abstracts
The semi-classical density sums (SCDS-Pixel) method [1] was used to study the intermolecular interaction energies in six polymorphs of phenobarbital, a model system for the barbiturate class of compounds.[2] Barbiturates display a rigid pyrimidinetrione ring whose two N-H and three C=O functions can be employed in H-bonding. The ensuing intermolecular N-H···O=C interactions result in a small set of standard H-bonded chain, layer and framework motifs.[3] Even though the average number of N-H···O=C bonds per molecule is always two, the standard barbiturate H-bond motifs differ in the number of intermolecular two-point and one-point N-H···O=C connections per molecule, i.e. (1; 0), (½; 1), (0; 2). For each polymorph (I, II, III, V, VI, X), cumulated Pixel energies, E(n), were calculated for the first n (n = 1, 2, 3, ...) interactions associated with the highest individual contributions to the lattice energy. The obtained sets of E(n) values were compared to one another to establish the differences associated with the formation of the alternate N-H···O=C motifs. Those polymorphs whose N-H···O bonded structures are dominated by two-point connections have superior E(n) values for small clusters of molecules (low n). However, this advantage diminishes gradually if larger clusters of molecules are considered and is completely compensated at n = 8. This indicates that crystal packing on the basis of one-point connection N-H···O=C motifs is viable only because the latter enable the formation of more advantageous weaker interactions which are dominated by dispersion forces. This case illustrates that an assessment of competing H-bond motifs cannot be restricted to just those molecules that are directly involved in H-bond interactions. Rather, the complete crystal packing has to be taken into account. [Figure: Evolution of the difference between E(n) (form V; one-point connections) and E(n)' (form III; two-point connections) with n].
abstracts
The formation of multi-component crystals with water (hydrates) is a widespread phenomenon among organic molecules. Hydrate formation is of high practical relevance for industrially used materials, as it affects their physicochemical properties. [1,2] To exclude water or moisture in industrial processes is often difficult. Therefore knowledge about the existence and stability of hydrates and the understanding and control of the anhydrate/hydrate balance is mandatory for avoiding manufacturing problems. In order to improve our understanding of hydrate formation we selected representative substances (morphine, codeine, ethylmorphine) from a class of molecules (morphinanes), which are prone to crystallize along with water. Stable hydrates of both, free bases and HCl salts, have been observed in this important class of drug compounds. This allowed us to investigate the influence of different functional groups, the role of water and the Cl- counterion on the structure and properties of these morphinanes. A crystallization screen on the six compounds considerably extended the total number of known solid forms from twelve [3] to 17 and the number of crystal structures from five to twelve. Anhydrous polymorphs were detected for all compounds except ethylmorphine (one anhydrate) and its HCl salt (no anhydrate). The relative stabilities of the hydrated and anhydrous forms differ considerably, which was evaluated by moisture sorption studies and thermal analytical experiments. Two different hydrates, a tri- and dihydrate, were found for morphine HCl. In the free bases, the substituents define the number of hydrogen bond donor groups and lead to differences in the sterical hindrance around polar groups, influencing the intermolecular interactions, packing and stability. Hydrate formation results in higher dimensional hydrogen bond networks, whereas salt formation decreases the packing variability of the structures among the different compounds. Calorimetric measurements and lattice energy calculations were employed to estimate the heat of hydrate/anhydrate phase transformation, showing an enthalpic stabilization of the hydrates over the anhydrates. The combination of a variety of experimental techniques with computational modelling allowed us to generate sufficient kinetic, thermodynamic and structural information to understand the principles of hydrate formation of morphinanes.
abstracts
Documentation in crystallographic science provides not only abstract information and data, but offers a particularly high a potential for aesthetical creations. This potential is evident by in the colourful cover images of some top crystallographic journals as well as the illustrations in the individual scientific articles. Those crystallographers who use polarized microscopic techniques in their daily research know about the fascinating and sometimes surprisingly aesthetic images in microscopic preparations of crystalline materials. The investigation of a vast number of organic compounds with hot stage microscopy over decades in our laboratories resulted in a collection of thousands of microphotographs. Although these images were recorded with the scientific purpose to document the morphology, textures, phase transitions etc. of crystalline phases, there are a many strikingly aesthetic pictures among this collection that would attract the attention of everyone. Therefore, we decided to make this material visible to the public and we started to produce high quality prints for back illuminated wall pictures and lampshades. On our opinion, one can "transform" practically every crystalline substance to appealing "paintings" with some scientific knowledge and preparation skills. This will be demonstrated with pictures produced from the products of an agrochemical company, which now decorate the walls of their building. The positive feedback and interest in the phenomena behind the colourful pictures allowed us to make our scientific work more visible to the public and to convey some understanding for the nature and properties of crystalline materials. The aim of this presentation is to report about past activities, our experience in public relations with crystal images and to inspire colleagues in crystallography to utilize the available potential for such purposes.