Competition between Halogen Atom and Ring of Halobenzenes as Hydrogen Bond Electron Donor Sites.

A halobenzene molecule contains several sites that are capable of acting in an electron-donating capacity within a H-bond.  One set of such sites comprise the lone electron pairs of the halogen (X) atoms on the periphery of the ring.  The π-electron system above the ring plane can also fulfill this function in many cases.  DFT calculations are applied to compare and contrast the propensity of these two site types to engage in such a H-bond within the context of mono, di, tri, tetra, and hexasubstituted halobenzenes.  The X atoms chosen for study comprise the full set: F, Cl, Br, and I.  It is found that even when the electrostatic potential of the X lone pair is more negative than that above the ring, it is the latter position which is the preferred binding site of HCl in most cases.  This preference switches over to the X lone pair only for higher order of substitution, with n=4 or 6.  This pattern is explained in large measure by the higher contribution of dispersion when the proton donor is located above the ring.