On the unusual properties of halogen bonds: a detailed ab initio study of X2-(H2O)(1-5) clusters (X = Cl and Br).

Halogen bonds have received a great deal of attention in recent years. Their properties, sometimes paralleled with those of hydrogen bonds, have not yet been fully understood. In this work, we investigate the nature of the intermolecular interactions between Cl(2) and Br(2) with water. Our analysis of several features of MP2/aug-cc-pVDZ-optimized stable clusters with different number and arrangement of water molecules shows that two different kinds of halogen-water coordination patterns are involved in the stability and properties found for these systems: halogen bonds (X-X...O) and halogen-hydrogen interactions, (X-X...H-O-H). Both types of interactions result in a large polarization of the halogen molecule, which leads to important cooperative effects on these structures. Although the general structural aspects of these clusters can be understood in terms of dipole-quadrupole forces at long range, where it is the dominant term, the SAPT analysis shows that factors such as polarization of pi densities and dispersion become increasingly important close to equilibrium. In particular, we show that the halogen-hydrogen interactions are weaker than halogen-oxygen interactions mainly due to the electrostatic and dispersion forces. We also calculate vibrational and electronic shifts that should be helpful for the interpretation of experimental results and for investigating the microsolvation phenomena for halogens in an aqueous environment.