In general, the relatively modest expansion experienced by most materials on heating is caused by increasing anharmonic vibrational amplitudes of the constituent atoms, ions or molecules. This phenomenon is called positive thermal expansion (PTE) and usually occurs along all three crystallographic axes. In very rare cases, structural peculiarities may give rise either to anomalously large PTE, or to negative thermal expansion (NTE, when lattice dimensions shrink with heating). As NTE and unusually large PTE are extremely uncommon for molecular solids, mechanisms that might give rise to such phenomena are poorly understood. Here we show that the packing arrangement of a simple dumbbell-shaped organic molecule, coupled with its intermolecular interactions, facilitates a cooperative mechanical response of the three-dimensional framework to changes in temperature. A series of detailed structural determinations at 15-K intervals has allowed us to visualize the process at the molecular level. The underlying mechanism is reminiscent of a three-dimensional (3D) folding trellis and results in exceptionally large and reversible uniaxial PTE and biaxial NTE of the crystal. Understanding such mechanisms is highly desirable for the future design of sensitive thermomechanical actuators.

A kinetically favoured crystal, with many molecules in the asymmetric unit, may be a fossil relic of the crystal nucleus of a more stable polymorph.

The catemer is an infinite one-dimensional pattern formed by the carboxylic acid group in crystals, and is constituted with O-H...O hydrogen bonds. The catemer is uncommon and may be contrasted with the ubiquitous carboxylic acid dimer, the favored mode of association of this functional group. Both catemers and dimers, however, have two O-H...O hydrogen bonds for each carboxy group, so the reasons for the rarity of the catemer must lie elsewhere. In this paper, we describe a group of around 25 phenylpropiolic acids in which the catemer is the default packing mode. Exceptionally, the particular catemer that is found in this family is of the very rare syn,anti variety. We show that a necessary ingredient in catemer formation is a supportive C-H...O hydrogen bond from a proximal C-H group, which is located on the phenyl ring, ortho to the ethynyl group, and suitably activated by electron withdrawing substituents. When steric factors become noteworthy, alternative patterns are adopted, such as the syn,syn catemer and, in one case, a rare cisoid dimer. When electron-donating groups, either through inductive effect such as methyl or through resonance such as halogens, are present on the phenyl ring, the dimer is formed in all but one case. Polymorphism seems not to be an issue in these carboxylic acids in that no compound would generally crystallize as both a dimer and a catemer. It may be concluded that a supporting interaction, in this case a C-H...O hydrogen bond, is the essential condition for the formation of any carboxylic acid catemer. Catemers are so rare because it is difficult to set up this type of supporting interaction in most carboxylic acids.

A variable temperature single crystal X-ray diffraction study revealed an unusual thermal expansion property of an organic salt, imidazolium 4-hydroxybenzene carboxylate, which exhibits colossal negative and positive axial thermal expansion along the crystallographic b axis and approximately along the a axis, respectively. The hydrogen bonded, two-dimensional square grid type of the flexible network in the crystal structure of the salt resembles a fencing structure that undergoes scissor-like motion resulting the abnormal thermal behavior. Thermal expansion induced by a scissor motion of the hydrogen bonded network in a multicomponent crystalline organic compound has not been reported before, although this mechanism is mentioned to elucidate colossal thermal expansion in some inorganic framework materials.

No abstract

A series of 4-substituted cubanecarboxylic acids and phenylpropiolic acids have been studied with the aim of elucidating steric and electronic factors exerted by the 4-substituent in the formation of the dimer, or alternatively, the rare syn-anti catemer patterns in their respective crystal structures. It is shown that catemer formation depends critically on the ability of a proximal C-H group to form a supportive C-H‚‚‚O bond. In turn, this means that the C-H group must be sufficiently activated toward hydrogen bond formation. Such activation is inherent to the cubyl group but must be present additionally from a suitable electron withdrawing group in the phenylpropiolic acids. In any event, while C-H activation is necessary for catemer formation it is not sufficient. The substituent group that is present in the 4-position must also be sufficiently bulky so as to form a close packed array that is compatible with the catemer geometry. These trends are justified by the crystal structures of the 12 acids in the two families wherein the 4-substituent group is H, F, Cl, Br, I, and CH 3 . Our results indicate that electronic and steric effects of functional groups may be distinguished in the solid state, in that the formation of either a dimer or catemer may be rationalized on the basis of these effects.

Two polymorphs of a substituted dihydroanthracene derivative were prepared by crystallization from different solvents. Each polymorph is reversibly interconvertible to the other by means of a single-crystal to single-crystal phase transformation.

A hexa-host compound yields at least four polymorphic forms from its melt phase. All four phases have been characterized using single-crystal X-ray diffraction; two of the phases were obtained by means of thermal stress, and the different forms exhibit conformational polymorphism.