Predicting points of zero charge of oxides and hydroxides

Pauling's electrostatic valence principle has been applied to describe the surface-charging mechanism of oxides in an aqueous environment. This approach has led to the development of an equation with which one can predict the points of zero charge (PZC) of oxides and hydroxides from crystallographic data. The equation proposed in the present work is an improved version of Parks' PZC equation. The improvements are the following: (i) The equation does not incorporate correction terms to take the cation coordination numbers into account; (ii) the PZC of a complex oxide can be predicted directly from crystallographic data instead of from “assumed” PZCs of its component oxides; and (iii) one can use the mean metal-oxygen bond distance of a crystal instead of the ionic radii tabulated by Parks, which are not consistent with the up-to-date values.     

Upconversion Luminescence through Cooperative and Energy-Transfer Mechanisms in Yb3+-Metal-Organic Frameworks

Lanthanide-doped metal–organic frameworks (Ln-MOFs) have versatile luminescence properties, however it is challenging to achieve lanthanide-based upconversion luminescence in these materials. Here, 1,3,5-benzenetricarboxylic acid (BTC) and trivalent Yb³⁺ ions were used to generate crystalline Yb-BTC MOF 1D-microrods with upconversion luminescence under near infrared excitation via cooperative luminescence. Subsequently, the Yb-BTC MOFs were doped with a variety of different lanthanides to evaluate the potential for Yb³⁺-based upconversion and energy transfer. Yb-BTC MOFs doped with Er³⁺, Ho³⁺, Tb³⁺, and Eu³⁺ ions exhibit both the cooperative luminescence from Yb³⁺ and the characteristic emission bands of these ions under 980 nm irradiation. In contrast, only the 497 nm upconversion emission band from Yb³⁺ is observed in the MOFs doped with Tm³⁺, Pr³⁺, Sm³⁺, and Dy³⁺. The effects of different dopants on the efficiency of cooperative luminescence were established and will provide guidance for the exploitation of Ln-MOFs exhibiting upconversion.     

Well‐Defined Amphiphilic C60‐PEG Conjugates: Water‐Soluble and Thermoresponsive Materials

For the preparation of well‐defined H₂O‐soluble C₆₀ polymers, several C₆₀‐PEG conjugates were prepared from a C₆₀ biscarboxylic acid derivative and monodisperse NH₂‐PEGs (NH₂‐EG_n, n = 4 – 36) via amide conjugation. When the relatively long PEGs (EG_n, n ≥ 12) were employed, the C₆₀‐PEG conjugates became completely H₂O‐soluble by forming micelle‐like structure shown by the data of surface tension, DLS, and cryo‐TEM. Interestingly, these H₂O‐soluble C₆₀‐PEG conjugates (C₆₀(EG_n)₂, n = 12 – 36) showed reversible thermoresponse to form larger aggregates (ca. 1 μm by DLS) at higher temperatures. The temperature for the aggregation was related to the lengths of PEGs attached to C₆₀; 29 °C (C₆₀(EG_n)₂, n = 12), 51 °C (n = 20), and 72 °C (n = 36). This thermoresponse was speculated to occur by dehydration of well‐organized PEG chains in the micelle‐type structure of monodisperse C₆₀‐PEG caused by gauche‐to‐anti conformational change of PEG anchors. This thermoresponse of well‐defined amphiphilic C₆₀‐PEG conjugates indicates potential applications in areas such as temperature sensors and thermoresponsive materials.     

Sulfur as a supplementary filler for bitumen-silica composites

Summary Dynamic-mechanical properties of bitumen-sulfur mixtures, containing more than 10% sulfur, change with time up to one month ageing. Composites were prepared by adding silica to these mixtures. In general, the time-temperature-superposition principle applies. Moreover, the viscoelastic properties of the bitumen matrix are not significantly altered by the presence of sulfur. However, the triple time-temperature-interfacial area equivalence principle could not be verified, in this instance, since the interfacial area between sulfur, the new filler, and bitumen could not be established.With 11 figures     

Cationic rhodium mono-phosphine fragments partnered with carborane monoanions [closo-CB11H6X6]- (X = H, Br). Synthesis, structures and reactivity with alkenes

Addition of the new phosphonium carborane salts [HPR(3)][closo-CB(11)H(6)X(6)] (R = (i)Pr, Cy, Cyp; X = H 1a-c, X = Br 2a-c; Cy = C(6)H(11), Cyp = C(5)H(9)) to [Rh(nbd)(mu-OMe)](2) under a H(2) atmosphere gives the complexes Rh(PR(3))H(2)(closo-CB(11)H(12)) 3 (R = (i)Pr 3a, Cy 3b, Cyp 3c) and Rh(PR(3))H(2)(closo-CB(11)H(6)Br(6)) 4 (R = (i)Pr 4a, Cy 4b, Cyp 4c). These complexes have been characterised spectroscopically, and for 4b by single crystal X-ray crystallography. These data show that the {Rh(PR(3))H(2)}(+) fragment is interacting with the lower hemisphere of the [closo-CB(11)H(6)X(6)](-) anion on the NMR timescale, through three Rh-H-B or Rh-Br interactions for complexes 3 and 4 respectively. The metal fragment is fluxional over the lower surface of the cage anion, and mechanisms for this process are discussed. Complexes 3a-c are only stable under an atmosphere of H(2). Removing this, or placing under a vacuum, results in H(2) loss and the formation of the dimer species Rh(2)(PR(3))(2)(closo-CB(11)H(12))(2) 5a (R = (i)Pr), 5b (R = Cy), 5c (R = Cyp). These dimers have been characterised spectroscopically and for 5b by X-ray diffraction. The solid state structure shows a dimer with two closely associated carborane monoanions surrounding a [Rh(2)(PCy(3))(2)](2+) core. One carborane interacts with the metal core through three Rh-H-B bonds, while the other interacts through two Rh-H-B bonds and a direct Rh-B link. The electronic structure of this molecule is best described as having a dative Rh(I) --> Rh(III), d(8)--> d(6), interaction and a formal electron count of 16 and 18 electrons for the two rhodium centres respectively. Addition of H(2) to complexes 5a-c regenerate 3a-c. Addition of alkene (ethene or 1-hexene) to 5a-c or 3a-c results in dehydrogenative borylation, with 1, 2, and 3-B-vinyl substituted cages observed by ESI-MS: [closo-(RHC[double bond, length as m-dash]CH)(x)CB(11)H(12-x)](-)x = 1-3, R = H, C(4)H(9). Addition of H(2) to this mixture converts the B-vinyl groups to B-ethyl; while sequential addition of 4 cycles of ethene (excess) and H(2) to CH(2)Cl(2) solutions of 5a-c results in multiple substitution of the cage (as measured by ESI-MS), with an approximately Gaussian distribution between 3 and 9 substitutions. Compositionally pure material was not obtained. Complexes 4a-c do not lose H(2). Addition of tert-butylethene (tbe) to 4a gives the new complex Rh(P(i)Pr(3))(eta(2)-H(2)C=CH(t)Bu)(closo-CB(11)H(6)Br(6)) 6, characterised spectroscopically and by X-ray diffraction, which show coordination of the alkene ligand and bidentate coordination of the [closo-CB(11)H(6)Br(6)](-) anion. By contrast, addition of tbe to 4b or 4c results in transfer dehydrogenation to give the rhodium complexes Rh{PCy(2)(eta(2)-C(6)H(9))}(closo-CB(11)H(6)Br(6)) 7 and Rh{PCyp(2)(eta(2)-C(5)H(7))}(closo-CB(11)H(6)Br(6)) 9, which contain phosphine-alkene ligands. Complex has been characterised crystallographically.     

Identification by LC-ESI-MS of flavonoids responsible for the antioxidant properties of Mallotus species from Vietnam

Several Mallotus species (Euphorbiaceae) are used in Vietnam as edible plants or as traditional medicines for different indications, some related to the treatment of inflammatory diseases. This study investigated the antioxidant activities of 33 samples from 17 Vietnamese Mallotus species. We also evaluated potential cytotoxic activity against human cervix carcinoma HeLa and human lung fibroblast WI-38 cells. Our aim is to develop safe dietary supplements with a protective effect against various diseases caused by tissue damage and the acceleration of the aging process linked to reactive oxygen species. These tests allowed the identification of non-cytotoxic plant species exhibiting significant antiradical properties. These antioxidant properties may be explained by their polyphenol composition. The antioxidant activity of the most active Mallotus species was further analyzed with and without tannins removal. We also identified by LC-ESI-MS some flavonoids responsible for a part of this activity.     

Anisotropic mesh adaptation: towards user‐independent,mesh‐independent and solver‐independent CFD. Part I: general principles

The present paper is the lead article in a three‐part series on anisotropic mesh adaptation and its applications to structured and unstructured meshes. A flexible approach is proposed and tested on two‐dimensional, inviscid and viscous, finite volume and finite element flow solvers, over a wide range of speeds. The directional properties of an interpolation‐based error estimate, extracted from the Hessian of the solution, are used to control the size and orientation of mesh edges. The approach is encapsulated into an edge‐based anisotropic mesh optimization methodology (MOM), which uses a judicious sequence of four local operations: refinement, coarsening, edge swapping and point movement, to equi‐distribute the error estimate along all edges, without any recourse to remeshing. The mesh adaptation convergence of the MOM loop is carefully studied for a wide variety of test cases. The mesh optimization generic coupling of MOM with finite volume and finite element flow solvers is shown to yield the same final mesh no matter what the starting point is. It is also shown that on such optimized meshes, the need for computational fluid dynamics (CFD) stabilization artifices, such as upwinding or artificial viscosity, are drastically reduced, if not altogether eliminated, in most well‐posed formulations. These two conclusions can be considered significant steps towards mesh‐independent and solver‐independent CFD. The structure of the three‐part series is thus, 1, general principles; 2, methodology and applications to structured and unstructured grids; 3, applications to three‐dimensional flows. Copyright © 2000 John Wiley & Sons, Ltd.     

Oxygen isotope effects on electron transfer to O2 probed using chemically modified flavins bound to glucose oxidase

Apo-glucose oxidase has been reconstituted with flavins modified in the 7 and 8 positions and characterized with regard to the catalytic rate of O(2) reduction and oxygen-18 isotope effects on this process. Kinetic studies as a function of driving force indicate a reorganization energy for electron transfer to O(2) of lambda = 28 kcal mol(-)(1) at optimal pH, which is similar to the value obtained earlier from temperature dependencies of rates (Roth, J. P.; Klinman, J. P. Proc. Natl. Acad. Sci. U.S.A. 2003, 100, 62-67). For the various enzyme-bound flavins, competitive oxygen-18 kinetic isotope effects fall within the narrow range of 1.0266(5) to 1.0279(6), apparently because of the dominant contribution of outer-sphere reorganization to the activation barrier; within the context of semiclassical and quantum mechanical electron transfer theories, the magnitude of the isotope effects reveals the importance of nuclear tunneling.     

Beyond structural motifs: the frontier of actinide-containing metal–organic frameworks

In this perspective, we feature recent advances in the field of actinide-containing metal–organic frameworks (An-MOFs) with a main focus on their electronic, catalytic, photophysical, and sorption properties. This discussion deviates from a strictly crystallographic analysis of An-MOFs, reported in several reviews, or synthesis of novel structural motifs, and instead delves into the remarkable potential of An-MOFs for evolving the nuclear waste administration sector. Currently, the An-MOF field is dominated by thorium- and uranium-containing structures, with only a few reports on transuranic frameworks. However, some of the reported properties in the field of An-MOFs foreshadow potential implementation of these materials and are the main focus of this report. Thus, this perspective intends to provide a glimpse into the challenges, triumphs, and future directions of An-MOFs in sectors ranging from the traditional realm of gas sorption and separation to recently emerging areas such as electronics and photophysics.

This perspective deviates from exclusively focusing on structural features of actinide-containing metal-organic frameworks and pivots towards their prospect as avant-garde materials with an emphasis on their physicochemical properties.     

Pyrochlore phases in the system ZnOBi2O3Sb2O5: II. Crystal structures of Zn2Bi3.08Sb2.92O14+δ and Zn2+xBi2.96−(x−y)Sb3.04−yO14.04+δ

Rietveld refinement of combined X-ray and neutron diffraction data has, within errors, confirmed the stoichiometries of two, cubic pyrochlore phases in the ZnOBi₂O₃Sb₂O₅ system. Neither phase has the ‘ideal’ stoichiometry, Zn₂Bi₃Sb₃O₁₄. One phase, P₁, is a Zn-rich, Bi-deficient solid solution Zn_2+xBi_{2.96−(x−y)}Sb_3.04−yO_14.04+δ. The other, P₂, is a Bi-rich line phase, stoichiometry Zn₂Bi_3.08Sb_2.92O_14+δ. Both structures have a mixture of Bi, Zn on the A-sites and Zn, Sb on the B-sites. However, Zn is displaced off-centre in the A-sites to achieve lower co-ordination number with realistic ZnO bond lengths. Additional structural complexities arise from: displacement of O(2) atoms; partial occupancies of O(1) and O(2) sites; partial occupancy of a third, interstitial oxygen site, O(3). Since the multiplicities of the off-centre sites are much higher than those of the ideal positions, there is considerable possibility for correlated short range order throughout the structures.