Hydrogen Bond versus Halogen Bond in Cation–Cation Complexes: Effect of the Solvent

Abstract

Competition between hydrogen- (HB) and halogen-bonded (XB) 4-ammoniumpyridine and halogenammonium (NHFX; n=0–3; X=F, Cl, Br, and I) cation–cation complexes are explored by means of DFT calculations. HB and XB minima structures are found for all systems in the gas phase. As the number of fluorine atoms increases, the HB complexes are more favored than those of XB. Proton transfer is generally observed in complexes with two, three, or four halogen atoms. The XB complexes evolve from traditional halogen bonds, to halogen-shared complexes, and to ionic complexes as the number of fluorine atoms increases. The dissociation transition states and their corresponding barriers are also characterized; the barriers increase as the number of fluorine atoms increases. The results if solvent effects are considered indicate that, even in an apolar solvent, such as n-hexane, most of the complexes have favorable binding energies. Atoms-in-molecules theory is used to analyze the complexes, and results in good correlations between electron density and total electron energy density (Η) values with the intermolecular bond length. According to the Η values obtained, the covalency of these interactions starts to manifest at distances around 72–74 % the sum of the van der Waals radii of the interacting atoms.