Radical recombination and association reactions

Rate coefficients for recombination reactions are related to those for dissociation via the equilibrium constant, which can generally be calculated from thermodynamical information with a high degree of precision, although the accuracy depends on the quality of the thermodynamic data. The rate coefficients are pressure dependent and the theoretical framework of unimolecular reactions can therefore be used to describe them. Because there is little or no activation energy for the recombination process, rates of radical association reactions can be measured over a wide range of temperatures and can be used, in combination with thermodynamic information, to calculate rate coefficients for unimolecular dissociations. The availability of data for a number of radical recombination reactions over a wide range of pressures and temperatures makes these reactions excellent test beds for theoretical models of pressure dependent reactions. 

In the following subsections we shall look in detail at reactions (31) and (32), from both experimental and theoretical viewpoints. 

If the forward and reverse reactions can be measured, then the relationship between rate coefficients and the equilibrium constant allows the thermodynamics of the reaction to be determined from kinetic measurements. This has been an extremely successful method of determining radical heats of formation and the results of a number of recent experiments are discussed in Section 2.5.5. 

We will conclude our discussion on recombination reactions with the self reaction of the HO2 radical. This reaction is of interest as it is an intermediate step in the conversion of the comparatively unreactive HO2 radical into OH radicals, via H2O2. 

Although not strictly an association or recombination reaction the HO2 self reaction is best understood via an intermediate complex mechanism and the reaction shows some of the properties of an association reaction. 

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For the methyl radical/hydrogen atom association reaction there are two degrees of freedom that become vibrations as reactants go over to the product. For the methyl recombination there are five such degrees of freedom. It is thus expected that the first factor, the decrease in the density of states as R approaches the product will be greater for methyl radical recombination, and as a consequence G E, R ) will be a weaker function of E. 

Association reactions can be further classified as simple and complex, similarly to the unimolecular decomposition reactions treated earlier. Simple association reactions involve the formation of a single bond, such as those observed in atom and radical recombinations, for example ... 

In association reactions of this type, where a new bond is formed, the intermediate has excess vibrational energy equal to the bond energy of the newly formed bond and is thus unstable with respect to dissociation back to reactants unless stabilized by collision. The situation is very similar to that prevailing in neutral systems for atom-atom or radical-radical recombinations, as such larger systems are analogous to those studied by Rabinovitch and co-workers241-243 by chemical-activation methods. Colli-sional stabilization or deactivation may result from V-T transfer if the third body, Mit is monoatomic (a rare-gas atom) or from V-V transfer if it is polyatomic. 

The interpretation of measured flame profiles by means of the continuity equations may be approached in one of two ways. The direct experimental approach involves the use of the measured profiles to calculate overall fluxes, reaction rates, and hence rate coefficients. Its successful application depends on the ability to measure the relevant profiles, including concentrations of intermediate products. This is not always possible. In addition, the overall fluxes in the early part of the reaction zone may involve large diffusion contributions, and these depend in turn on the slopes of the measured profiles. Thus accuracy may suffer. The lining up on the distance axis of profiles measured by different methods is also a problem, and, in quantitative terms, factor-of-two accuracy is probably about the best that may normally be expected from this approach at the position of maximum rate. Nevertheless, examination of the concentration dependence of reaction rates in flames may still provide useful preliminary information about the nature of the controlling elementary processes [119—121]. Some problems associated with flame profile measurements and their interpretation have been discussed by Dixon-Lewis and Isles [124]. Radical recombination rates in the immediate post-combustion zones of flames are capable of measurement with somewhat h her precision than above. 

Due to the corrosive nature of carboxylic acids, the oxidation reactor and associated peripherals must be constructed of corrosion-resistant materials, e. g., suitable stainless steels. As with all radical reactions, the surface to volume ratio of the reactor should be kept to a minimum to minimize radical recombination which always occurs on surfaces. 

Reverse processes, i.e. recombination (association) reactions of atoms and radicals with each other or with stable molecules, present a similar dependence of the reaction kinetics on pressure and temperature. The scheme of an recombination (association) reaction also comprises three elementary steps... 

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