulfonyl Fluoride in the Presence of Water Vapor and Electric Field

As a potential insulation gas to replace sulfur hexafluoride (SF₆) due to environmental concerns, trifluoromethanesulonyl fluoride (CF₃SO₂F) has attracted great interests in various high-voltage electric applications. Thermal stability of CF₃SO₂F plays an important role in the rational design of the gas-insulated electric equipment. Unimolecular decomposition of CF₃SO₂F was investigated using high-level ab initio methods including the explicitly correlated RCCSD(T)-F12, the composite ROCBS-QB3, and the multireference RS2 extrapolated to complete basis set limit on the basis of M06-2X-, B2PLYPD3-, and CCSD-optimized geometrical parameters. Rate coefficients and decomposition temperatures were simulated using master equations. CF₃SO₂F decomposes predominantly via a simple C-S bond cleavage to form CF₃ and SO₂F, accompanied by a roaming induced F-abstraction detour to release CF₄ and SO₂, or isomerizes via CF₃ migration to the more stable CF₃OSFO followed by the production of CF₂O and SOF₂. Various characteristic decomposition products (e.g., CF₄, C₂F₆, CF₂O, SO₂, SOF₂, SO₂F₂, CF₃H, and so on.) have been identified theoretically through secondary reactions and hydrolysis of CF₃SO₂F in the presence of water vapor. Electronic structures and stability of CF₃SO₂F could be affected significantly by the external electric field orientated along the S-C bond. The field-dependent electron-molecule capture rates support that CF₃SO₂F is superior to SF₆ in dielectric strength. The present computational findings shed light on the practical use of CF₃SO₂F as the replacement gas for SF₆.

High-Temperature Fluorocarbon Chemistry Revisited

The thermal dissociation reactions of C₂F₄ and C₂F₆ were studied in shock waves over the temperature range 1000-4000 K using UV absorption spectroscopy. Absorption cross sections of C₂F₄, CF₂, CF, and C₂ were derived and related to quantum-chemically modeled oscillator strengths. After confirming earlier results for the dissociation rates of C₂F₄, CF₃, and CF₂, the kinetics of secondary reactions were investigated. For example, the reaction CF₂ + CF₂ → CF + CF₃ was identified. Its rate constant of 10¹⁰ cm³ mol^-1 s^-1 near 2400 K is markedly larger than the limiting high-pressure rate constant of the dimerization CF₂ + CF₂ → C₂F₄, suggesting that the reaction follows a different path. When the measurements of the thermal dissociation CF₂ (+Ar) → CF + F (+Ar) are extended to temperatures above 2500 K, the formation of C₂ radicals was shown to involve the reaction CF + CF → C₂F + F (modeled rate constant 8.0 × 10¹² (T/3500 K)^1.0 exp(-4400 K/T) cm³ mol^-1 s^-1) and the subsequent dissociation C₂F (+Ar) → C₂ + F + (Ar) (modeled limiting low-pressure rate constant 3.0 × 10¹⁶ (T/3500 K)^-4.0 exp(-56880 K/T) cm³ mol^-1 s^-1). This mechanism was validated by monitoring the dissociation of C₂ at temperatures close to 4000 K. Temperature- and pressure-dependences of rate constants of reactions involved in the system were modeled by quantum-chemistry based rate theory.

Shock wave and modelling study of the dissociation kinetics of C2F5I

The thermal dissociation of C2F5I was studied in shock waves monitoring UV absorption signals from the reactant C2F5I and later formed reaction products such as CF, CF2, and C2F4. Temperatures of 950-1500 K, bath gas concentrations of [Ar] = 3 × 10-5-2 × 10-4 mol cm-3, and reactant concentrations of 100-500 ppm C2F5I in Ar were employed. Absorption-time profiles were recorded at selected wavelengths in the range 200-280 nm. It was found that the dissociation of C2F5I → C2F5 + I was followed by the dissociation C2F5 → CF2 + CF3, before the dimerization reactions 2CF2 → C2F4 and 2CF3 → C2F6 and a reaction CF2 + CF3 → CF + CF4 set in. The combination of iodine atoms with C2F5 and CF3 had also to be considered. The rate constant of the primary dissociation of C2F5I was analyzed in the framework of statistical unimolecular rate theory accompanied by a quantum-chemical characterization of molecular parameters. Rates of secondary reactions were modelled as well. Experimental rate constants for the dissociations of C2F5I and C2F5 agreed well with the modelling results. The comparably slow dimerization 2CF2 → C2F4 could be followed both by monitoring reactant CF2 and product C2F4 absorption signals, while CF3 dimerization was too fast to be detected. A competition between the dimerization reactions of CF2 and CF3, the recombination of CF2 and CF3 forming C2F5, and CF-forming processes like CF2 + CF3 → CF + CF4 finally was discussed.

Falloff curves and mechanism of thermal decomposition of CF3I in shock waves

The falloff curves of the unimolecular dissociation CF₃I (+Ar) → CF₃ + I (+Ar) are modelled by combining quantum-chemical characterization of the potential for the reaction, unimolecular rate theory, and experimental information on collisional energy transfer.

Shock wave studies of the pyrolysis of fluorocarbon oxygenates. I. The thermal dissociation of C3F6O and CF3COF

The thermal decomposition of hexafluoropropylene oxide, C₃F₆O, to perfluoroacetyl fluoride, CF₃COF, and CF₂ has been studied in shock waves in Ar between 630 and 1000 K. The subsequent decomposition of CF₃COF has been followed between 1400 and 1900 K.

Unimolecular isomerization of CH2FCD2Cl via the interchange of Cl and F atoms: assignment of the threshold energy to the 1,2-dyotropic rearrangement

The room-temperature gas-phase recombination of CH2F and CD2Cl radicals was used to prepare CH2FCD2Cl molecules with 91 kcal mol(-1) of vibrational energy. Three unimolecular processes are in competition with collisional deactivation of CH2FCD2Cl; HCl and DF elimination to give CHF═CD2 and CH2═CDCl plus isomerization to give CH2ClCD2F by the interchange of F and Cl atoms. The Cl/F interchange reaction was observed, and the rate constant was assigned from measurement of CHCl═CD2 as a product, which is formed by HF elimination from CH2ClCD2F. These experiments plus previously published results from chemically activated CH2ClCH2F and electronic structure and RRKM calculations for the kinetic-isotope effects permit assignment of the three rate constants for CH2FCD2Cl (and for CH2ClCD2F). The product branching ratio for the interchange reaction versus elimination is 0.24 ± 0.04. Comparison of the experimental rate constant with the RRKM calculated rate constant permitted the assignment of a threshold energy of 62 ± 3 kcal mol(-1) for this type-1 dyotropic rearrangement. On the basis of electronic structure calculations, the nature of the transition state for the rearrangement reaction is discussed. The radical recombination reactions in the chemical system also generate vibrationally excited CD2ClCD2Cl and CH2FCH2F molecules, and the rate constants for DCl and HF elimination were measured in order to confirm that the photolysis of CD2ClI and (CH2F)2CO mixtures was giving reliable data for CH2FCD2Cl.