Reverse activation barrier

Figure 4.4 An energy profile for the unimolecular decomposition of AB+, showing a reverse activation barrier (free) and a kinetic shift (fkjn)- Adapted from [65],...

This criterion was originally established for the fragmentation of alkanes by Stevenson [18] and was later demonstrated to be generally valid. [19,20] The rule can be rationalized on the basis of some ion thermochemical considerations (Fig. 6.4). Assuming no reverse activation barrier, the difference in thermodynamic stability as expressed in terms of the difference of heats of formation of the respective products determines the preferred dissociation pathway ... 

This equation is verified by the assumption that both the differences in the entropy changes for the competitive dissociation channels and the reverse activation barriers for the dissociation of the proton-bonded dimer are negligible or similar. In these conditions, gi = Qix and E — E2 = PA2 - PAi = APA. The entropy effects are generally comparable when the competing dissociations involve species that are similar in structure. 

Now the transition state, termed loose , is ill-defined and product-like. For ion-molecule reactions, reverse activation barriers are often absent because of the strong long-range ion-induced dipole or ion-dipole potential [1]. Further, for many systems of interest, the cleavage of the AB+ bond is heterolytic (i.e., the pair of electrons forming the bond is removed by one of the fragments). Quan-... 

It should be apparent that phase space fitting of kinetic energy release distributions yields important thermochemical information for exothermic reactions with no reverse activation barrier. As another example, Co+ ions decarbonylate acetone (reaction 10) yielding a dimethyl cobalt ion as... 

Figure 11. Kinetic energy release distribution for metastable loss of CH4 from nascent Co(C3Hg)+ collision complexes. The "unrestricted" phase space theory curve assumes the entrance channel contains only an orbiting transition state, the exit channel has only an orbiting transition state (no reverse activation barrier), and there are no intermediate tight transition states that affect the dynamics. The "restricted" phase space theory calculation includes a tight transition state for insertion into a C-H bond located 0.08 eV below the asymptotic energy of the reactants.

We can manipulate the size of the activation barrier by varying the cell voltage, which is a distinguishing feature of the electrochemical reaction. The forward activation barrier is decreased by a F, while the reverse activation barrier is increased by (1 — a)nF,. Here r] is the voltage... 

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