Retention of -4 Symmetry in Compounds Containing MAr₄ Molecules and Ions

Abstract

A search of the Cambridge Structural Database shows that MAr4 molecules usually retain the low-energy \overline 4 conformation in the crystal. MAr^{q}_{4} ions also usually occupy sites if the counterion can conform to \overline 4 (or higher) symmetry. The ability of the aryl groups to fit well around an empty \overline 4 site is crucial to the symmetry retention. Molecules (and ions) in the tetragonal structures pack in columns that parallel the c axis; the columnar cross sections are approximately square. Aryl groups in adjacent columns can interleave to form a sort of herringbone arrangement. The tetragonal packing of columns is sufficiently favorable that it sometimes occurs even if the molecules must be disordered. The columnar motif is also found in some lower-symmetry structures of MAr4 molecules and MAr^{q}_{4} ions; these structures may be pseudosymmetric.

A database survey of molecular and crystallographic symmetry

“…The vertical axis indicates the percentage of structures of each point group that belongs to the space-group/Wyckoffposition subset. this paper provide valuable information to crystallographers who`have come to expect the loss of molecular symmetry (except inversion) upon crystallization' (Lloyd & Brock, 1997). The following is a summary of our observations of crystal/molecule symmetry relationships determined from One-R, the data set of structures built from one type of molecule.…”

Unusual hydrogen bonded (OH)4 tetrahedral nests organize zinc(ii) coordination complexes in a non covalent diamondoid network

“…18 It has also been observed 19 that highly symmetrical space groups (i.e. tetragonal, hexagonal, cubic) often occur either if the molecule itself contains high order symmetry elements, or if particular supramolecular patterns of high symmetry are favourable, like in the case of MAr 4 compounds 17 or in the case of bulky monoalcohols. 20 In the present structures the observed tetragonal non-centrosymmetric packing is achieved by two different supramolecular association patterns.…”

“…Statistically, this occurs in about 10-12% of the CSD entries. 15 The crystallization of achiral molecules in non-centrosymmetric space groups has been rationalized by seeking supramolecular interactions that can generate polar chains 15,16 or other non-centrosymmetric packing motifs, 17 or it has been related to molecular rigidity. 18 It has also been observed 19 that highly symmetrical space groups (i.e.…”

“…2, by means of a four-fold phenyl embrace. 17,21 Experimental Syntheses Commercial a-(4-pyridyl)benzhydrol (LOH) is recrystallized from boiling CH 2 Cl 2 . Reagent grade solvents were used without further purification.…”

Bernauer′s Bands

“…At first glance, spherulites of TL and HA appear comparable, but careful examination reveals significant distinctions. [141,142] crystallizes in the space group P " 42 1 c, with lattice constants a = 12.1110(9) , c = 6.5430(4) , Z = 4, at 295 K. [4,143] The refractive indices are n o = 1.789 and n e = 1.730. [4] Banded spherulites of TL were described in GK but not before or after.…”