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.

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 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 modeling 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 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.

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.

Shock Wave and Theoretical Modeling Study of the Dissociation of CH2F2. I. Primary Processes

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.

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 = 10(15.97) 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.