Pentalene complexes of group 7 metal carbonyls: an organometallic mixed-valence system with very large metal-metal electronic coupling

The bimetallic complexes [M(CO)(3)](2)(mu:eta(5):eta(5)-Pn) (Pn = pentalene, C(8)H(6); M = Mn, Re) have been synthesized and characterized crystallographically; the Mn compound was isolated as solely the anti-isomer, while the Re analogue was formed as a mixture of anti- and syn-isomers. [Mn(CO)(3)](2)(mu:eta(5):eta(5)-Pn) may be reduced chemically to its mono- and dianions; the mixed-valence Mn(I)/Mn(0) monoanion is shown by ESR, vibrational, and electronic spectroscopies to be a Robin-Day class III system with an exceptionally large electronic coupling between the metal centers.     

Magnetic coupling and intermetallic electron transfer in the heterodinuclear bioctahedral complexes MW(III)Cl(9)(n-) (M = V(II), Cr(III), Mn(IV)): tweaking the balance between ferromagnetism and antiferromagnetism

Density functional theory (DFT) calculations have been used to investigate the effect of intermetallic electron transfer on the mode of magnetic coupling in the face-shared bimetallic complexes MWCl(9)(n-) (M = V, Cr, Mn; all with a nominal d(3) valence electronic configuration on each metal atom). These calculations illustrate a simple rule: when the oxidation state of M is lower than that of W, antiferromagnetic coupling is preferred, while ferromagnetism (via crossed exchange pathways) is favored when M has the higher oxidation state. This underlying trend in intermetallic interactions is seen to depend on the interplay among ligand field splitting, spin polarization splitting of alpha- and beta-spin orbitals, and the relative energies of the M and W valence d orbitals, and is mirrored in the results seen in a wider survey of mixed-metal, face-shared complexes.     

Vergleich der Me?genauigkeit der Szintillations-Z?hlung 3H- und/oder 14C-markierter Proben durch externe bzw. interne Standardisierung und Anwendung auf die Sauerstoff-Kolben-Verbrennung

Zusammenfassung Nach Verbrennung im Sauerstoffkolben zeigen die Szintillationsproben häufig über Stunden eine ansteigende Zählrate. Auch danach unterscheiden sich identisch präparierte Proben noch statistisch signifikant stärker, als aufgrund von Wäge- und Pipettierfehlern zu erwarten ist. Die zeitliche Inkonstanz und die Unterschiede von Probe zu Probe lassen sich durch Löschkorrekturkurven (Abhängigkeit der Zählausbeute und des externen Standardkanalverhältnisses von der Sauerstoffkonzentration) weitgehend eliminieren. Die so erhaltenen dpm-Werte haben geringere Standardabweichungen als die durch interne Standardisierung erhaltenen. Das Verfahren der externen Standardisierung wird einer Fehleranalyse für einzel- und doppelmarkierte Proben unterworfen. Ein Toluol/Methanol/Phenäthylamin-System absorbiert Kohlendioxid und Wasser quantitativ. Für einzelmarkierte Proben können mit diesem Gemisch für ³H 25% und für ¹⁴C 85% Zählausbeute erhalten werden. Bei ³H/¹⁴C-Markierung werden bei einer Kanaloptimierung nach Klein und Eisler noch 15% Zählausbeute für ³H neben 10% für ¹⁴C erhalten.

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Comparison of errors in liquid scintillation counting after quench correction by external or internal standardisation applied for oxygen flask combustion of ³H- and/or ¹⁴C-labelled samples

After oxygen flask combustion scintillation samples often show an increase in counting rates for several hours. Even after such a time identically prepared samples are statistically significantly more different as one should expect by weighing and pipetting errors. The inconstant cpm values during the first hours and the differences between the samples can mostly be eliminated by quench correction curves (dependence of counting efficiency and external channel ratio upon oxygen concentration). The dpm values achieved according to this method show smaller standard deviations than by internal standardisation. The procedure of the external standardisation was examined for error propagation. A system of toluene/methanol/phenethylamine absorbs carbon dioxide and water quantitatively. With this mixture the following counting efficiencies are achieved: 25% for ³H, 85% for ¹⁴C and for samples containing ³H and ¹⁴C 15% for ³H and 10% for ¹⁴C if the ³H channel is adjusted according to Klein and Eisler.

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Symbole und Abkürzungen ESKV Externes Standard-Kanal-Verhältnis - ESKV-ESKV₀ - ESKV₀ Beginn der Gültigkeit der Löschfunktion, stärkst gelöschte Probe - ZA auf 1,0 bezogene Zählausbeute - S Standardabweichung - cpm Impulsrate (min^–1) - dpm Zerfallsrate (min^–1) - A bzw. B nieder- bzw. höherenergetisches Nuklid - (A) Meßkanal A - k,l,m,n Eichfaktoren     

Influence of cysteine to cysteic acid oxidation on the collision-activated decomposition of protonated peptides: Evidence for intraionic interactions

Oxidation of cysteine residues to cysteic acids in C-terminal arginine-eontaining peptides (such as those derived by tryptic digestion of proteins) strongly promotes the formation of multiple members of the Y? series of fragment ions following low energy collision-activated decomposition (CAD) of the protonated peptides, Removal of the arginine residue abolishes the effect, which is also attenuated by conversion of the arginine to dimethylpyrim-idylornithine. The data indicate the importance of an intraionic interaction between the cysteic acid and arginine side-chains. Low energy CAD of peptides which include cysteic acid and histidine residues, also provides evidence for intraionic interactions. It is proposed that these findings are consistent with the general hypothesis that an increased heterogeneity (with respect to location of charge) of the protonated peptide precursor ion population is beneficial to the generation of a high yield of product ions via several charge-directed, low energy fragmentation pathways. Furthermore, these data emphasize the significance of gas-phase conformations of protonated peptides in determining fragmentation pathways.     

Characterizations of the Kerr metric

For stationary, asymptotically flat solutions of Einstein's equations, covariant functionals of the metric variables are defined which characterize the Kerr metric uniquely. For instance, we obtain a generalization of the Bach tensor to stationary metrics, which vanishes if and only if the solution is Kerr. We also give a new interpretation of the Schwarzschild-to-Kerr-transformation. Our results might be applicable to simplify the proof of the uniqueness theorem for stationary black holes.     

Na3Al2Nb34O64 und Na (Si,Nb) Nb10O19: Clusterverbindungen mit isolierten Nb6-Oktaedern

Na₃Al₂Nb₃₄O₆₄ and Na (Si, Nb) Nb₁₀O₁₉. Cluster Compounds with Isolated Nb₆-Octahedra Hexagonal ormolu coloured plates of the new compounds Na₃Al₂Nb₃₄O₆₄ ( I ) and Na(Si, Nb)Nb₁₀O₁₉ ( II ) were prepared by heating pellets of NaF, Al₂O₃, NbO₂ and NbO (3:1:8:2) and NaF, NbO₂ and NbO (1:4:2), respectively, at approx. 850°C. I was contained in a sealed gold capsule, II in a silica tube. The Si incorporated in II originates from the container material. Both compounds crystallize in R 3 , I with a = 784.4(1), c = 7065(1) pm, Z = 3 and II with a = 784.1(1), c = 4221.8(5) pm, Z = 6. I and II represent new structure types. They contain the same characteristic structural units, namely discrete Nb₆O₁₂ clusters (d_Nb–Nb = 283 ± 4 pm) and Nb₂O₁₀ units with Nb–Nb dumbells (d_Nb–Nb ≈? 269 pm) in edgesharing coordination octahedra. In addition NbO₆ octahedra containing Nb in the oxidation state + 5 and NaO₁₂ cube-octahedra occur in both compounds besides AlO₄ and SiO₄ tetrahedra in I and II , respectively. The structures can be described in terms of a common closepacking of O and Na atoms together with Nb₆ octahedra.     

Determination of proanthocyanidins in fresh grapes and grape products using liquid chromatography with mass spectrometric detection

Fresh grapes and grape products, such as grape wine and grape juice, were analyzed for proanthocyanidins (PACs) using liquid chromatography with electrospray ionization mass spectrometric (MS) detection. PACs were successfully separated and analyzed on the basis of their protonated molecules, allowing the identification of PACs in different degrees of polymerization from monomers to oligomers (up to 7 units), and in various isomeric forms. Using reversed-phase high-performance liquid chromatography (HPLC) combined with MS detection, the PAC monomers, (+)-catechin (C), (-)-epicatechin (EC), (-)-catechin gallate (CG), and (-)-epicatechin gallate (ECG), were successfully quantified using selected ion monitoring (SIM) mode. Standard curves were fitted for each PAC ranging from 43.8 to 5600 ng/mL for C, from 42.2 to 5400 ng/mL for EC, from 36.7 to 4700 ng/mL for CG, and from 39.8 to 5100 ng/mL for ECG. Good linearity (r2>0.999) was achieved for each analyte. The accuracy and precision (RSD) were within 10% (n=8) at the limit of detection. This method allows direct quantification of monomeric PACs in fresh grapes and grape-derived products. Additionally, flow injection analysis (FIA) was applied to estimate the concentration levels of PAC oligomers by comparing their FIA-MS peak areas, which were well correlated (r2=0.936) to the total concentrations of PAC monomers.     

Preparation of Neutral Ionophores for Alkali and Alkaline Earth Metal Cations and their application in ion selective membrane electrodes

The preparation of a series of non-cyclic, uncharged ligands able to selectively complex alkali and alkaline earth metal cations is described. These molecules are designed to be used as carriers for cations through membranes. Some of the compounds show high Ca²⁺ and Na⁺ selectivity, respectively, in liquid membrane electrodes.     

The first genuine observation of fluorescent mononuclear phthalocyanine aggregates

An initial photophysical study of a tetra-solketal-substituted zinc phthalocyanine is reported; at low temperature this compound exhibits very strong aggregation, and a new red shifted emission peak is observed, lambda max approximately 750 nm, attributed to a fluorescent phthalocyanine dimer.