A new parameter for classification of polycyclic aromatic hydrocarbon crystalline motifs: a Hirshfeld surface investigation

Abstract

The classical crystalline motif categorizations of polycyclic aromatic hydrocarbons (PAHs) have gained interest as of late as it was discovered that the “defining” crystallographic axis was not always the “shortest crystallographic axis” as previously defined and that systems under pressure would exhibit a motif's typical axis length but not its typical interplanar angle (θ). Here we use Hirshfeld surfaces to investigate the relative percent of intermolecular close contact interactions existing within the four established crystalline PAH motifs under ambient and high pressures. It was discovered that the fraction of C⋯C interactions (C⋯C%) coupled with θ could ultimately define the structural motifs. A new parameter, the pi-degree parameter (π°), which relates the amount of π⋯π interaction to the interplanar angle, was developed as a function of θ and C⋯C% in order to unambiguously categorize PAH motifs. A new motif, beta-herringbone (β-HB), defining a subset of PAH structures with σ-bound aromatic groups is defined. The new motif parameter was then used to investigate several high pressure and low temperature PAH structures and provided unambiguous motif categorization regardless of the “shortest crystallographic axis”, temperature, or pressure.