On the strength of the halogen bonds: Mutual penetration, atomic quadrupole moment and Laplacian distribution of the charge density analyses

Analyses of the atomic quadrupole moment and Laplacian distribution of the charge density were employed in the description of the strength of halogen bonds between ammonia as Lewis base and Lewis acids, DX (with X = F, Cl, Br, I and D = H, CN, F as halogen donor group). The geometries of all the complexes were fully optimized using the Møller–Plesset second-order perturbation theory with the 6-311++G(2d, 2p) basis set. For iodine atoms, an effective core potential (ECP) was used. Mutual penetration of electron charge densities, charge transference between Lewis base and Lewis acid, atomic dipole moment on nitrogen atom and change of atomic quadrupole moment on the halogen atom, were considered in this study. A significant charge density transference from the Lewis base to the Lewis acid is observed and the relation between the strength of the halogen bond and the amount of charge density transferred is explored. In addition, the analysis of the dipole moment at nitrogen atom in these complexes reflects the electronic rearrangement produced by the formation of the halogen bond and its strength. Moreover, we studied where the transferred electron charge is. We found that the Laplacian distribution and the atomic quadrupole moment on the halogen atom show that the electron density is not increased in the direction of the X⋯N bond, but it increases in the orthogonal direction to the bond forming (X⋯N). Finally, this work shows the important answers that the AIM analysis provides on the characteristics of the halogen bonds.