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.

Statistical theory for the reaction N + OH → NO + H: thermal low-temperature rate constants.

The reaction N + OH → NO + H involves the intermediate formation of NOH adducts which in part rearrange to HNO conformers. A statistical treatment of the process is developed in which an initial adiabatic channel capture of the reactants is accompanied by partial primary redissociation of the N⋯OH collision pairs. A criterion for the extent of this primary redissociation in competition to the formation of randomized, long-lived, complex of NOH is proposed.

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.

Gas-Phase Anionic Metal Clusters are Model Systems for Surface Oxidation: Kinetics of the Reactions of with O (M = V, Cr, Co, Ni; = 1-15).

The reactions of anionic metal clusters with O (M = V ( = 1-15), Cr ( = 1-15), Co ( = 1-12), and Ni ( = 1-14)) are investigated from 300 to 600 K using a selected-ion flow tube. All rate constants show a positive temperature dependence, well described by an Arrhenius equation. Rate constants exceed (or are extrapolated to exceed at higher temperatures) the Langevin-Gioumousis-Stevenson capture rate constant.

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.

Toward a quantitative analysis of the temperature dependence of electron attachment to SF.

New flowing afterglow/Langmuir probe investigations of electronic attachment to SF are described. Thermal attachment rate constants are found to increase from 1.5 × 10 cm s at 200 K to 2.

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.

On the Competition Between Electron Autodetachment and Dissociation of Molecular Anions.

We treat the competition between autodetachment of electrons and unimolecular dissociation of excited molecular anions as a rigid-/loose-activated complex multichannel reaction system. To start, the temperature and pressure dependences under thermal excitation conditions are represented in terms of falloff curves of separated single-channel processes within the framework of unimolecular reaction kinetics. Channel couplings, caused by collisional energy transfer and "rotational channel switching" due to angular momentum effects, are introduced afterward.

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.

Simplified Analysis and Representation of Multichannel Thermal Unimolecular Reactions.

Two-channel and multichannel thermal unimolecular reactions are analyzed by simple models, starting with the calculation of separated-channel rate constants and accounting for intrinsic channel coupling afterward. Reactions with rigid- and with loose-activated complex channels are distinguished. Weak-collision, energy-transfer, effects are suggested to govern the competition between rigid-activated complex channels, while angular-momentum, "rotational channel switching", effects dominate the competition between rigid- and loose-activated complex channels.

Electronically nonadiabatic mechanism of the vibrational relaxation of NO in Ar: Rate coefficients from ab initio potentials and asymptotic coupling.

In this paper, the electronically nonadiabatic Landau-Zener (LZ) mechanism for the vibrational relaxation v = 1 → v = 0 of NO(XΠ) in collisions with Ar(S01) is discussed. It corresponds to nonadiabatic transitions between two crossing vibronic potential energy surfaces (PESs) originating from vibrational states of the collision complex and supported by two coupled electronic PESs. The LZ rate coefficients k10LZ are calculated within the uniform Airy approach in the reaction coordinate approximation with parameters derived from ab initio PESs and an asymptotic estimation of the Franck-Condon factor in the nonadiabatic coupling region.

Kinetics in the real world: linking molecules, processes, and systems.

Unravelling elementary steps, reaction pathways, and kinetic mechanisms is key to understanding the behaviour of many real-world chemical systems that span from the troposphere or even interstellar media to engines and process reactors. Recent work in chemical kinetics provides detailed information on the reactive changes occurring in chemical systems, often on the atomic or molecular scale. The optimisation of practical processes, for instance in combustion, catalysis, battery technology, polymerisation, and nanoparticle production, can profit from a sound knowledge of the underlying fundamental chemical kinetics.

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.

Temperature and Pressure Dependences of the Reactions of Fe with Methyl Halides CHX (X = Cl, Br, I): Experiments and Kinetic Modeling Results.

The pressure and temperature dependences of the reactions of Fe with methyl halides CHX (X = Cl, Br, I) in He were measured in a selected ion flow tube over the ranges 0.4 to 1.2 Torr and 300-600 K.

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.