Twinning in chemical crystallography – a practical guide

Abstract

Abstract A non-negliglible percentage of crystal structures in chemical crystallography are derived from twinned crystals. The treatment of these data is dependent on the type of twinning. Twins by strict, pseudo or reticular merohedry may hamper space group determination and structure solution, while non-merohedral twins mainly hinder the data integration process. One structure determination of each type is described in detail. CsCp*Py (1) crystallizes in space group P65, mimicking 6/mmm symmetry by merohedral twinning. The data of IPr · AlI3 (2) (IPr=1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene) simulate mmm symmetry although the true space group is P21/n. The crystals of K[Au(CN)2] (3) are obverse/reverse twins. [Cu3(dppaO2)2((OSiMe2)2O)2(thf)2] (4) (ddpaO2=bis(diphenylphosphoryl)amine) crystallizes in space group P21/n and is an example of a non-merohedral twin.