An Atoms in Molecules Study of the Halogen Resonance Effect

We report a detailed study by means of the theory of atoms in molecules (AIM) of the resonance effect exhibited in systems where a halogen is adjacent to a carbon-carbon double bond. Moreover, we have carried out a comparable study of the respective saturated halohydrocarbons and hydrocarbons, as well as the related unsaturated hydrocarbons. The valence shell charge concentration (VSCC) of the atoms in systems that exhibit the halogen resonance effect is considerably different from that of the systems where only the electron withdrawing inductive effect is present. Our analysis of the bonded maximum charge concentration and the electronic properties at the bond critical points clearly indicate that the carbon-carbon double bond is strongly distorted as a result of the halogen resonance effect. Population analyses show that the halogen resonance effect is a donor effect, but the opposing electron-withdrawing inductive effect is stronger. Moreover, the analysis in terms of link points of the VSCCs of the carbons accounts for the observed position-dependence of electrophilic aromatic substitution in α- and β-halonaphthalenes.