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Dimensionality of intermolecular interactions in layered crystals by electronic-structure theory and geometric analysis.

Two-dimensional (2D) and layered structures gained a lot of attention in the recent years ("post-graphene era"). The chalcogen cyanides S(CN)(2) and Se(CN)(2) offer themselves as interesting model systems to study layered inorganic crystal structures; both are built up from cyanide molecules connected by chalcogen bonds (ChBs). Here, we investigate ChBs and their cooperativity directly within the layers of the S(CN)(2) and Se(CN)(2) crystal structures and, furthermore, in putative O(CN)(2) and Te(CN)(2) crystal structures derived therefrom. Moreover, we determine the energetic contributions of ChBs within the layers to the overall stabilization energy. To compare these structures not only energetically but also geometrically, we derive a direction-dependent root mean square of the Cartesian displacement, a possible tool for further computational investigations of layered compounds. The molecular chains connected by ChBs are highly cooperative but do not influence each other when combined to layers: the ChBs are nearly orthogonal in terms of energy when connected to the same chalcogen acceptor atom. Layers built up from ChBs account for 41% to 79% of the overall interaction energy in the crystal. This provides new, fundamental insight into the meaning of ChBs, and therefore directed intermolecular interactions, for the stability of crystal structures.