Applicability of Hirshfeld atom refinement for establishing the nature of chemical bonding in quinoic compounds

Hirshfeld atom refinement (HAR) has so far been explored almost exclusively for the determination of accurate and precise H-atom positions from X-ray diffraction experiments, neglecting other features of the resulting crystal structures and molecular wavefunctions. In contrast, here we compare the applicability of the HAR and transferable aspherical atom model (TAAM) approaches for the structure refinement, as well as for the reconstruction of electron-density distribution in a series of quinoic compounds, known for their pronounced single-double bond alternation. A set of five quinone-like compounds has been crystallized and investigated using single-crystal X-ray diffraction at standard resolution and subjected to various structure refinement approaches, namely, 2,3,5,6-tetrachlorocyclohexa-2,5-diene-1,4-dione (Cl4Q), C₆Cl₄O₂, 2,3,5,6-tetrafluorocyclohexa-2,5-diene-1,4-dione (F4Q), C₆F₄O₂, 2-[4-(dicyanomethylidene)cyclohexa-2,5-dien-1-ylidene]propanedinitrile (TCNQ), C₁₂H₄N₄, 2-[4-(dicyanomethylidene)-2,5-difluorocyclohexa-2,5-dien-1-ylidene]propanedinitrile (F2TCNQ), C₁₂H₂F₂N₄, and 2-[4-(dicyanomethylidene)-2,3,5,6-tetrafluorocyclohexa-2,5-dien-1-ylidene]propanedinitrile (F4TCNQ), C₁₂F₄N₄. The HAR results quantitatively reproduce the alternating electron-density effects in the studied compounds, while the electron densities from the TAAM approach, utilizing pseudoatom parameters averaged over a large number of chemical compounds, do not perform as well in capturing bond alternation. As a consequence of the differences in the electron-density models, the two refinement approaches also yield distinct atomic displacement parameters (ADPs).