L. Leiserowitz

Symmetry lowering in crystalline solid solutions: a study of cinnamamide–thienylacrylamide by X-ray and neutron diffraction and solid-state photochemistry

Principles are outlined for symmetry lowering of a mixed crystal composed of host and tailor-made additive molecules, based on selective occlusion of the latter through a subset of surface sites of the growing crystal, the symmetry of the surface generally being lower than that of the bulk. A survey is given of the various methods and approaches used to detect the reduction in symmetry. These include changes in crystal morphology, detection of enantiomeric segregation of chiral additives in ‘centrosymmetric’ crystals, generation of second-harmonic optical signals, optical birefringence, asymmetric photoreactions in the crystalline state and X-ray and neutron diffraction. The last two methods are applied to mixed crystals of cinnamamide (host) and thienylacrylamide (additive). The diffraction analysis demonstrated that the mixed crystals are composed of six sectors of reduced symmetry, from monoclinic centrosymmetric P21/c to triclinic P1 in four sectors and possibly Pc in the remaining two. The X-ray diffraction data were not sufficiently accurate to permit assignment of the absolute structures of the P1 sectors with the use of anomalous X-ray scattering. Thus, by this method one could not ascertain the absolute orientation of the guest molecules on the surface sites through which they were selectively occluded. This ambiguity was resolved by assignment of the absolute configuration of the chiral heterophotodimers, between host and guest, in enantiomeric excess in the P1 sectors, after irradiation with UV light. These results led to the definite conclusion that the selective occlusion of thienylacrylamide arises from a replacement of attractive C–H⋯π(electron) interactions between host molecules by repulsive sulfur (lone-pair electron)⋯π(electron) interactions between guest and host at the crystal surfaces.

Asymmetric photopolymerisation in chiral crystals. An example of a chiral resolved monomer packing in two quasi-enantiomeric phases

The ability of molecules, chiral by virtue of the presence of an s-butyl group, to display conformational polymorphism has been exploited for the performance of asymmetric syntheses of either sense on the same chiral molecule; this phenomenon is illustrated for the asymmetric solid-state polymerization of the monomer (R)-(–)-(1).

Molecular Discrimination in Crystal Growth in the Presence of “Tailor-Made” Inhibitors

The relation between the morphology of a crystal and its internal symmetry at a molecular level attracted the attention of scientists a long time ago. The most remarkable experiment in this context was that of Louis Pasteur [1], who in 1848 separated for the first time the two enantiomers of sodium ammonium tartrate, by utilizing the asymmetric habit of their crystals. However, it became evident over the years that the morphological characteristics of the crystals of a given compound depend not only on its crystal structure but also on external parameters of the overall crystallizing system such as solvent, supersaturation, temperature and, specifically impurities present in the system [2].

Enantiomeric 1-phenylpropyl and 1-phenylbutyl 9-anthroates as novel chiral inclusion complexing agents. Preparation and X-ray analysis of the 6:1 inclusion complexes with n-hexane

Resolved 1-phenylpropyl and 1-phenylbutyl 9-anthroates, when crystallised from n-hexane, form 6:1 inclusion molecular complexes which are isostructural; X-ray analysis of the propyl derivative demonstrates the formation of a channel structure by the anthracenes, in which the n-hexane is included.