Thioarsenides: a case for long-range Lewis acid–base-directed van der Waals interactions

Abstract

Electron density distributions, bond paths, Laplacian and local-energy density properties have been calculated for a number of As4S n (n = 3, 4 and 5) thioarsenide molecular crystals. On the basis of the distributions, the intramolecular As–S and As–As interactions classify as shared bonded interactions, and the intermolecular As–S, As–As and S–S interactions classify as closed-shell van der Waals (vdW) bonded interactions. The bulk of the intermolecular As–S bond paths link regions of locally concentrated electron density (Lewis-base regions) with aligned regions of locally depleted electron density (Lewis-acid regions) on adjacent molecules. The paths are comparable with intermolecular paths reported for several other molecular crystals that link aligned Lewis base and acid regions in a key–lock fashion, interactions that classified as long-range Lewis acid–base-directed vdW interactions. As the bulk of the intermolecular As–S bond paths (~70%) link Lewis acid–base regions on adjacent molecules, it appears that molecules adopt an arrangement that maximizes the number of As–S Lewis acid–base intermolecular bonded interactions. The maximization of the number of Lewis acid–base interactions appears to be connected with the close-packed array adopted by molecules: distorted cubic close-packed arrays are adopted for alacranite, pararealgar, uzonite, realgar and β-AsS and the distorted hexagonal close-packed arrays adopted by α- and β-dimorphite. A growth mechanism is proposed for thioarsenide molecular crystals from aqueous species that maximizes the number of long-range Lewis acid–base vdW As–S bonded interactions with the resulting directed bond paths structuralizing the molecules as a molecular crystal.