Synthesis, molecular and electronic structure, and TDDFT and TDDFT-PCM study of the solvatochromic properties of (Me2Pipdt)Mo(CO)4 complex (Me2Pipdt = N,N'-dimethylpiperazine-2,3-dithione)

The synthesis, spectroscopic, and structural characterization of the (Me2Pipdt)Mo(CO)4 complex (Me2Pipdt = N,N'-piperazine-2,3-dithione) are presented in this paper. The title complex crystallizes in the P2(1)/n space group with a = 25.541(3) A, b = 10.3936(14) A, c = 10.9012(12) A, beta = 92.261(9) degrees , V = 2891.6(6) A(3), and Z = 8. Gas- and solution-phase structural and electronic features of (Me2Pipdt)Mo(CO)4 and Me2Pipdt have been investigated using density functional theory. The molecular structure underscores the flexibility of the NC(S)C(S)N fragment in both the free ligand and the metal complex. On the basis of structural, spectroscopic, and theoretical results, the bidentate ligand in (Me2Pipdt)Mo(CO)4 is considered to be in the dithione, not dithiolate, form. Time-dependent density functional theory has been used for the investigation of the excited states and solvatochromic properties of (Me2Pipdt)Mo(CO)4. The calculated vertical excitation energies in solution are consistent with the experimental data, showing that the metal-to-ligand charge-transfer transitions, in both the visible and UV regions, dominate over the ligand-based pi-pi transitions.     

Towards accurate solvation dynamics of divalent cations in water using the polarizable amoeba force field: From energetics to structure

Molecular dynamics simulations were performed using a modified amoeba force field to determine hydration and dynamical properties of the divalent cations Ca2+ and Mg2+. The extension of amoeba to divalent cations required the introduction of a cation specific parametrization. To accomplish this, the Thole polarization damping model parametrization was modified based on the ab initio polarization energy computed by a constrained space orbital variation energy decomposition scheme. Excellent agreement has been found with condensed phase experimental results using parameters derived from gas phase ab initio calculations. Additionally, we have observed that the coordination of the calcium cation is influenced by the size of the periodic water box, a recurrent issue in first principles molecular dynamics studies.     

Study of the gamma irradiation effects on the PMMA/HA and PMMA/SW

The behavior of the poly(methyl methacrylate) (PMMA) under the action of gamma radiation has been sufficiently studied. In this work, we present results from melt flow index (MFI), thermogravimetric analysis (TGA), differential scanning calorimetry (DSC) and electron paramagnetic resonance (EPR) of PMMA composites with hydroxyapatite (HA) and seaweed residues (SW) irradiated with gamma rays at 1.08 kGy/h. Composites of PMMA/HA and PMMA/SW with 10%, 20% and 30% of the filler were prepared. The results show an increase in the MFI values with the integral dose of radiation, being consistent with chain-scission reactions. No EPR signal was observed in pure PMMA, while in the composites, the typical EPR signal of the PMMA radicals was observed, which increased with the amount of HA or SW. When comparing the relative intensities of the EPR signals for both types of composites, a slight increase in the concentration of free radicals generated in the sample with SW respect to that of PMMA/HA composite was obtained. A decay of the total free radical concentration was observed as time elapsed.     

流体力学半径对估算胶体微球聚集速率常数的影响

胶体粒子聚集速率常数实验值远低于理论值一直是被普遍关注的问题.聚集速率常数的理论推导是基于粒子的几何半径来考虑的,但决定粒子扩散速率及聚集速率的应该是粒子的流体力学半径(大于几何半径),因而它是使聚集速率常数实验值低于理论值的因素之一.影响流体力学半径的因素很多,其中,带电粒子在溶液中因表面存在双电层,会明显增大流体力学半径,造成聚集速率减慢.而双电层的厚度又随溶液中离子强度的不同而改变.本工作在聚集速率的公式中引入了修正因子,即几何半径与其流体力学半径之比,以修正由于用几何半径代替流体力学半径带来的误差.其中几何半径和流体力学半径可以分别用扫描电镜(SEM)和动态光散射(DLS)来测定.以两种粒径的聚苯乙烯带电微球为例,考察了在不同离子强度下,该误差的大小.结果发现,对于半径为30 nm的微球,用流体力学半径计算的慢聚集速率常数比理论值偏低约8%.该误差随离子强度增加而减少.对于快聚集情况,流体力学半径对聚集速率基本没有影响.     

An in situ Al K-edge XAS investigation of the local environment of H+- and Cu+-exchanged USY and ZSM-5 zeolites

Aluminum coordination in the framework of USY and ZSM-5 zeolites containing charge-compensating cations (NH4+, H+, or Cu+) was investigated by Al K-edge EXAFS and XANES. This work was performed using a newly developed in-situ cell designed especially for acquiring soft X-ray absorption data. Both tetrahedrally and octahedrally coordinated Al were observed for hydrated H-USY and H-ZSM-5, in good agreement with 27Al NMR analyses. Upon dehydration, water desorbed from the zeolite, and octahedrally coordinated Al was converted progressively to tetrahedrally coordinated Al. These observations confirmed the hypothesis that the interaction of water with Br?nsted acid protons can lead to octahedral coordination of Al without loss of Al from the zeolite lattice. When H+ is replaced with NH4+ or Cu+, charge compensating species that absorb less water, less octahedrally coordinated Al was observed. Analysis of Al K-edge EXAFS data indicates that the Al-O bond distance for tetrahedrally coordinated Al in dehydrated USY and ZSM-5 is 1.67 angstroms. Simulation of k3chi(k) for Cu+ exchanged ZSM-5 leads to an estimated distance between Cu+ and framework Al atoms of 2.79 angstroms.     

Coulometric titration of some phenolic steroids with bromine based on thin-layer hydrodynamic biamperometric end-point detection

Four phenolic steroids were titrated coulometrically with bromine in a methanol—water solvent containing hydrochloric acid and sodium bromide. End-point detection was achieved with a twin electrode thin-layer cell placed in a flow loop connected to the titration vessel. Linear-segmented biamperometric titration curves were recorded. Concentrations of 17-β-estradiol were determined in the range from 442 ppb to 8.84 ppm with relative inaccuracies of -1.3% and +2.2%, respectively. Part-per-million solutions of estrone and estriol were titrated with good accuracy and precision. Somewhat poorer precision and accuracy was found for the determination of 17-α-ethynyl estradiol.     

Thin-layer hydrodynamic biamperometric end-point detection: application to the coulometric titration of arsenic(III) with bromine

A sandwich-type thin-layer cell has been applied to hydrodynamic biamperometric end-point detection in coulometric titrations. The detector cell, constructed from two pieces of teflon, has platinum indicator electrodes and is placed in a flow loop attached to the coulometric titration cell. The thin-layer cavity has a volume of 6.5 μl and a thickness of 51μm, which are obtained using teflon tape as a spacer. A peristaltic pump maintains a continual flow through the loop. The titration cell is a closed vessel completely filled with the solution to be titrated. Operational amplifier circuits control the potential difference applied across the two thin-layer electrodes, and to measure the current developed in the thin-layer cell. Noise arising from the pulsed nature of the flow is reduced by a factor of 25 by using an electronic filter. The titration of arsenic(III) with electrogenerated bromine was used to study the performance of the system. Concentrations from 24 ppb to 1.6 ppm can be determined with relative inaccuracies of +4 and +0.1%, respectively.     

Eliminating absorbing interference using the H-point standard addition method: case of Griess assay in the presence of interferent heme enzymes such as NOS

Standard calibration methods used to determine trace analytes usually yield significant deviations from the actual analyte value in the presence of interferents in the assay media. These deviations become of particular concern when the concentration of the analyte is low, and when the results are used to draw mechanistic or kinetic conclusions, for instance in enzyme structure-function studies. In these circumstances, the H-point standard addition method (HPSAM) provides superior precision and accuracy. This method is developed here for the case of the spectrophotometric Griess assay used to determine nitrite in various enzymology investigations, such as nitrite determination in studies of nitrite reductases (NiR), or when determining nitrite as a breakdown product of nitric oxide synthesized by NOS enzymes. The results obtained by HPSAM are contrasted with those of the traditional calibration method.Electronic Supplementary Material Supplementary material is available for this article at     

Two concomitant polymorphs of 3,3′,4,4′-tetrakis(phenylethynyl)-1,1′-[2,2,2,-trifluoro- 1-(trifluromethyl)ethanediyl]bisbenzene

A representative compound of bis-ortho-diynyl arene (BODA), with a general formula [(m,p)R₂–Ph]₂ –X, where R: –C≡C–Ph and X: >C(CF₃)₂ has been structurally characterized. The compound is being investigated as a monomer for high-performance polyarylene networks and glassy carbon precursors. The bis(trifluoromethyl) derivative crystallizes in two concomitant polymorphic forms. The two polymorphs form a monotropic system with melting points of 436 and 463 K. The metastable form yields monoclinic crystals (P2₁/n, Z=4). a=12.677(3) ?, b=11.677(2) ?, c=24.026(5) ?, β=93.79(3)°. The thermodynamically stable form is monoclinic as well (P2₁/c, Z=8), a=10.7983(6) ?, b=30.628(2) ?, c=21.648(1) ?, β=94.737(1)° with small voids indicating less efficient packing. The two polymorphs contain different conformers of the rotationally flexible molecule.