Structure of Organic Crystals with Molecules Lying on Twofold Crystallographic Axes. Structural Class P21212, Z = 2(2)

Molecular arrangement in crystals of a rare structural class P2₁2₁2, Z = 2(2), with molecules occupying a system of special positions on 2 axes is considered. The Cambridge Structural Database (CSD) contains 58 organic crystal substances belonging to this class; 54 of these have clear-cut chains P_c2, Z = 1(2) (shashlyks) and form two subclasses. In crystals of the first subclass (23 representatives), there are layers possessing one of two remarkable features: molecules in a layer have an extraordinarily high molecular coordination number (m.c.n. is 8 in a layer), or molecules in a layer are linked by halogen...halogen specific short contacts (m.c.n. in a layer equals 4). In crystals of the second subclass (31 representative), layers are absent, and the structure is formed directly from chains.