Representation of “Broad” Falloff Curves for Dissociation and Recombination Reactions

Abstract

Expressions for representing the pressure dependence of unimolecular dissociation and the reverse recombination reactions are compared. Situations are considered where the broadening of the corresponding falloff curves is particularly pronounced, i.e. where broadening factors at the center of the falloff curves, cent , are very small and falloff curves correspondingly become very "broad". Such situations arise when unimolecular reactions of molecules with large numbers of low-frequency modes and high-temperature situations are considered. Recombination reactions of polyatomic species in atmospheric chemistry and dissociation reactions of large molecules in combustion are the fields of application of the present results. While previously proposed expressions for asymmetric broadening factors showed artifacts when cent decreased to values below about 0.4, alternative functions are described which avoid these problems for situations where cent decreases to values far below 0.1.

Reaction Kinetics of CaOH with H and O2 and O2CaOH with O: Implications for the Atmospheric Chemistry of Meteoric Calcium

“…(b) Dependence of the bimolecular rate constant of CaOH removal by O 2 on the number density of the buffer gas (N 2 ). Red circles, experimental data at 298 K; blue dots and black squares, master equation calculations at 298 and 200 K, respectively; and thin and thick lines, fits to the master equation results to the Troe expression for termolecular reactions, using the conventional broadening factor 39 and the new broadening factors proposed in ref (41), respectively.…”

“…The best fit yields F c = 0.136 ± 0.005, which is smaller than the value of 0.6 often assumed. 40 The O 2 CaOH adduct has a relatively large number of low-frequency vibrational modes (see Table S1 of the Supporting Information), which tends to cause F c in the conventional falloff expression to be less than 0.2, resulting in unphysical inflection points in the falloff curves 41 (thin lines in Figure 5b). We have therefore used the modified broadening factor expression of Troe (eqs 7 and 8 in ref (41), with parameters x 0 = 0.9, b = 0.25, and F c = 0.2), which produces a better fit to the master equation data, as shown by the thick lines in Figure 5b.…”

Barrierless association of CF 2 and dissociation of C 2 F 4 by variational transition-state theory and system-specific quantum Rice–Ramsperger–Kassel theory

“…In this work, the necessary physical parameters for treating the pressure dependence of collisional activation (43)(44)(45)(46)(47)(48)(49)(50) are directly taken from the literature; in the high-pressure limit, no empirical parameters are adjusted to the experimental data, and our calculations are from first principles. At finite pressure, the rate constant depends on the average energy transferred per collision <ΔE> all , which has been fitted to experiments in refs.…”

“…6. A simplified version of Troe's (46) master equation-based treatment of collision effects is used for computing the collisional efficiency β c (44), and the unimolecular F E factor is computed using the Whitten-Rabinovitch method (49,50,65). For these calculations, Lennard-Jones parameters σ and «/k B for C 2 F 4 are 4.68 Å and 235.9 K (66), respectively, and for Ar are 3.4 Å and 120 K, respectively (67).…”

Investigation of the Growth Behaviour of Cobalt Thin Films from Chemical Vapour Deposition, Using Directly Coupled X-ray Photoelectron Spectroscopy

“…To understand the relations between desired properties of the materials and the boundary conditions of their formation, highly interdisciplinary knowledge is desired, typically involving -besides material science -fluid dynamics and, in particular, physical chemistry that can address thermodynamic and kinetic properties of such complex systems and may make use of spectroscopic methods for their characterisation. As often on the way from elementary processes to application, mechanistic considerations based on fundamental knowledge from kinetics and thermodynamics as well as on advanced techniques to analyse important properties, have been instrumental to understand complex reactive systems such as in atmospheric chemistry, combustion or astrochemistry [1][2][3]. Such approaches have often pertained to the gas phase, and respective concepts for gassolid systems are an active area of research [4].…”