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121.
122.
Schimmel HG Kearley GJ Nijkamp MG Visser CT de Jong KP Mulder FM 《Chemistry (Weinheim an der Bergstrasse, Germany)》2003,9(19):4764-4770
Single-walled carbon nanotubes (SWNT) were reported to have record high hydrogen storage capacities at room temperature, indicating an interaction between hydrogen and carbon matrix that is stronger than known before. Here we present a study of the interaction of hydrogen with activated charcoal, carbon nanofibers, and SWNT that disproves these earlier reports. The hydrogen storage capacity of these materials correlates with the surface area of the material, the activated charcoal having the largest. The SWNT appear to have a relatively low accessible surface area due to bundling of the tubes; the hydrogen does not enter the voids between the tubes in the bundles. Pressure-temperature curves were used to estimate the interaction potential, which was found to be 580+/-60 K. Hydrogen gas was adsorbed in amounts up to 2 wt % only at low temperatures. Molecular rotations observed with neutron scattering indicate that molecular hydrogen is present, and no significant difference was found between the hydrogen molecules adsorbed in the different investigated materials. Results from density functional calculations show molecular hydrogen bonding to an aromatic C[bond]C that is present in the materials investigated. The claims of high storage capacities of SWNT related to their characteristic morphology are unjustified. 相似文献
123.
Phase transitions that occurred in perovskite BaPbO3 have been investigated using high-resolution time-of-flight neutron powder diffraction. The structure at room temperature is orthorhombic (space group Imma), which is derived from the cubic aristotype by tilting the PbO6 octahedra around the two-fold axis (tilt system a0b−b−). The orthorhombic structure shows anisotropic line broadening attributed to the presence of micro twins. At above about 573 K, BaPbO3 undergoes a discontinuous phase transition to a tetragonal structure (space group I4/mcm) with the tilting of the PbO6 octahedra being about the four-fold axis of the cubic aristotype (tilt system a0a0c−). With further increasing the temperature, BaPbO3 experiences a continuous phase transition to a simple cubic structure (space group Pm3¯m) at above about 673 K. The later phase transition is characterised by a critical exponent of β=0.36, depicted by the three-dimensional Heisenberg universality class. The earlier reported Imma→I2/m phase transition above room temperature has not been observed. 相似文献
124.
J. R. de Dobbelaere J. W. de Haan H. M. Buck G. J. Visser 《Theoretical chemistry accounts》1973,31(1):95-99
CNDO/2 and INDO calculations have been carried out in order to construct a suitable model for the activated complex during the reaction. In this reaction model the migrating hydrogen atom moves along an edge of the cyclopentadiene ring. An analysis of this situation suggests a partial electron transfer from the migrating hydrogen to the nascent cyclopentadienyl system. This charge transfer is discussed in terms of aromaticity. The calculated activation enthalpies are 10 kcal/mole (CNDO/2) and 17 kcal/mole (INDO), whereas the experimental value is ca. 24 kcal/mole [1]. 相似文献
125.
Ben-Daniel R de Visser SP Shaik S Neumann R 《Journal of the American Chemical Society》2003,125(40):12116-12117
We have demonstrated that a polyfluorinated alcohol, 2,2,2-trifluoroethanol, solvent enables haloperoxidase type activity and the oxychlorination of arenes (benzene and its alkylated derivatives) without a metal catalyst. The polyfluorinated alcohol has a dual function; it catalyzes electrophilic chlorination of less reactive arenes by several orders of magnitude and oxidation of chloride at lower H+ concentrations. DFT calculations show that a complementary charge template in the transition state explains the catalysis of the electrophilic chlorination. 相似文献
126.
Rob G. Visser 《Accreditation and quality assurance》2006,10(10):521-526
Guidelines are given for the evaluation of proficiency test (PT) results in order to increase the effectivity of PT participation. For better understanding, some statistical background is given along with some examples to show the effects of the choices made by the PT provider. The calculation method of the assigned value and the selection of the standard deviation both affect the z-score that is used by the participating laboratory to judge the quality of its performance in the PT. Therefore, the participating laboratory is advised to use the PT results with care and, if necessary, to recalculate the z-scores. Finally, advice is given on how not to follow up bad PT results along with some valuable steps that could be part of an effective follow-up procedure. 相似文献
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The preparation of phosphatidyl-β-glucosyl diglyceride is described. The synthesis of glycophospholipid was accomplished by using: (a) the levulinoyl group for the temporary protection of the glucose hydroxyl functions of , which could then be converted into the dioleoyl substituted derivative ; (b) the tetraisopropyldisiloxane-1,3-diyl (TIPS) group to protect the 3'- and 4'-hydroxyl groups of , in a two step procedure, to afford compound ; (c) a 2,4-dichlorophenyl protected phosphatidic acid derivative . Compound could be selectively coupled to the primary hydroxyl function of to afford the fully protected glycophospholipid . Finally, removal of the 2,4-dichlorophenyl and TIPS protecting groups from was performed with syn-4-nitrobenzaldoximate and fluoride ions, respectively, to afford glycophospholipid . 相似文献
130.
(2+2)- and (4+2)-cycloadducts are formed by thermal reaction of xanthenethione with allenes. Activation parameters for ths “concerted two step” (π2s+π2s+π2s) reaction are presented. 相似文献