Structural Variability in the Monofluoroethynylbenzenes Mediated through Interactions Involving ``Organic'' Fluorine

Competition among weak intermolecular interactions can lead to polymorphism, the appearance of various crystalline forms of a substance with comparable cohesive energies. The crystal structures of 2-fluorophenylacetylene (2FPA) and 3-fluorophenylacetylene (3FPA), both of which are liquids at ambient conditions, have been determined by in situ cryocrystallization. Both compounds exhibit dimorphs, with one of the forms observed in common, P2(1), Z = 2 and the other form being Pna2(1), Z = 4 for 2FPA and P2(1)/c, Z = 12 for 3FPA. Variations in the crystal structures of the dimorphs of each of these compounds arise from subtle differences in the way in which weak intermolecular interactions such as C-H center dot center dot center dot pi and C-H center dot center dot center dot F are manifested. The interactions involving ``organic'' fluorine, are entirely different from those in the known structure of 4-fluorophenylacetylene (4FPA), space group P2(1)/c, Z = 4. The commonalities and differences in these polymorphs of 2FPA and 3FPA have been analyzed in terms of supramolecular synthons and extended long-range synthon aufbau module (LSAM) patterns. These structures are compared with the three polymorphs of phenylacetylene, in terms of the T-shaped C-H center dot center dot center dot pi interaction, a feature common to all these structures.

Onset of sphalerite to wurtzite transformation in ZnS nanoparticles

In this work, the incubation period for the onset of sphalerite to wurtzite transformation in isolated ZnS nanoparticles 2 to 7 nm in size was determined via the in situ isothermal annealing of as-synthesized sphalerite nanoparticles in a transmission electron microscope (TEM). Nanoparticles sitting on the TEM grid were well separated from each other in order to minimize particle sintering during the annealing operation. The phase transformation onset was observed at 300 degrees C, 350 degrees C, and 400 degrees C after 90, 10, and 4 min, respectively. These time-temperature data for the phase transformation onset were then used to calculate the activation energy for the nucleation of the wurtzite phase in 2 to 7 nm sphalerite particles. The activation energy determined was 24 Kcal/mol. (C) 2011 American Institute of Physics. [doi: 10.1063/1.3622625]

Rhodium Carbenoid Mediated C-H Activation of a Tertiary Methyl Group: An Enantiospecific Approach to the Angular Triquinanes Norsilphiperfolane and Norcameroonanes

An enantiospecific synthesis of the angular triquinane system present in the sesquiterpenes cameroonanes and silphiperfolanes has been accomplished, starting from 5-isopropenyl-2-methylcyclopent-1-ene-1-methanol [readily available in three steps from (R)-limonene] employing an intramolecular rhodium carbenoid insertion into the C-H bond of a tertiary methyl group for the construction of the triquinane system.

Characterization of phase transformation behaviour and microstructural development of electroless Ni-B coating

Phase transformation behaviour of amorphous electroless Ni-B coating with a targeted composition of Ni-6wt% B is characterized in conjunction with microstructural development and hardness. Microscopic observations of the as-deposited coating display a novel microstructure which is already phase separated at multiple length scales. Spherical colonies of similar to 5 mu m consist of 2-3 mu m nodular regions which are surrounded by similar to 2-3 mu m region that contains fine bands ranging from 10 to 70 nm in width. The appearance of three crystalline phases in this binary system at different stages of heat treatment and the concomitant variation in hardness are shown to arise from nanoscale fluctuations in the as-deposited boron content from 4 to 8 wt%. High temperature annealing reveals continuous crystallization up to 430 degrees C, overlapping with the domain of B loss due to diffusion into the substrate. The implications of such a microstructure for optimal heat treatment procedures are discussed. (C) 2011 Elsevier B.V. All rights reserved.

Inhibition of peroxynitrite- and peroxidase-mediated protein tyrosine nitration by imidazole-based thiourea and selenourea derivatives

In the present study, the synthesis and characterization of a series of N-methylimidazole-based thiourea and selenourea derivatives are described. The new compounds were also studied for their ability to inhibit peroxynitrite (PN)- and peroxidase-mediated nitration of protein tyrosine residues. It has been observed that the selenourea derivatives are more efficient than the thiourea-based compounds in the inhibition of protein nitration. The higher activity of selenoureas as compared to that of the corresponding thioureas can be ascribed to the zwitterionic nature of the selenourea moiety. Single crystal X-ray diffraction studies on some of the thiourea and selenourea derivatives reveal that the C S bonds in thioureas possess more of double bond character than the C=Se bonds in the corresponding selenoureas. Therefore, the selenium compounds can react with PN or hydrogen peroxide much faster than their sulfur analogues. The reactions of thiourea and selenourea derivatives with PN or hydrogen peroxide produce the corresponding sulfinic or seleninic acid derivatives, which upon elimination of sulfurous/selenous acids produce the corresponding N-methylimdazole derivatives.

Evolution of crystallinity of free gold agglomerates and shape transformation

We report the shape evolution of free gold agglomerates with different morphologies that transform to ellipsoidal and then to spherical shapes during the heating cycle. The shape transformation is associated with a structural transition from polycrystalline to single crystalline. The structural transition temperature is shown to be dependent on the final size of the particles and not on the initial morphologies of the agglomerates. It is also shown that the transition occurs well below the melting temperature which is in contrast with the melt-freeze process reported in the literature.

Size and temperature dependent stability and phase transformation in single-crystal zirconium nanowire

A novel size dependent FCC (face-centered-cubic) -> HCP (hexagonally-closed-pack) phase transformation and stability of an initial FCC zirconium nanowire are studied. FCC zirconium nanowires with cross-sectional dimensions < 20 are found unstable in nature, and they undergo a FCC -> HCP phase transformation, which is driven by tensile surface stress induced high internal compressive stresses. FCC nanowire with cross-sectional dimensions > 20 , in which surface stresses are not enough to drive the phase transformation, show meta-stability. In such a case, an external kinetic energy in the form of thermal heating is required to overcome the energy barrier and achieve FCC -> HCP phase transformation. The FCC-HCP transition pathway is also studied using Nudged Elastic Band (NEB) method, to further confirm the size dependent stability/metastability of Zr nanowires. We also show size dependent critical temperature, which is required for complete phase transformation of a metastable-FCC nanowire.

Nanostructuration via solid state transformation as a strategy for improving the thermoelectric efficiency of PbTe alloys

The approach taken in this paper in order to modify the scattering features of electrons and phonons and improve the figure of merit (ZT) of thermoelectric PbTe is to alter the microstructure at constant chemistry. A lamellar pattern of PbTe/GeTe at the nano- and microscale was produced in Pb(0.36)Ge(0.64)Te alloy by the diffusional decomposition of a supersaturated solid solution. The mechanism of nanostructuration is most likely a discontinuous spinodal decomposition. A simple model relating the interface velocity to the observed lamellar spacing is proposed. The effects of nanostructuration in Pb(0.36)Ge(0.64)Te alloy on the electrical and thermal conductivity, thermopower and ZT were investigated. It was shown that nanostructuration through the formation of a lamellar pattern of PbTe/GeTe is unlikely to provide a significant improvement due to the occurrence of discontinuous coarsening. However, the present study allows an analysis of possible strategies to improve thermoelectric materials via optimal design of the microstructure and optimized heat treatment. (C) 2011 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

Highly efficient dialkylphosphate-mediated syntheses of hydantoins and a bicyclohydantoin under solvent-free conditions

Diversely substituted hydantoins have been synthesized by new strategy from cyanamide based precursor, that is, methyl N-cyano-N-alkyl/arylaminoacetate. Dialkylphosphates were employed as the mild reagent to hydrolyze and cyclize the substrate in one step to give quantitative yields of the desired products. Syntheses of multivalent hydantoins viz bis-hydantoin, bicyclohydantoin have potentially widened the scope and applicability of the present method. Solvent-free conditions and very easy work-up procedure make the reaction convenient and eco-friendly. Single crystal structures of some of the representative compounds are also reported. (C) 2011 Elsevier Ltd. All rights reserved.

Production of niobium powder by electronically mediated reaction (EMR) using calcium as a reductant

Explored in this study is an electronically mediated reaction (EMR) route for the production of niobium powder using calcium as a reductant for niobium oxide (Nb2O5). Feed material, Nb2O5, and reductant calcium alloy containing aluminum and nickel were charged into electronically isolated locations in a molten salt (e.g. CaCl2) at 1173 K. The current flow through an external path between the feed and reductant locations was monitored. A current approximately 0.4 A was measured during the reaction in the external circuit connecting cathode and anode location. Niobium powder with low aluminum and nickel content was obtained although liquid Ca–Al–Ni alloy was used as the reductant. This clearly demonstrates that niobium metal powder can be produced by an electronically mediated reaction (EMR), without direct physical contact between feed (Nb2O5) and reductant (calcium). Mechanism of calciothermic reduction of Nb2O5 in the molten salt is discussed using an isothermal chemical potential diagram.

Usefulness of in Situ Single Crystal to Single Crystal Transformation (SCSC) Studies in Understanding the Temperature-Dependent Dimensionality Cross-over and Structural Reorganization in Copper-Containing Metal-Organic Frameworks (MOFs)

Two copper-containing compounds [Cu(3)(mu(3)-OH)(2)-(H(2)O)(2){(SO(3))-C(6)H(3)-(COO)(2)}(CH(3)COO)] , I, and [Cu(5)(mu(3)-OH)(2)(H(2)O)(6){(NO(2))-C(6)H(3)-(COO)(2)}(4)]center dot 5H(2)O, II, were prepared using sulphoisophthalic and nitroisophthalic acids. The removal of the coordinated water molecules in the compounds was investigated using in situ single crystal to single crystal (SCSC) transformation studies, temperature-dependent powder X-ray diffraction (PXRD), and thermogravimetric analysis (TGA). The efficacy of SCSC transformation studies were established by the observation of dimensionality cross-over from a two-dimensional (I) to a three-dimensional structure, Cu(6)(mu(3)-OH)(4){(SO(3))-C(6)H(3)-(COO)(2)}(2)(CH(3)COO)(2), Ia, during the removal of the coordinated water molecules. Compound H exhibited a structural reorganization forming Cu(5)(mu(2)-OH)(2){(NO(2))C(6)H(3)-(COO)(2))(4)], Ha, possessing trimeric (Cu(3)O(12)) and dimeric (Cu(2)O(8)) copper clusters. The PXRD studies indicate that the three-dimensional structure (Ia) is transient and unstable, reverting back to the more stable two-dimensional structure (I) on cooling to room temperature. Compound Ha appears to be more stable at room temperature. The rehydration/dehydration studies using a modified TGA setup suggest complete rehydration of the water molecules, indicating that the water molecules in both compounds are labile. A possible model for the observed changes in the structures has been proposed. Magnetic studies indicate changes in the exchanges between the copper centers in Ha, whereas no such behavior was observed in Ia.

Influence of Tempered Microstructures on the Transformation Behaviour of Cold Deformed and Intercritically Annealed Medium Carbon Low Alloy Steel

This research is focused on understanding the role of microstructural variables and processing parameters in obtaining optimised dual phase structures in medium carbon low alloy steels. Tempered Martensite structures produced at 300, 500, and 650 degrees C, were cold rolled to varied degrees ranging from 20 to 80% deformation. Intercritical annealing was then performed at 740, 760, and 780 degrees C for various time duration ranging from 60 seconds to 60 minutes before quenching in water. The transformation behaviour was studied with the aid of optical microscopy and hardness curves. From the results, it is observed that microstructural condition, deformation, and intercritical temperatures influenced the chronological order of the competing stress relaxation and decomposition phase reactions which interfered with the rate of the expected alpha -> gamma transformation. The three unique transformation trends observed are systematically analyzed. It was also observed that the 300 and 500 degrees C tempered initial microstructures were unsuitable for the production of dual structures with optimized strength characteristics.

On the athermal nature of the beta to omega transformation

One characteristic feature of the athermal beta -> omega transformation is the short time scale of the transformation. So far, no clear understanding of this issue exists. Here we construct a model that includes contributions from a Landau sixth-order free energy density, kinetic energy due to displacement, and the Rayleigh dissipation function to account for the dissipation arising from the rapid movement of the parent product interface during rapid nucleation. We also include the contribution from omega-like fluctuations to local stress. The model shows that the transformation is complete on a time scale comparable to the velocity of sound. The estimated nucleation rate is several orders higher than that for diffusion-controlled transformations. The model predicts that the athermal omega phase is limited to a certain range of alloying composition. The estimated nucleation rate and the size of ``isothermal'' particles beyond 17% Nb are also consistent with experimental results. The model provides an explanation for the reprecipitation process of the omega particles in the ``cleared'' channels formed during deformation of omega-forming alloys. The model also predicts that acoustic emission should be detectable during the formation of the athermal phase. (C) 2011 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

Retention Time Variability as a Mechanism for Animal Mediated Long-Distance Dispersal

Long-distance dispersal (LDD) events, although rare for most plant species, can strongly influence population and community dynamics. Animals function as a key biotic vector of seeds and thus, a mechanistic and quantitative understanding of how individual animal behaviors scale to dispersal patterns at different spatial scales is a question of critical importance from both basic and applied perspectives. Using a diffusion-theory based analytical approach for a wide range of animal movement and seed transportation patterns, we show that the scale (a measure of local dispersal) of the seed dispersal kernel increases with the organisms' rate of movement and mean seed retention time. We reveal that variations in seed retention time is a key determinant of various measures of LDD such as kurtosis (or shape) of the kernel, thinkness of tails and the absolute number of seeds falling beyond a threshold distance. Using empirical data sets of frugivores, we illustrate the importance of variability in retention times for predicting the key disperser species that influence LDD. Our study makes testable predictions linking animal movement behaviors and gut retention times to dispersal patterns and, more generally, highlights the potential importance of animal behavioral variability for the LDD of seeds.