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81.
Darbost U Rager MN Petit S Jabin I Reinaud O 《Journal of the American Chemical Society》2005,127(23):8517-8525
The host-guest properties of calix[6]tren 1 have been evaluated. The receptor is based on a calix[6]arene that is covalently capped at the narrow rim by a tren unit. As a result, the system presents a concave hydrophobic cavity with, at its bottom, a grid-like nitrogenous core. Despite its well-defined cavity and opening to the outside at the large rim, 1 did not behave as a good receptor for neutral molecules in chloroform. However, it exhibited efficient endo-complexation of ammonium guests. By contrast, the per-protonated host, 1.4H(+), behaved as a remarkable receptor for small organic molecules. The complexation is driven by a strong charge-dipole interaction and hydrogen bonds between the polar guest and the tetracationic cap of the calixarene. Finally, coordination of Zn(2+) to the tren core led to the asymmetrization of calixarene cavity and to the strong but selective endo-binding of neutral ligands. This study emphasizes the efficiency of a receptor presenting a concave hydrophobic cavity that is polarized at its bottom. The resulting combination of charge-dipole, hydrogen bonding, CH-pi, and van der Waals interactions highly stabilizes the supramolecular architectures. Also, importantly, the tren cap allows the tuning of the polarization, offering either a basic (1), a highly charged and acidic (1.4H(+)), or a coordination (1.Zn(2+)) site. As a result, the system proved to be highly versatile, tunable, and interconvertible in solution by simple addition of protons, bases, or metal ions. 相似文献
82.
Uniform, well-dispersed silver particles of various morphologies have been prepared by reducing highly acidic silver nitrate solutions with ascorbic acid in the presence of a sodium naphthalene sulfonate-formaldehyde copolymer as dispersing agent. By varying the temperature of the reaction, the free acid content, the addition rate of the reductant, and the aging time, both isometric and anisotropic silver particles could be obtained. It was found that the latter were formed by aggregation of nanosize subunits, which were identified by electron microscopy and X-ray diffractometry. 相似文献
83.
In this brief work we express the cycle index of the molecular point groups as a function of a limited number of initial geometrical parameters. Such parameters are the number
of elements composing the domain D of sites of substitutions in the molecule belonging to the point group G, and the numbers
of sites of D lying on the symmetry elements
for G. 相似文献
84.
The kinetics of the reaction of manganese(III) with oxalic acid (OA) has been studied in H2SO4 solutions. Under the experimental conditions of 6 × 10–3 <>0 < 0.4=" mol=">–3 and [H2SO4]0 0.2 mol dm–3 the observed pseudo-first order rate constant k
obs follows the expression
相似文献
85.
Jaroslav Koutecký 《Theoretical chemistry accounts》1967,8(3):244-248
Approximate pairing properties are proved for the Hartree-Fock solutions for highly symmetric heteroatomic molecules. These molecules have an alternant topology and are built from two kinds of atoms in such a way that by an operation the atoms of one kind go over into the atoms of the second kind and vice versa.
Zusammenfassung Es werden genäherte alternierende Eigenschaften für Hartree-Fock-Lösungen hochsymmetrischer heterocyclischer Moleküle abgeleitet. Diese Moleküle haben eine alternierende Topologie, sie sind aus zwei Sorten von Atomen in solcher Weise aufgebaut, daß die Atome der ersten Sorte in die Atome der zweiten Sorte — und umgekehrt — transformiert werden können. Résumé Des propriétés d'accouplement orbital approché sont prouvées pour les solutions de type Hartree-Fock des molécules hétéroatomiques hautement symétriques. Ces molécules ont une topologie alternante et sont formées de deux classes d'atomes de telle sorte qu'il existe une opération interchangeant les atomes des deux classes. Supported in part by a grant to The Johns Hopkins University from the Petroleum Research Fund of the American Chemical Society. 相似文献 86.
The solubility products for some rare earth hydroxides and their temperature dependence have been shown. Greater solubility is shown for larger atomic radii and for lower temperature. 相似文献
87.
Maprotiline determination in blood serum samples was investigated by the use of both capillary isotachophoresis and spectrofluorimetric techniques. The analyte had to be enriched before determination and extraction with n-heptane was used for this purpose. The preliminary separation enabled the determination of maprotiline in blood serum at therapeutic concentration levels. 相似文献
88.
Laura S. Zamorano Francisco Gavilanes Ivan Yu. Sakharov Robert B. van Huystee Valery L. Shnyrov 《Thermochimica Acta》2004,417(1):67-73
Detailed differential scanning calorimetry (DSC), steady-state tryptophan fluorescence and far-UV circular dichroism (CD) studies, together with enzymatic assays, were carried out to monitor the thermal stability of anionic peanut peroxidase (aPrx) at pH 3.0. The spectral parameters were seen to be good complements to the highly sensitive but integral method of DSC. Thus, changes in far-UV CD corresponded to changes in the overall secondary structure of the enzyme, while changes in intrinsic tryptophan fluorescence emission corresponded to changes in the tertiary structure of the enzyme. The results, supported with data concerning changes in enzymatic activity with temperature, show that thermally induced transitions for aPrx are irreversible and strongly dependent upon the scan rate, suggesting that denaturation is under kinetic control. It is shown that the process of aPrx denaturation can be interpreted with sufficient accuracy in terms of the simple kinetic scheme, , where k is a first-order kinetic constant that changes with temperature, as given by the Arrhenius equation; N is the native state, and D is the denatured state. On the basis of this model, the parameters of the Arrhenius equation were calculated. 相似文献
89.
Ivan Gutman Sabina Gojak Boris Furtula Slavko Radenkovi? Andrej Vodopivec 《Monatshefte für Chemie / Chemical Monthly》2006,46(2):1127-1138
Within classes of isomeric benzenoid hydrocarbons various Kekulé- and Clar-structure-based parameters (Kekulé structure count, Clar cover count, Herndon number, Zhang–Zhang polynomial) are all mutually correlated. This explains why both the total π-electron energy (E), the Dewar resonance energy (DRE), and the topological resonance energy (TRE) are well correlated with all these parameters. Nevertheless, there exists an optimal value of the variable of the Zhang–Zhang polynomial for which it yields the best results. This optimal value is negative-valued for E, around zero for TRE, and positive-valued for DRE. A somewhat surprising result is that TRE and DRE considerably differ in their dependence on Kekulé- and Clar-structure-based parameters. 相似文献
90.
A protocol has been established for the transformation of chiral ortho 1-hydroxyethyl propenyl benzenes under both anaerobic and oxidative mercury(II) mediated conditions to produce chiral isochromanes. Further transformations of the former products yielded chiral isochromanquinones, while the latter afforded the corresponding chiral 4-hydroxyisochromanquinones. 相似文献
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