A “Blind Area” in Geometrical Parameters Taken from Crystal Diffraction Data

Symmetrically independent geometrical parameters of six molecules, viz. benzene C6H6 (D6h), pyridine C5H5N (C2v), diphenyl C6H5–C6H5 (D2), 2,2′-dipyridine C5H4N–C5H4N (C2), naphthalene C10H8 (D2h), ferrocene (C5H5)2Fe (D5), and substituted (η5-C5H5)FeCp′ molecular fragment, were calculated from crystal structures deposited in the Cambridge Structural Database (CSD). Average CSD interatomic distances from low-temperature (LT) studies are systematically shortened by 0.01–0.02 Å in comparison with gas electron diffraction results for free molecules; room temperature CSD bond lengths are further reduced relative to LT ones. Bicyclic diphenyl and 2,2′-dipyridine display a similar distribution of planar (mostly in special positions) and nonplanar molecules with the interring dihedral angle varying up to 49.6° and 46.0°, respectively. A strong positive correlation between C–C and Fe–C bond lengths was observed in (η5-C5H5)Fe fragments. Bond angle values at neighboring atoms in planar cycles are negatively correlated. In all cases, distributions of CSD data contain a “blind area” where data points are randomly scattered within 0.02–0.04 Å (bond distances) and 2–3° (bond angles) independent of R-factor when R < 0.06. This area broadens with increasing temperature of a study. These observations correspond well to the model of a static and/or dynamic disorder of molecules between discrete sets of atomic positions in a crystal.