Pulsed field-gradient spin echo N.M.R. studies of flow in fluids

An analysis is presented of the effects of various flow patterns on spin echo shapes and amplitudes in the presence of both static and pulsed gradients in the applied magnetic field. Two approaches are used, one based on the Bloch equations, the other on a molecular average picture. In the case of flow patterns with velocity gradients the effects of self-diffusion across these on the N.M.R. experiments is pointed out and shown to be negligible in most cases of interest. Extension of a multipulse sequence for the study of flow to include pulsed field gradients is outlined. Experiments are described which largely confirm the theoretical predictions (except in the case of echo shapes) for plug flow and laminar flow in a circular pipe. The problem of loss of labelled nuclei from the receiver coil during the experiment is treated in detail for the case of laminar flow. The method is applied to a brief study of the flow properties of agar gel as a function of agar concentration. It is shown by using both phase sensitive and diode detectors that the flow pattern changes from laminar at low agar concentration to a sheared plug flow at higher concentrations.     

Group-type quantitative analysis of flavor compounds in ripening avocados

Avocados are a superfood gaining popularity in people's diet. Profiling and quantifying the volatiles associated with flavor can further help in understanding the fruit. However, this is challenging due to relatively low abundance of volatile compounds. The complex mixtures inherent to avocado flavor can result in coelutions using classical chromatographic techniques. To overcome these challenges, solid-phase microextraction was used to extract and preconcentrate volatiles, then separated and quantified using two-dimensional gas chromatography with a flame ionization detector. This technique enhances separation power and produces well-ordered chromatograms, allowing for templated groupings of compounds of similar chemical composition into regions. Using the flame ionization detector, an average response factor was determined and used for quantification of these templated group-type regions, as well as individual compounds. This group-type quantification improved the overall precision of compound classes in 50 avocados by at least a factor of 2, when compared to that of the individual components. Overall, the abundance of associated flavor groups, such as terpenes and alcohols decreased, whereas aldehyde groups remained constant throughout ripening. The combination of solid-phase microextraction with two-dimensional gas chromatography and group-type quantification allows for an overall better understanding of the volatiles associated with flavor of avocados.     

Formation and evaporation of nonsingular black holes

Regular (nonsingular) space-times are given that describe the formation of a (locally defined) black hole from an initial vacuum region, its quiescence as a static region, and its subsequent evaporation to a vacuum region. The static region is Bardeen-like, supported by finite density and pressures, vanishing rapidly at large radius and behaving as a cosmological constant at small radius. The dynamic regions are Vaidya-like, with ingoing radiation of positive-energy flux during collapse and negative-energy flux during evaporation, the latter balanced by outgoing radiation of positive-energy flux and a surface pressure at a pair creation surface. The black hole consists of a compact space-time region of trapped surfaces, with inner and outer boundaries that join circularly as a single smooth trapping horizon.     

Structural consequences of the one-electron reduction of d4 [Mo(CO)2(eta-PhC[triple bond]CPh)Tp']+ and the electronic structure of the d5 radicals [M(CO)L(eta-MeC[triple bond]CMe)Tp'] {L = CO and P(OCH2)3CEt}

Reduction of [M(CO)2(eta-RC[triple bond]CR')Tp']X {Tp' = hydrotris(3,5-dimethylpyrazolyl)borate, M = Mo, X = [PF6]-, R = R' = Ph, C6H4OMe-4 or Me; R = Ph, R' = H; M = W, X = [BF4]-, R = R' = Ph or Me; R = Ph, R' = H} with [Co(eta-C5H5)2] gave paramagnetic [M(CO)2(eta-RC[triple bond]CR')Tp'], characterised by IR and ESR spectroscopy. X-Ray structural studies on the redox pair [Mo(CO)2(eta-PhC[triple bond]CPh)Tp'] and [Mo(CO)2(eta-PhC[triple bond]CPh)Tp'][PF6] showed that oxidation is accompanied by a lengthening of the C[triple bond]C bond and shortening of the Mo-C(alkyne) bonds, consistent with removal of an electron from an orbital antibonding with respect to the Mo-alkyne bond, and with conversion of the alkyne from a three- to a four-electron donor. Reduction of [Mo(CO)(NCMe)(eta-MeC[triple bond]CMe)Tp'][PF6] with [Co(eta-C5H5)2] in CH2Cl2 gives [MoCl(CO)(eta-MeC[triple bond]CMe)Tp'], via nitrile substitution in [Mo(CO)(NCMe)(eta-MeC[triple bond]CMe)Tp'], whereas a similar reaction with [M(CO){P(OCH2)3CEt}(eta-MeC[triple bond]CMe)Tp']+ (M = Mo or W) gives the phosphite-containing radicals [M(CO){P(OCH2)3CEt}(eta-MeC[triple bond]CMe)Tp']. ESR spectroscopic studies and DFT calculations on [M(CO)L(eta-MeC[triple bond]CMe)Tp'] {M = Mo or W, L = CO or P(OCH2)3CEt} show the SOMO of the neutral d5 species (the LUMO of the d4 cations) to be largely d(yz) in character although much more delocalised in the W complexes. Non-coincidence effects between the g and metal hyperfine matrices in the Mo spectra indicate hybridisation of the metal d-orbitals in the SOMO, consistent with a rotation of the coordinated alkyne about the M-C2 axis.     

The cation-deficient Ruddlesden-Popper oxysulfide Y2Ti2O5S2 as a layered sulfide: topotactic potassium intercalation to form KY2Ti2O5S2

Potassium intercalation into the cation-deficient n = 2 Ruddlesden-Popper oxysulfide Y(2)Ti(2)O(5)S(2) to form KY(2)Ti(2)O(5)S(2) has been carried out by reaction of the oxysulfide with potassium vapor in sealed metal tubes at 400 degrees C, potassium naphthalide in THF at 50 degrees C, or potassium in liquid ammonia at temperatures as low as -78 degrees C. Insertion of potassium is topotactic, and although a site 12-coordinate by oxide ions is vacant in the perovskite-type oxide slabs of the structure, potassium is too large to enter this site via the 4-coordinate window, and instead enters the rock-salt-type sulfide layers of the structure which necessitates a 30% increase in the lattice parameter c normal to the layers. In contrast with one of the sodium intercalates of Y(2)Ti(2)O(5)S(2) (beta-NaY(2)Ti(2)O(5)S(2)) in which sodium occupies a tetrahedral site in the sulfide layers, potassium favors an 8-coordinate site which necessitates a relative translation of adjacent oxide slabs. KY(2)Ti(2)O(5)S(2) is tetragonal: P4/mmm, a = 3.71563(4) A, c = 14.8682(2) A (at 298 K), Z = 1. Although the resistivity (3.4(1) x 10(3) Omega cm) is larger than would be expected for a metal, temperature independent paramagnetism dominates the magnetic susceptibility, and the material is electronically very similar to the analogous sodium intercalate beta-NaY(2)Ti(2)O(5)S(2) which features reduced-titanium-containing oxide layers of very similar geometry and electron count.     

Selective deintercalation of apex over face-shared oxide ions in the topotactic reduction of Sr7Mn4O15 to Sr7Mn4O12

Sodium hydride selectively deintercalates the apex rather than face-shared oxide ions within the structure of Sr(7)Mn(4)O(15) leading to the formation of the structurally related reduced phase Sr(7)Mn(4)O(12).     

Shear rheology of lyotropic liquid crystals: a case study

We have investigated the rheological properties of lyotropic liquid crystals (LCs) formed by self-assembled neutral lipids and water, their relationship with the topology of the structure, and their dependence on temperature and water content. The phase diagram of a representative monoglyceride-water system, determined by combining cross-polarized optical microscopy and small-angle X-ray scattering (SAXS), included four structures: lamellar, hexagonal, gyroid bicontinuous cubic (Ia3d), and double diamond bicontinuous cubic (Pn3m), as well as several regions of two-phase coexistence of some of the above structures. Rheology in the linear viscoelastic regime revealed a specific signature that was characteristic of the topology of each structure considered. The order-order transitions lamellar-to-cubic and cubic-to-hexagonal, as well as the order-disorder transitions from each LC to an isotropic fluid, were easily identified by following the development of the storage and loss moduli, G' and G', respectively. The viscoelastic properties of both bicontinuous cubic phases were shown to be strongly frequency-dependent, following a pseudo-Maxwell behavior, with multiple relaxation times. Cubic-to-cubic transitions were nicely captured by scaling the longest relaxation time, tau, with either temperature or water volume fraction. Therefore, the set of the three main parameters used to establish the rheological behavior of the structure, that is, G', G', and relaxation time, tau, constitutes a consistent ensemble to identify the structures of the liquid crystal. Finally, relaxation spectra, extracted for all liquid crystalline phases, allowed an additional possible identification criterion of the various structures considered.