Atomistic in situ investigation of the morphogenesis of grains during pressure-induced phase transitions: molecular dynamics simulations of the B1-B2 transformation of RbCl

The mechanistic details of the pressure-induced B1-B2 phase transition of rubidium chloride are investigated in a series of transition path sampling molecular dynamics simulations. The B2→B1 transformation proceeds by nucleation and growth involving several, initially separated, nucleation centers. We show how independent and partially correlated nucleation events can function within a global mechanism and explore the evolution of phase domains during the transition. From this, the mechanisms of grain boundary formation are elaborated. The atomic structure of the domain-domain interfaces fully support the concept of Bernal polyhedra. Indeed, the manifold of different grain morphologies obtained from our simulations may be rationalized on the basis of essentially only two different kinds of Bernal polyhedra. The latter also play a crucial role for the B1→B2 transformation and specific grain boundary motifs are identified as preferred nucleation centers for this transition.     

Binary boron-rich borides of magnesium: single-crystal investigations and properties of MgB(7) and the new boride Mg(∼5)B(44)

Single crystals of dark-red MgB(7) were grown from the elements in a Cu-melt. The crystal structure (Pearson symbol oI64; space group Imma; a = 10.478(2) ?, b = 5.977(1) ?, c = 8.125(2) ?, 2842 reflns, 48 params, R(1)(F) = 0.018, R(2)(I) = 0.034) consists of a hexagonal-primitive packing of B(12)-icosahedra and B(2)-units in trigonal-prismatic voids. According to the UV-vis spectra and band structure calculations MgB(7) is semiconducting with an optical gap of 1.9 eV. The long B-B distance of 2.278 ? within the B(2)-unit can be seen as a weak bonding interaction. The new Mg(～5)B(44) occurs beside the well-known MgB(12) as a byproduct. Small fragments of the black crystals are dark-yellow and transparent. The crystal structure (Pearson symbol tP196, space group P4(1)2(1)2, a = 10.380(2) ?, c = 14.391(3) ?, 4080 reflns, 251 params, R(1)(F) = 0.025, R(2)(I) = 0.037) is closely related to tetragonal boron-II (t-B(192)). It consists of B(12)-icosahedra and B(19+1)-units. With a charge of -6 for the B(19+1)-units and a Mg-content of ～20 Mg-atoms per unit cell the observed Mg content in Mg(～5)B(44) is quite close to the expected value derived from simple electron counting rules. All compositions were confirmed by EDXS. The microhardness was measured on single crystals for MgB(7) (H(V) = 2125, H(K) = 2004) and MgB(12) (H(V) = 2360, H(K) = 2459).     

A modular approach towards functional decoration of peptide-polymer nanotapes

Self-assembled peptide-polymer nanotapes of poly(ethylene oxide)-peptide conjugates are modified by a simple amine-azide transfer to create azide-containing nanofibres, which provide a platform for modular functionalization as demonstrated by the introduction of different carboxyl bearing entities to modulate the calcium binding properties of the nanotapes.     

A multisyringe flow-through sequential extraction system for on-line monitoring of orthophosphate in soils and sediments

A fully automated flow-through microcolumn fractionation system with on-line post-extraction derivatization is proposed for monitoring of orthophosphate in solid samples of environmental relevance. The system integrates dynamic sequential extraction using 1.0 mol l⁻¹ NH₄Cl, 0.1 mol l⁻¹ NaOH and 0.5 mol l⁻¹ HCl as extractants according to the Hieltjes-Lijklema (HL) scheme for fractionation of phosphorus associated with different geological phases, and on-line processing of the extracts via the Molybdenum Blue (MB) reaction by exploiting multisyringe flow injection as the interface between the solid containing microcolumn and the flow-through detector. The proposed flow assembly, capitalizing on the features of the multicommutation concept, implies several advantages as compared to fractionation analysis in the batch mode in terms of saving of extractants and MB reagents, shortening of the operational times from days to hours, highly temporal resolution of the leaching process and the capability for immediate decision for stopping or proceeding with the ongoing extraction. Very importantly, accurate determination of the various orthophosphate pools is ensured by minimization of the hydrolysis of extracted organic phosphorus and condensed inorganic phosphates within the time frame of the assay. The potential of the novel system for accommodation of the harmonized protocol from the Standards, Measurement and Testing (SMT) Program of the Commission of the European Communities for inorganic phosphorus fractionation was also addressed. Under the optimized conditions, the lowest detectable concentration at the 3σ level was ≤0.02 mg P l⁻¹ for both the HL and SMT schemes regardless of the extracting media. The repeatability of the MB assay was better than 2.5% and the dynamic linear range extended up to 7.0 mg P l⁻¹ in NH₄Cl and NaOH media and 15 mg P l⁻¹ whenever HCl is utilized as extractant for both the HL and SMT protocols.     

On‐Surface Domino Reactions: Glaser Coupling and Dehydrogenative Coupling of a Biscarboxylic Acid To Form Polymeric Bisacylperoxides

Herein we report the on‐surface oxidative homocoupling of 6,6′‐(1,4‐buta‐1,3‐diynyl)bis(2‐naphthoic acid) (BDNA) via bisacylperoxide formation on different Au substrates. By using this unprecedented dehydrogenative polymerization of a biscarboxylic acid, linear poly‐BDNA with a chain length of over 100 nm was prepared. It is shown that the monomer BDNA can be prepared in situ at the surface via on‐surface Glaser coupling of 6‐ethynyl‐2‐naphthoic acid (ENA). Under the Glaser coupling conditions, BDNA directly undergoes polymerization to give the polymeric peroxide (poly‐BDNA) representing a first example of an on‐surface domino reaction. It is shown that the reaction outcome varies as a function of surface topography (Au(111) or Au(100)) and also of the surface coverage, to give branched polymers, linear polymers, or 2D metal–organic networks.     

Characterization and optimization of liquid electrodes for lateral dielectrophoresis

Using the concept of insulator-based "electrodeless" dielectrophoresis, we present a novel geometry for shaping electric fields to achieve lateral deviation of particles in liquid flows. The field is generated by lateral planar metal electrodes and is guided along access channels to the active area in the main channel. The equipotential surfaces at the apertures of the access channels behave as vertical "liquid" electrodes injecting the current into the main channel. The field between a pair of adjacent liquid electrodes generates the lateral dielectrophoretic force necessary for particle manipulation. We use this force for high-speed deviation of particles. By adding a second pair of liquid electrodes, we focus a particle stream. The position of the focused stream can be swept across the channel by adjusting the ratio of the voltages applied to the two pairs. Based on conformal mapping, we provide an analytical model for estimating the potential at the liquid electrodes and the field distribution in the main channel. We show that the simulated particle trajectories agree with observations. Finally, we show that the model can be used to optimize the device geometry in different applications.     

Synthesis and characterization of (E)- and (Z)-3-mercapto-2-propenenitrile. Microwave spectrum of the Z-isomer

The kinetically unstable compound 3-mercapto-2-propenenitrile (HS-CH=CH-C[triple bond]N) has been prepared for the first time by flash vacuum pyrolysis at 800 degrees C of 3-(tert-butylthio)-2-propenenitrile with a yield of 77% and a Z:E ratio of 8:1. Several deuterium and 15N isotopologues were also prepared using isotopically enriched compounds. Quantum chemical calculations of the structural and conformational properties of the Z- and E-isomers were undertaken at the B3LYP/6-311++G(3df,2pd), MP2/6-311++G(3df,2pd), MP2/aug-cc-pVTZ, and G3 levels of theory. These methods all predict that the Z- and the E-forms each have two "stable" planar rotameric forms with the H-S-C=C link of atoms in either a synperiplanar or an antiperiplanar conformation, with the synperiplanar form of the Z-isomer as the global minimum. The Z-isomer has been investigated by means of Stark-modulation microwave spectroscopy. Spectra attributable to the parent and three deuterium-substituted isotopologues of a single conformer were recorded and assigned. Additionally, the spectrum belonging to the first excited state of the lowest bending vibration was assigned. The ground-state rotational constants obtained by the least-squares analysis of these transitions were found to be in excellent agreement with the corresponding approximate equilibrium values generated by the MP2/aug-cc-pVTZ calculations. The preferred conformer of this molecule was found to have a synperiplanar arrangement of the H-S-C=C chain of atoms and a planar or nearly planar geometry, with a stabilizing intramolecular hydrogen bond formed between the H atom of the thiol group and pi-electron density associated with the C[triple bond]N triple bond. The possible astrochemical/astrobiological significance of this compound is discussed.