Shock wave study and theoretical modeling of the thermal decomposition of c-C4F8

The thermal dissociation of octafluorocyclobutane, c-C4F8, was studied in shock waves over the range 1150-2300 K by recording UV absorption signals of CF2. It was found that the primary reaction nearly exclusively produces 2 C2F4 which afterwards decomposes to 4 CF2. A primary reaction leading to CF2 + C3F6 is not detected (an upper limit to the yield of the latter channel was found to be about 10 percent). The temperature range of earlier single pulse shock wave experiments was extended. The reaction was shown to be close to its high pressure limit. Combining high and low temperature results leads to a rate constant for the primary dissociation of k1 = 1015.97 exp(-310.5 kJ mol-1/RT) s-1 in the range 630-1330 K, over which k1 varies over nearly 14 orders of magnitude. Calculations of the energetics of the reaction pathway and the rate constants support the conclusions from the experiments. Also they shed light on the role of the 1,4-biradical CF2CF2CF2CF2 as an intermediate of the reaction.Fil: Cobos, Carlos Jorge. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - La Plata. Instituto de Investigaciones Fisicoquímicas Teóricas y Aplicadas. Universidad Nacional de La Plata. Facultad de Ciencias Exactas. Instituto de Investigaciones Fisicoquímicas Teóricas y Aplicadas; ArgentinaFil: Hintzer, K.. Dyneon Gmbh; AlemaniaFil: Sölter, L.. Universität Göttingen; AlemaniaFil: Tellbach, E.. Universität Göttingen; AlemaniaFil: Thaler, A.. Dyneon Gmbh; AlemaniaFil: Troe, J.. Universität Göttingen; Alemania. Max-Planck-Institut fu¨r biophysikalische Chemie; Alemani

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 characterizations of the potential energy surface for the reaction, standard unimolecular rate theory, and experimental information on the average energy transferred per collision between excited CF3I and Ar. The (essentially) parameter-free theoretical modelling gives results in satisfactory agreement with data deduced from earlier shock wave experiments employing a variety of reactant concentrations (between a few ppm and a few percent in the bath gas Ar). New experiments recording absorption–time signals of CF3I, I2, CF2 and (possibly) IF at 450–500 and 200–300 nm are reported. By analysing the decomposition mechanism, besides the unimolecular dissociation of CF3I, these provide insight into the influence of secondary reactions on the experimental observations.Fil: Cobos, Carlos Jorge. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - La Plata. Instituto de Investigaciones Fisicoquímicas Teóricas y Aplicadas. Universidad Nacional de La Plata. Facultad de Ciencias Exactas. Instituto de Investigaciones Fisicoquímicas Teóricas y Aplicadas; ArgentinaFil: Sölter, L.. Universität Göttingen; AlemaniaFil: Tellbach, E.. Universität Göttingen; AlemaniaFil: Troe, J.. Universität Göttingen; Alemania. Institut Max Planck fuer Bioanorganische Chemie; Alemani

Shock wave study and theoretical modeling of the thermal decomposition of c-C4F8

The thermal dissociation of octafluorocyclobutane, c-C4F8, was studied in shock waves over the range 1150-2300 K by recording UV absorption signals of CF2. It was found that the primary reaction nearly exclusively produces 2 C2F4 which afterwards decomposes to 4 CF2. A primary reaction leading to CF2 + C3F6 is not detected (an upper limit to the yield of the latter channel was found to be about 10 percent). The temperature range of earlier single pulse shock wave experiments was extended. The reaction was shown to be close to its high pressure limit. Combining high and low temperature results leads to a rate constant for the primary dissociation of k1 = 1015.97 exp(-310.5 kJ mol-1/RT) s-1 in the range 630-1330 K, over which k1 varies over nearly 14 orders of magnitude. Calculations of the energetics of the reaction pathway and the rate constants support the conclusions from the experiments. Also they shed light on the role of the 1,4-biradical CF2CF2CF2CF2 as an intermediate of the reaction.Instituto de Investigaciones Fisicoquímicas Teóricas y Aplicada

Shock wave study and theoretical modeling of the thermal decomposition of c-C4F8

The thermal dissociation of octafluorocyclobutane, c-C4F8, was studied in shock waves over the range 1150-2300 K by recording UV absorption signals of CF2. It was found that the primary reaction nearly exclusively produces 2 C2F4 which afterwards decomposes to 4 CF2. A primary reaction leading to CF2 + C3F6 is not detected (an upper limit to the yield of the latter channel was found to be about 10 percent). The temperature range of earlier single pulse shock wave experiments was extended. The reaction was shown to be close to its high pressure limit. Combining high and low temperature results leads to a rate constant for the primary dissociation of k1 = 1015.97 exp(-310.5 kJ mol-1/RT) s-1 in the range 630-1330 K, over which k1 varies over nearly 14 orders of magnitude. Calculations of the energetics of the reaction pathway and the rate constants support the conclusions from the experiments. Also they shed light on the role of the 1,4-biradical CF2CF2CF2CF2 as an intermediate of the reaction.Instituto de Investigaciones Fisicoquímicas Teóricas y Aplicada

Shock wave and modelling study of the unimolecular dissociation of Si(CH₃)₂F₂: an access to spectroscopic and kinetic properties of SiF₂

The thermal dissociation of Si(CH₃)₂F₂ was studied in shock waves between 1400 and 1900 K. UV absorption-time profiles of its dissociation products SiF₂ and CH₃ were monitored. The reaction proceeds as a unimolecular process not far from the high-pressure limit. Comparing modelled and experimental results, an asymmetric representation of the falloff curves was shown to be most realistic. Modelled limiting high-pressure rate constants agreed well with the experimental data. The UV absorption spectrum of SiF₂ was shown to be quasi-continuous, with a maximum near 222 nm and a wavelength-integrated absorption cross section of 4.3 (±1) × 10⁻²³ cm³ (between 195 and 255 nm, base e), the latter being consistent with radiative lifetimes from the literature. Experiments over the range 1900-3200 K showed that SiF₂ was not consumed by a simple bond fission SiF₂ →SiF + F, but by a bimolecular reaction SiF₂ + SiF₂ → SiF + SiF₃ (rate constant in the range 10¹¹-10¹² cm³ mol⁻¹ s⁻¹), followed by the unimolecular dissociation SiF₃ → SiF₂ + F such that the reaction becomes catalyzed by the reactant SiF₂. The analogy to a pathway CF₂ + CF₂ → CF + CF₃, followed by CF₃ → CF₂ + F, in high-temperature fluorocarbon chemistry is stressed. Besides the high-temperature absorption cross sections of SiF₂, analogous data for SiF are also reported.Facultad de Ciencias ExactasInstituto de Investigaciones Fisicoquímicas Teóricas y Aplicada

Shock wave and modeling study of the thermal decomposition reactions of pentafluoroethane and 2-H-heptafluoropropane

The thermal decomposition reactions of CF3CF2H and CF3CFHCF3 have been studied in shock waves by monitoring the appearance of CF2 radicals. Temperatures in the range 1400–2000 K and Ar bath gas concentrations in the range (2–10) × 10−5 mol cm−3 were employed. It is shown that the reactions are initiated by C–C bond fission and not by HF elimination. Differing conclusions in the literature about the primary decomposition products, such as deduced from experiments at very low pressures, are attributed to unimolecular falloff effects. By increasing the initial reactant concentrations in Ar from 60 to 1000 ppm, a retardation of CF2 formation was observed while the final CF2 yields remained close to two CF2 per C2F5H or three CF2 per C3F7H decomposed. This is explained by secondary bimolecular reactions which lead to comparably stable transient species like CF3H, releasing CF2 at a slower rate. Quantum-chemical calculations and kinetic modeling help to identify the reaction pathways and provide estimates of rate constants for a series of primary and secondary reactions in the decomposition mechanism.Instituto de Investigaciones Fisicoquímicas Teóricas y Aplicada

Falloff Curves of the Reaction CF₃ (+M) → CF₂ + F (+M)

The thermal dissociation reaction CF₃ (+Ar) → CF₂ + F (+Ar) was studied in incident and reflected shock waves by monitoring UV absorption signals of the primary dissociation product CF₂. CF₃ radicals were produced by thermal decomposition of CF₃I. Accounting for secondary reactions of F atoms, rate constants for the unimolecular dissociation were derived. Experimental parts of the falloff curves were obtained over the ranges 1544 – 2106 K and 1.0 × 10⁻⁵ ≤ [Ar] ≤ 9.3 × 10⁻⁵ mol cm⁻³. Theoretical modelling allowed for a construction of the full falloff curves connecting the limiting low pressure rate constants k₀ = [Ar] 2.5 × 10¹⁸ (T/2000 K)-5.1 exp (-42 450 K/T) cm³ mol⁻¹ s⁻¹ with the limiting high pressure rate constants k∞ = 1.6 × 10¹⁶ (T/2000 K)-1.3 exp (-43 250 K/T) s⁻¹ (center broadening factors of Fcent= 0.25, 0.22, and 0.20 at 1500, 2000, and 2500 K, respectively, were used). The influence of simplifications of falloff expressions and of limiting rate constants on the representation of experimental data is discussed.Facultad de Ciencias ExactasInstituto de Investigaciones Fisicoquímicas Teóricas y Aplicada

Shock wave and modelling study of the dissociation pathways of (C2F5)3N

The thermal decomposition of perfluorotriethylamine, (C2F5)3N, was investigated in shock waves by monitoring the formation of CF2. Experiments were performed over the temperature range of 1120-1450 K with reactant concentrations between 100 and 1000 ppm of (C2F5)3N in the bath gas Ar and with [Ar] in the range of (0.7-5.5) × 10-5 mol cm-3. The experiments were accompanied by quantum-chemical calculations of the energies of various dissociation paths and by rate calculations, in particular for the dissociation of C2F5via C2F5 → CF3 + CF2. The overall reaction can proceed in different ways, either by a sequence of successive C-N bond ruptures followed by fast C2F5 decompositions, or by a sequence of alternating C-C and C-N bond ruptures. A cross-over between the two pathways can also take place. At temperatures below about 1300 K, yields of less than one CF2 per (C2F5)3N decomposed were observed. On the other hand, at temperatures around 2000 K, when besides the parent molecule, CF3 also dissociates, yields of six CF2 per (C2F5)3N decomposed were measured. The rate-delaying steps of the dissociation mechanism at intermediate temperatures were suggested to be the processes (C2F5)NCF2 → (C2F5)N + CF2 and (CF2)N → N + CF2. The reduction of the CF2 yields at low temperatures was tentatively attributed to a branching of the mechanism at the level of (C2F5)2NCF2, from where the cyclic final product perfluoro-N-methylpyrrolidine, (C4F8)NCF3, is formed which was identified in earlier work from the literature.Universidad Nacional de La Plat

Shock Wave and Modeling Study of the Pyrolysis of Pentafluoroethane and 2-H-Heptafluoropropane

The pyrolysis of C2HF5 and C3HF7 have been studied in shock waves by monitoring die UV-absorption of CF2 at 248nm and temperatures in the range of 1400 - 2000 K. Ar was used as bath gas with concentrations in the range (2-10) x 10^-5 mol cm^-3. It is shown that the reactions are initiated by C-C bond fission and differing conclusions in the literature can be explained with falloff effects. Quantum-chemical calculations were employed to model the kinetic of primary and secondary reactions in the decomposition mechanisms