SURFACE ELECTROSTATIC POTENTIALS OF HALOGENATED METHANES AS INDICATORS OF DIRECTIONAL INTERMOLECULAR INTERACTIONS

Using an ab initio self-consistent-field molecular orbital approach, we have computed 6-31G*//STO-3G* electrostatic potentials for CH3F, CF4, CH3Cl, CCl4, CH3Br, and CBr4. It is demonstrated that the potentials along the carbon-halogen bonds of these systems can explain the observed directional preferences of the halogens' intermolecular interactions. Our surface potentials for the chlorinated and brominated molecules favor the observed ''side-on'' and ''head-on'' interactions with electrophiles and nucleophiles, respectively, of C-Cl and C-Br in crystals, whereas the potentials of CH3F and CF4 are indicative of fluorine interacting only with electrophiles, as is found experimentally. The strongly positive potentials at the ends of the chlorines and bromines in CCl4 and CBr4 are consistent with complexes that these form with electron donors, e.g., the pi regions of benzene and p-xylene and the lone pairs of pyridine and tetrahydrofuran.