Melting point, molecular symmetry and aggregation of tetrachlorobenzene isomers: the role of halogen bonding

Tetrachlorobenzenes represent one of the best known, but not yet fully understood, group of isomers of the structure-melting point relationship. The differences in melting temperatures of these structurally related compounds were rationalized in terms of the hierarchy and nature of formed noncovalent interactions, and the molecular aggregation that is influenced by molecular symmetry. The highest melting point is associated with the highly symmetric 1,2,4,5-tetrachlorobenzene isomer. The structures of less symmetrical 1,2,3,4-tetrachlorobenzene and 1,2,3,5-tetrachlorobenzene, determined at 270 and 90 K, show a distinct pattern of halogen bonds, characterized by the different numbers and types of interactions. The evolution of Cl...Cl/H distances with temperature indicates the attractive character of intermolecular interactions and their importance to the structural and thermodynamic parameters of isomeric compounds. The favoured Cl...Cl halogen bonds were found to play a decisive role in differentiating the melting temperatures of tetrachlorobenzene isomers. It was also found that, besides the molecular symmetry and ability to form specific intermolecular interactions, both the type and the distribution of interactions are the important factors responsible for the melting behaviour of the studied isomers. The observed preferences, in tetrachlorobenzenes, for the formation of specific noncovalent interactions correspond to the distribution of calculated partial atomic charges and to the magnitudes of electrostatic potential on the molecular surfaces as well as correlate with the enthalpy of melting parameters.

A thermodynamically consistent phase diagram of a trimorphic pharmaceutical, l-tyrosine ethyl ester, based on limited experimental data

Crystalline polymorphs possess different physical properties, and phase changes between those polymorphs may affect the properties of engineered materials such as drugs. This is very well illustrated by the large effort that is put into the capability to predict phase behaviour of pharmaceuticals to avoid the unexpected appearance of different crystal forms. Much progress has been made, but one of the remaining challenges is (the accuracy in) the prediction of phase behaviour as a function of temperature. Obviously, predictions should at a certain point be verified against experimental data; however, it may not always be easy to elucidate the phase behaviour of a given compound experimentally, because thermodynamically and kinetically controlled phenomena occur in a convoluted fashion in experimental data. The present paper discusses the trimorphism of l-tyrosine ethyl ester as an example case of how experimental data in combination with the thermodynamic tenets lead to a consistent phase diagram, which can be used as the basis for pharmaceutical formulations and for comparison with polymorph predictions by computer. The positions of the two-phase equilibria I-II, I-III, and I-L have been obtained experimentally. Using the Clapeyron equation and the alternation rule, it has been shown how the positions of the other equilibria II-L, III-L, and II-III can be deduced in combination with the stability rankings of the phases and the phase equilibria. The experimental data have been obtained by synchrotron X-ray diffraction, Raman spectroscopy, and thermal analysis as a function of pressure and temperature. Furthermore, laboratory X-ray diffraction as a function of temperature and differential scanning calorimetry have been used. At room temperature, form II is the most stable phase, which remains stable with increasing pressure, as it possesses the smallest specific volume. Form I becomes stable above 33 °C (306 K), but with increasing pressure it turns into form III. On thermodynamic grounds, form III is expected to have a stable domain at very low temperatures.

Loose crystals engineered by mismatched halogen bonds in hexachloroethane

The shortest intermolecular contacts in the engineered loose crystal of hexachloroethane are longer than the sum of van der Waals radii, reached only at the pressure of 1.2 GPa.

Orientational relaxations in solid (1,1,2,2)tetrachloroethane

We employ dielectric spectroscopy and molecular dynamic simulations to investigate the dipolar dynamics in the orientationally disordered solid phase of (1,1,2,2)tetrachloroethane. Three distinct orientational dynamics are observed as separate dielectric loss features, all characterized by a simply activated temperature dependence. The slower process, associated to a glassy transition at 156 ± 1 K, corresponds to a cooperative motion by which each molecule rotates by 180° around the molecular symmetry axis through an intermediate state in which the symmetry axis is oriented roughly orthogonally to the initial and final states. Of the other two dipolar relaxations, the intermediate one is the Johari-Goldstein precursor relaxation of the cooperative dynamics, while the fastest process corresponds to an orientational fluctuation of single molecules into a higher-energy orientation. The Kirkwood correlation factor of the cooperative relaxation is of the order of one tenth, indicating that the molecular dipoles maintain on average a strong antiparallel alignment during their collective motion. These findings show that the combination of dielectric spectroscopy and molecular simulations allows studying in great detail the orientational dynamics in molecular solids.

Thermodynamics by synchrotron X-ray diffraction: phase relationships and crystal structure of l-tyrosine ethyl ester form III

Structure, transition enthalpy and equilibrium curve were obtained by X-ray diffraction for the commercial form and a new crystalline high-pressure form.