SPP 1178: Experimental Charge Density as the Key to Understand Chemical Interactions
Final Report Abstract
The Deutsche Forschungsgemeinschaft funded the corresponding priority program SPP 1178 to amalgamate scientists from quite different fields to benefit from the recent tremendous developments concerning the determination and analyses of the electronic structure of molecules and solids even under extreme conditions and in the excited state. Hence this priority program provided a unique platform for understanding the nature of chemical interactions. The generous funding supported leading methodical groups specialized in high resolution diffraction experiments on the one hand and advanced quantum mechanical calculations on the other hand to work on challenging compounds displaying complex electronic structures and their immediately feed-back results fuelled new synthetic endeavours within the preparative groups. Still most of our understanding of the chemical bond is deduced from simple geometrical criteria such as the analysis of bond lengths and valence angles, which are commonly obtained by routine structure determination methods. Modern charge density analyses, however, overcome the limitation of these rather empirical and intuitive concepts by providing a direct and local insight into the electronic structure of matter. Especially, advanced charge density concepts like the Quantum Theory of Atoms in Molecules (QTAIM) or the usage of Electron Localization Functions (e.g. ELF or ELI-D) supply tools to elucidate the nature of the chemical bond in detail by revealing even subtle electronic features at subatomic resolution. The charge density distribution as a physical observable property also provided an ideal experimental benchmark for the complementing theoretical studies and projects of the SPP 1178. Indeed, the close exchange among preparative, structural and theoretical chemists and physicists in this priority program helped tremendously to identify misconceptions in chemistry and set the scene for new unconventional synthetic approaches. The design of compounds with improved chemical or physical properties has always been the major driving force of innovation in chemistry. Especially the control and understanding of their electronic structures is one of the most fundamental objectives in this cross-sectional science discipline. While the various synthetic targets and chemical/physical questions might differ among the various SPP1178 projects, the overarching issue is always the same: selectively analyze and design the electronic structures within a molecule or material to finetune its physical or chemical properties such as reactivity or functionality. These aims could largely be accomplished due to enormous methodical and experimental efforts and the permanent instrumental and scientific innovation involved within this discipline. The priority program therefore also provided the ideal platform to train the next generation of scientists planning challenging experiments at large scale facilities like spallation sources, free-electron lasers, and advanced synchrotron sources.
