High Pressure Studies Of Intermolecular Interations In Supramolecular Crystals

Intermolecular interation is the force between two molecules; it is that negative gradient of the potential energy between the interacting molecules, if energy is a function of the distance between the centers of the molecules.?Supermolecule is a well defined complex of molecules held together by noncovalent bonds.?The properties of the molecular and supramolecular crystals are determined, on the one hand, by the chemical structure of the molecules and, on the other, by the three-dimensional arrangement in the crystal lattice. The intermolecular interactions between the individual building blocks, i.e. the molecules, and the resulting lattice architecture substantially determine the mechanical, electrical and optical properties of the substances. If these intermolecular interactions are modified the structure and related properties vary. This gives an insight into the structure-property relationships and implies a possible property tuning. For study of the influences of the intermolecular interactions on the structure and properties for specific molecules without changing their structure, it is more appropriate to preserve the chemical structure. Thus, a better way is the application of high pressures. Here, the chemical structure of the molecule is maintained, only the distances between the structural units are changed and therefore the corresponding intermolecular interactions.? For our investigation we focused on the high pressure behavior of intermolecular interactions. In this thesis, we report a combined experimental and computational study of three kinds of hydrogen and halogen bonded supramolecular crystals as a function of pressure, and obtained the following original results:Single crystal samples of the 1:1 adduct between cyanuric acid and melamine (CA·M), an outstanding case of noncovalent synthesis, have been studied by Raman spectroscopy and synchrotron X-ray diffraction in a diamond anvil cell up to pressures of 15 GPa. The abrupt changes in Raman spectra around 4.4 GPa have provided convincing evidence for pressure-induced structural phase transition. This phase transition was confirmed by angle dispersive X-ray diffraction (ADXRD) experiments to be a space group change from C2/m to its subgroup P2₁/m. On release of pressure, the observed transition was irreversible and the new high-pressure phase was fully preserved at ambient conditions. We propose that this phase transition was due to supramolecular rearrangements brought about by changes in the hydrogen bonding networks.The effects of high pressure on the structural stability of melamine–boric acid adduct (C₃N₆H₆·2H₃BO₃, M·2B), a three-dimensional hydrogen-bonded supramolecular architecture, were studied by in situ synchrotron X-ray diffraction and Raman spectroscopy. M·2B exhibited a high compressibility and a strong anisotropic compression, which can be explained by the layer like crystal packing. Furthermore, evolution of XRD patterns and Raman spectra indicated M·2B crystal undergoes a reversible pressure-induced amorphization (PIA) at 18 GPa. The mechanism for the PIA was attributed to the competition between close packing and long range order. Ab initio calculations were also performed to account for the behavior of hydrogen bonding under high pressure.We have performed high-pressure studies of cyanuric chloride using Raman spectroscopy and synchrotron X-ray diffraction. The experimental results indicated that halogen bonding is an effective non-covalent interaction to stabilize the crystal structure. Moreover, cyanuric chloride exhibited a high compressibility and a strong anisotropic compression, which can be explained by the layered crystal packing. In addition, most of Raman peaks exhibited blue shift with increasing pressure whereas the peaks corresponding to out of plane ring vibrations showed red shift. This red shift was believed to be related to the"fish-scale"arrangement of halogen-bonded networks under high pressures. Ab initio calculations were also performed to account for the high pressure Raman spectra and the high pressure behavior of halogen bonding. We hope these findings will contribute to achieving more insight into the nature of halogen bonding and the structural properties of halogen-bonded systems at high pressure.