Cooperativity between the Halogen Bond and the Hydrogen Bond in H3N•••XY•••HF Complexes (X, Y = F, Cl, Br)

Ab initio calculations are used to provide information on H3N center dot center dot center dot XY center dot center dot center dot HF triads (X, Y=F Cl, Br) each having a halogen bond and a hydrogen bond. The investigated triads include H3N center dot center dot center dot Br-2-HF, H3N center dot center dot center dot Cl-2 center dot center dot center dot HF, H(3)A center dot center dot center dot BrCl center dot center dot center dot HF, H3N center dot center dot center dot BrF center dot center dot center dot HF, and H3N center dot center dot center dot ClF center dot center dot center dot HF. To understand the properties of the systems better, the corresponding dyads are also investigated. Molecular geometries, binding energies, and infrared spectra of monomers, dyads, and triads are studied at the MP2 level of theory with the 6-311 ++ G(d,p) basis set. Because the primary aim of this study is to examine cooperative effects, particular attention is given to parameters such as cooperative energies, many-body interaction energies, and cooperativity factors. The cooperative energy ranges from -1.14 to -4.64 kcal mol(-1), the three-body interaction energy from -2.17 to -6.71 kcal mol(-1), and the cooperativity factor from 1.27 to 4.35. These results indicate significant cooperativity between the halogen and hydrogen bonds in these complexes. This cooperativity is much greater than that between hydrogen bonds. The effect of a halogen bond on a hydrogen bond is more pronounced than that of a hydrogen bond on a halogen bond.