Long-range behavior of noncovalent bonds. Neutral and charged H-bonds, pnicogen, chalcogen, and halogen bonds

Abstract

Ab initio calculations show the drop in interaction energy with bond stretch ΔR can be fit to a common power n, in the functional form ΔR−n. This exponent is smaller for charged H-bonds, as compared to neutral systems, where n varies in the order pnicogen<chalcogen<halogen bond. The decay is slowest for the electrostatic term, followed by induction and then by dispersion. The halogen bond has the greatest sensitivity to bond stretching in terms of all three components. The values of the exponent n are smaller for electrostatic energy than would be expected if it arose purely as a result of classical multipole interactions, such as dipole–dipole for the neutral systems. The exponents are larger when the fitting is done with respect to intermolecular distance R, rather than to its stretch relative to equilibrium length, although still not precisely matching what might be expected on classical grounds.