| Abstract: |
GalC8 [l‐threo‐d‐galacto‐octitol, systematic name (2S,3R,4R,5R,6R,7S)‐octane‐1,2,3,4,5,6,7,8‐octol] crystallizes with a pseudo‐tetragonal lattice featuring a long (∼88 Å) c axis. The bulk of the structure possesses P43212 (Z′ = 1) pseudo‐symmetry, whereby molecules are arranged linearly with eight molecules in a translation period, thus explaining the long c axis. According to this pseudo‐symmetry, adjacent molecules are alternately related by 21 screw rotations (in ⟨100⟩ directions) and twofold rotations (in ⟨110⟩ directions). Due to an asymmetrical hydrogen‐bonding network, symmetry is broken in the vicinity of the twofold rotation axes, leading to an overall P21 (Z′ = 4) space‐group symmetry. The lost symmetry is retained as a twin operation. The structure can be classified as a 'non‐classical' OD polytype, in the sense that there are nonequivalent layer contacts, yet all members of the polytype family are locally equivalent. The main polytype is not of a maximum degree of order (MDO) and can be described as an alternation of fragments of the two MDO polytypes with P43 and P212121 symmetry, respectively. [ABSTRACT FROM AUTHOR] |