Collision complex reactive Chapter

A consequence of assumption I is that the collision complex does not undergo collision with a third body during its motion over the barrier. This will be analyzed in more detail in Chapter 5. This assumption implies that the characteristic time Tiy of motion over the barrier is short compared to the collision time of non-reactive collisions. The latter has to be small compared with the overall reaction time r/. 

For ion-molecule collision complexes this has been experimentally implemented by cooling the complex through a supersonic expansion (Levy, 1981, 1984). Even further, one can reactivate these stable complexes either by collisions or by IR multiphoton absorption, as discussed in Chapter 7. The non-covalent bonding in the complex is shown for example by the inequivalence of the two Cl atoms in a CICH3 Cl complex, that dissociates very preferentially to the attacking Cl isotopomer. 

We use in Chapter 2 the Kohn variational 5—matrix formalism to probe the sensitivity of H-fH2 cross sections to small changes in the PES, to help resolve a discrepancy between experiment and theory over a possible H3 collision complex. We find the reactive scattering calculations to be very robust, and thus trust their predictions. 

Elementary reactions on solid surfaces are central to heterogeneous catalysis (Chapter 8) and gas-solid reactions (Chapter 9). This class of elementary reactions is the most complex and least understood of all those considered here. The simple quantitative theories of reaction rates on surfaces, which begin with the work of Langmuir in the 1920s, use the concept of sites, which are atomic groupings on the surface involved in bonding to other atoms or molecules. These theories treat the sites as if they are stationary gas-phase species which participate in reactive collisions in a similar manner to gas-phase reactants. 

Binary and ternary spectra. We will be concerned mainly with absorption of electromagnetic radiation by binary complexes of inert atoms and/or simple molecules. For such systems, high-quality measurements of collision-induced spectra exist, which will be reviewed in Chapter 3. Furthermore, a rigorous, theoretical description of binary systems and spectra is possible which lends itself readily to numerical calculations, Chapters 5 and 6. Measurements of binary spectra may be directly compared with the fundamental theory. Interesting experimental and theoretical studies of various aspects of ternary spectra are also possible. These are aimed, for example, at a distinction of the fairly well understood pairwise-additive dipole components and the less well understood irreducible three-body induced components. Induced spectra of bigger complexes, and of reactive systems, are also of interest and will be considered to some limited extent below. 

In this Chapter, we consider the theory of collision-induced absorption by rare gas mixtures. We look at various theoretical efforts and compare theoretical predictions and computations with measured spectra and other experimental facts. The theory of induced absorption is based on quantum mechanics, but in certain cases, the use of classical physics may be justified, or indeed be the only viable choice. The emphasis will be on the computation of induced absorption by non-reactive, small atomic systems in the infrared. Diatomic and triatomic systems show most of the features of collisional absorption without requiring complex theory for their treatment. The theory of induced absorption of small clusters involving molecules will be considered in Chapter 6. 

Transition-state theory is based on the assumption of chemical equilibrium between the reactants and an activated complex, which will only be true in the limit of high pressure. At high pressure there are many collisions available to equilibrate the populations of reactants and the reactive intermediate species, namely, the activated complex. When this assumption is true, CTST uses rigorous statistical thermodynamic expressions derived in Chapter 8 to calculate the rate expression. This theory thus has the correct limiting high-pressure behavior. However, it cannot account for the complex pressure dependence of unimolecular and bimolecular (chemical activation) reactions discussed in Sections 10.4 and 10.5. 

If one considers the van der Waals complexes as a way to study binary collisions, the possibility of the formation of clusters of given size is a way to probe the role of the environment of other molecules on the reactivity. It is well known that solvent effects play an important role, not only in the kinetics but also in the results of chemical reaction. The study of molecular clusters in supersonic jet experiments allows step-by-step solvation of reactants as will be shown in this chapter, most of the reactions which have been studied occur when a finite number of molecules is reached—this number being often small (less than ten molecules). 

The purpose of this chapter is to review the kinetics and mechanisms of photochemical reactions in amorphous polymer solids. The classical view for describing the kinetics of reactions of small molecules in the gas phase or in solution, which involves thermally activated collisions between molecules of approximately equivalent size, can no longer be applied when one or more of the molecules involved is a polymer, which may be thousands of times more massive. Furthermore, the completely random motion of the spherical molecules illustrated in Fig. la, which is characteristic of chemically reactive species in both gas and liquid phase, must be replaced by more coordinated motion when a macromolecule is dissolved or swollen in solvent (Fig. b). Furthermore, a much greater reduction in independent motions must occur when one considers a solid polymer matrix illustrated in Fig. Ic. According to the classical theory of thermal reactions the collisional energy available in the encounter must be suificient to transfer at least one of the reacting species to some excited-state complex from which the reaction products are derived. The random thermal motion thus acts as an energy source to drive chemical reactions. 

In order for a reaction to occur, there must be a collision between the reactants at the reactive site of the molecule (see How Do Reactions Occur Collision Theory, earlier in this chapter). The larger and more complex the reactant molecules, the less chance there is of a collision at the reactive site. Sometimes, in very complex molecules, the reactive site is totally blocked off... 

The first of the theoretical chapters (Chapter 9) treats approaches to the calculation of thermal rate constants. The material is familiar—activated complex theory, RRKM theory of unimolecular reaction, Debye theory of diffusion-limited reaction—and emphasizes how much information can be correlated on the basis of quite limited models. In the final chapt, the dynamics of single-collision chemistry is analyzed within a highly simplified framework the model, based on classical mechanics, collinear collision geometries, and naive potential-energy surfaces, illuminates many of the features that account for chemical reactivity. 

---