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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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

The thermal unimolecular dissociation of CH3F was studied in shock waves by monitoring the UV absorption of a dissociation product identified as CH2F. It is concluded that, under conditions applied, the formation of this species corresponds to a minor, spin-allowed, dissociation channel of about 3% yield. Near to the low-pressure limit of the reaction, on the other hand, the energetically more favourable dissociation leads to 3CH2 + HF on a dominant, spin-forbidden, pathway. By considering the multichannel character of the reaction, it is shown that, in contrast to the low-pressure range, the high-pressure range of the reaction should be dominated by CH2F formation. The channel-switching probably takes place at pressures higher than those applied in the present work. In addition to the two dissociation channels of CH3F producing 3CH2 + HF and CH2F + H, a third, spin-allowed, dissociation channel leading to 1CHF + H2 was also considered and estimated to proceed with a yield smaller than 0.5%. Besides the dissociation of CH3F, the dissociation of CH2F was studied by monitoring the UV spectrum of CH2F. Details of this spectrum were investigated. Similar to CH3F, the dissociation of CH2F can proceed on several dissociation channels, under the present conditions either to CHF + H or to CF + H2. After modelling single-channel falloff curves for all reaction pathways, coupling effects of multichannel dissociations were interpreted in the framework of multichannel unimolecular rate theory. [ABSTRACT FROM AUTHOR]

The thermal decomposition of octafluorooxalane, C4F8O, to C2F4 + CF2 + COF2 has been studied in shock waves highly diluted in Ar between 1300 and 2200 K. The primary dissociation was shown to be followed by secondary dissociation of C2F4 and dimerization of CF2. The primary dissociation was found to be in its falloff range and falloff curves were constructed. The limiting low and high pressure rate constants were estimated and compared with modelling results. Quantum-chemical calculations identified possible reaction pathways, either leading directly to the final products of the reaction or passing through an open-chain CF2CF2CF2 intermediate which dissociates in a second step. [ABSTRACT FROM AUTHOR]

The thermal decomposition of hexafluoropropylene oxide, C3F6O, to perfluoroacetyl fluoride, CF3COF, and CF2 has been studied in shock waves highly diluted in Ar between 630 and 1000 K. The measured rate constant k1 = 1.1 × 1014 exp(−162(±4) kJ mol−1/RT) s−1 agrees well with literature data and modelling results. Using the reaction as a precursor, equimolar mixtures of CF3COF and CF2 were further heated. Combining experimental observations with theoretical modelling (on the CBS-QB3 and G4MP2 ab initio composite levels), CF3COF is shown to dissociate on two channels, either leading to CF2 + COF2 or to CF3 + FCO. By monitoring the CF2 signals, the branching ratio was determined between 1400 and 1900 K. The high pressure rate constants for the two channels were obtained from theoretical modelling as k5,∞(CF3COF → CF2 + COF2) = 7.1 × 1014 exp(−320 kJ mol−1/RT) s−1 and k6,∞(CF3COF → CF3 + FCO) = 3.9 × 1015 exp(−355 kJ mol−1/RT) s−1. The experimental results obtained at [Ar] ≈ 5 × 10−6 mol cm−3 were consistent with modelling results, showing that the reaction is in the falloff range of the unimolecular dissociation. The mechanism of secondary reactions following CF3COF dissociation has been analysed as well. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

The possibilities to extract incompletely characterized molecular parameters from experimental thermal rate constants for dissociation and recombination reactions are explored. The reaction system CF 4 (+M) ⇄ CF 3 + F (+M) is chosen as a representative example. A set of falloff curves is constructed and compared with the available experimental database. Agreement is achieved by minor (unfortunately not separable) adjustments of reaction enthalpy and collisional energy transfer parameters.

Monitoring UV absorption signals of SiF 2 and SiF, the thermal dissociation reactions of SiF 4 and SiF 2 were studied in shock waves. Rationalizing the experimental observations by standard unimolecular rate theory in combination with quantum-chemical calculations of the reaction potentials, rate constants for the thermal dissociation reactions of SiF 4 , SiF 3 , and SiF 2 and their reverse recombination reactions were determined over broad temperature and pressure ranges. A comparison of fluorosilicon and fluorocarbon chemistry was finally made.

The thermal dissociation of Si(CH 3 ) 2 F 2 was studied in shock waves between 1400 and 1900 K. UV absorption-time profiles of its dissociation products SiF 2 and CH 3 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 SiF 2 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 cm 3 (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 SiF 2 was not consumed by a simple bond fission SiF 2 →SiF + F, but by a bimolecular reaction SiF 2 + SiF 2 → SiF + SiF 3 (rate constant in the range 10 11 -10 12 cm 3 mol -1 s -1 ), followed by the unimolecular dissociation SiF 3 → SiF 2 + F such that the reaction becomes catalyzed by the reactant SiF 2 . The analogy to a pathway CF 2 + CF 2 → CF + CF 3 , followed by CF 3 → CF 2 + F, in high-temperature fluorocarbon chemistry is stressed. Besides the high-temperature absorption cross sections of SiF 2 , analogous data for SiF are also reported.

The thermal dissociation reaction CF 3 (+Ar) → CF 2 + F (+Ar) was studied in incident and reflected shock waves by monitoring UV absorption signals of the primary dissociation product CF 2 . CF 3 radicals were produced by thermal decomposition of CF 3 I. 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 modeling allowed for a construction of the full falloff curves connecting the limiting low-pressure rate constants k 0 = [Ar] 2.5 × 10 18 ( T /2000 K) -5.1  exp (-42 450 K/ T ) cm 3 mol -1 s -1 with the limiting high-pressure rate constants k ∞ = 1.6 × 10 16 ( T /2000 K) -1.3  exp (-43 250 K/ T ) s -1 (center broadening factors of F cent = 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.

The falloff curves of the unimolecular dissociation CF 3 I (+Ar) → CF 3 + 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 CF 3 I 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 CF 3 I, I 2 , CF 2 and (possibly) IF at 450-500 and 200-300 nm are reported. By analysing the decomposition mechanism, besides the unimolecular dissociation of CF 3 I, these provide insight into the influence of secondary reactions on the experimental observations.

The thermal unimolecular dissociation of CH 3 F was studied in shock waves by monitoring the UV absorption of a dissociation product identified as CH 2 F. It is concluded that, under conditions applied, the formation of this species corresponds to a minor, spin-allowed, dissociation channel of about 3% yield. Near to the low-pressure limit of the reaction, on the other hand, the energetically more favourable dissociation leads to 3 CH 2 + HF on a dominant, spin-forbidden, pathway. By considering the multichannel character of the reaction, it is shown that, in contrast to the low-pressure range, the high-pressure range of the reaction should be dominated by CH 2 F formation. The channel-switching probably takes place at pressures higher than those applied in the present work. In addition to the two dissociation channels of CH 3 F producing 3 CH 2 + HF and CH 2 F + H, a third, spin-allowed, dissociation channel leading to 1 CHF + H 2 was also considered and estimated to proceed with a yield smaller than 0.5%. Besides the dissociation of CH 3 F, the dissociation of CH 2 F was studied by monitoring the UV spectrum of CH 2 F. Details of this spectrum were investigated. Similar to CH 3 F, the dissociation of CH 2 F can proceed on several dissociation channels, under the present conditions either to CHF + H or to CF + H 2 . After modelling single-channel falloff curves for all reaction pathways, coupling effects of multichannel dissociations were interpreted in the framework of multichannel unimolecular rate theory.

The unimolecular dissociation of CH 2 F 2 leading to CF 2 + H 2 , CHF + HF, or CHF 2 + 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 CF 2 . It is shown that the energetically most favorable dissociation channel leading to CF 2 + H 2 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 CH 2 F 2 → CHF + HF is followed by the reaction CHF + HF → CF 2 + H 2 . The experimental value of the rate constant for the latter reaction indicates that it does not proceed by an addition-elimination process involving CH 2 F 2 * intermediates, as assumed before.

The reaction of CF 2 with H 2 was studied in shock waves by monitoring UV absorption signals. CF 2 was prepared by thermal dissociation of C 2 F 4 (or of c-C 3 F 6 ). The rate constant of the reaction CF 2 + H 2 → CHF + HF near 2000 K was found to be close to 10 11 cm 3 mol -1 s -1 , consistent with earlier information on the reverse reaction CHF + HF → CF 2 + H 2 and a modeled equilibrium constant. The kinetic studies were accompanied by spectroscopic investigations. Absorption cross sections of C 2 F 4 between 190 and 220 nm were measured near 1000 K and compared with room-temperature values from the literature. Likewise, absorption cross sections of CF 2 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 CH 2 F and CF radicals. Absorptions at longer wavelengths, reaching up to 510 nm, were postulated to arise from C 2 radicals formed at later stages of the reaction.

The thermal dissociation of CH 2 F 2 and the reaction of CF 2 with H 2 was studied in shock waves over the temperature range 1800-2200 K, monitoring the absorption-time profiles at 248 nm. Besides contributions from CF 2 , the signals showed strong absorptions from secondary reaction products, probably mostly CH 2 F formed by the reaction CHF + H 2 → CH 2 F + H. Rate constants of a series of possible secondary reactions were modeled, including falloff curves for the thermal dissociations of CHF, CHF 2 , and CH 2 F and rate constants of the reactions CHF + CH 2 F 2 → CHF 2 + CH 2 F, CHF + H 2 → CH 2 F + H, H + CH 2 F 2 → CHF 2 + H 2 , H + CF 2 → CF + HF, and H + CF → C + HF. On this basis, concentration-time profiles were simulated and compared with experimental absorption-time profiles.

The thermal decomposition of octafluorooxalane, C 4 F 8 O, to C 2 F 4 + CF 2 + COF 2 has been studied in shock waves highly diluted in Ar between 1300 and 2200 K. The primary dissociation was shown to be followed by secondary dissociation of C 2 F 4 and dimerization of CF 2 . The primary dissociation was found to be in its falloff range and falloff curves were constructed. The limiting low and high pressure rate constants were estimated and compared with modelling results. Quantum-chemical calculations identified possible reaction pathways, either leading directly to the final products of the reaction or passing through an open-chain CF 2 CF 2 CF 2 intermediate which dissociates in a second step.

The thermal decomposition of hexafluoropropylene oxide, C 3 F 6 O, to perfluoroacetyl fluoride, CF 3 COF, and CF 2 has been studied in shock waves highly diluted in Ar between 630 and 1000 K. The measured rate constant k 1 = 1.1 × 10 14  exp(-162(±4) kJ mol -1 /RT) s -1 agrees well with literature data and modelling results. Using the reaction as a precursor, equimolar mixtures of CF 3 COF and CF 2 were further heated. Combining experimental observations with theoretical modelling (on the CBS-QB3 and G4MP2 ab initio composite levels), CF 3 COF is shown to dissociate on two channels, either leading to CF 2 + COF 2 or to CF 3 + FCO. By monitoring the CF 2 signals, the branching ratio was determined between 1400 and 1900 K. The high pressure rate constants for the two channels were obtained from theoretical modelling as k 5,∞ (CF 3 COF → CF 2 + COF 2 ) = 7.1 × 10 14  exp(-320 kJ mol -1 /RT) s -1 and k 6,∞ (CF 3 COF → CF 3 + FCO) = 3.9 × 10 15  exp(-355 kJ mol -1 /RT) s -1 . The experimental results obtained at [Ar] ≈ 5 × 10 -6 mol cm -3 were consistent with modelling results, showing that the reaction is in the falloff range of the unimolecular dissociation. The mechanism of secondary reactions following CF 3 COF dissociation has been analysed as well.

The thermal decomposition of CF4 (+Ar) → CF3 + F (+Ar) was studied in shock waves over the temperature range 2000-3000 K varying the bath gas concentration [Ar] between 4 × 10(-6) and 9 × 10(-5) mol cm(-3). It is shown that the reaction corresponds to the intermediate range of the falloff curve. By combination with room temperature data for the reverse reaction CF3 + F (+He) → CF4 (+He) and applying unimolecular rate theory, falloff curves over the temperature range 300-6000 K are modeled. A comparison with the reaction system CH4 (+M) ⇔ CH3 + H (+M) is made.