Facile, aprotic degradation of η-CpZrCl3·dme by ternary sodium/group 14 tert-butoxides: From η-CpZrCl3·dme back to NaCp in two easy steps

CpZrCl₃·dme was treated with Na[El(O^tBu)₃], El = Ge, Sn, Pb, respectively. The addition of Na[Sn(O^tBu)₃] to CpZrCl₃·dme caused rapid cyclopentadienide loss and the equally rapid appearance of CpSnCl, half of which crystallized as the trinuclear complex {[ZrCl(O^tBu)₃]₂·CpSnCl}. Pristine CpSnCl reacted almost instantly with NaO^tBu to give NaCp and Na[Sn(O^tBu)₃], which co-crystallized as a coordination polymer. Na[Ge(O^tBu)₃] also displaced Cp from zirconium, but with a different product distribution, giving Cp₂Ge, fac-[Ge(μ-^tBuO)₃ZrCl(O^tBu)₂], and ZrCl(O^tBu)₃. By contrast, Na[Pb(O^tBu)₃] only exchanged its tert-butoxide groups with zirconium to furnish CpZr(O^tBu)₃ and PbCl₂. The solid-state structures of {[ZrCl(O^tBu)₃]₂·CpSnCl}, fac-[Ge(μ-^tBuO)₃ZrCl(O^tBu)₂], and {NaCp·Na[Sn(O^tBu)₃]}_n were determined.     

Holographic investigations of azobenzene-containing low-molecular-weight compounds in pure materials and binary blends with polystyrene

This paper reports on the synthesis and the thermal and optical properties of photochromic low-molecular-weight compounds, especially with respect to the formation of holographic volume gratings in the pure materials and in binary blends with polystyrene. Its aim is to provide a basic understanding of the holographic response with regard to the molecular structure, and thus to show a way to obtain suitable rewritable materials with high sensitivity for holographic data storage. The photoactive low-molecular-weight compounds consist of a central core with three or four azobenzene-based arms attached through esterification. Four different cores were investigated that influence the glass transition temperature and the glass-forming properties. Additional structural variations were introduced by the polar terminal substituent at the azobenzene chromophore to fine-tune the optical properties and the holographic response. Films of the neat compounds were investigated in holographic experiments, especially with regard to the material sensitivity. In binary blends of the low-molecular-weight compounds with polystyrene, the influence of a polymer matrix on the behavior in holographic experiments was studied. The most promising material combination was also investigated at elevated temperatures, at which the holographic recording sensitivity is even higher.     

Experimental investigation of incipient equilibrium conditions for the formation of semi-clathrate hydrates from quaternary mixtures of (CO2 + N2 + TBAB + H2O)

The three-phase (vapour + liquid + solid) equilibrium conditions for semi-clathrates formed from three mixtures of (CO₂ + N₂), in aqueous solutions of tetra-butyl ammonium bromide (TBAB), were measured in an isochoric reactor. The experiments were conducted at temperatures between (281 and 290) K, at pressures between (1.9 and 5.9) MPa and in aqueous TBAB solutions of w_TBAB = (0.05, 0.10, and 0.20). The experimental results obtained in this study were compared with previously obtained results for gas hydrates, formed from the same three mixtures of (CO₂ + N₂) and it was observed that semi-clathrates formed at a substantially lower pressure than did gas hydrates.     

Synthesis of a difunctional orthogonal coupler for the preparation of carbohydrate-functionalized sP(EO-stat-PO) hydrogels

The synthesis and characterization of a new difunctional coupler (4) based on trimethylolpropane (TMP) are described. The coupler is used to connect biologically active N-acetylglucosamine (GlcNAc) on amino-reactive microtiter plates and on star-shaped poly[(ethylene oxide)-stat-(propylene oxide)] hydrogel layers in microtiter plates. The biological activity of immobilized carbohydrates is determined using an enzyme-linked lectin assay.     

Baltimore PM2.5 Supersite: highly time-resolved organic compounds—sampling duration and phase distribution—implications for health effects studies

As part of the Baltimore PM2.5 Supersite study, intensive three-hourly continuous PM2.5 sampling was conducted for nearly 4 weeks in summer of 2002 and as well in winter of 2002/2003. Close to 120 individual organic compounds have been quantified separately in filter and polyurethane foam (PUF) plug pairs for 17 days for each sampling period. Here, the focus is on (1) describing briefly the new sampling system, (2) discussing filter/PUF plugs breakthrough experiments for semi-volatile compounds, (3) providing insight into phase distribution of semi-volatile organic species, and (4) discussing the impact of air pollution sampling time on human exposure with information on maximum 3- and 24-h averaged ambient concentrations of potentially adverse health effects causing organic pollutants. The newly developed sampling system consisted of five electronically controlled parallel sampling channels that are operated in a sequential mode. Semi-volatile breakthrough experiments were conducted in three separate experiments over 3, 4, and 5 h each using one filter and three PUF plugs. Valuable insight was obtained about the transfer of semi-volatile organic compounds through the sequence of PUF plugs and a cut-off could be defined for complete sampling of semi-volatile compounds on only one filter/PUF plug pair, i.e., the setup finally used during the seasonal PM2.5 sampling campaign. Accordingly, n-nonadecane (C19) with a vapor pressure (vp) of 3.25 × 10⁻⁴ Torr is collected with > 95% on the filter/PUF pair. Applied to phenanthrene, the most abundant the PAH sampled, phenanthrene (vp, 6.2 × 10⁻⁵ Torr) was collected completely in wintertime and correlates very well with three-hourly PM2.5 ambient concentrations. Valuable data on the fractional partitioning for semi-volatile organics as a function of season is provided here and can be used to differentiate the human uptake of an organic pollutant of interest via gas- and particle-phase exposure. Health effects studies often relay on PM2.5 exposure measurements taken over 24 h or longer. We found that maximum 3-h concentrations are frequently two to five times higher than that found for maximum 24-h concentrations, an important aspect when considering that short-term exposure to higher air pollution levels are more likely to overpower defense mechanisms in the human lung with subsequent adverse effects even at lower pollutant levels.     

Recursion relations for the extended Krylov subspace method

The evaluation of matrix functions of the form f(A)v, where A is a large sparse or structured symmetric matrix, f is a nonlinear function, and v is a vector, is frequently subdivided into two steps: first an orthonormal basis of an extended Krylov subspace of fairly small dimension is determined, and then a projection onto this subspace is evaluated by a method designed for small problems. This paper derives short recursion relations for orthonormal bases of extended Krylov subspaces of the type K^m,mi+1(A)=span{A^-m+1v,…,A^-1v,v,Av,…,A^miv}, m=1,2,3,…, with i a positive integer, and describes applications to the evaluation of matrix functions and the computation of rational Gauss quadrature rules.     

Mn2+ complexes with 12-membered pyridine based macrocycles bearing carboxylate or phosphonate pendant arm: crystallographic, thermodynamic, kinetic, redox, and 1H/17O relaxation studies

Mn(2+) complexes represent an alternative to Gd(3+) chelates which are widely used contrast agents in magnetic resonance imaging. In this perspective, we investigated the Mn(2+) complexes of two 12-membered, pyridine-containing macrocyclic ligands bearing one pendant arm with a carboxylic acid (HL(1), 6-carboxymethyl-3,6,9,15-tetraazabicyclo[9.3.1] pentadeca-1(15),11,13-triene) or a phosphonic acid function (H(2)L(2), 6-dihydroxyphosphorylmethyl-3,6,9,15-tetraazabicyclo[9.3.1]pentadeca-1(15),11,13-triene). Both ligands were synthesized using nosyl or tosyl amino-protecting groups (starting from diethylenetriamine or tosylaziridine). The X-ray crystal structures confirmed a coordination number of 6 for Mn(2+) in their complexes. In aqueous solution, these pentadentate ligands allow one free coordination site for a water molecule. Potentiometric titration data indicated a higher basicity for H(2)L(2) than that for HL(1), related to the electron-donating effect of the negatively charged phosphonate group. According to the protonation sequence determined by (1)H and (31)P pH-NMR titrations, the first two protons are attached to macrocyclic amino groups whereas the subsequent protonation steps occur on the pendant arm. Both ligands form thermodynamically stable complexes with Mn(2+), with full complexation at physiological pH and 1:1 metal to ligand ratio. The kinetic inertness was studied via reaction with excess of Zn(2+) under various pHs. The dissociation of MnL(2) is instantaneous (at pH 6). For MnL(1), the dissociation is very fast (k(obs) = 1-12 × 10(3) s(-1)), much faster than that for MnDOTA, MnNOTA, or the Mn(2+) complex of the 15-membered analogue. It proceeds exclusively via the dissociation of the monoprotonated complex, without any influence of Zn(2+). In aqueous solution, both complexes are air-sensitive leading to Mn(3+) species, as evidenced by UV-vis and (1)H NMRD measurements and X-ray crystallography. Cyclic voltammetry gave low oxidation peak potentials (E(ox) = 0.73 V for MnL(1) and E(ox) = 0.68 V for MnL(2)), in accordance with air-oxidation. The parameters governing the relaxivity of the Mn(2+) complexes were determined from variable-temperature (17)O NMR and (1)H NMRD data. The water exchange is extremely fast, k(ex) = 3.03 and 1.77 × 10(9) s(-1) for MnL(1) and MnL(2), respectively. Variable-pressure (17)O NMR measurements have been performed to assess the water exchange mechanism on MnL(1) and MnL(2) as well as on other Mn(2+) complexes. The negative activation volumes for both MnL(1) and MnL(2) complexes confirmed an associative mechanism of the water exchange as expected for a hexacoordinated Mn(2+) ion. The hydration number of q = 1 was confirmed for both complexes by (17)O chemical shifts. A relaxometric titration with phosphate, carbonate or citrate excluded the replacement of the coordinated water molecule by these small endogenous anions.     

Collision-induced absorption by H2 pairs: from hundreds to thousands of kelvin

An interaction-induced dipole surface (IDS) and a potential energy surface (PES) of collisionally interacting molecular hydrogen pairs H(2)-H(2) was recently obtained using quantum chemical methods (Li, X.; et al. Computational Methods in Science and Engineering, ICCMSE. AIP Conf. Proc. 2009, ; see also Li, X.; et al. Int. J. Spectrosc. 2010, ID 371201). The data account for substantial rotovibrational excitations of the H(2) molecules, as encountered at temperatures of thousands of kelvin (e.g., in the atmospheres of "cool" stars). In this work we use these results to compute the binary collision-induced absorption (CIA) spectra of dense hydrogen gas in the infrared at temperatures up to several thousand kelvin. The principal interest of the work is in the spectra at such higher temperatures, but we also compare our computations with existing laboratory measurements of CIA spectra of dense hydrogen gas and find agreement.