Dependences of Space-Group Type Incidences of Organic and Metal-Organic Compounds on Reduced Unit-Cell Volumes

Abstract

The dependences of occurrences of the most frequent space-group types P, P2 1 , P2 1 /c, C2/c, P2 1 2 1 2 1 and Pbca on reduced (primitive) unit-cell volumes up to 8000 Å3 were investigated for organic and metal-organic compounds with different number of residues (1–3), i.e. for the compounds with different number of molecular constituents. The dependences for these space-group types are similar, single-peaked; their maxima are proportional to the number of asymmetric units (a compact symmetry-independent part of the unit cell) affected by Z′ (number of formula units per asymmetric unit) in the pertinent space-group types. The dependences of Z′ < 1, Z′ = 1, Z′ > 1 within each space group type on reduced unit-cell volumes are also similar in shape. From these dependences it can be inferred that ability for crystallization of organic and metal-organic molecules ceases for the structures with the reduced unit-cell volumes above 8000 Å 3 . This volume corresponds roughly to 450 non-hydrogen atoms.