The acidity constants of some pyrimidine bases in various water-organic solvent media

The acid ionization constants of some pyrimidine bases of nucleic acids were determined pH-metrically at 25 degrees C and at the constant ionic strength I = 0.10 mol l(-1) (KNO3) in pure water as well as in aqueous media containing variable mole percentages (5-30%) of organic solvents. The organic solvents used were methanol, ethanol, N,N-dimethylformamide (DMF), dimethylsulfoxide (DMSO), acetonitrile, acetone and dioxane. The results obtained indicated that the acidity constants are generally decreased as the content of an organic solvent in the medium is increased. It was deduced that the hydrogen bonding interactions and the solvent basicity in addition to the electrostatic effect are the major effects influencing significantly the acid ionization process of pyrimidine bases in the different water-organic solvent media. Some thermodynamic parameters (deltaH, deltaG degrees, deltaS degrees) of the ionization process over the temperature range 5-45 degrees C in pure water were also determined and discussed.     

Ab initio calculations and vibrational energy level fits for the lower singlet potential-energy surfaces of C3

Ab initio multireference configuration interaction potential energy surfaces are computed for the eight lowest singlet surfaces of C(3). These reveal several important features, including several conical intersections in linear, nonlinear, and equilateral triangle geometries. These intersections are important because, particularly for the excited A (1)Pi(u) state, reasonable ab initio results could only be obtained by including nearby, near degenerate, (1)Sigma(u) (-) and (1)Delta(u) states that cross the A (1)Pi(u) state around 4500 cm(-1) above the equilibrium geometry, and a (1)Pi(g) state whose potential in turn crosses the other states about 2000 cm(-1) further up. These states are probably responsible for the complexity of the shorter wavelength UV absorption spectrum of C(3). The computed potential energy surface for the ground, X (1)Sigma(g) (+), state and for the lowest two excited singlet surfaces (which both correlate with the A (1)Pi(u) state in a collinear geometry) are fitted to analytic functional forms. Vibrational energy levels are calculated for both states, taking account of the Renner-Teller coupling in the excited A (1)Pi(u) state. The potential parameters for both states are then least-squares fitted to experimental data. The ground-state fit covers a range of approximately 8500 cm(-1) above the lowest level, and reproduces 100 observed vibrational levels with an average error of 2.8 cm(-1). The A (1)Pi(u) state surfaces cover a range of 3250 cm(-1) above the zero-point level, and reproduce the 44 observed levels in this range with an average error of 2.8 cm(-1).     

Enhanced Photocatalytic H2-production Activity of CdS Nanoflower using Single Atom Pt and Graphene Quantum Dot as Dual Cocatalysts

Single-atom catalysts have high catalytic activity due to their unique quantum size effects and optimal atom utilization. Herein, visiblelight-responsive photocatalysts were designed by coupling Cd S with graphene quantum dots（GQDs） and platinum single atoms（Pt SAs）. GQDs and Pt SAs were successively loaded on ultrathin Cd S nanosheets through freeze-drying and in-situ photocatalytic reduction. The synergistic effect between Pt SAs and GQDs results in superior photocatalytic activity with a hydr...     

Total synthesis of (+)-zaragozic acid C

A total synthesis of (+)-zaragozic acid C is described. Key features of the synthesis are the use of a double Sharpless asymmetric dihydroxylation reaction of diene 6 to control stereochemistry at four contiguous stereocenters from C3 to C6; the introduction of the C1-side chain by reaction between the anion derived from the dithiane monosulfoxide 27 and the core aldehyde 12; a high yielding, acid-mediated simultaneous acetonide deprotection-dithiane removal-ketalization procedure leading exclusively to the 2, 8-dioxabicyclo[3.2.1]octane core 34; and a novel triple oxidation procedure allowing installation of the tricarboxylic acid.     

A comprehensive and compact n-heptane oxidation model derived using chemical lumping

A detailed reaction mechanism for n-heptane oxidation has been compiled and subsequently simplified. The model is based on a kinetic model for C1-C4 fuel oxidation of Hoyermann et al. [Phys. Chem. Chem. Phys., 2004, 6, 3824] and a detailed mechanism for n-heptane oxidation developed by Curran et al. [Combust. Flame, 1998, 114, 149]. The generated mechanism is kept compact by limiting the application of the low temperature oxidation pathways to the fuel molecule. The first reaction steps and the complex low temperature paths in the oxidation process have been simplified and reorganized by linear chemical lumping. The reported procedure allows a decrease in number of species and reactions with only a minor loss of model accuracy. The simplified model is of very compact size and gives an advantageous starting point for further model reduction. By this chemically lumped general mechanism without further adjustments the large set of experimental data for the high and low temperature oxidation (ignition delay times, species concentration profiles, heat release and engine pressure profiles, flame speeds and flame structure data) for conditions ranging from very low to high temperatures (550-2300 K), very lean to extremely fuel rich (0.22 < phi < 3) mixtures and pressures between 1 and 42 bar is consistently described providing a basis for reliable predictions for future applications, (i) building reaction mechanisms for similar but chemically more complex fuels (e.g. iso-octane, n-decane,...) and (ii) calculating complex flow fields ("fluid dynamics") after further simplification with advanced reduction tools.     

Synthesis and characterization of Mn(II), Au(III) and Zr(IV) hippurates complexes

Mn(II), Au(III) and Zr(III) complexes with N-benzoylglycine (hippuric acid) (abbreviation hipH) were synthesized and characterized by elemental analysis, molar conductivity, magnetic measurements, spectral methods (mid-infrared, (1)H NMR, mass, X-ray powder diffraction and UV/vis spectra) and simultaneous thermal analysis (TG and DTG) techniques. The molar conductance measurements proved that all hippuric acid complexes are non-electrolytes. The electronic spectra and magnetic susceptibility measurements were used to infer the structures. The IR spectra of the ligand and its complexes are used to identify the type of bonding. The kinetic thermodynamic parameters such as: E*, DeltaH*, DeltaS* and DeltaG* are estimated from the DTG curves. The free ligand and its complexes have been studied for their possible biological antifungal activity.     

氮硫共掺杂多孔碳材料的制备及其在锂硫电池中的应用

锂硫电池因其较高的理论容量和对环境友好等优势被视为极具发展潜力的储能装置,但是多硫化物的穿梭效应极大地限制了锂硫电池的实际应用。本文以葡萄糖为碳源,离子液体为氮源和硫源,KCl和ZnCl₂为模板剂,KOH为活化剂,通过热解工艺合成了氮硫共掺杂多孔碳（NSPC）。XPS和极性吸附实验表明N、S杂原子成功引入并且提高了碳材料对多硫化物的吸附能力,有效缓解多硫化物的穿梭效应,而较高的比表面积（1290.67 m²·g^-1）有助于提高硫负载量。负载70.1wt.%的硫后（S@NSPC）作为锂硫电池的正极材料表现出了良好的电化学性能。在167.5 mA·g^-1的电流密度下S@NSPC的首次放电容量为1229.2 mAh·g^-1,远高于S@PC的861.6 mAh·g^-1,且S@NSPC循环500圈后容量为328.1 mAh·g^-1。当电流密度从3350 mA·g^-1恢复至167.5 mA·g^-1时,可逆容量达到首圈放电比容量的80%,几乎恢复至其初始值。     

Impact of composition ratio on the structure and optical properties of (1-x)MnFe2O4/(x)ZnMn2O4 nanocomposite

($1-x$)MnFe$_{2}$O$_{4}$ (MFO)/$x$ZnMn$_{2}$O$_{4}$ (ZMO) ($x=0$, 0.2, 0.5, 0.8, and 1.0) nanocomposite samples were prepared using co-precipitation procedure. The phase percentage, cell parameters, and crystallite size of MFO and ZMO phases in each nanocomposite sample were calculated using Rietveld refinement procedure. The x-ray diffraction (XRD) analysis and Fourier-transform infrared spectroscopy techniques established the variation in the lattice parameters of each phase are due to permutation of all cations among the octahedral and tetrahedral sites of MFO and ZMO. The different oxidation states of different ions in all samples were determined using x-ray photoelectron spectroscopy (XPS) technique. The variation in absorbance of the nanocomposite samples with composition parameter ($x$) is dependent on the wavelength region. The optical bandgap of the nanocomposite samples is decreased as the content of ZMO phase increased. The effect of alloying on the refractive index, extinction coefficient, dielectric constant, optical conductivity, and the nonlinear optical behaviors of all samples were studied in detail. The nanocomposite sample $x=0.5$ disclosed upgraded optical parameters with the highest refractive index, optical conductivity, and PL intensity, which nominate it to be functional in various application fields.     

Search for heavy particles decaying into electron-positron pairs in pp collisions

We present results of searches for technirho (rho(T)), techniomega (omega(T)), and Z' particles, using the decay channels rho(T),omega(T),Z'-->e(+)e(-). The search is based on 124.8 pb(-1) of data collected by the D0 detector at the Fermilab Tevatron during 1992-1996. In the absence of a signal, we set 95% C.L. upper limits on the cross sections for the processes pp-->rho(T),omega(T),Z'-->e(+)e(-) as a function of the mass of the decaying particle. For certain model parameters, we exclude the existence of degenerate rho(T) and omega(T) states with masses below about 200 GeV. We exclude a Z' with mass below 670 GeV, assuming that it has the same couplings to fermions as the Z boson.     

A regularized stiffened-gas equation of state

An alternative to the stiffened-gas equation of state is proposed that corrects for unphysical behavior, namely the returning of negative pressure values for densities lower than the reference density. The question of generalizing a given isentropic equation of state is first addressed. Then, a regularized stiffened-gas equation of state is derived, which is the main goal of this work. Finally, the use of the new equation of state is briefly illustrated for a classical computational fluid dynamics benchmark concerning the evaluation of liquid impact on a wall.