On the nature of the stabilization of benzene···dihalogen and benzene···dinitrogen complexes: CCSD(T)/CBS and DFT-SAPT calculations.

The structure and stabilization energies of benzene (and methylated benzenes)···X(2) (X=F, Cl, Br, N) complexes were investigated by performing CCSD(T)/complete basis set limit and density functional theory/symmetry-adapted perturbation theory (DFT-SAPT) calculations. The global minimum of the benzene···dihalogen complexes corresponds to the T-shaped structure, whereas that of benzene···dinitrogen corresponds to the sandwich one. The different binding motifs of these complexes arise from the different quadrupole moments of dihalogens and dinitrogen. The different sign of the quadrupole moments of these diatomics is explained based on the electrostatic potential (ESP). Whereas all dihalogens, including difluorine, possess a positive σ hole, such a positive area of the ESP is completely missing in the case of dinitrogen. Moreover, benzene···X(2) (X=Br, Cl) complexes are stronger than benzene···X(2) (X=F, N) complexes. When analyzing DFT-SAPT electrostatic, dispersion, induction, and δ(Hartree-Fock) energies, we recapitulate that the former complexes are stabilized mainly by dispersion energy, followed by electrostatic energy, whereas the latter complexes are stabilized mostly by the dispersion interaction. The charge-transfer energy of benzene···dibromine complexes, and surprisingly, also of methylated benzenes···dibromine complexes is only moderate, and thus, not responsible for their stabilization. Benzene···dichlorine and benzene···dibromine complexes can thus be characterized merely as complexes with a halogen bond rather than as charge-transfer complexes.