FA3-MS20 Charge, spin and momentum densities for the prediction of physical properties

Material properties of siloxanes are governed by the inherently low basicity of the siloxane linkage Si-O-Si and thus its inability to form hydrogen bonds with polar molecules like water. Due to the importance of the linkage in minerals (e.g. quartz, feldspars) and polymers (e.g. silicones), it is a point of major interest to find ways to tune the properties of the siloxane linkage to develop new classes of materials. The idea is to influence the chemical properties of the siloxane linkage – basicity serves as the test case in this study – by variation of the Si-O-Si bond angle.[1] The vast amount of siloxane compounds exhibit Si-O-Si angles between 130° and 180°. Tetrahedral angles do not occur in nature, but only in a few exceptional synthetical cases. Various quantum-chemical calculations were carried out on disiloxane H3SiOSiH3. The Si-O-Si angle was varied from small angles to linearity in a many-step potential-energy surface scan with small angle increments to observe the development of chemical properties dependent on the bond angle. Electron density and the electron localizability indicator (ELI, [2]) were obtained at each angle. It resulted that disiloxane is indeed not able to bind hydrogen-bonding donors at large Si-O-Si angles; only for Si-O-Si angles smaller than 165° hydrogen bonding is generally feasible; only for angles smaller than 130° the hydrogen-bond energy becomes competitive; at angles smaller than 100°, siloxanes are even better hydrogen-bond acceptors than ethers. Discontinuous changes of the shapes of the oxygen lone pairs are crucial to explain this behaviour, which can be visualised by the Laplacian of the electron density and the ELI. Catastrophe theory describes these cusps and allows to classify different structural stability domains of disiloxane with different chemical properties like basicity.[3] To examine this finding experimentally, four siloxane compounds were synthesised and crystallised. Two of them comprise exceptionally strained siloxane linkages, which was achieved by forcing the linkage into a five-membered ring system. The other two comprise relaxed siloxane linkages, which can be found in minerals or polymers. High-resolution X-ray measurements at low temperatures were performed on synchrotron beamlines F1 and D3 of HASYLAB at DESY in Hamburg. Silanol-siloxane hydrogen bonds with the siloxane linkage as acceptor could indeed be found in the crystal structures of the compounds with small Si-O-Si angles. Electronic properties obtained by means of electron density and ELI varied significantly between compounds with small and with large Si-O-Si angles, so that the predictions from the potential-energy surface scans could be confirmed for compounds with potential for rational material development.