Modulation of optical properties of dissolved humic substances by their molecular complexity

In this study, we show that several UV-Vis absorbance, steady-state and time-resolved fluorescence parameters of a series of dissolved humic substances (DHS) from different sources (e.g. terrestrial fulvic and humic acids, and humic acid-like molecules extracted from composted and vermicomposted wastes) correlate with the molar absorptivity at 280 nm per mole of organic carbon (ε(280)), which in turn is proportional to the molecular complexity (e.g. molecular size, aromaticity and oxidation degree) of the DHS. Both absorbance and fluorescence spectral responses were sensitive to the molecular complexity associated with the maturation degree of the DHS. Depending on the DHS, different emitting responses by excitation at the UVA (340 nm) and VIS (460 nm) regions of the absorption spectra were observed. The results were explained in terms of the extent of intramolecular electronic interactions between electron donor groups, such as polyhydroxylated aromatics and indoles, and more oxidized acceptor groups (e.g. quinones or other oxidized aromatics) as the molecular complexity of the DHS increased.     

Synthesis and Crystal Structure of Ammonium,Strontium Hexacyanoferrate(II) Dihydrate

Abstract  

The molecular structure of Sr(NH₄)₂[Fe(CN)₆]·2H₂O, was determined by X-ray diffraction methods. It crystallizes in the orthorhombic Pnma space group with a = 13.1420(3), b = 14.1656(4), c = 7.3364(2) ?, and Z = 4. The structure was solved and refined. The iron(II) complex is located on a crystallographic inversion center with the metal in a nearly perfect octahedral environment [Fe–C bond distances from 1.919(4) to 1.939(4) ?]. The Sr(II) ion lies on a crystallographic mirror plane, sandwiched between two nearly square Archimedean anti-prism basis, angled 45o from each other. One base is formed by the two water molecules located on the mirror plane [Sr···Ow distances of 2.741(4) and 2.636(4) ?] and by a pair of mirror-related cyanide N-atom [d(Sr···N) = 2.676(3) ?], the other one by two pairs of mirror-related cyanide nitrogen atoms [Sr···N distances of 2.649(3) and 2.640(4) ?] of neighboring [Fe(CN)₆]⁴⁻ anions. The infrared absorption spectrum of the compound was also recorded and briefly discussed.     

Co3(RL)2(hfac)6 ladder complex of 5-[4-(N-tert-butyl-N-aminoxyl)phenyl]pyrimidine

5-[4-(N-tert-butyl-N-aminoxyl)phenyl]pyridimine (4NITPhPyrim = RL) forms a 1-D ladder polymer complex with Co(hfac)2 of stoichiometry Co3(RL)2(hfac)6, having antiparallel [Co(II)RL]n linear chains (rails) that are cross-linked by Pyrim-Co(hfac)2-Pyrim rungs. The magnetic behavior above 100 K is consistent with contributions from one high-spin Co(II) ion (the cross-link, S = 3/2) plus two Co-ON units with strongly antiferromagnetic (AFM) metal-radical exchange (each S = 1). The chiT data show an AFM downturn as the temperature drops. Assuming weak exchange along chain portions of the polymer due to poor spin polarization across the phenyl-pyrimidine bond in RL, a linear three-spin (S = 1, 3/2, and 1) fit to the T > 18 K data yields an AFM cross-linker (rung) effective exchange of J(CL)/k = (-)5.3 K = (-)3.7 cm(-)(1). Superexchange (sigma-orbital overlap) is a likely mechanism for the effective AFM exchange between CoON and Co spin sites in the three-spin groupings.     

High pressure polymorphs of LiCoPO4 and LiCoAsO4

Olivine-LiCoXO₄ (X = P, As) compounds might transform to the denser spinel-type and Na₂CrO₄-type structures under pressure. In this work, the relative energetic stability of the three polymorphs and the pressure of the possible polymorphic transformations are investigated combining experiments and first principles calculations. Olivine-LiCoAsO₄ is predicted to transform to the Na₂CrO₄-like structure at 0.4 GPa and to the spinel structure at 5.8 GPa (0 K). Quenching HP/HT experiments show that olivine-LiCoAsO₄ treated at 6 GPa/1173 K transforms to the spinel-like structure. Computational results indicate that olivine-LiCoPO₄ transforms to the Na₂CrO₄-like form at around 4 GPa (0 K), the latter being the stable form till very high pressures (21.6 GPa). In good agreement with this, olivine-LiCoPO₄ when subjected to 6 GPa/1173 K and 15 GPa/1173 K is converted to the Na₂CrO₄-type polymorph. Crystallographic data of the new compound LiCoPO₄ within the Na₂CrO₄ structural type are provided.     

Presence of in-plane anisotropy and oscillatory magnetoresistive behavior in epitaxial SrRuO3 thin films with orthogonal equivalent axes

In-plane magnetic hysteresis measurements performed on thin films of SrRuO₃ (SRO) deposited on (001)-oriented SrTiO₃ substrates showed orthogonal equivalent axes. This finding, nevertheless, was not conclusive argument to discard the presence of in-plane anisotropy in these samples. Certainly, measurements of the in-plane magnetoresistance (MR) featured anisotropic behavior with a well-defined angular dependence. The observed 180° periodicity of the function MR(θ) corresponded to that expected for the standard anisotropic magnetoresistance phenomenon (AMR). On the other hand, the longitudinal MR (zero Lorentz force) and transverse MR (nonzero Lorentz force), recorded at low temperatures and magnetic fields, displayed positive MR with a relatively broad maximum for the first field ramp up to 4 T. For the subsequent field sweep down, MR was negative for all field orientations. The described behavior was symmetric upon reversal of the applied magnetic field leading to a strong hysteretic behavior of MR.