Intralayer Molecular Packing Coefficient as One Packing Characteristic of Planar Layer-Stacked Crystals and Its Dominators

Planar layer-stacking, as a special stacking mode, is attracting much attention in the fields of some molecular solid materials. In the present work, we propose a special index, the intralayer molecular packing coefficient (ILMPC), as the ratio of the sum of sectional areas of all molecules on a layer to that of the layer, to deepen the understanding of the planar layer-stacking. Besides, methods for calculating the sectional area (Sm,s) and perimeter (Cm,s) of a planar molecule along its plane are proposed to describe molecular size to account for its compact on describing ILMPC by intralayer intermolecular interactions (ΔEintralayer). Regarding 65 planar layer-stacking, it shows a certain correlation of ILMPC with ΔEintralayer/Cm,s (R2 = 0.30) or ΔEintralayer/Sm,s (R2 = 0.23) and a significantly linear one of packing coefficient with ILMPC/dinterlayer (R2 = 0.86) in which dinterlayer represents the interlayer distance. Hopefully, the ILMPC will be an effective geometric descriptor of intralayer molecular compactness and contribute to a deeper understanding of planar layer-stacking.