Hydrogen Bond versus Anti-Hydrogen Bond:  A Comparative Analysis Based on the Electron Density Topology

The theory of atoms in molecules is used to examine the nature of anti-hydrogen bond (anti-H bond) interaction. Contrary to what is found in normal hydrogen bond (H bond) complexes, which are characterized by lengthening of the X−H bond and a red shift of its stretching frequency, the anti-H bond leads to a shortening of the X−H bond length and a blue shift of its vibrational frequency. The topological properties of the electron density have been determined for a series of C−H···π complexes, which exhibit either anti-H bond or normal H bond character, as well as for the complexes C₆H₅F···HCCl₃ and C₆H₆···HF, which are representative cases of anti- and normal H bonds. Inspection of the set of topological criteria utilized to characterize conventional H bonds shows no relevant difference in the two classes of H···π complexes. Analysis of the results suggests that the specific features of the anti-H bond originates from the redistribution of electron density in the C−H bond induced upon complexation, which in turn evidences the different response − dispersion versus electrostatic− of the interacting monomer for stabilizing the complex.