129Xe NMR spectroscopy study of porous cyanometallates

Zinc and cadmium hexacyanocobaltates(III) were prepared, and their porous networks were explored using 129Xe spectroscopy. The crystal structures of these two compounds are representative of porous hexacyanometallates, cubic (Fm-3m) for cadmium and rhombohedral (R-3c) for zinc. In the cubic structure, the porosity is related to systematic vacancies created from the elemental building block (i.e., the hexacyanometallate anion), whereas the rhombohedral (R-3c) structure is free of vacant sites but has tetrahedral coordination for the zinc atom, which leads to relatively large ellipsoidal pores communicated by elliptical windows. According to the Xe adsorption isotherms, these porous frameworks were found to be accessible to the Xe atom. The structure of the higher electric field gradient at the pore surface (Fm-3m) appears and is accompanied by a stronger guest-host interaction for the Xe atoms and a higher capacity for Xe sorption. For cadmium, the 129Xe NMR signal is typical of isotropic movement for the Xe atom, indicating that it remains trapped within a spherical cavity. From spectra recorded for different amounts of adsorbed Xe, the cavity diameter was estimated. For the zinc complex, 129Xe NMR spectra are asymmetric because of the Xe atom movement within an elongated cavity. The line-shape asymmetry changes when the Xe loading within the porous framework increases, which was ascribed to Xe-Xe interactions through the cavity windows. The Xe adsorption revealed additional structural information for the studied materials.     

Multiresidue analysis of three groups of pesticides in washing waters from olive processing by solid-phase extraction-gas chromatography with electron capture and thermionic specific detection

A multiresidue solid phase extraction-gas chromatography (SPE-GC) method is developed for the analysis of 28 organochlorine (OCPs), organophosphorus (OPPs) and organonitrogen (ONPs) pesticides at ng l^− 1 levels in washing water from olive processing. Pesticides are separated from the water matrix, containing estimable amounts of olive oil, by eluting them from a C₁₈ SPE cartridge using dichloromethane. The target compounds are determined in the final extract by gas chromatography using thermionic specific (TSD) and electron capture (ECD) detection. Recoveries range from 79% to 129% in spiked washing water samples and detection limits are between 1 and 4 ng l^− 1. The method was successfully applied to determine OCPs, OPPs and ONPs in 25 samples of water collected directly in the inlet and outlet of the washing devices of three olive mills at different locations in Jaén (Spain). The most frequently encountered pesticide residues were terbuthylazine, simazine and diuron. In some of the water samples coming from the inlet of the washing devices, terbuthylazine and diuron measured concentrations were above the concentration limits scheduled by law.     

Families of arcs on rational surface singularities

We consider the spaceH of arcs on a rational surface singularity (S, P), with the proalgebraic structure induced by the truncation maps. We introduce some sets of arcs by imposing valuative conditions and we prove that they are closed subsets ofH. This leads to give a sufficient condition in order to have an affirmative answer for the problem of Nash. We conclude the solution of the problem for the minimal surface singularities. Supported by D.G.I.C.Y.T. PB91-0210-C02-01     

Adsorption and separation of propane and propylene by porous hexacyanometallates

The separation capability for mixtures of propane and propylene by porous frameworks representatives of transition metal hexacyanometallates was studied from adsorption data under equilibrium conditions at 273.15 K and from inverse gas chromatography profiles at different column temperatures. Samples of two porous solids were considered; Cd₃[Co(CN)₆]₂, which is representative of Prussian blue analogues (cubic structure) with a porous framework related to vacancies for building block, and Zn₃[Co(CN)₆]₂ (rhombohedral phase) where the porous framework results from the tetrahedral coordination for the Zn atoms. The two materials were found to be able for the mixtures separation, with the highest separation ability for the rhombohedral phase under equilibrium conditions but, in dynamic conditions the cubic one shown a better separation, which was ascribed to a kinetic contribution related to a smaller windows size.     

Sharp Békollé estimates for the Bergman projection

We prove sharp estimates for the Bergman projection in weighted Bergman spaces in terms of the Békollé constant. Our main tools are a dyadic model dominating the operator and an adaptation of a method of Cruz-Uribe, Martell and Pérez.     

Singularities with the highest Mather minimal log discrepancy

This paper characterizes singularities with Mather minimal log discrepancies in the highest unit interval, i.e., the interval between $d-1$ and $d$ , where $d$ is the dimension of the scheme. The class of these singularities coincides with one of the classes of (1) compound Du Val singularities, (2) normal crossing double singularities, (3) pinch points, and (4) pairs of non-singular varieties and boundaries with multiplicities less than or equal to 1 at the point. As a corollary, we also obtain one implication of an equivalence conjectured by Shokurov for the usual minimal log discrepancies.     

Viral self-assembly as a thermodynamic process

The protein shells, or capsids, of nearly all spherelike viruses adopt icosahedral symmetry. In the present Letter, we propose a statistical thermodynamic model for viral self-assembly. We find that icosahedral symmetry is not expected for viral capsids constructed from structurally identical protein subunits and that this symmetry requires (at least) two internal "switching" configurations of the protein. Our results indicate that icosahedral symmetry is not a generic consequence of free energy minimization but requires optimization of internal structural parameters of the capsid proteins.     

Fusion of the extended modified liquid drop model for nucleation and dynamical nucleation theory

We present a new phenomenological approach to nucleation, based on the combination of the "extended modified liquid drop" model and dynamical nucleation theory. The new model proposes a new cluster definition, which properly includes the effect of fluctuations, and it is consistent both thermodynamically and kinetically. The model is able to predict successfully the free energy of formation of the critical nucleus, using only macroscopic thermodynamic properties. It also accounts for the spinodal and provides excellent agreement with the result of recent simulations.     

Role of the anion in the alkali halides interaction with D-ribose: a 1H and 13C NMR spectroscopy study

The alkali halides interaction with d-ribose in D2O solutions was studied by 1H and 13C NMR spectroscopy. The observed changes in the NMR spectra are interpreted according to a model in which the hydroxyls rich region, from C1 to C4, interacts with the cation while the CH2 group at C5 on the opposite side of the sugar interacts with the anion. It seems, during the salt-sugar interaction, cation and anion preserve, at least partially, their ion-pair character. The cooperative interaction of the sugar hydroxyl groups with the cation leads to a polarization within the sugar molecule, which favors the anion interaction with its most positive region. A correlation between the chemical shift of C5 atom and the atomic number of the anion was observed, which is discussed as a neighboring paramagnetic effect; as higher is the halogen atom more pronounced is the resulting shift of the C5 signal. The anion effect is weak but also observed in the 13C signals of those carbon atoms bound to hydroxyl groups where the interaction is predominant with the cation. The 1H signal of the anomeric protons and the relative population of isomers in the alkali halide solution also show an anion dependence.     

Thermal evolution of microporous nitroprussides on their dehydration process

Divalent transition metal nitroprussides form a family of microporous materials which lose their crystallization water (coordinated and zeolitic) below 100°C and then remain stable up to above 150°C. The dehydration process of representative samples in their stable phases was studied by thermo-gravimetry (TG) and differential scanning calorimetry (DSC). The copper complex dehydrates in a single step through a practically irreversible process. For cadmium and cobalt complexes the water evolution on heating takes place in two stages. The first one, where only zeolitic waters are removed, is dominated by a diffusion mechanism while, during the loss of the strongly bonded waters (second stage) the material framework effect is added. The involved activation energy and its dependence on the conversion degree were estimated evaluating the thermo-gravimetric data according to an isoconversion model.