Gas-phase chemical models

Before reviewing the results of pseudo-time-dependent models of homogeneous cloud interiors in detail, it is worthwhile to mention five successful predictions/ex- 

A second successful prediction is that many so-called metastable species (i.e. isomers) are abundant even if they are quite reactive in the laboratory.66 Perhaps the simplest interstellar molecule in this class is HNC, but large numbers of others can be seen in Table 1. It is assumed that most metastable species are formed in dissociative recombination reactions along with their stable counterparts at approximately equal rates, and that both are destroyed by ion-molecule reactions so that the laboratory reactivity, which is normally determined by reactions with neutral species, is irrelevant. Both HCN and HNC, for example, are thought to derive from the dissociative recombination reaction involving a linear precursor ion  

Not all metastables derive from dissociative recombination reactions the metastable ion HOC+ is produced in a variety of normal ion-molecule reactions along with the lower energy formyl ion. 

DCO+ is detected in space with an abundance with respect to HCO+ of = 2% in the 10 K source TMC-1, in good agreement with model results that consider a wider variety of exchange reactions than discussed here.70 

A fourth success concerns the high degree of unsaturation found in the observed list of molecules. Very few highly saturated molecules are detected, and those that are saturated tend to be found in highly localized sources known as hot cores, where they are probably formed via H-atom hydrogenation on grain surfaces.54 The reason that ion-molecule reactions do not produce more saturated polyatomic species is, as discussed above, the small number of reactions between hydrocarbon ions and H2 that can occur rapidly. 

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We have previously focused attention on purely gas-phase chemical models, gas-phase models with accretion onto grains, and gas-phase models with both accretion and desorption. Gas-grain models, such as that of Aikawa et briefly mentioned above, differ from these models in that they include surface chemistry. For models with large numbers of surface reactions, either the simple rate equation treatment is used, or the rate coefficients /cab (see Eq. (1.74)) are modified in a semi-empirical manner to handle fractional average adsorbate abundances to an extent. The so-called modified rate treatment has been tested against stochastic methods in small systems of equations. 

Fig. 4.1 Abundances (compared to total H) of species computed by a pure gas-phase chemical model for dense cloud conditions as a function of time...

Grote R F and Hynes J T 1980 The stable states picture of chemical reactions. II. Rate constants for condensed and gas phase reaction models J. Chem. Phys. 73 2715-32... 

Chemical vapor deposition (CVD) of carbon from propane is the main reaction in the fabrication of the C/C composites [1,2] and the C-SiC functionally graded material [3,4,5]. The carbon deposition rate from propane is high compared with those from other aliphatic hydrocarbons [4]. Propane is rapidly decomposed in the gas phase and various hydrocarbons are formed independently of the film growth in the CVD reactor. The propane concentration distribution is determined by the gas-phase kinetics. The gas-phase reaction model, in addition to the film growth reaction model, is required for the numerical simulation of the CVD reactor for designing and controlling purposes. Therefore, a compact gas-phase reaction model is preferred. The authors proposed the procedure to reduce an elementary reaction model consisting of hundreds of reactions to a compact model objectively [6]. In this study, the procedure is applied to propane pyrolysis for carbon CVD and a compact gas-phase reaction model is built by the proposed procedure and the kinetic parameters are determined from the experimental results. 

Exchange reactions of H + H2(or Hj) have provided the testing ground for theoretical methods which are used to understand gas-phase chemical dynamics . Interest in modeling the reaction of hydrogen with metal surfaces is therefore not unexpected. In addition, hydrogen often plays an important role in reactions associated with catalysis, so studies of this type also have practical application. 

At the energies required for conformational conversions and other exchange processes which are amenable to study by NMR spectroscopy, the reacting molecules have state densities which are much lower than those of molecules undergoing isomerization and decomposition reactions which are generally found to obey RRKM kinetics. Whether these systems can be modeled with RRKM theory is a question of current interest. Table 8 lists molecules for which pressure-dependent gas-phase chemical exchange rate constants have been obtained. 

An impressive number of gas phase chemical studies of ions have emerged during the last fifty years. Most of these studies were experimental, and a wide range of instrumentation methods, mostly mass spectrometric ones, have been used. More recently, these studies have been complemented by high level quantum chemical and other model calculations, providing firm connection between experiment and theory. 

RADM2 The gas-phase chemical mechanism of Regional Acid Deposition Model, version 2... 

J. Wolfrum, H.-R. Volpp, J. Warnatz cind R. Ranneicher, Ekl., Gas Phase Chemical Reaction Systems Experiments and Models 100 Years after Max Bodenstein, Springer Series in Chemical Physics, Vol. 61 (Springer, Heidelberg, 1996). 

The bimolecular reaction rate for particles constrained on a planar surface has been studied using continuum diffusion theory " and lattice models. In this section it will be shown how two features which are not taken account of in those studies are incorporated in the encounter theory of this chapter. These are the influence of the potential K(R) and the inclusion of the dependence on mean free path. In most instances it is expected that surface corrugation and strong coupling of the reactants to the surface will give the diffusive limit for the steady-state rate. Nevertheless, as stressed above, the initial rate is the kinetic theory, or low-friction limit, and transient exp)eriments may probe this rate. It is noted that an adaptation of low-density gas-phase chemical kinetic theory for reactions on surfaces has been made. The theory of this section shows how this rate is related to the rate of diffusion theory. 

The detailed reaction mechanism proposed by Yetter [26] and augmented by Lin and co-workers [4] is employed to treat the gas-phase chemical kinetics. The model is developed based on a hierarchical approach for collecting kinetic... 

Using a kinetic modeling methodology similar to that described in section 8.3, acetaldehyde photodegradation can be modeled assuming that carbon dioxide and acetaldehyde are the only gas phase chemical species (Figure 8.4). 

R. F. Grote and J. T. Hynes, ]. Chem. Phys., 73, 2715 (1980). The Stable States Picture of Chemical Reactions. II. Rate Constants for Condensed and Gas Phase Reaction Models. 

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