Hydrido-Carbonyl Rhenium Clusters with a Square Geometry of the Metal Core. Synthesis and X-Ray Characterization of the Novel [Re4(μ-H)3(CO)16]− Anion

Three and tetranuclear ring clusters have been obtained by treatment of [Re₂(CO)₈(THF)₂] with carbonyl-rhenates containing two terminal hydrides. The reaction with [ReH₂(CO)₄]^- provided a selective route to the previously known [Re₃(-H)₂(CO)₁₂]^- triangular cluster anion 1. The reaction with [Re₂H₂(-H)(CO)₈]^- gave the novel [Re₄(-H)₃(CO)₁₆]^- anion 2, containing a rare example of a puckered-square metal cluster. Protonation of 1 is known to afford the neutral [Re₃(-H)₃(CO)₁₂] species 3. Analogously the reaction of 2 with a strong acid afforded the previously known square metal clusters [Re₄(-H)₄(CO)₁₆] 4. The reaction could not be reversed by treatment with bases. Photolysis of 4 gave the unsaturated complex [Re₂(-H)₂(CO)₈] 5: this is the reverse of the dimerization reaction, that in THF at room temperature produces 4 from 5. Thermal treatment (reflux in cyclohexane for 24 h) left 4 almost unchanged. A single crystal X-ray analysis of [NEt₄]2 showed a s/e/s/s (e=eclipsed, s=staggered) conformation of the Re(CO)₄ units, leading to a puckered geometry of the ring, at variance with the square-planar geometry of 4 (all eclipsed). Two of the three hydrides of 2 have been located as bridging the Re–Re edges from inside the metal ring, as previously observed in 4. Density functional computations indicated a puckered conformation as the most stable for both 2 and 4, with very low activation energies for ring inversion (6.6 and 2.2 kcal·mol^-1, respectively), but ruled out solid state fluxionality for 4, whose observed planar geometry must be attributed to packing stabilization.     

Complete characterization of almost Moore digraphs of degree three

It is well known that Moore digraphs do not exist except for trivial cases (degree 1 or diameter 1), but there are digraphs of diameter two and arbitrary degree which miss the Moore bound by one. No examples of such digraphs of diameter at least three are known, although several necessary conditions for their existence have been obtained. In this paper, we prove that digraphs of degree three and diameter k ≥ 3 which miss the Moore bound by one do not exist. © 2004 Wiley Periodicals, Inc. J Graph Theory 48: 112–126, 2005     

New Results on EX Graphs

By the extremal number ex(n; t) = ex(n; {C ₃, C ₄, . . . , C _t }) we denote the maximum size (that is, number of edges) in a graph of order n > t and girth at least g ≥ t + 1. The set of all the graphs of order n, containing no cycles of length ≥ t, and of size ex(n; t), is denoted by EX(n; t) = EX(n; {C ₃, C ₄, . . . , C _t }), these graphs are called EX graphs. In 1975, Erdős proposed the problem of determining the extremal numbers ex(n; 4) of a graph of order n and girth at least 5. In this paper, we consider a generalized version of this problem, for t ≥ 5. In particular, we prove that ex(29; 6) = 45, also we improve some lower bounds and upper bounds of ex _u (n; t), for some particular values of n and t.     

Stability of Hereditary Graph Classes Under Closure Operations

If is a subclass of the class of claw‐free graphs, then is said to be stable if, for any , the local completion of G at any vertex is also in . If is a closure operation that turns a claw‐free graph into a line graph by a series of local completions and is stable, then for any . In this article, we study stability of hereditary classes of claw‐free graphs defined in terms of a family of connected closed forbidden subgraphs. We characterize line graph preimages of graphs in families that yield stable classes, we identify minimal families that yield stable classes in the finite case, and we also give a general background for techniques for handling unstable classes by proving that their closure may be included into another (possibly stable) class.     

[Re5(μ-H)4(CO)20]− and [Re5(μ-H)5(CO)20], Two Isolobal Analogues of Cyclopentane

The first five-membered rings of metal atoms connected by M–M or M-H-M bonds only have been obtained by a Re₂+Re₃ condensation in which a polyhydride acts as a bridging bidentate ligand toward a coordinatively unsaturated fragment (see scheme below). In spite of the octahedral coordination of the Re centers, the Re₅ rings display conformations (twisted and envelope) comparable with those observed for organic five-membered rings of tetrahedral carbon atoms.     

Primary Reaction Dynamics of Proteorhodopsin Mutant D97N Observed by Femtosecond Infrared and Visible Spectroscopy†

Femtosecond time‐resolved spectroscopy in the visible and IR range was utilized to study the primary reaction dynamics of the proteorhodopsin (PR) D97N mutant in comparison with wild type PR at different pH values. The analysis of the data obtained in the mid‐IR closely resembles the results for wild type PR. The observation of the first ground state intermediate K is initially obscured by a complex reaction scheme of vibrational relaxation and heating effects, but its spectral signature clearly emerges at long delay times. In the visible range, a biexponential decay of the excited state within 30 ps and the formation of the K photoproduct is observed. The decay time constants derived for the D97N mutant in D₂O are slightly larger than in H₂O due to H/D exchange. This kinetic isotope effect is even less pronounced than for wild type PR at pH 6. These results support the current notion of a pH dependent hydrogen bonding network in the retinal binding pocket of PR and a weaker interaction between the retinal Schiff base and the counter ion complex compared to bacteriorhodopsin.     

On bipartite graphs of diameter 3 and defect 2

We consider bipartite graphs of degree Δ≥2, diameter D=3, and defect 2 (having 2 vertices less than the bipartite Moore bound). Such graphs are called bipartite (Δ, 3, ?2) ‐graphs. We prove the uniqueness of the known bipartite (3, 3, ?2) ‐graph and bipartite (4, 3, ?2)‐graph. We also prove several necessary conditions for the existence of bipartite (Δ, 3, ?2) ‐graphs. The most general of these conditions is that either Δ or Δ?2 must be a perfect square. Furthermore, in some cases for which the condition holds, in particular, when Δ=6 and Δ=9, we prove the non‐existence of the corresponding bipartite (Δ, 3, ?2)‐graphs, thus establishing that there are no bipartite (Δ, 3, ?2)‐graphs, for 5≤Δ≤10. © 2009 Wiley Periodicals, Inc. J Graph Theory 61: 271–288, 2009     

Extraction of Alkylphenols and Nonylphenol Mono- and Diethoxylates from Water Using Magnetically Modified Adsorbents

An LC-DAD-ESI-MS⁽²⁾ method was developed for simultaneous analysis of major chemical constituents in Shexiang Baoxin Pill (SBP), an important multiherbal formula in traditional Chinese medicine widely used for treating cardiovascular diseases. The chromatographic separation was performed on a Symmetry C₁₈ column (5 μm, 250 × 4.6 mm), whilst water with 0.5% formic acid and acetonitrile were used as mobile phase. On the basis of the characteristic UV absorption profile, the information of molecular weight and structure provided by ESI-MS⁽²⁾, 57 constituents from moschus, radix rhizoma ginseng, calculus bovis, cortex cinnamomi, styrax and venenum bufonis, were detected and 47 of them were identified in this study. The method can be the basis for the quality control of SBP.     

Preparation of samples of airborne particles for determination of metals

Summary A Soxhlet extraction procedure for the preparation of samples of airborne particulate matter for analysis by atomic-absorption spectrophotometry has been developed and tested by addition of known amounts of lead, iron, manganese, copper and cadmium and by analysing portions of atmospheric samples processed by (1) Soxhlet extraction, (2) acid digestion and (3) autoclave digestion. Soxhlet extraction proved to be superior to the other two procedures. It is simple and reliable, and offers less possibility of contamination by, or loss of, trace metals.

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Probengewinnung von Schwebstaub zur Metallbestimmung

Zusammenfassung Ein Verfahren der Soxhlet-Extraktion zur Probengewinnung für die Analyse von Luftverunreinigungsteilchen durch Atomarabsorption wurde ausgearbeitet und durch Zusatz bekannter Mengen Blei, Eisen, Mangan, Kupfer und Cadmium geprüft. Die Soxhlet-Extraktion, die saure Veraschung und die Veraschung im Autoklaven wurde zwecks Analyse von Luftproben verglichen, wobei sich das erstgenannte Verfahren als überlegen erwies. Es ist einfach, zuverlässig und bietet geringe Gefahr des Verlustes oder der Einschleppung von Spurenmetallen.