Comparison of halogen bonds in M-X•••N contacts (M=C, Si, Ge and X=Cl, Br)

The halogen bonds (XB) formed between some Si-X- and Ge-X- (X is Cl and Br) containing molecules and NCH (as a Lewis base) have been investigated and compared with C-X center dot center dot center dot N halogen bond. Although, in all cases, the existence of a positive electrostatic potential (sigma-hole) along the extension of M-X was responsible for halogen bond formation, multipole expansion of electrostatic potential reveals that these positive potentials originate from different atomic multipole moments. Indeed, in addition to the monopole moment of M atoms, the quadrupole moment of X in C-X molecules, the dipole moment of the halogen in Si-X molecules and both the dipole and quadrupole moments of X in Ge-X molecules are mainly responsible for the existence of the sigma-hole along the extension of the M-X bond. From a different point of view, the distribution of the Laplacian of electron density shows that all the studied M-X center dot center dot center dot N halogen bonds can be regarded as lump-hole interactions; a region of charge depletion (hole) in the valence shell charge concentration (VSCC) of the halogen atom interacts with a region of charge concentration (lump) in the VSCC of nitrogen and forms a halogen bond. On the other hand, interacting quantum atoms (IQA) analysis of atomic energies indicates that, in contrast to C-X center dot center dot center dot N contacts, in which the interaction between halogen and nitrogen is attractive, there is a net repulsive interaction between X and N in Si-X center dot center dot center dot N and Ge-X center dot center dot center dot N complexes. Indeed, the attraction between Si/Ge and nitrogen is mainly responsible for the formation of these halogen bonds.