Double-Bond Character of Conjugated Carbon-Chlorine Bonds

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Abstract
The π electron loss of a chlorine atom conjugated with a resonating system can be obtained at least approximately from experimental measurements of the asymmetry of the chlorine nuclear electric quadrupole interaction. Using molecular orbital theory the π electron loss ρ can be simply expressed in terms of the chlorine Coulomb integral, the carbon‐chlorine resonance integral, and the energies and wave functions of the system with which the chlorine atom is conjugated. Comparison of theoretical and experimental values of ρ show that the carbon‐chlorine resonance integral is about one‐third of the carbon‐carbon resonance integral. The basic conclusion is that chlorine atoms in general resonate only weakly with attached aromatic systems. Most of the effects previously ascribed to double bond character can alternatively be explained by an increased electronegativity of the carbon sp2 orbital.