Electronic charge distribution in the intermetallic compound MnHg

The Electron Density Distribution (EDD) of the Mn and Hg atoms in MnHg has been elucidated for the first time using the statistical approach MEM (Maximum Entropy Method). Reported powder X‐ray data have been used for the present analysis. Presently, MEM studies are gaining momentum because of the feasibility of mapping the refined electron densities with very high accuracy, well resolved and clear electron density distributions obtainable from this method, etc. Moreover, the MEM results resemble very nearly the true structure/electron densities unlike the conventional Fourier methods, which give biased information due to many reasons, the major one being the series termination error. In the present analysis, the bonding between Mn and Hg, and the electron content for different radii of the two atoms has been investigated. The thermal vibration of the two atoms has also been analyzed and reported. The thermal parameter of Hg has been more accurately determined than that of Mn, which is in consistent with the electron density distribution of the atoms.     

New model for ion neutralization at surfaces

We investigate the neutralization of low energy He⁺ ions in close collisions with metal surface atoms. In order to describe the neutralization process as completely as possible, we consider Auger neutralization (AN), resonant neutralization (RN) and resonant ionization (RI). Our calculation agrees well with experimental data and shows that in some metals (like Pd) AN is the dominant process, whereas in others (like Al) RN and RI contribute significantly for energies above the threshold for reionization.     

Signal enhancement for gradient reverse-phase high-performance liquid chromatography-electrospray ionization mass spectrometry analysis with trifluoroacetic and other strong acid modifiers by postcolumn addition of propionic acid and isopropanol

Trifluoroacetic acid (TFA) and other volatile strong acids, used as modifiers in reverse-phase high-performance liquid chromatography, cause signal suppression for basic compounds when analyzed by electrospray ionization mass spectrometry (ESI-MS). Evidence is presented that signal suppression is caused by strong ion pairing between the TFA anion and the protonated sample cation of basic sample molecules. The ion-pairing process “masks” the protonated sample cations from the ESI-MS electric fields by rendering them “neutral”. Weakly basic molecules are not suppressed by this process. The TFA signal suppression effect is independent from the well-known spray problem that electrospray has with highly aqueous solutions that contain TFA. This previously reported spray problem is caused by the high conductivity and surface tension of aqueous TFA solutions. A practical method to enhance the signal for most basic analytes in the presence of signal-suppressing volatile strong acids has been developed. The method employs postcolumn addition of a solution of 75% propionic acid and 25% isopropanol in a ratio 1:2 to the column flow. Signal enhancement is typically 10-50 times for peptides and other small basic molecules. Thus, peptide maps that use ESI-MS for detection can be performed at lower levels, with conventional columns, without the need to use capillary chromatography or reduced mass spectral resolution to achieve satisfactory sensitivity. The method may be used with similar results for heptafluorobutyric acid and hydrochloric acid. A mechanism for TFA signal suppression and signal enhancement by the foregoing method, is proposed.     

NMR spectroscopy and relaxation studies on liquid-crystalline side-chain polysiloxanes

Two polysiloxanes with mesogenic side chains were investigated with polarization microscopy, DSC, ¹H and ¹³C NMR. From proton NMR the global order parameter of one sample (the other does not orient itself in the magnetic field of 2.1 T) and some information about phase transitions was obtained. ¹³C NMR yielded selective order parameters with respect to different atomic positions. The order parameter varies slightly also within the mesogenic unit, but much stronger within the spacer (odd-even effect). With the help of T_1ρ experiments some molecular motions had been detected and characterized (main-chain motion, over-head rotation of the mesogenic unit, methyl rotation, side-chain rotation).     

Isolation and Identification of Pennogenin Tetraglycoside from Cestrum nocturnum (Solanaceae) and Its Antifungal Activity against Fusarium kuroshium,Causal Agent of Fusarium Dieback

Antifungal assay-guided fractionation of the methanolic crude extract of Cestrum nocturnum (Solanaceae), popular known as ‘lady of the night’, led the isolation and identification of the steroidal saponin named pennogenin tetraglycoside, which was identified for the first time in this plant species by spectroscopic means. The crude extract, fractions and pennogenin tetraglycoside exhibited mycelial growth inhibition of Fusarium solani and F. kuroshium. F. solani is a cosmopolitan fungal phytopathogen that affects several economically important crops. However, we highlight the antifungal activity displayed by pennogenin tetraglycoside against F. kuroshium, since it is the first plant natural product identified as active for this phytopathogen. This fungus along with its insect symbiont known as Kuroshio shot hole borer (Euwallacea kuroshio) are the causal agents of the plant disease Fusarium dieback that affects more than 300 plant species including avocado (Persea americana) among others of ecological relevance. Scanning electron microscopy showed morphological alterations of the fungal hyphae after exposure with the active fractions and 12 phenolic compounds were also identified by mass spectrometry dereplication as part of potential active molecules present in C. nocturnum leaves.     

Dilute oral iron solutions as gastrointestinal contrast agents for magnetic resonance imaging; initial clinical experience

Delineation of the gastrointestinal tract in magnetic resonance imaging (MRI) remains a problem. Ferric ammonium citrate is paramagnetic, producing a high MRI signal intensity by virtue of its spin-lattice (T1) relaxation rate enhancement properties. Water is diamagnetic, producing a low MRI signal intensity, especially with short TR and TE times. To compare efficacy for gastrointestinal contrast alteration, ferric ammonium citrate was administered to 18 patients and water was given to 10 patients. Spin-echo imaging at 0.35T was performed after administration of these agents. Ferric ammonium citrate produced high signal intensity within the esophagus, stomach, duodenum, and small intestine that aided in the differentiation of the gastrointestinal tract from adjacent tumors, vessels, and viscera. Delineation of the gut wall was superior using ferric ammonium citrate compared to that produced by water. Delineation of the margins of the pancreas, liver, and kidney from adjacent gastrointestinal tract was also better with ferric ammonium citrate. Optimal distinction between bowel and fat was better with water. Longer TE times (75 to 200 ms) may allow improved contrast between gut and intrabdominal fat using ferric ammonium citrate.     

The linkage between reversible Friedel–Crafts acyl rearrangements and the Scholl reaction

Structural Chemistry - Friedel–Crafts acyl rearrangements in PPA (at 80–240 °C) and Scholl reactions in AlCl3/NaCl (at 140–220 °C) of...     

Odd-Even Effect on Luminescence Properties of Europium Aliphatic Dicarboxylate Complexes

The odd–even effect in luminescent [Eu₂(L)₃(H₂O)_x]⋅y(H₂O) complexes with aliphatic dicarboxylate ligands (L: OXA, MAL, SUC, GLU, ADP, PIM, SUB, AZL, SEB, UND, and DOD, where x=2–6 and y=0–4), prepared by the precipitation method, was observed for the first time in lanthanide compounds. The final dehydration temperatures of the Eu³⁺ complexes show a zigzag pattern as a function of the carbon chain length of the dicarboxylate ligands, leading to the so-called odd-even effect. The FTIR data confirm the ligand–metal coordination via the mixed mode of bridge–chelate coordination, except for the Eu³⁺-oxalate complex. XRD results indicate that the highly crystalline materials belong to the monoclinic system. The odd–even effect on the 4 f–4 f luminescence intensity parameters (Ω₂ and Ω₄) is explained by using an extension of the dynamic coupling mechanism, herein named the ghost-atom model. In this method, the long-range polarizabilities ( ) were simulated by a ghost atom located at the middle of each ligand chain. The values of were estimated using the localized molecular orbital approach. The emission intrinsic quantum yield ( ) of the Eu³⁺ complexes also presented an the odd-even effect, successfully explained in terms of the zigzag behavior shown by the Ω₂ and Ω₄ intensity parameters. Luminescence quenching due to water molecules in the first coordination sphere is also discussed and rationalized.     

Hydrogenolysis of Dinuclear PCNR Ligated PdII μ-Hydroxides and Their Mononuclear PdII Hydroxide Analogues

The hydrogenolysis of mono- and dinuclear Pd^II hydroxides was investigated both experimentally and computationally. It was found that the dinuclear μ-hydroxide complexes {[(PCN^R)Pd]₂(μ-OH)}(OTf) (PCN^H=1-[3-[(di-tert-butylphosphino)methyl]phenyl]-1H-pyrazole; PCN^Me=1-[3-[(di-tert-butylphosphino)methyl]phenyl]-5-methyl-1H-pyrazole) react with H₂ to form the analogous dinuclear hydride species {[(PCN^R)Pd]₂(μ-H)}(OTf). The dinuclear μ-hydride complexes were fully characterized, and are rare examples of structurally characterized unsupported singly bridged μ-H Pd^II dimers. The {[(PCN^Me)Pd]₂(μ-OH)}(OTf) hydrogenolysis mechanism was investigated through experiments and computations. The hydrogenolysis of the mononuclear complex (PCN^H)Pd-OH resulted in a mixed ligand dinuclear species [(PCN^H)Pd](μ-H)[(PCC)Pd] (PCC=a dianionic version of PCN^H bound through phosphorus P, aryl C, and pyrazole C atoms) generated from initial ligand “rollover” C−H activation. Further exposure to H₂ yields the bisphosphine Pd⁰ complex Pd[(H)PCN^H]₂. When the ligand was protected at the pyrazole 5-position in the (PCN^Me)Pd−OH complex, no hydride formed under the same conditions; the reaction proceeded directly to the bisphosphine Pd⁰ complex Pd[(H)PCN^Me]₂. Reaction mechanisms for the hydrogenolysis of the monomeric and dimeric hydroxides are proposed.