Influence of Molecular Parameters on Rate Constants of Thermal Dissociation/Recombination Reactions: The Reaction System CF ⇄ CF + F.

The possibilities to extract incompletely characterized molecular parameters from experimental thermal rate constants for dissociation and recombination reactions are explored. The reaction system CF (+M) ⇄ CF + F (+M) is chosen as a representative example. A set of falloff curves is constructed and compared with the available experimental database.

The Thermal Dissociation-Recombination Reactions of SiF, SiF, and SiF: A Shock Wave and Theoretical Modeling Study.

Monitoring UV absorption signals of SiF and SiF, the thermal dissociation reactions of SiF and SiF 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, SiF, and SiF and their reverse recombination reactions were determined over broad temperature and pressure ranges. A comparison of fluorosilicon and fluorocarbon chemistry was finally made.

Shock wave and modelling study of the unimolecular dissociation of Si(CH)F: an access to spectroscopic and kinetic properties of SiF.

The thermal dissociation of Si(CH)F was studied in shock waves between 1400 and 1900 K. UV absorption-time profiles of its dissociation products SiF and CH were monitored. The reaction proceeds as a unimolecular process not far from the high-pressure limit.

High-Temperature Fluorocarbon Chemistry Revisited.

The thermal dissociation reactions of CF and CF were studied in shock waves over the temperature range 1000-4000 K using UV absorption spectroscopy. Absorption cross sections of CF, CF, CF, and C were derived and related to quantum-chemically modeled oscillator strengths. After confirming earlier results for the dissociation rates of CF, CF, and CF, the kinetics of secondary reactions were investigated.

Shock wave and modelling study of the dissociation kinetics of CFI.

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.

Falloff Curves of the Reaction CF (+M) → CF + F (+M).

The thermal dissociation reaction CF (+Ar) → CF + F (+Ar) was studied in incident and reflected shock waves by monitoring UV absorption signals of the primary dissociation product CF. CF radicals were produced by thermal decomposition of CFI. Accounting for secondary reactions of F atoms, rate constants for the unimolecular dissociation were derived.

Falloff curves and mechanism of thermal decomposition of CFI in shock waves.

The falloff curves of the unimolecular dissociation CFI (+Ar) → CF + 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 CFI 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 CFI, I, CF and (possibly) IF at 450-500 and 200-300 nm are reported.

Shock wave and modelling study of the dissociation pathways of (CF)N.

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.

Experimental and modelling study of the multichannel thermal dissociations of CHF and CHF.

The thermal unimolecular dissociation of CHF was studied in shock waves by monitoring the UV absorption of a dissociation product identified as CHF. 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 CH + HF on a dominant, spin-forbidden, pathway.

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

The unimolecular dissociation of CHF leading to CF + H, CHF + HF, or CHF + 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. It is shown that the energetically most favorable dissociation channel leading to CF + H has a higher threshold energy than the energetically less favorable one leading to CHF + HF.

Shock Wave and Theoretical Modeling Study of the Dissociation of CHF. II. Secondary Reactions.

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

Kinetic and Spectroscopic Studies of the Reaction of CF with H in Shock Waves.

The reaction of CF with H was studied in shock waves by monitoring UV absorption signals. CF was prepared by thermal dissociation of CF (or of c-CF). The rate constant of the reaction CF + H → CHF + HF near 2000 K was found to be close to 10 cm mol s, consistent with earlier information on the reverse reaction CHF + HF → CF + H and a modeled equilibrium constant.

Shock wave studies of the pyrolysis of fluorocarbon oxygenates. II. The thermal dissociation of CFO.

The thermal decomposition of octafluorooxalane, CFO, to CF + CF + COF 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 CF and dimerization of CF. The primary dissociation was found to be in its falloff range and falloff curves were constructed.

Shock wave studies of the pyrolysis of fluorocarbon oxygenates. I. The thermal dissociation of CFO and CFCOF.

The thermal decomposition of hexafluoropropylene oxide, CFO, to perfluoroacetyl fluoride, CFCOF, and CF has been studied in shock waves highly diluted in Ar between 630 and 1000 K. The measured rate constant k = 1.1 × 10 exp(-162(±4) kJ mol/RT) s agrees well with literature data and modelling results.

Shock wave and modeling study of the reaction CF4 (+M) ⇔ CF3 + F (+M).

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.

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).

Shock wave study of the thermal dissociations of C3F6 and c-C3F6. I. dissociation of hexafluoropropene.

The thermal dissociation of C3F6 was studied between 1330 and 2210 K in shock waves monitoring the UV absorption of CF2. CF2 yields of about 2.6 per parent C3F6 were obtained at reactant concentrations of 500-1000 ppm in the bath gas Ar.

Shock wave study of the thermal dissociations of C3F6 and c-C3F6. II. dissociation of hexafluorocyclopropane and dimerization of CF2.

The thermal dissociation of c-C3F6 has been studied in shock waves over the range 620-1030 K monitoring the UV absorption of CF2. The reaction was studied close to its high-pressure limit, but some high-temperature falloff was accounted for. Quantum-chemical and kinetic modeling rationalized the experimental data.

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.

Experimental and modeling study of the reaction C2F4 (+ M) ⇔ CF2 + CF2 (+ M).

The thermal dissociation reaction C2F4(+ M) → 2CF2(+ M) was studied in shock waves monitoring CF2 radicals by their UV absorption. The absorption coefficients as functions of wavelength and temperature were redetermined and are represented in analytical form. Dissociation rate constants as functions of bath gas concentration [M] and temperature, from previous and the present work, are presented analytically employing falloff expressions from unimolecular rate theory.

Role of water complexes in the reaction of propionaldehyde with OH radicals.

There has been considerable debate and speculation about the role of weakly bound complexes in radical-molecule reactions in the gas phase, especially in atmospheric chemistry. Among the significant number of potentially important molecular aggregates in chemical reactions, water complexes are of particular interest. Beyond the well-known energy transfer role of water in complex-forming reactions, it has been shown that water may also have a catalytic effect on the kinetics of radical-molecule reactions because of reduced reaction barrier heights for the complexes.