Collecting Reactant Molecules

In the examples above, one or both of the reaction centers are already attached to the metal center. In many cases, the reactants are free before reaction occurs. If a metal ion or complex is to promote reaction between A and B, it is obvious that at least one species must coordinate to the metal for an effect. It is far from obvious whether both A and B enter the coordination sphere of the metal in a particular instance. A number of metal-oxygen complexes can oxygenate a variety of substrates (SOj, CO, NO, NO2, phosphines) in mild conditions. Probably the substrate and O2 are present in the coordination sphere of the metal during these so-called autoxidations. In the reaction of oxygen with transition metal phosphine complexes, oxidation of metal, of phosphine or of both, may result. The initial rate of reaction of O2 with Co(Et3P)2Cl2 in tertiary butylbenzene. 

Another example, where both reactants coordinate for an effective neighboring group participation, is in the Cu(II) catalysis of the reactions (Prob. 2, Chap. 1)  

The metal appears to function by complexing with E1202 or H02 (sometimes this is detected) the resulting species then interacts with another molecule of H2O2 or other H donor, such as N2H4. Copper(II) complexes with no coordinated water appear inactive, and an interesting comparison of the catalytic activity of 1 1 Cu complexes with en, dien, and trien is shown in Fig. 6.3. The type of detailed mechanism envisaged for (6.10) is 

N — N might represent bipyridine, which will retain the metal in solution, even in alkaline conditions. 

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The beams of reactant molecules A and B intersect in a small scattering volume V. The product molecule C is collected in the detector. The detector can be rotated around the scattering centre. Various devices may be inserted in the beam path, i.e. between reactants and scattering volume and between scattering volume and product species to measure velocity or other properties. The angular distribution of the scattered product can be measured by rotating the detector in the plane defined by two molecular beams. The mass spectrophotometer can also be set to measure a specific molecular mass so that the individual product molecules are detected. 

Reaction (3.1) is the initiation step, where M is a reactant molecule forming a radical R. Reaction (3.2) is a particular representation of a collection of propagation steps and chain branching to the extent that the overall chain branching ratio can be represented as a. M is another reactant molecule and a has any value greater than 1. Reaction (3.3) is a particular chain propagating step forming a product R It will be shown in later discussions of the hydrocarbon-air... 

Structure is an average structure which is taken up by molecular systems as they pass from reactant to product it cannot be studied in the same way as for a molecule because it is not a discrete species and cannot be isolated even in principle (see Chapter 1). It is impossible to measure attributes of the transition structure in the same way that can be done for regular collections of molecules. Since the transition state can be considered as if it were an equilibrium state it is possible to define its effective charges in the same way as those just considered for reactant and product molecules in equilibrium reactions. Equation (28) has rate constants for breakdown of the transition state species (represented as J ) forward kf) and return kf) which are essentially invariant because they register the collapse of the transition structure. These rate constants are independent of substituent changes and are therefore associated with zero P values. The equilibrium constants for formation of the transition state kfk and for its breakdown to products kjk f vary only according to changes in k and A , . 

Reactants can be lumped into a single or several so-called supercomponents or "lumps." A variant of this approach, potentially useful if the number of reactants of the same kind is very large, is to treat their mixture as a continuum rather than as a collection of discrete components. Alternatively, lumping can be done by reactive configurations regardless of what reactant molecules they are part of. 

The average kinetic energy of a collection of molecules is proportional to the absolute temperature. At a particular temperature, Tj, a definite fraction of the reactant molecules have sufficient kinetic energy, KE >E, to react to form product molecules on collision. At a higher temperature, T2, a greater fraction of the molecules possess the necessary activation energy, and the reaction proceeds at a faster rate. This is depicted in Figure 16-13 a. 

We start the next section with a discussion of self repair of the catalytic site after reaction to restore it to its initial state when the reaction cycle has been completed. Self repair in a catalytic system is the lowest level of self organization. It is an intrinsic property of a catalytic system and occurs locally at each catalytic site. In the two sections that follow we will introduce the general features of self organization, that result from collective cooperative effects, due to the interaction of catalytic reaction cycles of reactant molecules occuring at different catalytic centers. The example chosen is CO oxidation on a reconstructing Pt surface. It will appear that fundamental studies in computer science and the cellular automata have contributed in an essential way to understanding such phenomena. 

The minima in our 3n-dimensional contour map (or potential surface) will correspond to possible stable arrangements of the atoms, i.e., to a stable molecule or collection of molecules that can be formed from them. In particular, one such minimum (R) will correspond to the reactants in our reaction and another (P) to the products. The situation is indicated schematically in Fig. 5.1 for a simplified case with just two geometric variables. 

For reactants having complex intramolecular structure, some coordinates Qk describe the intramolecular degrees of freedom. For solutions in which the motion of the molecules is not described by small vibrations, the coordinates Qk describe the effective oscillators corresponding to collective excitations in the medium. Summation rules have been derived which enable us to relate the characteristics of the effective oscillators with the dielectric properties of the fi edium.5... 

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