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Thethermal dissociation of C3F6was studiedbetween 1330 and 2210 K in shock waves monitoring the UV absorptionof CF2. CF2yields of about 2.6 per parent C3F6were obtained at reactant concentrations of500–1000 ppm in the bath gas Ar. These yields dropped to about1.8 when reactant concentrations were lowered to 60 ppm. The increaseof the CF2yield with increasing concentration was attributedto bimolecular reactions between primary and secondary dissociationproducts. Quantum-chemical and kinetic modeling calculations helpedto estimate the contributions from the various primary dissociationsteps. It was shown that the measurements correspond to unimolecularreactions in their falloff range. Falloff representations of the rateconstants are given, leading to an overall high pressure rate constant k∞= 2.0 × 1017(−104kcal mol–1/RT) s–1and a relative rate of about 2/3:1/3 for the reactions C3F6→ CF3CF + CF2versus C3F6→ C2F3+ CF3. [ABSTRACT FROM AUTHOR]

The thermal dissociation of c-C3F6has beenstudied in shock waves over the range 620–1030 K monitoringthe UV absorption of CF2. The reaction was studied closeto its high-pressure limit, but some high-temperature falloff wasaccounted for. Quantum-chemical and kinetic modeling rationalizedthe experimental data. The reaction is suggested to involve the 1,3biradical CF2CF2CF2intermediate.CF2formed by the dissociation of c-C3F6dimerizes to C2F4. The measured rateof this reaction is also found to correspond to the falloff range.Rate constants for 2CF2→ C2F4as a function of temperature and bath gas concentration [Ar] aregiven and shown to be consistent with literature values for the high-pressurerate constants from experiments at lower temperatures and dissociationrate constants obtained in the falloff range at higher temperatures.The onset of falloff at intermediate temperatures is analyzed. [ABSTRACT FROM AUTHOR]

The unimolecular dissociation of CH2F2 leading to CF2 + H2, CHF + HF, or CHF2 + H is investigated by quantum-chemical calculations and unimolecular rate theory. Modeling of the rate constants is accompanied by shock wave experiments over the range of 1400-1800 K, monitoring the formation of CF2. It is shown that the energetically most favorable dissociation channel leading to CF2 + H2 has a higher threshold energy than the energetically less favorable one leading to CHF + HF. Falloff curves of the dissociations are modeled. Under the conditions of the described experiments, the primary dissociation CH2F2 → CHF + HF is followed by the reaction CHF + HF → CF2 + H2. The experimental value of the rate constant for the latter reaction indicates that it does not proceed by an addition-elimination process involving CH2F2* intermediates, as assumed before. [ABSTRACT FROM AUTHOR]

The reaction of CF2 with H2 was studied in shock waves by monitoring UV absorption signals. CF2 was prepared by thermal dissociation of C2F4 (or of c-C3F6). The rate constant of the reaction CF2 + H2 → CHF + HF near 2000 K was found to be close to 1011 cm³ mol-1 s-1, consistent with earlier information on the reverse reaction CHF + HF → CF2 + H2 and a modeled equilibrium constant. The kinetic studies were accompanied by spectroscopic investigations. Absorption cross sections of C2F4 between 190 and 220 nm were measured near 1000 K and compared with room-temperature values from the literature. Likewise, absorption cross sections of CF2 near 2000 K were measured between 210 and 300 nm and compared with room-temperature data. Additional superimposed absorption signals were recorded during the reaction and identified by their time dependence and by quantum-chemical calculations employing time-dependent density functional theory. A previously unknown absorption spectrum of CHF radicals near 200 nm was identified and its wavelength dependence determined. Further strong absorptions between 190 and 300 nm were attributed to CH2F and CF radicals. Absorptions at longer wavelengths, reaching up to 510 nm, were postulated to arise from C2 radicals formed at later stages of the reaction. [ABSTRACT FROM AUTHOR]

The thermal dissociation of CH2F2 and the reaction of CF2 with H2 was studied in shock waves over the temperature range 1800-2200 K, monitoring the absorption-time profiles at 248 nm. Besides contributions from CF2, the signals showed strong absorptions from secondary reaction products, probably mostly CH2F formed by the reaction CHF + H2 → CH2F + H. Rate constants of a series of possible secondary reactions were modeled, including falloff curves for the thermal dissociations of CHF, CHF2, and CH2F and rate constants of the reactions CHF + CH2F2 → CHF2 + CH2F, CHF + H2 → CH2F + H, H + CH2F2 → CHF2 + H2, H + CF2 → CF + HF, and H + CF → C + HF. On this basis, concentration-time profiles were simulated and compared with experimental absorption-time profiles. [ABSTRACT FROM AUTHOR]