Hydrogen-hydrogen bonding: the current density perspective

Current density plots of closed-shell intermolecular H-H interactions characterized by a bond critical point (BCP) show two vortices separated by a saddle, a pattern which allows for a clear definition of a pair current strength. This H-H current strength turns out to be roughly related to the potential energy density at the BCP and then to the dissociation energy. The same pattern is also recognizable, at least for an azimuthal orientation of a field perpendicular to the H-H line, for the intramolecular interactions previously investigated to propose the H-H bonding. In the case of the H atoms of the bay region of polycyclic aromatic hydrocarbons, the current of the H-H delocalized diatropic vortex gives a quantitative indication of stabilization; however, on rotation of the field and the subsequent onset of a bay-delocalized paratropic vortex (a typical signature of antiaromaticity), the diatropic vortex can be reshaped or it can even disappear, consistently with its smallness, and thus showing the effect of other more relevant interactions.