High Magnetic Field Consequences on the NMR Hyperfine Shifts in Solution

Pseudocontact shifts arise from the isotropic reorientational average of the dipolar coupling between unpaired electron and nuclei, in the presence of magnetic susceptibility anisotropy. The effect of residual orientation due to high magnetic fields on pseudocontact shifts is evaluated here. The effect is found to be smaller and of opposite sign with respect to another novel effect of high magnetic fields on hyperfine shifts due to saturation of the electron spin magnetic moment as described by the Brillouin equation.     

Two polymorphs of 5‐cyclohexyl‐5‐ethylbarbituric acid and their packing relationships with other barbiturates

Polymorph (Ia) (m.p. 474 K) of the title compound, C₁₂H₁₈N₂O₃, displays an N—H...O=C hydrogen‐bonded layer structure which contains R₆⁶(28) rings connecting six molecules, as well as R₂²(8) rings linking two molecules. The 3‐connected hydrogen‐bonded net resulting from these interactions has the hcb topology. Form (Ib) (m.p. 471 K) displays N—H...O=C hydrogen‐bonded looped chains in which neighbouring molecules are linked to one another by two different R₂²(8) rings. Polymorph (Ia) is isostructural with the previously reported form II of 5‐(2‐bromoallyl)‐5‐isopropylbarbituric acid (noctal) and polymorph (Ib) is isostructural with the known crystal structures of four other barbiturates.     

Aggregation and dispersion of silver nanoparticles in exposure media for aquatic toxicity tests

Silver nanoparticles (AgNPs) are currently being very widely used in industry, mainly because of their anti-bacterial properties, with applications in many areas. Once released into the environment, the mobility, bioavailability, and toxicity of AgNPs in any ecosystem are dominated by colloidal stability. There have been studies on the stability or the aggregation of various nanoparticles (NPs) under a range of environmental conditions, but there is little information on fully characterised AgNPs in media used in (eco)toxicity studies. In this study, monodisperse 7, 10 and 20 nm citrate-stabilised AgNPs were synthesised, characterised and then fractionated and sized by flow field-flow fractionation (FFF) and measured with dynamic light scattering (DLS) in different dilutions of the media recommended by OECD for Daphnia magna (water flea) toxicity testing. Stability of NPs was assessed over 24 h, and less so over 21 days, similar time periods to the OECD acute and chronic toxicity tests for D. magna. All particles aggregated quickly in the media with high ionic strength (media1), resulting in a loss of colour from the solution. The size of particles could be measured by DLS in most cases after 24h, although a fractogram by FFF could not be obtained due to aggregation and polydispersity of the sample. After diluting the media by a factor of 2, 5 or 10, aggregation was reduced, although the smallest NPs were unstable under all media conditions. Media diluted up to 10-fold in the absence of AgNPs did not induce any loss of mobility or fecundity in D. magna. These results confirm that standard OECD media causes aggregation of AgNPs, which result in changes in organism exposure levels and the nature of the exposed particles compared to exposure to fully dispersed particles. Setting aside questions of dose metrics, significant and substantial reduction in concentration over exposure period suggests that literature data are in the main improperly interpreted and nanoparticles are likely to have far greater biological effects than suggested thus far by poorly controlled exposures. We recommend that the standard OECD media is diluted by a factor of ca. 10 for use with these NPs and this test media, which reduces AgNP aggregation without affecting the viability of the text organism.     

A multianalytical approach to investigate stone biodeterioration at a UNESCO world heritage site: the volcanic rock-hewn churches of Lalibela, Northern Ethiopia

A multianalytical approach combining Optical Microscopy (OM), Backscattered Variable Pressure Scanning Electron Microscopy + Energy Dispersive X-ray Spectroscopy (VP-BSEM + EDS), Powder X-ray Diffractometry (PXRD), Raman Spectroscopy, and Microbiological techniques has been applied to characterize decay products and processes occurring at the surface of two rock-hewn churches (Bete Gyorgis and Bete Amanuel) at the UNESCO’s World Heritage site of Lalibela, Northern Ethiopia. The two churches were carved into volcanic scoria deposits of basaltic composition. In their geological history, the Lalibela volcanic rocks underwent late to post-magmatic hydrothermal alteration together with partial laterization and are therefore characterized by a decay-prone highly vesicular microtexture with late stage to post-magmatic precipitation of secondary mineral phases (calcite–zeolite–smectite). The main objective of the study was to gain a better insight into the weathering products and mechanisms affecting the surface of the stone monuments and to assess the relative contribution of natural “geological” weathering processes versus biological/salt attack in stone decay at this unique heritage site. Results indicate that while the main cause of bulk rock deterioration and structural failure could be related to the stone inherited “geological” features, biological attack by micro- (bacteria) and/or macro- (lichens) organisms is currently responsible for severe stone surface physical and chemical weathering leading to significant weakening of the stone texture and to material loss at the surface of the churches walls. A prompt and careful removal of the biological patinas with the correct biocidal treatment is therefore recommended.     

Übergangsmetallkomplexe mit Schwefelliganden. XLIV. Ruthenium(II)-Komplexe mit dem sterisch anspruchsvollen Thioether-Thiolat-Liganden ‚buS4’︁2− (= 1,2-Bis(3,5-di(tertiärbutyl)-2-mercaptophenylthio)-ethan (2-)) sowie den Koliganden PPh3, PMe3, CO,NH3 und N2H4

Transition Metal Complexes with Sulfur Ligands. XLIV. Ruthenium(II) Complexes with the Sterically Demanding Thioether-thiolate Ligand ?^buS₄’?^2?(= 1,2-Bis(3,5-di(tertiarybutyl)-2-mercaptophenylthio) ethane (2-)) and PPh₃, CO, PMe₃, NH₃, and N₂H₄ Coligands The coordination properties of the sterically demanding thioether-thiolate ligand ‘^buS₄’^2? (= 1,2-Bis(3,5-di(tertiarybutyl)-2-mercaptophenylthio)ethane (2-)) towards Ruthenium were investigated. [Ru(PPh₃)₂ (‘^buS₄’)], 1 , was obtained from [RuCl₂(PPh₃)₃] and ‘^buS₄’? Li₂. One PPh₃ ligand in 1 is labile towards substitution and can be exchanged by L ? CO ( 2 ), PMe₃ ( 3 ), or NH₃ ( 5 ) yielding [Ru(L)(PPh₃)(‘^buS₄’)]. The PMe₃ complex [Ru(PMe₃)₂(‘^buS₄’)], 4 , is thermically inert as well as 2, 3 , and [Ru(CO)₂(‘^buS₄’)], 6 , which was obtained from [RuCl₂(CO)₃THF] and ‘^buS₄’? Li₂. Considering the thermical reaction inertness of 6 , its fast reaction with N₂H₄ yielding [Ru(N₂H₄) (CO) (‘^buS₄’)], 7 , is remarkable; the reaction probably takes place via 19e- intermediates. All ‘^buS₄’ complexes are better soluble in organic solvents than the corresponding [Ru(‘S₄’)] parent compounds, their ν(CO)frequencies or ³¹PNMR shifts, however, are nearly identical, allowing the conclusion that the influence of the t-butyl groups is topological and not electronic. All now complexes were characterized by elemental analyses as well as IR, NMR, and mass spectroscopy.