Density Polarization and Chemical Reactivity

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Abstract
A qualitative analysis of the main polarization effects in molecular systems is presented. Both the single and multiple site reactivity cases are discussed and illustrative applications are given for selected molecular systems: acrylonitrile, propene, formamide, and the trans-nitrous acid. The polarization effects are expected to determine the directional specificity of an early approach (before the substantial charge transfer and the intramolecular rearrangements) of both reactants in a chemical reaction. The preferred direction of such an approach is therefore postulated as that generating the minimum energy change of reactants in the Field of their respective reaction partners, including the stabilization due to the relaxation of their densities in a changed environment. The example of the electrophilic substitution in the five membered heterocycles is examined. It demonstrates the importance of the density polarization effects for explaining the observed α-preference in these reactions for furan, pyrrole and N-methyl pyrrole. Next we discuss the relation between the density relaxation and the internal stability of molecular systems. Finally, we briefly comment on the Atoms-in-a-Molecule description.

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