Dissociation activation barriers

Schroeder J and Troe J 1993 Soivent effects in the dynamics of dissociation, recombination and isomerization reactions Activated Barrier Crossing ed G R Fieming and P Hanggi (Singapore Worid Scientific) p 206... 

Although the thermodynamic aspects of acylotropy are well documented, there have been few kinetic studies of the process. The activation barrier is much higher than for prototropy and only Castells et al. (72CC709) have succeeded in observing a coalescence phenomenon in H NMR spectra. At 215 °C in 1-chloronaphthalene the methyl groups of N-phenyl-3,5-dimethylpyrazole-l-carboxamide coalesce. The mechanism of dissociation-combination explains the reversible evolution of the spectra (Scheme 9). 

Homolytic bond dissociation, the breaking of a covalent bond with two radicals resulting, generally occurs without any extra activation barrier (see also Chapter 3, Problem 1). Thus, the rate of bond dissociation (kinetics) is directly related to the stabilities of the resulting radicals (thermodynamics). 

Finally, we should also exploit one more key experimental fact—the I activation barrier for the dissociation of the R-O bond in the protonated R-OH+R molecule is available from kinetic studies of the so-called 1 specific acid catalysis reaction. 

Whereas the adsorption energies of the adsorbed molecules and fragment atoms only slightly change, the activation barriers at step sites are substantially reduced compared to those at the terrace. Different from activation of a-type bonds, activation of tt bonds at different sites proceeds through elementary reaction steps for which there is no relation between reaction energy and activation barrier. The activation barrier for the forward dissociation barrier as weU as for the reverse recombination barrier is reduced for step-edge sites. 

Class 111-type behavior is the consequence of this impossibihty to create step-edge-type sites on smaller particles. Larger particles wiU also support the step-edge sites. Details may vary. Surface step directions can have a different orientation and so does the coordinative unsaturation of the atoms that participate in the ensemble of atoms that form the reactive center. This wiU enhance the activation barrier compared to that on the smaller clusters. Recombination as well as dissociation reactions of tt molecular bonds will show Class 111-type behavior. 

If we move the chemisorbed molecule closer to the surface, it will feel a strong repulsion and the energy rises. However, if the molecule can respond by changing its electron structure in the interaction with the surface, it may dissociate into two chemisorbed atoms. Again the potential is much more complicated than drawn in Fig. 6.34, since it depends very much on the orientation of the molecule with respect to the atoms in the surface. For a diatomic molecule, we expect the molecule in the transition state for dissociation to bind parallel to the surface. The barriers between the physisorption, associative and dissociative chemisorption are activation barriers for the reaction from gas phase molecule to dissociated atoms and all subsequent reactions. It is important to be able to determine and predict the behavior of these barriers since they have a key impact on if and how and at what rate the reaction proceeds. 

Fig. 20. Possible dissociation channels of allyl radical and their standard heats of formation relative to allyl. The loss of H2 generally proceeds via a high activation barrier and is thus considered unlikely. (From Fischer et ai.14B)...

As depicted in Scheme 1, reductive and oxidative cleavages may follow either a concerted or a stepwise mechanism. How the dynamics of concerted electron transfer/bond breaking reactions (heretofore called dissociative electron transfers) may be modeled, and particularly what the contribution is of bond breaking to the activation barrier, is the first question we will discuss (Section 2). In this area, the most numerous studies have concerned thermal heterogeneous (electrochemical) and homogeneous reactions. 

However, recently Inderwildi et al.28 showed by density functional theory (DFT) calculations that hydrogenation of CO leading to formyl (oxomethyli-dyne) and subsequent conversion toward CH2 show lower activation barriers than CO dissociation. 

The rate enhancement for cyclohexane dehydrogenation observed for submonolayer copper deposits may result from changes in the geometric (6) and the electronic (8) properties of the copper overlayer relative to bulk copper. Alternatively, the two metals may catalyze different steps of the reaction cooperatively. For example, dissociative adsorption on bulk copper is unfavorable because of an activation barrier of approximately 5 kcal/mol (33). 

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