929 research outputs found
A Guided Tour Through Modern Charge Density Analysis
No full textA concise summary is provided on the basic aspects of charge density (CD) analysis and an overview of the charge density research and developments over the last 10 years. A glimpse is given to the issues which are treated in more details in the remaining chapters of this book and to those few that, although of some importance, could not be covered. Advances in experimental methodologies and in the charge density model refinements, along with progresses in quantum mechanical methods and in the interpretation/understanding of chemical bonding and interactions are briefly summarized. The increasingly stronger connection between charge density research and challenging questions of relevance to chemistry and to materials and life science is overviewed
Resonant structures and electron density analysis
No full textIn perfect resonance: The electron density distribution is typically used to analyze chemical bonding, even if strong electron delocalization occurs. The electron densities and electric potentials of picoline and the picolyl anion (see picture) have been analyzed and a detailed description of picolyl lithium complexes has been derived. The next goal is to predict chemical reactivity from electron density. © 2009 Wiley-VCH Verlag GmbH & Co. KGaA
The future of topological analysis in experimental charge-density research
No full textIn a recent paper, Dittrich (2017) critically discussed the benefits of analysing experimental electron density within the framework of the quantum theory of atoms in molecules, often called simply the topological analysis of the charge density. The point he raised is important because it challenges the scientific production of a very active community. The question whether this kind of investigation is still sensible is intriguing and it fosters a multifaceted answer. Granted that none can predict the future of any field of science, but an alternative point of view emerges after answering three questions: Why should we investigate the electron charge (and spin) density? Is the interpretative scheme proposed by the quantum theory of atoms in molecules useful? Is an experimental charge density necessary?The state of the art and the future of the topological analysis of experimental electron charge density is the topic of this perspective article, within a debate stimulated by a recent communication
Electron Density distribution in Organometallic materials
Get PDFThe electron density distribution is afundamental property that provides information on the way in which atoms are held together to form molecules, polymers or supramolecular aggregates. Particular attention should be dedicated to the investigation of inorganic or organometallic materials, for their application in several fields. The goal of the research in this area is to find the inherent relationship between the actual electron density distribution of a molecule or a solid and its properties, including reactivity. This review summarizes the most recent results of electron density analysis and gives some personal perspective on future developments, focusing on applications in material science. © Schweizerische Chemische Gesellschaft
Modern charge density studies: the entanglement of experiment and theory
Get PDFThis tutorial review article is intended to provide a general guidance to a reader interested to learn about the methodologies to obtain accurate electron density mapping in molecules and crystalline solids, from theory or from experiment, and to carry out a sensible interpretation of the results, for chemical, biochemical or materials science applications. The review mainly focuses on X-ray diffraction techniques and refinement of experimental models, in particular multipolar models. Neutron diffraction, which was widely used in the past to fix accurate positions of atoms, is now used for more specific purposes. The review illustrates three principal analyses of the experimental or theoretical electron density, based on quantum chemical, semi-empirical or empirical interpretation schemes, such as the quantum theory of atoms in molecules, the semi-classical evaluation of interaction energies and the Hirshfeld analysis. In particular, it is shown that a simple topological analysis based on a partition of the electron density cannot alone reveal the whole nature of chemical bonding. More information based on the pair density is necessary. A connection between quantum mechanics and observable quantities is given in order to provide the physical grounds to explain the observations and to justify the interpretations
Crystallographic approaches for the investigation of molecular materials: structure property relationships and reverse crystal engineering
Get PDFThis article discusses the connection between crystallography and material science. It sheds light on some of the research opportunities that are currently available and it critically reviews the directions taken by the scientific community in the field of crystal engineering. The focus is on materials formed by the assembly of organic and organometallic molecular building blocks. © Schweizerische Chemische Gesellschaft
Comments on `Hydrogen bonds in crystalline D-alanine: diffraction and spectroscopic evidence for differences between enantiomers'
Get PDFThe recent paper by Belo, Pereira, Freire, Argyriou, Eckert & Bordallo [(2018), IUCrJ, 5, 6–12] reports observations that may lead one to think of very strong and visible consequences of the parity-violation energy difference between enantiomers of a molecule, namely alanine. If proved, this claim would have an enormous impact for research in structural chemistry. However, alternative, more realistic, explanations of their experiments have not been ruled out by the authors. Moreover, the theoretical calculations carried out to support the hypothesis are unable to differentiate between enantiomers (molecules or crystals). Therefore, the conclusions drawn by Belo et al. (2018) are deemed inappropriate as the data presented do not contain sufficient information to reach such a conclusion
The polarizability of organometallic bonds
Get PDFThe distributed atomic polarizabilities enable the investigation of the coordination of organic ligands to transition metals giving new insight of some organometallic complexes used in catalytic processes. This approach is useful to appreciate the enormous changes occurring upon complexation to a metal, which is mainly responsible for the augmented reactivity of these species. The method we have developed allows calculating the polarizability of an atom in a molecule upon a partitioning of the electron density, for example using the Quantum Theory of Atoms in Molecules. The polarizabilities result from numerical differentiation of the atomic dipole moments with respect to the electric field. From the polarizability tensors, other useful quantities derive, for example a mathematically precise definition of bond polarizability. In terms of chemical reactivity, the distributed polarizabilities are complementary to the traditional analysis of the electrostatic potential, which informs only on susceptibility toward hard, charge-controlled chemical reactions. On the contrary, atomic polarizabilities enable addressing the sites more keen on soft, orbital-controlled reactions
Cryo-Crystallography: Diffraction at Low Temperature and More
No full textThis chapter comments on the motivations and the methods of crystallographic studies at low temperature. Cry-crystallography is a brunch of Crystallography, a science that is too often confused with a technique. On the other hand, the scientific background to study crystal phases at low temperature is here provided, together with a survey of many possible techniques that provide complementary or supplementary information. Several applications are discussed, in particular in relation with highly accurate studies like electron density determination or phase transition mechanisms. © 2011 Springer-Verlag Berlin-Heidelberg
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