Reactivity descriptors Relative Energy

Figures 19.2 through 19.5 show a relatively justified correlation in trends of the energy and different global reactivity descriptors with variation of molecular geometry. Figure 19.2 illustrates the presence of (1) at the global minimum on...

In our second example we look at the reduction of chlorinated ethenes at a nickel electrode and at the surfaces of two zero-valent metals [Fe(0), Zn(0)]. To gain insight into the rate-limiting process(es) in these cases, we consider how the relative overall reduction rates (relative to PCE) of PCE, TCE, and the three DCE isomers (see Fig. 14.15 for structures) vary as a function of two common descriptors used in QSARs, the one-electron reduction potential (EJ Fig- 14.17a) and the bond dissociation energy (DR X Fig. 14.176). In all these systems, the reduction rates were found to be significantly slower than diffusion of the compounds to the respective surfaces. Therefore, the large differences in the relative reactivities of the compounds between the systems reflect differences in the actual reaction at the metal surface. 

Reaction indices have been frequently used to analyze and predict positional selectivity of S Ar reactions. The average local ionization energy, 7(r), seems to have the best predictive power for this reaction type among the commonly used descriptors. Local minima in /(r) reflect both the positional selectivity for S Ar and the relative reactivity in aromatic and heteroaromatic systems. The 7(r) approach generally has quantitative predictive capacity for nitrations and halogenation, but it has problems for systems where steric hindrance is of key importance for the regioselectivity. 

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