Investigation of the Preyssler phosphotungstate heteropolyanion, [NaP5W30O110]14−, properties with different counter ions

Preyssler heteropolytungstophosphate anions with different counter ions, X _x M _y [NaP₅W₃₀O₁₁₀], where X = H and M = Cs, Ba, La, Ce, Hg, Tl, Pb and Bi, were prepared and the effect of the counter ion on their behavior was investigated by i.r. spectroscopy, surface area measurements (BET), simultaneous thermal analysis (STA), cyclic voltammetry and X-ray photoelectron spectroscopy (XPS). Thermal analyses indicate that the Ce derivative has the highest and the Hg derivative the lowest thermal stability. Only the Cs, Ba and Tl salts show measurable surface area. All heteropolyanions with similar i.r. spectra and electrochemical behavior demonstrate that the heteropolyanion structures are retained. The X-ray photoelectron spectra of the heteropolyacid salts reveal high binding energies for counter ions in comparison with classic salts and strong interaction of counter ion with the oxo oxygen's for H⁺ and Cs⁺ derivatives.     

A simple graphical approach to predict local residue conformation using NMR chemical shifts and density functional theory

The dependency of amino acid chemical shifts on φ and ψ torsion angle is, independently, studied using a five‐residue fragment of ubiquitin and ONIOM(DFT:HF) approach. The variation of absolute deviation of ¹³C^α chemical shifts relative to φ dihedral angle is specifically dependent on secondary structure of protein not on amino acid type and fragment sequence. This dependency is observed neither on any of ¹³C^β, and ¹H^α chemical shifts nor on the variation of absolute deviation of ¹³C^α chemical shifts relative to ψ dihedral angle. The ¹³C^α absolute deviation chemical shifts (ADCC) plots are found as a suitable and simple tool to predict secondary structure of protein with no requirement of highly accurate calculations, priori knowledge of protein structure and structural refinement. Comparison of Full‐DFT and ONIOM(DFT:HF) approaches illustrates that the trend of ¹³C^α ADCC plots are independent of computational method but not of basis set valence shell type. © 2016 Wiley Periodicals, Inc.     

Retention time prediction of polycyclic aromatic hydrocarbons in gas chromatography–mass spectrometry using QSPR based on random forests and artificial neural network

Structural Chemistry - In this study, a quantitative structure–property relationship (QSPR) was proposed using the random forests (RF) and artificial neural network (ANN) for determining the...     

Crossed Andreev reflection in graphene normal-superconductor-normal structures with pseudo-diffusive interfaces

In this Letter graphene normal-superconductor-normal heterostructures are modeled for studying the crossed Andreev reflection. A thin layer of undoped graphene with Fermi energy at the Dirac point at is assumed the interface between superconductor layer and each normal lead. The resulting contribution of the crossed Andreev reflection to the nonlocal conductance equals that of the electron elastic cotunneling. We explain this as another figure of merit for pseudodiffusive conduction at the Dirac point of the undoped layers. Also structures with only one undoped layer at the interface between the superconductor and one of the normal leads, as well as structures in which one of the leads is ferromagnetic, show pseudodiffusive conduction at the Dirac points.     

Theoretical study on the atmospheric formation of sulfur trioxide as the primary agent for acid rain

The reaction mechanism of SO₂ with O₃ on the singlet potential energy surface has been investigated theoretically at the G3MP2B3//B3LYP/6-311+G(3df) level of theory. The reactants are initially associated with adducts IN1(O₂S–OOO) and IN2(OS-cyclic O₄) in a barrier-less process. Subsequently, these adducts undergo isomerization and dissociation processes to produce cis-OSOO + ³O₂, SO₃(C _s ) + ³O₂ and SO₃(D _3h ) + ³O₂ products. The SO₃(D _3h ) + ³O₂ is major product and the cis-OSOO + ³O₂ and SO₃(C _s ) + ³O₂ are minor products. No stable pathway has been found for the formation of trans-OSOO and cyclic-SOOO isomers in the reaction of SO₂ + O₃. For major product, the rate constant of SO₂ + O₃ reaction is 2.30 × 10⁻²³ cm³ molecule⁻¹ s⁻¹, at room temperature and atmospheric pressure.     

Reversed-phase dispersive liquid-liquid microextraction with multivariate optimization for sensitive HPLC determination of tyrosol and hydroxytyrosol in olive oil

A reversed-phase dispersive liquid-liquid microextraction (RP-DLLME) method coupled to HPLC was developed for the extraction of hydroxytyrosol (HTy) and tyrosol (Ty) from virgin olive oil. In this first application of the RP-DLLME method to non-polar samples, the phenolic compounds were directly extracted into an aqueous micro-drop, which could be injected into a chromatography column without any further pretreatment. A glass test tube with lengthened conical bottom was fitted inside a centrifuge tube in this work for more efficient withdrawal of the sedimented phase with a microsyringe. The volumes of water and ethyl acetate, the pH of water and the centrifuge time as four effective parameters on the extraction were optimized by a central composite design (response surface) method. Five replicated analyses under the optimized conditions (i.e., 0.2 mL ethyl acetate as disperser and 100 μL water at pH 11 as the extraction solvent) resulted in recoveries of 104.3 and 97.6%, and relative standard deviations of 5.75 and 4.57 for HTy and Ty, respectively. The detection limit of the method (3σ) was 0.043 mg L(-1) for HTy and 0.032 mg L(-1) for Ty. The method was successfully applied to the determination of HTy and Ty in five olive oil samples.     

Adsorption of polyfunctional 5‐fluorouracil and 2,4‐dithio‐5‐fluorouracil on Au(111) surface: Structure,energy, and electronic transmission

Adsorption of 5‐fluorouracil (5‐FU) and 2,4‐dithio‐5‐fluorouracil (2,4‐DT‐5‐FU) on Au(111) surface at low coverage is studied by using periodic‐slab‐density functional theory calculation. Isolated 5‐FU molecule adsorbs preferentially at bridge site in a vertical configuration via N? H group by forming the N? H···Au nonconventional H‐bond. The formation of the anchor Au? O bond is not observed. Substitution of oxygen atoms of 5‐FU with sulfur strongly influences the nature of adsorption and leads to the Au? S anchor bond and the N? H···Au nonconventional H‐bond of single 2,4‐DT‐5‐FU molecule on Au(111) surface. The adsorption site and orientation of 2,4‐DT‐5‐FU molecule on the surface are similar to those of 5‐FU. The metal–molecule coupling effects at asymmetric Au/S(N? H)S/mol/C? H/Au and Au/N? H/mol/O/Au transport junctions and symmetric Au/S(N? H)S/mol/mol/S(N? H)S/Au and Au/O/mol/mol/O/Au transport junctions are also investigated. The electronic structure is analyzed in detail, and the obtained results are used for illustrating the electron transmission in metal–molecule–metal systems. © 2011 Wiley Periodicals, Inc. Int J Quantum Chem, 2011     

An ab initio study on the nature of σ-hole interactions in pnicogen-bonded complexes with carbene as an electron donor

ABSTRACT

A theoretical study of the complexes formed between ZH₂X (Z = P, As, Sb, Bi; X = F, Cl, Br, CN, NC, OH, NH₂) and an N-heterocyclic carbene (imidazol-2-ylidene) is carried out by means of ab initio calculations. According to molecular electrostatic potential analysis, it is inferred that the divalent C atom of the carbene can act as a Lewis base with the pnicogen atom Z of ZH₂X. The pnicogen bond distances (Z–C) are in the range of 2.050–2.911 for these complexes. While the Z?X bonds are longer than the corresponding Z?C bonds in the X = Cl and Br complexes, most of the Z?X bonds are short enough to suggest that they should be considered as covalent bonds which have lost some degree of covalency. For a given Z, the ZH₂Br forms the strongest complex, followed by ZH₂Cl and ZH₂F. On the other hand, the binding energy in the halogenated ZH₂X complexes follows the reverse ranking expected based on the values of the σ-hole of the isolated ZH₂X monomers. The nature of the pnicogen bond interaction in these complexes is analysed by quantum theory of atoms in molecules (QTAIM) and natural bond orbital methods. According to QTAIM analysis, a partially covalent character can be attributed to the pnicogen bonds studied here.     

Numerical modeling of 3-D flow on porous broad crested weirs

Gabion weirs with optional design as a broad crested weirs are suitable structures to reduce flash flood with a minimal negative impact on the water environment. In the present study, the 3-D flow was simulated around gabion weirs with respect to free-surface water. The Reynolds-averaged Navier–Stokes equations are solved to predict water surface over the gabion weir. The VOF method with the geometric reconstruction scheme was applied to treat the complex free-surface flow. Simulations were performed using three variants of the k–ε and the RSM models to find the water level and velocity distribution profile and results are compared with several experimental data available in the literature. The structured mesh was used for all domains with high dense mesh near the solid region. A comparison between experimental data and simulations indicates that the k–ε model can be used to predict the complex flow and water level with high accuracy.