Organic molecular compounds with modulated crystal structures

Abstract

Abstract. Even though the higher-dimensional superspace approach is an established method to describe aperiodic crystal structures, it is not yet fully considered and applied by a larger community of scientists. In this contribution the structural description of incommensurately and commensurately modulated molecular compounds is discussed and it is shown, that the higher-dimensional superspace approach is an elegant way for an exact structural description and an exact crystal-chemical analysis of such structures. While discussing several modulated molecular compounds, the idea behind the higher-dimensional superspace approach is shown. On the examples treated in the discussion it is explained, how to understand and how to interprete modulated molecular compounds. Also a short introduction is given to the higher-dimensional superspace approach itself, to the basic principles and to the applied nomenclature.

Confined linear molecules inside an aperiodic supramolecular crystal: The sequence of superspace phases in n-hexadecane/urea

“…Apart from quasicrystals, 13 a small number of other aperiodic crystals have also required 5D formalism. 15,57 For example, work performed by Ren and coworkers concerned aperiodic composites in an inorganic misfit layer family. 58 There, however, the five dimensional character is not induced by a solid-solid phase transition.…”

How Accurate Do X-ray Data Need To Be To Obtain a Reliable Order of Stability for Polymorphs? The Case Study of p-Hydroxyacetophenone Polymorphs

“…Therefore, one may raise a question: as energies for the orthorhombic form of hydroxyacetophenone with two molecules in asymmetric unit (asu) remains the same as for ten molecules in asu, why do we observe a modulation at low temperature? In the case of modulated systems, very often modulation can be rationalised as result of a compromise between the steric demands of the molecule and the optimal arrangement of hydrogen bonding moieties 35 . We decided to investigate dimer interaction energies, and verify, whether due to modulation some interactions for the structure with Z'=10 are becoming significantly stronger than in the case of averaged structure, with Z'=2.…”

Antiviral Agents Derived from Novel 1-adamantyl Singlet Nitrenes

“…One monoclinic crystal revealed an incommensurately modulated crystal phase [35][36][37][38]. With this intriguing structure solved, the question arises as to why this nitrene in 3 is stable at all.…”

“…Proceeding further, 3 was crystallized from aqueous ethanol, and the obtained colourless needles were subjected to X-ray crystallographic analysis. One monoclinic crystal revealed an incommensurately modulated crystal phase [35][36][37][38]. The underlying average structure was solved by computational refinement and consists of a salt adduct (1:1) of 1 and 4-fluorobenzoic acid (3 ox ; Figures 3A-3C and Additional file 1).…”