Experimental electron density of halogen bonds and interhalogen contacts

• Applicant: Professor Dr. Ullrich Englert
• Subject Area: Inorganic Molecular Chemistry - Synthesis and Characterisation ; Physical Chemistry of Solids and Surfaces, Material Characterisation
• Term: from 2015 to 2019
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 278999726

Halogen bonds rejoice growing popularity in the field of crystal engineering. A halogen bond occurs between the electrophilic region of a heavy halogen opposite to a covalent bond, often referred to as "sigma hole", and a suitable donor atom with a lone pair as nucleophile. Contacts between neighbouring halogen atoms in suitable geometry can be considered a special case of a halogen bond. Most of the discussions which address such interactions in the solid state are based on geometric considerations. We will combine our expertise in crystal engineering and charge density studies to obtain more in-depth information and probe the halogen bond at the level of electron density. In the context of our proposal, we will investigate crystals in which heavy halogens and lone pair donor atoms subtend short contacts between molecular or ionic residues. These target features may occur in chemically quite different solids: We envisage to study crystals of neutral coordination compounds, of suitably substituted pyridium halides or halogenometallates, and of neutral organic molecules. Co-crystals between halogen bond donors and acceptors offer a particularly versatile approach. In addition to the synthesis and structural chracterization of new compounds we will also consider high resolution studies of such crystals for which only data at standard resolution have been reported. X-ray diffraction on single crystals represents a straightforward method for studying intermolecular contacts, but we will not only investigate our target solids with respect to geometry: In order to assess the electron density and properties such as the electrostatic potential, we will study high quality crystals of compounds featuring interesting packing characteristics at low temperature and up to high resolution. Phase transitions, twinning and disorder represent potential obstacles which must be avoided. From a successful experiment we will deduce the electron density in critical points and its derived quantities such as the Laplacian, energy densities or the much less exploited source function. We expect to correlate these charge density results with the geometry and the nature of the halogen bonds and the halogen...halogen interactions encountered in our solids. We are confident to contribute reliable experimental data to the ongoing discussion about the relevance of specific short contacts versus global structural features acting at longer distances, such as electrostatic contributions or relative orientation of dipoles.

• DFG Programme: Research Grants