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Soft Acid (soft + acid)
Selected AbstractsAdsorption of Small Molecules in Zeolites: A Local Hard,Soft Acid,Base Approach.CHEMINFORM, Issue 48 2003Ramesh Ch. No abstract is available for this article. [source] Acid-Base Interactions in Energetic Materials: I. The Hard and Soft Acids and Bases (HSAB) Principle,Insights to Reactivity and Sensitivity of Energetic MaterialsPROPELLANTS, EXPLOSIVES, PYROTECHNICS, Issue 1 2005Ernst-Christian Koch Abstract The chemistry of energetic materials can be described applying acid-base reaction formalism. Addressing the HSAB concept, the number of electrons transferred, ,N, in an acid-base reaction, allows for description and prediction of properties of composite and homogeneous materials. At first ,N helps in estimating the rate of reaction of binary systems with either given fuel or oxidizer. Nevertheless ,N is only a relative number thus the range of comparability remains narrow. At second ,N can be used as a measure for the sensitivity of homogeneous explosives. The increased reactivity of hypothetical fragments to recombine in a reaction such as R3C.+.NO2=R3C,NO2 given by ,N correlates very well with experimentally determined reduced impact sensitivity of 1,3,5-trinitrobenzene compounds. On the contrary the rising impact sensitivity of metal azides correlates with rising values of ,N of Mn+/N3, reaction because increased reactivity, that is increased electron transfer from the azide anion to the metal cation triggers formation of the azide radical (.N3). The latter then decomposes rapidly to give dinitrogen. This increased reactivity/sensitivity of metral azides coincides with covalent bonding whereas ionic azides are relatively insensitive. [source] Local HSAB principle in the conjugate addition of p -substituted thiophenols to cyclohexenoneINTERNATIONAL JOURNAL OF QUANTUM CHEMISTRY, Issue 1 2005Rocío Meza Abstract In the formation of new bonds through the Michel-type addition of thiolates to a ,,,-conjugate system, it is observed that the reaction takes place faster if the thio donor compound contains an electron-rich group or the enone acceptor is highly electron deficient. In contrast, the hard,soft acid,base (HSAB) principle predicts that this reaction is favored when a soft,soft interaction between the reactants takes place. Taking into account that softness is related to a barely electronegative atom of high polarizability, we consider it of interest to analyze the effect of charge transfer of a p -substituent on the softness of sulfur in thiophenols, as well as its impact in the conjugate addition to 2-cyclohexen-1-one. Experiments-in-competition, net charge of X-groups at the p -position of the aromatic ring, the global and local softness at sulfur, and the electrophilicity, obtained by density functional theory (DFT), led to the observation that the reaction is faster for electron attractor thiophenols. The softness at sulfur increases by delocalization of charge through the aromatic ring. © 2005 Wiley Periodicals, Inc. Int J Quantum Chem, 2005 [source] Selectivity of nitro versus fluoro substitution in arenes in their reactions with charged O - and S -nucleophilesJOURNAL OF PHYSICAL ORGANIC CHEMISTRY, Issue 6 2007Irina A. Khalfina Abstract The relative mobility of nitro and fluoro substituents in 1,3-dinitro- and 1-fluoro-3-nitrobenzenes, 3,5-dinitro- and 3-fluoro-5-nitrobenzotrifluorides under the action of the nucleophiles (2ArYH)·K2CO3 and ArY,K+ in solution and the nucleophiles ArYH·K2CO3 (Y,=,O, S) under heterogeneous conditions was studied by a competitive method in DMF at 40,140,°C. The unique dependences of ,,H, on ,,S, and ,,H, on ,,G, were determined for all the substrates and nucleophiles. The dependence of the mechanistic pathway on the nucleophile is discussed. Two results are relevant to the reactions studied: (i) substituent effects in the nucleophiles (2ArYH)·K2CO3 and ArYH·K2CO3 on the activation entropies suggest that the entropy favours the displacement of nitro group; (ii) the negative signs of ,,H, and ,,S, for the reactions of the nucleophiles ArY,K+ indicate that the enthalpy determines the displacement of nitro group. It is concluded that the selectivity of the reactions with aryloxide and arylthioxide ions cannot be explained by the hard,soft acid,base principle only. Copyright © 2007 John Wiley & Sons, Ltd. [source] Short communication: An alternative and effective catalyst for the silastannation of arylacetylenes with Me3SiSnBu3 at room temperatureAPPLIED ORGANOMETALLIC CHEMISTRY, Issue 2 2004Taichi Nakano Abstract The palladium-catalyzed silastannation of acetylenes with tributyl(trimethylsilyl)stannane in the presence of triethylphosphite is reported for the first time. The reaction occurs at room temperature to give (Z)-silyl(stannyl)ethenes in high yields. The protodemetallation of the resulting adducts with HCl,tetraethylammonium chloride is described first, which demonstrates that the reaction is governed only by the stability of a carbonium ion arising from the protonation to (Z)-silyl(stannyl)ethenes rather than the hard and soft acid and base principle, i.e. the ,-cation stabilization effect (,,, stabilization one) of a stannyl group in the carbonium ion is rather significant. Copyright © 2004 John Wiley & Sons, Ltd. [source] Adsorption of Cadmium Ion and Gallium Ion to Immobilized Metallothionein Fusion ProteinBIOTECHNOLOGY PROGRESS, Issue 6 2002Masaaki Terashima A fusion protein made from maltose binding protein (pmal) and human metallothionein (MT) was expressed using E. coli. The purified recombinant protein (pmal-MT) was immobilized on Chitopearl resin, and characteristics of pmal-MT for metal binding were evaluated. As expected from the tertiary structure of metallothionein, the pmal-MT ligand adsorbed 12.1 cadmium molecules per one molecule of the ligand at pH 5.2. The pmal-MT ligand also bound 26.6 gallium molecules per one molecule of the ligand at pH 6.5. Neither cadmium ion nor gallium ion bound to a control protein bovine serum albumin (BSA). Adsorption isotherms for both ions were correlated by Langmuir-type equations. Two types of binding sites have been elucidated on the basis of HSAB (hard and soft acid and base) theory. It was suggested that gallium ion specifically binds to amino acid residues containing oxygen and nitrogen atoms, while cadmium ion binds to specific binding sites formed by multiple cysteine residues. The pmal-MT ligand bound these metals in the concentration range of 0.2,1.0 mM, and the bound metal ions could be eluted under relatively mild conditions (pH 2.0). The pmal-MT Chitopearl resin was stable and could be used repeatedly without loss of binding activity. Thus, this new ligand would be useful for recovery of toxic heavy metals and/or valuable metal ions from various aqueous solutions. [source] DFT-HSAB Prediction of Regioselectivity in 1,3-Dipolar Cycloadditions: Behavior of (4-Substituted)benzonitrile Oxides towards Methyl Propiolate,CHEMISTRY - A EUROPEAN JOURNAL, Issue 4 2006Alessandro Ponti Dr. Abstract The regioselectivity of 1,3-dipolar cycloadditions between (4-substituted)benzonitrile oxides and methyl propiolate cannot be rationalized on the basis of the electron demand of the reactants or frontier molecular-orbital theory. To this problem, we have applied a quantitative formulation of the hard,soft acid,base principle developed within the density functional theory. Global and local reactivity indices were computed at B3LYP/6-311+G(d,p) level. The details of charge transfer upon the reactive encounter have been elucidated, and the computed regioselectivity has been shown to be in good agreement with experimental data. [source] Adsorption of Transition Metals in Aqueous Solutions by Fluted Pumpkin (Telfairia occidentalisHook f),WasteCHEMISTRY & BIODIVERSITY, Issue 9 2005Michael Horsfall Jr. The adsorption of some divalent transition metal (Hg, Rh, Pt, and Pd) ions in aqueous solution onto fluted pumpkin waste biomass has been investigated. The data were discussed in terms of ionic radii, surface area, and the hard,soft acid,base (HSAB) concept. The monolayer sorption capacities as obtained by the Langmuir adsorption isotherm model were determined to be ca. 9.89,mg/g, 9.81,mg/g, 10.59,mg/g, and 6.84,mg/g for for HgII, RhII, PtII, and PdII, respectively. The results are relevant for the optimal design of a wastewater treatment plant and for prediction of model parameters of sorbate,sorbent interactions. [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] Living cationic polymerization of amide-functional vinyl ethers: Specific properties of SnCl4 -based initiating systemJOURNAL OF POLYMER SCIENCE (IN TWO SECTIONS), Issue 18 2008Motomasa Yonezumi Abstract Living cationic copolymerization of amide-functional vinyl ethers with isobutyl vinyl ether (IBVE) was achieved using SnCl4 in the presence of ethyl acetate at 0 °C: the number,average molecular weight of the obtained polymers increased in direct proportion to the monomer conversion with relatively low polydispersity, and the amide-functional monomer units were introduced almost quantitatively. To optimize the reaction conditions, cationic polymerization of IBVE in the presence of amide compounds, as a model reaction, was also examined using various Lewis acids in dichloromethane. The combination of SnCl4 and ethyl acetate induced living cationic polymerization of IBVE at 0 °C when an amide compound, whose nitrogen is adjacent to a phenyl group, was used. The versatile performance of SnCl4 especially for achieving living cationic polymerization of various polar functional monomers was demonstrated in this study as well as in our previous studies. Thus, the specific properties of the SnCl4 initiating system are discussed by comparing with the EtxAlCl3,x systems from viewpoints of hard and soft acids and bases principle and computational chemistry. © 2008 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 6129,6141, 2008 [source] Marcus-Analyse ambidenter Reaktivität,ANGEWANDTE CHEMIE, Issue 30 2010Martin Breugst Die wirklichen Ursachen ambidenter Reaktivität: Das HSAB-Konzept kann nicht einmal das Verhalten der im Bild gezeigten Prototypen ambidenter Nucleophile erklären (HSAB: hard and soft acids and bases). Eine bessere Alternative ist die Marcus-Theorie, die die freie Aktivierungsenthalpie als Kombination eines intrinsischen und eines thermodynamischen Beitrags beschreibt. [source] | ||||||||||