High Level Theoretical Study of Benzene-Halide Adducts: The Importance of C-H-Anion Hydrogen Bonding

High level ab initio calculations were performed on the interaction of halide anions (F-, Cl-, Br-, and I-) to benzene. For these systems recent experimental and theoretical data are rather scarce, in spite of their growingly acknowledged importance for binding in complex biological systems. We have thus explored the complete basis set limit and the effect of counterpoise basis set superposition error corrections on the minimum geometries and energies of benzene-halide adducts in their possible interaction modes. The binding energy and enthalpy values (ranging from -15.3 kcal/mol for fluoride to -6.1 kcal/inol for iodide) show that the hydrogen bonding occurring in these complexes cannot be described as a weak interaction. We have furthermore investigated the topology of the minima and of other selected sections of the potential energy surface, so to gain further insight on the nature of the halide-benzene interaction. In particular, the geometry corresponding to the C-6v symmetry, although being overall repulsive, has displayed the unprecedented presence of a small flex (a minimum in C-6v symmetry) with interaction energy close to zero or slightly attractive.