Intermolecular Forces

Abstract

Three types of intermolecular interactions are considered: van der Waals (vdW), donoracceptor and hydrogen bonds. Are vdW forces caused by instantaneous dipoles created by electrons’ orbiting the nuclei (London)—or by accumulation of electron density between atoms (Feynman, later Bader), broadly similar to covalent bonding? Is crystal packing governed by the closest contacts—or by diffuse interactions of more distant atoms? What is the physical meaning of vdW radii? Are they additive, iso- or anisotropic, and how they relate to atom-atom potential curves? What is special about condensed helium? These and other issues are discussed invoking structures of crystals and of gas-phase dimers (vdW molecules), equations of state, thermal motion, electron density maps, and polarizabilities. There is a correspondence, both in energy and geometry, between covalent and vdW bonding, whereas donor-acceptor interactions are intermediate between them. Hydrogen bonds are reviewed briefly, clearing some common misconceptions which over-estimate their importance.