A molecular dynamics study of the thermal response of crystalline cellulose Iβ

Molecular dynamics simulations were performed to better understand the atomic details of thermal induced transitions in cellulose Iβ. The latest version of the GLYCAM force field series (GLYCAM06) was used for the simulations. The unit cell parameters, density, torsion angles and hydrogen-bonding network of the crystalline polymer were carefully analyzed. The simulated data were validated against the experimental results obtained by X-ray diffraction for the crystal structure of cellulose Iβ at room and high temperatures, as well as against the temperature-dependent IR measurements describing the variation of hydrogen bonding patterns. Distinct low and high temperature structures were identified, with a phase transition temperature of 475–500 K. In the high-temperature structure, all the origin chains rotated around the helix axis by about 30° and the conformation of all hydroxymethyl groups changed from tg to either gt on origin chains or gg on center chains. The hydrogen-bonding network was reorganized along with the phase transition. Compared to the previously employed GROMOS 45a4 force field, GLYCAM06 yields data in much better agreement with experimental observations, which reflects that a cautious parameterization of the nonbonded interaction terms in a force field is critical for the correct prediction of the thermal response in cellulose crystals.     

A vibrational study of the metal—olefin bond in norbornadiene complexes of Rh(I), Pd(II) and Pt(II)

Vibrational assignments of the complexes [RhClC₇H₈]₂, PtCl₂C₇H₈, and PdCl₂C₇H₈ have been undertaken. A reinvestigation of the norbornadiene spectrum was necessary and a new set of assignments is given. The shift in energy of bands I and II and the out-of-plane olefinic CH wagging modes, as well as the relative energies of the bands associated with the metal—olefin motions are consistent with the total metal—norbornadiene interaction following the order Rh>Pt>Pd and the extent of the π component being ordered as Rh⋍Pt>Pd.     

Protamine-stabilized gold nanoclusters as a fluorescent nanoprobe for lead(II) via Pb(II)–Au(I) interaction

The authors report on a one-pot approach for synthesizing highly fluorescent protamine-stabilized gold nanoclusters. These are shown to be a viable nanoprobe for selective and sensitive fluorometric determination of lead(II) via quenching of fluorescence via Pb(II)-Au(I) interaction. Under optimized conditions, fluorescence measured at excitation/emission peaks of 300/599 nm drops in the 80 nM–15 μM lead(II) concentration range. The detection limit is 24 nM, and relative standard deviations (for n?=?11) at concentrations of 0.10, 4.0 and 15 μM are 1.6, 2.5 and 1.9%, respectively. The relative recoveries of added lead(II) in the water samples ranged from 97.9?±?2.29% to 101.2?±?1.83%.

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Graphical abstract Lead(II) ions are found to be able to selectively and sensitively quench the fluorescence of the protamine-gold nanoclusters (PRT-AuNCs). Thereby, an inexpensive, selective and sensitive lead(II) assay was established.

     

Quantum chemical study of the effects of interaction of polyhydrated Hg(II) with methylcobalamin — A derivative of vitamin B<Subscript>12</Subscript>

Quantum chemical methods were used to investigate the mechanism of interaction of polyhydrated Hg(II) with methylcobalamin — a derivative of vitamin B₁₂. Migration of the methyl group in the B₁₂·Hg(II) nH₂O cluster, initiating the formation of methylmercury, is considered. The reaction of substitution of methylmercury for the cobalt atom in the corrin fragment of B₁₂ has been studied. The thermodynamic and activation characteristics of these processes have been obtained. The mercury analog of vitamin B₁₂ is shown to exist in two radically different conformations.     

Structural organization and dimensionality at the hands of weak intermolecular Au···Au, Au···X and X···X (X = Cl, Br, I) interactions

The salts K[AuCl(2)(CN)(2)]·H(2)O (1), K[AuBr(2)(CN)(2)]·2H(2)O (2) and K[AuI(2)(CN)(2)]·?H(2)O (3) were synthesized and structurally characterized. Compound 1 crystallizes as a network of square planar [AuCl(2)(CN)(2)](-) anions separated by K(+) cations. However, 2 and 3 feature 2-D sheets built by the aggregation of [AuX(2)(CN)(2)](-) anions via weak, intermolecular X···X interactions. The mixed anion double salts K(3)[Au(CN)(2)](2)[AuBr(2)(CN)(2)]·H(2)O (4) and K(5)[Au(CN)(2)](4)[AuI(2)(CN)(2)]·2H(2)O (5) were also synthesized by cocrystallization of K[Au(CN)(2)] and the respective K[AuX(2)(CN)(2)] salts. Similarly to 2 and 3, the [Au(CN)(2)](-) and [AuX(2)(CN)(2)](-) anions form 2-D sheets via weak, intermolecular Au(I)···X and Au(I)···Au(I) interactions. In the case of 5, a rare unsupported Au(I)···Au(III) interaction of 3.5796(5) ? is also seen between the two anionic units. Despite the presence of Au(I) aurophilic interactions of 3.24-3.45 ?, neither 4 nor 5 exhibit any detectable emission at room temperature, suggesting that the presence of Au(I)···X or Au(I)···Au(III) interactions may affect the emissive properties.     

Characterization and spectroscopic study of new complexes of Cd(II), Hg(II) and Pb(II) with the sodium salt of morin-5′-sulfonic acid

Solid compounds of Cd(II), Hg(II) and Pb(II) with the sodium salt of morin-5′-sulfonic acid (NaMSA) were obtained. The molecular formula of the complexes are: Cd(C₁₅H₈O₁₀SNa)₂?·?6H₂O, CdOH(C₁₅H₈O₁₀SNa)?·?4H₂O, Hg(C₁₅H₈O₁₀S)?·?4H₂O and Pb(C₁₅H₈O₁₀S)?·?3H₂O. Some of their physicochemical properties such as UV-Vis, infrared, ¹³C NMR and mass spectra, thermogravimetric analysis, and solubility were studied. On the basis of spectroscopic data NaMSA was bound to Cd²⁺ via 4C=O and 3C?–?oxygen and the Hg²⁺ and Pb²⁺ ions by 5C–OH, 4C=O and 3C–OH.     

A Silver(I)–Gold(II) Hexanuclear Guanidinate–Benzoate Cluster with Short Au–Au Bonds

Abstract  Attempts to remove the halide atoms from [Au₂(hpp)₂Cl₂], 1, Hhpp = 1,3,4,6,7,8-hexahydro-2H-pyrimido[1,2-a]pyrimidine, with Ag(I) benzoate lead to the formation of the Au(I)–Ag(I) product, [(PhCOO)₂Au₄(hpp)₄Ag₂(PhCOO)₄], 2. This material is stable to air and light at room temperature and shows a UV–vis spectrum in THF with absorbances at 575, 440, 345, and 273 nm. The mixed metal product crystallizes as green crystals in the monoclinic space group P2₁/n. The Au–Au distances of 2.4473(19) ? are the shortest gold–gold distances reported to date. The gold···silver distance is 3.344(3) ? and the silver···silver distance is 2.771(6) ?. This latter distance is short compared with the Ag···Ag distance of 2.902(3) ? in the eight-membered silver benzoate dimer starting material. The Au(II) hpp and Ag(I) benzoate components are linked by carboxylate groups and two gold-silver interactions. This result stands in structural contrast to terminal carboxylate products observed with Au(II) ylides and amidinates wherein the carboxylate is not bridging to another metal atom. Index Abstract  Three equivalents of silver benzoate react with [Au₂(hpp)₂Cl₂], 1, Hhpp = 1,3,4,6,7,8-hexahydro-2H-pyrimido[1,2-a]pyrimidine, to form the gold(II)-silver(I) product, 2, [(PhCOO)₂Au₄(hpp)₄Ag₂(PhCOO)₄]. The gold–gold distance of 2.4473(19) ? is the shortest gold–gold distance reported to date. The gold–silver distance is 3.344(3) ? and the silver–silver distance is 2.771(6) ?. Dedicated to the memory of F. Albert Cotton (1930–2007).     

A molecular simulation study of the distribution of cation in zeolites

NVT Monte Carlo simulations are first used to describe the distribution of Na cations in Faujasite for several Si/Al ratios. These calculations were performed by combining two different sets of potential parameters combined with both T-atoms and explicit Si,Al models. Grand Canonical Monte Carlo simulations are then employed to investigate the influence of water adsorption on the distribution of cations in the case of a Faujasite sample with 56 cations (NaY56). These simulations data are compared to available experimental data and the influence of the choice of the forcefield for describing the cation/zeolite interaction on these results is discussed.     

C–H···Onitrate synthon assisted molecular assembly of hydrogen bonded Ni(II) and Cu(II) complexes

Two Schiff base metal complexes [Cu–SPETN·NO₃ (1) and Ni–SPETN·NO₃ (2) [SPETN?=?2,2′-[propane,1,3-diylbis(nitrilomethyldyne)pyridyl,phenolate]] with hydrogen bonding groups have been synthesized and characterized by single-crystal X-ray diffraction. In both of the compounds nitrates occupy a crystallographic general position. In 1 the lattice nitrates are on the 2₁ screw axis while in 2 they are at the crystallographic inversion center. C–H···O_nitrate synthons (formed by the nitrate anions and peripheral hydrogen bonding groups of the metal complexes) are non-covalent building blocks in molecular-assembly and packing of the cationic Schiff base metal complexes (M?=?Ni²⁺, Cu²⁺), resulting in 2-D hydrogen bonded networks. The Cu···Cu non-bonding contact in 1 is 3.268?Å while the Ni–Ni bonding distance in 2 is 3.437?Å.     

Observations of carbon–carbon coupling of 4,4'-dibromo-p-terphenyl on Cu(110) surface at molecular level

The carbon–carbon couplings of 4,4'-dibromo-p-terphenyl(DBTP) on Cu(110) surface have been investigated at a single molecular level by scanning tunneling microscopy. After annealing at 353 K, a mixture of parallel non-organometallic and organometallic intermediates of DBTP molecules along the[1–10] direction of the surface has been observed. Further annealing at 393 K causes one group of molecules to form oligomers with para-para and para-meta motifs via Ullmann reaction and the other group of molecules to synthesize oligomers with meta-meta motifs via direct carbon–carbon coupling reaction. Statistical results directly reveal that the occurrence of reaction type is strongly related to the initial binding configuration of DBTP molecules.     

Degradation of cellulose at the wet-dry interface: I—study of the depolymerization

The degradation of cellulose in paper due to the formation of a tideline at the wet-dry interface when paper is suspended in water was explored. SEC/MALS was used to assess the molar mass, while ICP/MS, SEM/EDS and CE/UV provided a qualitative and quantitative analysis of the elements and inorganic ions present in the paper. Immediately after the formation of the tideline, no significant depolymerization was observed at the wet-dry interface, despite the accumulation of water soluble brown and/or fluorescent degradation compounds and salts containing sodium, chlorine, sulfur and calcium. Various artificial aging configurations were applied to the paper with tidelines to evaluate the effect of the material accumulated at the wet-dry interface on the long-term stability of paper. The decrease in the molar mass of cellulose (above, at and below the tideline) differed depending on the type of aging, i.e. whether the entire sheet of paper was aged or whether small amounts of paper from the different areas were sampled and aged, individually or together, which evidences a complex degradation pathway. In the former aging configuration the material accumulated in the tideline affected the degradation of the tideline area to the same extent or more than the other areas. When the different areas of the paper sheet were sampled and aged together, it was found that the presence of the tideline clearly affected the degradation of the other paper areas. Conversely, in that case, cellulose within the tidelines was the least degraded. The area below the tideline, through which the water migrated, showed the most significant degradation.     

A kinetic study of the aqueous phase oxidation of di-μ-oxo-bis(L-cysteine)-oxomolybdenum(V) by molecular oxygen

The molecular oxygen-mediated decomposition of the binuclear complex, prepared from oxomolybdate(V) and L-(+)-cysteine, was studied spectrophotometrically at pH 3.5–5.6. The formation of MoVI was detected. The effects of pH and [O2] on the decomposition kinetics are given by the equation:

Solvation of cations in the LiNO3-Ca(NO3)2-H2O system at 25°C. A molecular dynamics simulation study

A series of solutions of the ternary LiNO₃-Ca(NO₃)₂-H₂O system is simulated under standard conditions by means of classical molecular dynamics (MD). Several variants of the potentials of interparticle interactions predicting different structures and compositions of the lithium cation hydration shell are used in the calculations. The coordination numbers of ions, the mean residence times of water molecules and nitrate anions in the solvation shells of cations, and the self-diffusion coefficients of solution components are estimated for all of the investigated systems. The structural features of the solvation shells of cations in multicomponent aqueous solutions are described on the basis of the obtained data.     

The influence of molecular interaction on polymerization—11: A study of the influence of the solvent on the tacticities of poly(2-naphthyl methacrylated) and poly(cumylphenyl methacrylate) obtained in various solvents

Polymerizations of 2-naphthyl methacrylate and 4(1-methylphenethyl)phenyl methacrylate (cumylphenyl methacrylate) have been carried out in solvents with different dielectric constants and donor numbers (DN). By means of ¹H-NMR and fluorescence spectroscopy, the tacticities of the polymers were studied. It was found that the structures of the polymers depend on the dielectric constant of the solvent and not on its donor number. In solvents with high dielectric constant, the contents of isotactic triads are higher than for solvents with low dielectric constants.     

Electrocatalytic reduction of molecular oxygen at electrodes coated with a new cobalt(II)–platinum(II) bimetallic porphyrin

Cobalt(II) inserts into 5-(4-pyridyl)-10,15,20-(3,4-dimethoxyphenyl)porphyrin (1) by reaction of the porphyrin with the cobalt(II) acetate salt in refluxing N,N′-dimethylformamide solutions. When the porphyrin and the cobalt porphyrin are reacted with PtCl₂(DMSO)₂ in dichloromethane at ambient temperatures the platinum complex coordinates to the peripheral pyridyl group of the porphyrin. Roughened EPG electrodes coated with the cobalt(II)–platinum(II) bimetallic porphyrin, complex (4), show a 500?mV shift in the reduction of molecular oxygen in acidic media when compared to the bare electrode. Oxidation of the bimetallic porphyrin coated electrodes at 1.20?V versus saturated calomel electrode (SCE) results in an additional shift of ca 100?mV for the reduction of molecular oxygen when compared to the bimetallic porphyrin coated electrode prior to oxidation. In addition the oxidized surface shows the ability to reduce approximately 50% of the oxygen to water and the other 50% to hydrogen peroxide according to rotating disk electrode measurements.     

Halogenation of Fluoro-Substituted Zinc(II) Tetraphenylporphyrins at the β-Position

Russian Journal of Organic Chemistry - [2,3,7,8,12,13,17,18-Octabromo-5,10,15,20-tetrakis(2,6-difluorophenyl)porphyrinato]zinc(II) was synthesized by reaction of...     

Highly selective ion probe for Al3+ based on Au(I)···Au(I) interactions in a bis-alkynyl calix[4]arene Au(I) isocyanide scaffold

A bis-alkynyl calix[4]arene Au(I) isocyanide complex has been synthesized and demonstrated to be a selective ion probe for Al(3+) based on Au(I)Au(I) interactions.     

Luminescent copper(I) halide adducts of [Au(im(CH2py)2)2]PF6 exhibiting short Au(I)···Cu(I) separations and unusual semibridging NHC ligands

The picolyl-substituted NHC complex [Au(im(CH(2)py)(2))(2)]PF(6) (1) reacts with two equivalents of copper(I) halides, affording compounds [Au(im(CH(2)py)(2))(2)(CuX)(2)]PF(6) (X = Cl, 2; Br, 3; I, 4). Each complex contains a nearly linearly coordinated [Au(NHC)(2)](+) center where the two picolyl groups on each im(CH(2)py)(2) ligand chelate a single copper atom. The Cu(I) center resides in a distorted tetrahedral environment and is coordinated to two pyridyl groups, a halide ion, and a gold metalloligand. The Au(I)-Cu(I) separations measure 2.7030(5), 2.6688(9), and 2.6786(10) ? for 2-4, respectively. Additionally, each Cu(I) center is further coordinated by a semibridging NHC ligand with short Cu-C separations of ~2.3 ?. In solution, these complexes dissociate the Cu(I) ion. In the solid state, 2-4 are photoluminescent with respective emission maxima of 512, 502, and 507 nm. The reaction of [Au(im(CH(2)py)(2))(2)]PF(6) with four equivalents of CuBr afforded the coordination polymer {[AuCu(2)Br(2)(im(CH(2)py)(2))(2)]Br·3CH(3)CN}(n) (5). This polymeric complex contains [Au(NHC)(2)](+) units interconnected by Cu(2)Br(2) dimers. In 5, the Au-Cu separations are long at 4.23 and 4.79 ?, while the Cu-Cu distance is considerably shorter at 2.9248(14) ?. In the solid state, 5 is photoluminescent with a broad band appearing at 533 nm.     

Luminescence of cesium dicyanoaurate(I). Evidence for extended Au(I)Au(I) interactions in two dimensions

The luminescence of microcrystalline samples of Cs[Au(CN)₂] is reported and ascribed to planar AuAu interactions. Low-temperature measurements from 1.6 to 100 K of the luminescence decay rate of Cs[Au(CN)₂] for the band centered at 458 nm reveal the presence of at least two emitting levels separated by 46 cm⁻¹. A simple orbital model is proposed to account for the luminescence, and spin—orbit coupling is invoked to explain the splitting of the two lowest states. The analogies and differences between this luminescence and that observed for salts of Pt(CN)₄²⁻ are briefly discussed.