THE ROLE PLAYED BY INTERMOLECULAR VAN DER WAALS LOCAL CONCENTRATIONS AND DEPLETIONS OF ELECTRON DENSITY IN THE DETERMINATION OF DIRECTIONAL ‘KEY-LOCK' INTERACTIONS AND MOLECULAR STRUCTURE

The molecules in molecular crystals are expected to fill space as efficiently as possible, adopting a cheek by jowl dense close-packed periodic structure. Arsenolite, As₂O₃, is an exception. Rather than its molecules being close-packed, they are arranged like the C atoms in diamond, occupying about one-third of the space occupied by a close-packed structure. The tetrahedral array of the molecules can be understood in terms of directed van der Waals (vdW) intermolecular As-O bond paths that connect Lewis acid domains on As and Lewis base domains on O likened to a Lewis acid-base reaction. In a study of molecules, Feynman demonstrated that vdW interactions can arise by dint of the accumulation and the deformation of the electron density (ED) distribution between the nuclei of a pair of bonded atoms. Using equations formulated to evaluate the forces for a pair of bonded atoms at a separation, R, large compared with the radii of atoms, he not only demonstrated that the ED for each of the atoms is deformed from central symmetry, but that a dipole moment is induced on each with the center of gravity of the ED distribution for the pair displaced slightly toward one another. On the basis of this result, he concluded that “It is not the interaction of these dipoles which leads to vdW’s forces, but rather the attraction of each nucleus for the distorted charge distribution of its own electrons that gives the attractive 1/R⁷ force”. Slater has since concluded that there is no fundamental difference between vdW and covalent interactions, an assertion that indicates that intermolecular vdW interactions are directional like intramolecular covalent interactions but substantially weaker. As such, a continuum of directed bonded interactions exist between covalent and vdW interactions. It will be demonstrated that molecular crystals that have defied an understanding in terms of the forces that are believed to arise by dint of non-directional electronic dipole-dipole coupling between the molecules can be understood in terms of directed Lewis acid-base intermolecular connecting bond paths. As observed by Bader (1998), the accumulation the ED between a vdW bonded pair of atoms naturally results in a bond path that runs between the pair, a line in the case of a van der Waals interaction that characterizes the directed nature of the bonded interaction.