Characteristics and nature of halogen bonds in linear clusters of NCX (X = Cl, and Br): an ab initio, NBO and QTAIM study

A theoretical study was performed to examine intermolecular halogen bond properties in one-dimensional NCX chains (X = Cl and Br). Geometries, binding energies, and n(N) -> sigma*(C-X) charge-transfer of linear (NCCl)(2-10) and (NCBr)(2-10) chains were investigated by means of second-order Moller-Plesset perturbation theory (MP2) and DFT methods. All MP2, B3LYP and M06/aug-cc-pVTZ results indicate that the magnitudes of the interaction energies increase with increasing halogen size. Considering MP2/aug-cc-pVTZ results, it can be seen that the (NCBr)(10) cluster is bound -12.72 kcal/ mol more strongly than (NCCl)(10). The n-dependent trend of nN -> sigma*(C-X) charge-transfer is reasonably correlated with cooperative effects in halogen bond energies. According to the quantum theory of atoms in molecules, the capability of the (NCX)(2-10) clusters for electron localization, at the C-X . . . N bond critical point (BCPs), depends on the cluster size and thereby leads to cooperative changes in the C-X . . . N strength, and n(N) -> sigma*(C-X) charge-transfer. It is also revealed that for all intermolecular C-X . . . N interactions, total electronic energy densities are all greater than zero, suggesting that the interactions are significantly electrostatic in nature.d