Pressure-induced polymorphism in Al3BC3: A first-principles study

Al₃BC₃, an isostructural phase to Mg₃BN₃, experienced no pressure-induced phase transformation that occurred in the latter material (J. Solid State Chem. 154 (2000) 254-256). The discrepancy is not clear yet. Using the first-principles density functional calculations, we predict that Al₃BC₃ undergoes a hexagonal-to-tetragonal structural transformation at 24 GPa. The predicted phase equilibrium pressure is much higher than the previously reported pressure range, i.e., 2.5-5.3 GPa, conducted on phase stability of Al₃BC₃. A homogeneous orthorhombic shear strain transformation path is proposed for the phase transformation. The transformation enthalpy barrier is estimated to yield a low value, i.e., 0.129 eV/atom, which ensures that the transformation can readily take place at the predicted pressure.     

Synthesis and characterization of vanadium oxides nanorods

Vanadium oxides nanorods with high crystallinity and high surface area were synthesized by hydrothermal method using laurylamine hydrochloride, metal alkoxide and acetylacetone. The samples characterized by XRD, nitrogen adsorption isotherm, SEM, TEM, and SAED. Uniformly sized B phase VO₂ nanorods had widths about 40-80 nm and lengths reaching up to 1 μm. V₂O₅ rodlike structured with the widths about 100-500 nm and the lengths of 1-10 μm were obtained by calcination at 400 °C for 4 h. This synthesis method provides a new simple route to fabricate one-dimensional nanostructured metal oxides under mild conditions.     

Ab initio molecular dynamics simulation of pressure-induced phase transition in MgS

Pressure-induced phase transition in MgS is studied using a constant pressure ab initio molecular dynamics method, and a solid evidence of existence of its high-pressure phase is provided. As predicted by total energy calculations, MgS undergoes a structural phase transformation from the rocksalt structure to a CsCl-type structure under hydrostatic pressure. The transformation mechanism is characterized, and two intermediate phases having P4/nmm and P2₁/m symmetries for the rocksalt-to-CsCl-type phase transformation of MgS are proposed, which is different from the previously proposed mechanisms. We also study this phase transition using the total energy calculations. Our predicted transition parameters and bulk properties are in good agreement with the earlier first principle simulations.     

Computation of some thermodynamic properties of helium–neon and helium–krypton fluid mixtures using molecular dynamics simulation

We have performed molecular dynamics simulations to obtain internal energy and pressure of helium–neon and helium–krypton mixtures at different densities using accurate recently two-body ab initio potentials supplemented by quantum corrections following the Feynman–Hibbs approach. The significance of this work is that the three-body expression of Wang and Sadus [22] was used to improve prediction of the pressures and internal energies of helium + krypton and helium + neon mixtures without requiring an expensive three-body calculation. Our results show a good agreement with the corresponding experimental data.     

Structural and vibrational characterization of the organic semiconductor tetracene as a function of pressure and temperature

Neutron powder diffraction and inelastic neutron scattering measurements were performed on crystalline tetracene, a molecular semiconductor of triclinic crystal structure that adopts a herringbone layered motif, as a function of pressure up to 358 MPa. In combination with theoretical and simulated computations, these measurements permit detailed characterization of the structural and vibrational changes of tetracene as a function of pressure. Powder diffraction at 295 K reveals anisotropic modification of the crystal structure with increasing pressure. Particularly, the unit cell parameters associated with the two-dimensional herringbone layers of the solid state structure displayed continuous change at all measured pressures, whereas perpendicular to the herringbone layers the structure remains relatively unchanged. The measured compressibilities along the [1 0 0], [0 1 0], and [0 0 1] crystal axes are −3.8 × 10⁻⁴, −1.9 × 10⁻⁴, and −3.4 × 10⁻⁴ Å/MPa, respectively. Inelastic neutron scattering spectra were collected at several pressures in the 25–75 and 0–25 meV energy ranges using a filter analyzer and a Fermi chopper time-of-flight spectrometer, respectively. Assignment of the spectral peaks to specific intramolecular vibrational modes has been accomplished using ab initio density functional theory calculations and the low energy lattice phonon modes were interpreted from the results of molecular dynamics simulations at 1 atm and 358 MPa. Anisotropic behavior parallel to that observed in the structural measurements is also apparent in both the intramolecular and lattice phonon vibrational dynamics. Intramolecular vibrations having atomic displacements entirely within the plane of the molecule’s aromatic ring remain unchanged with increasing pressure while vibrations with atomic displacements perpendicular to the molecular plane shift to higher energy. The lattice phonons display a similar anisotropy with increasing pressure. Phonon modes propagated within the herringbone layer are significantly shifted to higher energy with increasing pressure relative to the modes with displacements primarily perpendicular to the layers. Overall, both the planar internal geometry and the layered arrangement of the tetracene molecules significantly influence the observed structural and vibrational behavior with increasing pressure.     

Simultaneous determination of aflatoxins B1, B2, G1, G2, M1 and M2 in peanuts and their derivative products by ultra-high-performance liquid chromatography-tandem mass spectrometry

A reliable ultra-high-performance liquid chromatography-tandem mass spectrometry (UHPLC-MS/MS) method for simultaneous determination of aflatoxins B1, B2, G1, G2, M1 and M2 in peanuts and their derivative products was developed. The sample was extracted by 84% of acetonitrile aqueous solution and the extract was purified by a reliable solid phase extraction-based clean-up method. Then, the analytes were separated on Acquity UPLC HSS T3 column (100 mm × 2.1 mm, 1.8 μm particle size), and eluted with a mobile phase consisting of (A) water containing 0.1% formic acid and (B) acetonitrile/methanol (50/50, v/v). The separated compounds were detected with a Waters Micromass Quattro Ultima Pt tandem quadrupole mass spectrometer operating in positive electro-spray ionization using multiple reaction monitoring mode. The established method was extensively validated by determining the linearity (R² ≥ 0.9990), average recovery (74.7-86.8%) and precision (relative standard deviation ≤ 10.9%). It was shown to be a suitable method for simultaneous determination of the six aflatoxins in peanuts and their derivative products. Finally, a total of 73 samples randomly collected from different areas in Zhejiang province were screened for aflatoxins with the proposed method. The results showed that 31 samples of peanut butter, 14 samples of fresh peanut and 5 samples of musty peanut were contaminated with aflatoxins. Meanwhile, this was the first report on aflatoxins M1 and M2, which were found in unprocessed peanuts and their derivative products.     

Synthesis of titanate, TiO2 (B), and anatase TiO2 nanofibers from natural rutile sand

Titanate nanofibers were synthesized by hydrothermal method (150 °C for 72 h) using natural rutile sand as the starting materials. TiO₂ (B) and anatase TiO₂ (high crystallinity) nanofibers with the diameters of 20-100 nm and the lengths of 10-100 μm were obtained by calcined titanate nanofibers for 4 h at 400 and 700 °C (in air), respectively. The samples characterized by XRD, SEM, TEM, SAED, HRTEM, and BET surface area. This synthesis method provides a simple route to fabricate one-dimensional nanostructured TiO₂ from low cost material.     

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.     

Micelle-mediated cloud point extraction and spectrophotometric determination of rhodamine B using Triton X-100

A new micelle-mediated cloud point extraction method is described for sensitive and selective determination of trace amounts of rhodamine B by spectrophotometry. The method is based on the cloud point extraction of rhodamine B from aqueous solution using Triton X-100 in acidic media. The extracted surfactant rich phase is diluted with water and its absorbance is measured at 563 nm by a spectophotometer. The effects of different operating parameters such as concentration of surfactant and salt, temperature and pH on the cloud point extraction of rhodamine B were studied in details and a set of optimum conditions were obtained. Under optimum conditions a linear calibration graph in the range of 5-550 ng mL⁻¹ of rhodamine B in the initial solution with r = 0.9991 (n = 15) was obtained. Detection limit based on three times the standard deviation of the blank (3S_b) was 1.3 ng mL⁻¹ (n = 10) and the relative standard deviation (R.S.D.) for 50 and 350 ng mL⁻¹ of rhodamine B was 2.40 and 0.87% (n = 10), respectively. The method was applied for the determination of rhodamine B in soft pastel, hand washing liquid soap, matches tip and textile dyes mixture samples.     

Zeolite synthesis by simulation of their natural formation conditions: from macroscopic to nanosized crystals

A successful route to prepare zeolites with controlled crystal size from the millimeter range down to nanometer scale by simulating the natural formation conditions is presented. Water is used as the solution and reaction medium at high temperatures and pressures (T=120-400°C, p=1 kbar); aluminosilicate glasses with identical chemical composition as the zeolite products are used as precursors. The synthesis of heulandite is reported as representative example.     

Palladium-free Suzuki-Miyaura cross-coupling at elevated pressures

Elevated pressure (15 kbar) in the liquid phase improves yields and permits the use of cheaper Fe(III), Co(II), and Ni(II) metal salts as catalysts for the Suzuki-Miyaura cross-coupling of aryl halides and aryl boronic acids.     

The Hydrophobic Gap at High Hydrostatic Pressures

We have gained new insight into the so‐called hydrophobic gap, a molecularly thin region of decreased electron density at the interface between water and a solid hydrophobic surface, by X‐ray reflectivity experiments and molecular dynamics simulations at different hydrostatic pressures. Pressure variations show that the hydrophobic gap persists up to a pressure of 5 kbar. The electron depletion in the interfacial region strongly decreases with an increase in pressure, indicating that the interfacial region is compressed more strongly than bulk water. The decrease is most significant up to 2 kbar; beyond that, the pressure response of the depletion is less pronounced.     

Separation of amino acids in glucose isomerase crystal: Insight from molecular dynamics simulations

The separation of amino acids (Arg, Phe and Trp) in a liquid chromatography is investigated using molecular dynamics simulations. A bioorganic nanoporous material – glucose isomerase crystal – is used as the stationary phase and water as the mobile phase. The transport velocities of amino acids decrease in the order Arg > Phe > Trp, consistent with experimental measurement. The elution order is not affected by the solute concentration or by the flowing rate of mobile phase. Arg is highly hydrophilic and charged, interacts with water the most strongly, and thus moves with flowing water the fastest. Trp has the largest van der Waals volume and encounters the largest steric hindrance, leading to the slowest velocity. From the number distributions of amino acids around protein surface, Trp and Phe are found to stay closer to protein than Arg. The solvent-accessible surface areas of amino acids and the numbers of hydrogen bonds between amino acids and water further elucidate the observed velocity difference. The simulation results provide useful microscopic insight into the retention mechanisms in chromatographic separation process and suggest that glucose isomerase crystal has the capability to separate amino acids.     

Synthesis,crystal structure and catalytic activity of ruthenium(II) carbonyl complexes containing ONO and ONS donor ligands

Diamagnetic ruthenium(II) complexes of the type [Ru(L)(CO)(B)(EPh₃)] [where E = As, B = AsPh₃; E = P, B = PPh₃, py (or) pip and L = dibasic tridentate ligands dehydroacetic acid semicarbazone (abbreviated as dhasc) or dehydroacetic acid phenyl thiosemicarbazone (abbreviated as dhaptsc)] were synthesized from the reaction of [RuHCl(CO)(B)(EPh₃)₂] (where E = As, B = AsPh₃; E = P, B = PPh₃, py (or) pip) with different tridentate chelating ligands derived from dehydroacetic acid with semicarbazide or phenylthiosemicarbazide. All the complexes have been characterized by elemental analysis, FT-IR, UV–Vis and ¹H NMR spectral methods. The coordination mode of the ligands and the geometry of the complexes were confirmed by single crystal X-ray crystallography of one of the complexes [Ru(dhaptsc)(CO)(PPh₃)₂] (5). All the complexes are redox active and are monitored by cyclic voltammetric technique. Further, the catalytic efficiency of one of the ruthenium complexes (5) was determined in the case of oxidation of primary and secondary alcohols into their corresponding aldehydes and ketones in the presence of N-methylmorpholine-N-oxide.     

From unsaturated dodecahedranes to C40 cages?

(2+2)-Dimerization of unsaturated, highly pyramidalized dodecahedranes (Φ = 43-45°) is explored as a route to C₄₀-cage molecules and non-classical ions derived thereby. The cycloadducts (X-ray structure) formed under vigorous conditions either in solution or in the solid phase above 300 °C did not undergo 2σ → 2π isomerization. The MS fragmentation patterns, in line with calculations (B3LYP/6-31G*), suggest the formation of ‘open’ C₄₀ dications.     

n-Heptane under pressure: structure and dynamics from molecular simulations

Atomistic molecular dynamics simulations have been performed in the isothermal-isobaric ensemble to explore the phase behavior of n-heptane. Motivated by recent high-pressure spectroscopic experiments on n-heptane, the present work aims at understanding the liquid-solid and the alluded to solid-solid transitions upon increasing pressure. Starting from the stabilized solid phase at 300 K and 10 kbar, we have investigated the range of these two transitions by a gradual decrease and increase of pressure, respectively. Although the solid-liquid transition has clear signatures such as the formation of gauche defects along the molecular backbone, the present model does not show any sign of a first-order solid-solid transition at high pressures. However, interesting changes in the environment around methyl groups and in their dynamics are observed. These have been substantiated by calculations of the vibrational density of states obtained from a normal-mode analysis and from the simulation trajectory.     

Metamagnetic phase transition in a diruthenium compound with interpenetrating sublattices

The diruthenium compound [Ru₂(O₂CMe)₄]₃[Cr(CN)₆] may be the only known material that contains two weakly-coupled, magnetically-ordered sublattices occupying the same three-dimensional volume. Due to the strong easy-plane anisotropy on each Ru₂ complex, the moment of each sublattice is constrained to one of the eight cubic diagonals. At low fields, the two sublattices are antiferromagnetically aligned by weak dipolar and deformation energies. But above a metamagnetic critical field of about 1000 Oe, the sublattice moments become ferromagnetically aligned and the net magnetization increases dramatically. We have successfully modeled this metamagnetic transition by assuming that the individual sublattice spin configurations are only weakly distorted by the magnetic field. This model suggests that the ground state of each sublattice undergoes a phase transition at a pressure of about 7 kbar. The drop in the sublattice moment and the rise in the sublattice susceptibility above 7 kbar can be explained by a high- to low-spin transition (S = 3/2 to 1/2) on the mixed-valent diruthenium complexes.     

Dilatometric study on the melting of polypropylene at elevated pressure

Summary Using high pressure dilatometers, the specific volume of isotactic polypropylene has been measured as a function of temperature and pressure up to 3.5 kbar. Up to 1 kbar this has been done not only for a commercial whole polymer, but also for sharp fractions. The slope of the curve of the melting temperature as a function of pressure has been determined accurately for low pressure (38 K/kbar) and has been found to remain constant almost up to 1 kbar. Because of uncertainties of the published data on crystal density, the accuracy of the volume change at the melting point is low, hence the enthalpy of fusion has been evaluated invoking theClausius-Clapeyron equation with an accuracy of only 15 %. The transition data volume, enthalpy and entropy are also given as function of pressure. The pronounced dependence of the melting point on the molecular weight has been explained by imperfect tacticity.

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Zusammenfassung Mit Hochdruckdilatometern wurde das spezifische Volumen von isotaktischem Polypropylen in Abhängigkeit von der Temperatur and bei Drucken bis 3,5 kbar gemessen. Bis 1 kbar wurde dies nicht nur an einem technischen Polymer durchgeführt, sondern auch für scharfe Fraktionen. Die Anfangssteigerung der Schmelzpunktskurve caber dem Druck konnte recht genau zu 38 K/kbar bestimmt werden. Sie bleibt bis beinahe 1 kbar konstant. Wegen der experimentellen Unsicherheit der veröffentlichten Kristalldichten kann der Volumensprung Kristall-Schmelze nur ungenau ausgewertet werden. Daher ist auch unsere nachClausius-Clapeyron bestimmte Schmelzentropie und entbalpie von nur auf etwa 15% genau. Die Größen Schmelztemperatur, -volumen, -entropie und-enthalpy werden bis 3,5 kbar als Funktion des Drucks angegeben. Die beobachtete starke Molekulargewichtsabhängigkeit des Schmelzpunkts wird durch unvollständige Taktizität begründet.

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With 7 figures and 3 tables     

Novel spectrofluorimetric method for the determination of sulfite with rhodamine B hydrazide in a micellar medium

A novel spectrofluorimetric method for the determination of sulfite using rhodamine B hydrazide as fluorogenic reagent in the presence of polyoxyethylene sorbitan monooleate (Tween 80) surfactant micelles is described. The method is based on the mixture of sulfite with rhodamine B hydrazide, a colorless, non-fluorescent substance in Tween 80 surfactant micelles which gives rhodamine B-like fluorescence emission. The fluorescence intensity increase is linearly related to the concentration of sulfite in the range 5-800 ng ml⁻¹ with a detection limit of 1.4 ng ml⁻¹ (3σ). The optimal conditions for the detection of sulfite are evaluated and the possible detection mechanism is also discussed. The method has been successfully applied to the determination of total sulfite in wines and compares well with the standard iodimetric method.