Reaction rate theory atmospheric chemistry

The next chapter reviews the reactions of free atoms and radicals which play an important role in the modeling of complex processes occurring in the polluted atmosphere and in combustion chemistry. J. Jodkowski discusses the computational models of the reaction rate theory most frequently used in the theoretical analysis of gas-phase reaction kinetics and presents examples of the reactions of reactive components of the polluted atmosphere, such as 02, NOx, OH, NH2, alkyl radicals, and halogen atoms. Kinetic parameters of the reactions under investigation are provided in an analytical form convenient for kinetic modeling studies. The presented expressions allow for a successful description of the kinetics of the reaction systems in a wide temperature range and could be used in kinetic studies of related species. 

The twentieth century saw an enormous amount of experimental and theoretical research on elementary chemical reactions, an effort which continues today. The fruits of this work are extensive kinetics databases, and molecular theories from which to make estimates when experimental data are not available. Equally important are parallel developments in thermochemistry. All of this information makes possible the development of detailed chemical kinetics models of overall chemical reactions. Models have been constructed and applied to such diverse topics as atmospheric chemistry, combustion, low temperature oxidation, chemical vapor deposition, and reactions in traditional chemical process industries. The rate of each elementary reaction in a model is expressed as... 

Part IV again is a theoretical one, in which different approaches for the calculation of state-specific and thermal rate data are described. The article by A.F. Wagner presents a new approach to describe the influence of hindered rotations on recombination/dissociation kinetics in the framework of transition state theory. In the papers by D.C. Clary and G. Nyman an approximate quantum mechanical method is described and used to calculate thermal rate coefficients for gas phase reactions of interest in atmospheric chemistry which involve polyatomic molecules. Finally, different approaches to describe vibrational relaxation of diatoms in thermal collisions are discussed by E,E, Nikitin,... 

The calculation of theoretical rate constants for gas-phase chemical reactions involved in atmospheric chemistry is a subject of great interest. Theoretical kinetic methodologies utilize the quantum chemical characterization of the stationary points along the PES of a reaction to calculate the rate constants and product distributions. These methods allow for the elucidation of rate constants over the temperature and pressure range in the atmosphere. Various theoretical methods are available for rate constant calculations. Here, we focus on transition state theory (TST) and its variants to calculate the reaction rate constants. 

Is best used for representing the pressure dependence of the termolecular reaction rate constant. This equation is based on the curve fitting to the pressure dependence of the unimolecular decomposition rate by the Kassel theory. In Eq. (2.54), is called a broadening factor, and it is a good experimental fitting for many termolecular reactions in atmospheric chemistry has been obtained by taking e.g. Fp = 0.6. The curve (b) in Fig. 2.9 shows the schematic pressure dependence... 

In large part, the chemistry we meet in practice takes place in a solution of some kind, but a quantitative description of the chemical kinetics involved is much more complex than for gaseous reactions. The key difference lies in the interparticle distances. In a gas at atmospheric pressure, the particles occupy less then 1 % of the total volume and, effectively, move independently of each other. In a solution the solute and solvent molecules, with the latter being in the majority, take up more than 50% of the available space, the distances between the various species are relatively small, and each particle is in continuous contact with its neighbours. It is these interactions which greatly complicate the formulation of a satisfactory theory of chemical kinetics in solution. Indeed, the rate of an elementary reaction and for that matter a composite reaction, can be significantly influenced by the choice of solvent. 

At the beginning of LACTOZ there was a well developed theory of the fast photochemistry governing tropospheric free radical concentrations. However, the theory had not been validated by field measurements. Successful development of instruments for field measurements of OH and RO2 radicals have led to the anticipated capability to observe atmospheric free radical chemistry and validate the models describing their production and loss, and the related production rates of ozone. A number of field campaigns for the study of radical chemistry have been conducted in Europe. Work in LACTOZ has made a substantial contribution to the data base for gas-phase reactions which control OH, HO2 and related radical concentrations in the daytime troposphere this information is needed to interpret the results of these experiments. 

In this chapter, we describe the current status of theoretical kinetics for chemical reactions at low temperature, i.e., from 1 to 200K. The desire to understand the chemistry of interstellar space and of low temperature planetary atmospheres provides the general motivation for studying chemical kinetics at such temperatures. For example, the chemistry of Titan s atmosphere is currently a topic of considerable interest. This motivation led to the development of novel experimental techniques, such as the CRESU (cinetique de reaction en ecoulement supersonique uniforme) method, which allows for the measurement of rate coefficients at temperatures as low as lOK (see Chapter 2 by Canosa et ai). Such measurements provide important tests for theory and have sparked a renewed interest in theoretical analyses for this temperature range. ... 

Reactions between neutrals include atom/radical + radical and atom/radical + molecule reactions. As discussed above, the intermolecular forces are shorter range than is the case with ion-molecule reactions, so that it is necessary to consider chemical interactions explicitly when modelling a reaction. After a section on experimental methods, the ideas behind transition state (TS) theory and its variational modification are discussed, together with theories of reactions where the TS switches, as the temperature increases, from A-B distances mainly controlled by the potential arising from electrostatic interaction to shorter distances where chemical forces are important. While the pressure in the ISM is too low for pressure dependent reactions, this topic is important in the conditions used to measure rate coefficients and in the chemistry of planetary atmospheres, including those of the exoplanets (see Chap. 5). This topic is discussed in Sect. 3.4.4, which also introduces the ideas that lie behind master equation models, which are widely used for such reactions. These models can also be used for reactions in which the adduct AB from an A + B reaction dissociates into several products, and these ideas are discussed in Sect. 3.4.5. Section 3.4 concludes with discussion of two examples of neutral + neutral reactions. 

Theory and calculations on the chemical reactions of polyatomic molecules are very active areas of research, " There are several reasons for this. The most modem experimental techniques using lasers and molecular beams are being applied to study the microscopic details of such chemical reactions including how different vibrational modes of polyatomic molecules influence reactivity," and measurements of the lifetimes of reaction complexes. State-selected experiments of this type require detailed quantum reactive scattering theory in their interpretation. Furthermore, there is a need for the accurate calculation of kinetic data such as rate constants of polyatomic reactions that are sometimes difficult to study in the laboratory but are important in areas such as atmospheric, combustion, and interstellar chemistry. 

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