Understanding chemistry from combined X-ray and quantum mechanical studies

Abstract

Since the theoretical prediction [1] and experimental verification of Charge Concentrations (CCs) in the valence shell of transition metals several attempts have been undertaken to understand their origin and relevance in chemistry and physics [2]. In pioneering studies we could demonstrate that these CCs not only influence the geometry of coordination compounds [3] and solids but also serve as controlling parameters for important chemical reactions like the activation of chemical bonds in catalytic reactions [4]. Furthermore, in covalent solids such as transition metal oxides and carbides they appear to signal electron localization phenomena which are reflected by metal-to-insulator transitions or the suppression of superconductivity [5]. The complex interplay of valence shell charge concentrations with the physical and chemical properties of molecules and solids will be the central topic of this contribution. Much work has been dedicated to phase transitions characterizing broken symmetries and their associated critical fluctuations. In this talk, we will present two up to date research axes in the fields of phase transitions in crystals. The first one will concern ultra-fast phase transitions induced by femto-second pulsed lasers [1,2]. These transformations use a new degree of freedom, the ultra-short time, coupling coherence, cooperativity and out of equilibrium. The second axis will concern symmetry breakings which take place in crystallographic superspaces. There, the new degrees of freedom come from the aperiodicity of the materials allowing very original structural instabilities [3]. X-ray diffraction has developed structural chemistry, hence the understanding of the matter and its behavior. 3D molecular pictures and their accurate geometries provide fundamental information about bonding between atoms and the associated chemical properties. Besides being beautiful, molecules raise thought-provoking questions about functionality. For this, it is important to establish proper structure-property relationships in terms of the electronic distribution and, in this respect, quantum mechanical theories have been useful even at some lower level of approximation. Today reciprocal validation of crystallographic results and electronic structures are possible by standard DFT approaches, which afford very good structural simulations. Not necessarily, the latter obtained with the introduction of most accurate physical-mathematical treatments (reductionism) mean also good understanding. This is the key to gain proper capabilities of controlling the matter, such as for example the improvement of a known chemical process or devising new potential ones. Fortunately, crystallography with the myriad of catalogued structures (data banks) allows horizontal comparisons of the chemistries and even a small difference can be enlightening. For instance, the geometric perturbation introduced by a different substituent can affect the electron density and this helps redirecting the chemical evolution of the species in a desired dire...