Role of Directed van der Waals Bonded Interactions in the Determination of the Structures of Molecular Arsenate Solids

Bond paths, local energy density properties, and Laplacian, L(r) = -del(2)rho(r), composite isosurfaces of the electron density distributions were calculated for the intramolecular and intermolecular bonded interactions for molecular solids of AS(2)O(3) and ASO(2) composition, an As2O5 crystal, a number of arsenate molecules, and the arsenic metalloid, arsenolamprite. The directed intermolecular van der Waals As-O, O-O, and As-As bonded interactions are believed to serve as mainstays between the individual molecules in each of the molecular solids. As-O bond paths between the bonded atoms connect Lewis base charge concentrations and Lewis acid charge depletion domains, whereas the O-O and As-As paths connect Lewis base pair and Lewis acid pair domains, respectively, giving rise to sets of intermolecular directed bond paths. The alignment of the directed bond paths results in the periodic structures adopted by the arsenates. The arrangements of the As atoms in the claudetite polymorphs of As2O3 and the As atoms in arsenolamprite are similar. Like the As2O3 polymorphs, arsenolamprite is a molecular solid connected by relatively weak As-As intermolecular directed van der Waals bond paths between the layers of stronger As-As intramolecular bonded interactions. The bond critical point and local energy density properties of the intermolecular As-As bonded interactions in arsenolamprite are comparable with the As-As interactions in claudetite I. As such, the structure of claudetite I can be viewed as a stuffed derivative of the arsenolamprite structure with O atoms between pairs of As atoms comprising the layers of the structure. The cubic structure adopted by the arsenolite polymorph can be understood in terms of sets of directed acid-base As-O and base-base O-O pair domains and bond paths that radiate from the tetrahedral faces of its constituent molecules, serving as face-to-face key-lock mainstays in forming a periodic tetrahedral array of molecules rather than one based on some variant of close packing. The relatively dense structure and the corrugation of the layers in claudetite I can also be understood in terms of directed van der Waals As-O bonded interactions. Our study not only provides a new basis for understanding the crystal chemistry and the structures of the arsenates, but it also calls for a reappraisal of the concept of van der Waals bonded interactions, how the structures of molecular solids are viewed, and how the molecules in these solids are bonded in a periodic structure.