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Chemical Concepts (chemical + concept)
Selected AbstractsA New View on the Spectrochemical and Nephelauxetic Series on the Basis of Spin-Polarized Conceptual DFTCHEMPHYSCHEM, Issue 5 2009Jan Moens Abstract In a row: Chemical concepts from the spin-polarized conceptual DFT field are used to explain the spectrochemical and nephelauxetic series within a group of ruthenium complexes. The spectrochemical and nephelauxetic series are analyzed within the context of spin-polarized conceptual DFT. For a series of different [RuL6]xcomplexes, the local spin-philicitycondensed on the metal ion shows a remarkable analogy with some semi-empirical scales of the spectrochemical series. The localFukui function in turn can be linked to the nephelauxetic effect. Herein, we present a non-empirical, unified approach for a quantitative discussion of both series. [source] On the performance of some aromaticity indices: A critical assessment using a test setJOURNAL OF COMPUTATIONAL CHEMISTRY, Issue 10 2008Ferran Feixas Abstract Aromaticity is a central chemical concept widely used in modern chemistry for the interpretation of molecular structure, stability, reactivity, and magnetic properties of many compounds. As such, its reliable prediction is an important task of computational chemistry. In recent years, many methods to quantify aromaticity based on different physicochemical properties of molecules have been proposed. However, the nonobservable nature of aromaticity makes difficult to assess the performance of the numerous existing indices. In the present work, we introduce a series of fifteen aromaticity tests that can be used to analyze the advantages and drawbacks of a group of aromaticity descriptors. On the basis of the results obtained for a set of ten indicators of aromaticity, we conclude that indices based on the study of electron delocalization in aromatic species are the most accurate among those examined in this work. © 2008 Wiley Periodicals, Inc. J Comput Chem, 2008 [source] Density Functional Study of the Complexation Reaction of Sn(CH3)3X (X = F, Cl, Br and I) with Halide AnionsEUROPEAN JOURNAL OF INORGANIC CHEMISTRY, Issue 20 2003Frank De Proft Abstract The Lewis acid-base reaction between Sn(CH3)3X and Y, (with X, Y = F, Cl, Br and I) has been studied using quantum chemical calculations. Complexation energies were calculated at the Density Functional Theory (DFT) level and rationalized on the basis of a local application of the hard and soft acids and bases principle. It was observed that smaller differences in the local softness of the interacting sites in the Lewis acid and base correspond to stronger interactions. Moreover, the calculated sequences in complexation energies can be reproduced using equations containing chemical concepts introduced within the framework of conceptual density functional theory and rooted in the hard and soft acids and bases principle and referring only to the reactants. A method of treating the electronegativity and softness of the halide anions is presented based on a Taylor expansion of the electronegativity of the neutral halogens and the softness-polarizability proportionality. Experimental evidence for the calculated sequences was gathered from measured 117Sn chemical shifts and 1J (13C- 119/117Sn) coupling constant changes upon complexation. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2003) [source] Microbial life in glacial ice and implications for a cold origin of lifeFEMS MICROBIOLOGY ECOLOGY, Issue 2 2007P. Buford Price Abstract Application of physical and chemical concepts, complemented by studies of prokaryotes in ice cores and permafrost, has led to the present understanding of how microorganisms can metabolize at subfreezing temperatures on Earth and possibly on Mars and other cold planetary bodies. The habitats for life at subfreezing temperatures benefit from two unusual properties of ice. First, almost all ionic impurities are insoluble in the crystal structure of ice, which leads to a network of micron-diameter veins in which microorganisms may utilize ions for metabolism. Second, ice in contact with mineral surfaces develops a nanometre-thick film of unfrozen water that provides a second habitat that may allow microorganisms to extract energy from redox reactions with ions in the water film or ions in the mineral structure. On the early Earth and on icy planets, prebiotic molecules in veins in ice may have polymerized to RNA and polypeptides by virtue of the low water activity and high rate of encounter with each other in nearly one-dimensional trajectories in the veins. Prebiotic molecules may also have utilized grain surfaces to increase the rate of encounter and to exploit other physicochemical features of the surfaces. [source] Are the Hirshfeld and Mulliken population analysis schemes consistent with chemical intuition?INTERNATIONAL JOURNAL OF QUANTUM CHEMISTRY, Issue 9 2009Soumen Saha Abstract In the present article, we report a comparative study between the Hirshfeld and Mulliken population analysis schemes (abbreviated as HPA and MPA, respectively). Trends of atomic charges derived from these two population analysis schemes are compared with those expected from other commonly used chemical concepts like electronegativity, inductive effects, and resonance effects. Although previous studies on intramolecular reactivity sequences demonstrated that HPA generates reliable and non-negative (and thus physically more realistic) condensed Fukui function (FF) values, the present study reveals problems with the HPA charge partitioning technique. Specifically, HPA fails to reproduce reliable intermolecular and intramolecular charge trends in several systems. Reasons for the success and failure of HPA are discussed and a method for improving the Hirshfeld charge partitioning is proposed. © 2008 Wiley Periodicals, Inc. Int J Quantum Chem, 2009 [source] Facilitating student understanding of buffering by an integration of mathematics and chemical concepts,BIOCHEMISTRY AND MOLECULAR BIOLOGY EDUCATION, Issue 2 2004Robert Curtright Abstract We describe a simple undergraduate exercise involving the titration of a weak acid by a strong base using a pH meter and a micropipette. Students then use their data and carry out graphical analyses with a spreadsheet. The analyses involve using mathematical concepts such as first-derivative and semi-log plots and provide an opportunity for collaboration between biochemistry and mathematics instructors. By focusing on titration data, rather than the titration process, and using a variety of graphical transformations, we believe that students achieve a deeper understanding of the concept of buffering. [source] Bond Orders between Molecular FragmentsCHEMISTRY - A EUROPEAN JOURNAL, Issue 8 2006Adam J. Bridgeman Dr. Abstract An extension of the Mayer bond order for the interaction between molecular fragments is presented. This approach allows the classical chemical concepts of bond order and valence to be utilised for fragments and the interactions between the fragments and symmetry-adapted linear combinations to be analysed. For high-symmetry systems, the approach allows the contribution from each irreducible representation to be assessed and provides a semiquantitative measure of the role of each bonding mode to interfragment interactions. The utility of this tool has been examined by a study of the bonding in symmetrical sandwich complexes. The validity of the frontier-orbital approach and the contributions from each frontier-orbital interaction can also be assessed within this model. As demonstrated by a study of a number of mixed-sandwich complexes, the model proves to be especially useful for low-symmetry systems in which separation of the ,, , and , roles in bonding of the ligand is difficult to assess. The fragment bond order describes the interaction between preoptimized fragment orbitals and is independent of the charges that are placed on these fragments. Although the method allows the chemist to define fragments in any way they choose, most insight is gained by using the same frontier orbitals employed so successfully in perturbational molecular-orbital approaches. The results are free from the influence of the electron-counting method used to describe fragments, such as the rings and metals in sandwich complexes. [source] | ||||||||||