Structural similarities in tetraaryltins described by virtual non-crystallographic rotations or translations: Kitaigorodskii's morphotropism is revisited

Recently Kálmán [(2005), Acta Cryst. B61, 536–547] revealed that semirigid molecules or their patterns held together e.g. by hydrogen bonds may perform non-crystallographic rotations (through 180, 90° etc.) around themselves whenever a substitution, ring enlargement or isomerization destroys the existing close packing, i.e. the novel substituent or the enlarged ring can no longer fit in the hollows formed between the molecules. In other words, the old and new arrangements of such chemically similar molecules can be converted into each other by virtual rotations. However, when a semirigid molecule without substitution, but under the influence of solvents, temperature etc., is fully or partly rearranged in the solid state, the corresponding non-crystallographic rotation (hereinafter ncr) is real and gives rise to polymorphism. Such polymorphs are hallmarked by full or partial isostructurality and show that ncrs always occur together with isostructurality. First Kitaigorodskii [(1961), Organic Chemical Crystallography, New York: Consultants Bureau] reported on the structural similarity of three tetraaryltins, (p-RC₆H₄)₄Sn, R = H, CH₃, CH₃O, which is terminated by the larger C₂H₅O group. A revisit to these structures revealed that the tetragonal → monoclinic conversion termed by Kitaigorodskii as a `morphotropic step' is also performed by an ncr. Similarly, other tetraaryltins in the literature are related by ncrs or the nc translation of the semirigid tetrahedra, or they remain isostructural. Since one of the definitions of morphotropism, a word of Greek origin, is `turn of form', the ncrs of semirigid molecules can be denoted – following Kitaigorodskii – by this word, whereas its alternative definition in the morphological crystallography of `unidirectional changes' [applied by Groth (1870). Ber. Chem. Ges. 3, 449–457] covers the non-crystallographic translations described first in this work.