The Flooding Time in Random Graphs

Based on our analysis of the hopcount of the shortest path between two arbitrary nodes in the class G _p (N) of random graphs, the corresponding flooding time is investigated. The flooding time T _N (p) is the minimum time needed to reach all other nodes from one node. We show that, after scaling, the flooding time T _N (p) converges in distribution to the two-fold convolution ^(2*) of the Gumbel distribution function (z)=exp (–e ^–z ), when the link density p _N satisfies Np _N /(log N)³ if N .     

Analysis of the pi-pi stacking interactions between the aminoglycoside antibiotic kinase APH(3')-IIIa and its nucleotide ligands

A key contact in the active site of an aminoglycoside phosphotransferase enzyme (APH(3')-IIIa) is a pi-pi stacking interaction between Tyr42 and the adenine ring of bound nucleotides. We investigated the prevalence of similar Tyr-adenine contacts and found that many different protein systems employ Tyr residues in the recognition of the adenine ring. The geometry of these stacking interactions suggests that electrostatics play a role in the attraction between these aromatic systems. Kinetic and calorimetric experiments on wild-type and mutant forms of APH(3')-IIIa yielded further experimental evidence of the importance of electrostatics in the adenine binding region and suggested that the stacking interaction contributes approximately 2 kcal/mol of binding energy. This type of information concerning the forces that govern nucleotide binding in APH(3')-IIIa will facilitate inhibitor design strategies that target the nucleotide binding site of APH-type enzymes.     

Synthesis and reactivity of a ferrocene-derived PCP-pincer ligand

The 1,3-bis(diphosphinomethyl)ferrocene 3 readily reacts with [(C2H4)2RhCl]2 to form an equilibrating pair of diastereomers 8a and 8b by C-H insertion into the ferrocene.     

Kinetic and thermodynamic preferences for the diastereoselective oxidative addition of H(2) to trans-Ir(PR(3))(2)(CO)Cl: monodentate chiral phosphines may impart exceptional degrees of diastereoselectivity

Studies on square planar iridium complexes of the type trans-Ir(PR(3))(2)(CO)Cl, where PR(3) is PhP[(C(5)Me(4))](2), PhP[Me(2)C(4)H(6)], or PhP[Pr(i)(2)C(4)H(6)], demonstrate that monodentate chiral phosphines impart exceptional degrees of diastereoselectivity in the oxidative addition of H(2). Thus, the oxidative addition of H(2) to the two faces of the meso isomer (R,S)-trans-Ir(PR(3))(2)(CO)Cl proceeds with a kinetic diastereoselectivity which exceeds that for related square planar iridium complexes employing bidentate chiral phosphine ligands. Furthermore, the kinetically favored dihydride is not favored thermodynamically, and the magnitude of the inversion of the kinetic and thermodynamic selectivities is greater than has previously been observed using bidentate phosphines.     

Longitudinal focusing and cooling of a molecular beam

A neutral polar molecule experiences a force in an inhomogeneous electric field. This electric field can be designed such that a beam of polar molecules is exposed to a harmonic potential in the forward direction. In this potential the longitudinal phase-space distribution of the ensemble of molecules is rotated uniformly. This property is used to longitudinally focus a pulsed beam of ammonia molecules and to produce a beam with a longitudinal velocity spread of 0.76 m/s, corresponding to a temperature of 250 mu K.     

Structure determination of gas-phase niobium and tantalum carbide nanocrystals via infrared spectroscopy

Niobium and tantalum carbide clusters have been isolated in the gas phase and irradiated with intense tunable infrared (IR) light. Stable neutral clusters are selectively ionized and subsequently detected in a mass spectrometer. By tuning the IR frequency, infrared multiphoton absorption spectra are obtained for a whole range of clusters. These mass-selective IR spectra lead to insights into the structures of small niobium and tantalum carbide clusters and nanocrystals.     

Caracte´risation ete´tude structurale du bichromate mixte AgKCr2O7

The crystal structure of AgKCr₂O₇ has been determined by single crystal X-ray diffraction. The structure is orthorhombic, space groupPnam, with cell constantsa = 15.175(7),b = 7.414(4), andc = 6.014(2)A? forZ = 4. The structure was refined to a finalR of 0.072 for 934 reflections. The configuration of the dichromate ion, pseudo-C_2v, is similar to that found in most other dichromate structures. The potassium ion is surrounded by eight oxygen atoms and the silver ion has a square planar coordination.     

Is asymptotic freedom enough?

Quantum field theories with strong interactions are usually required to be not only renormalizable but also asymptotically free, in order to avoid diseases such as the Landau ghost. In this paper we suggest an even more restrictive requirement: “asymptotic convergence”, which means that at high energies it must be possible to formulate a convergent resummation procedure for the perturbation expansion. Such a convergent resummation technique exists in QCD in the infinite color limit (N → ∞). We give an outline of a proof of this statement, and a brief account of possible consequences of our asymptotic convergence condition on model building.     

Di-2-Pyridylketoxime,a new reagent for the spectrophotometric determination of iron(II)

Di-2-pyridylketoxime is proposed for the spectrophotometric determination of iron(II). The complex is stable and exhibits an absorption maximum at 534 nm in an alkaline citrate medium. Beer's law is obeyed and the molar absorptivity is 1.5·10³. Large amounts of copper and nickel can be tolerated. The method is simple, convenient and reproducible.     

Chimie organométallique du niobium VIII. Etude détaillée de la réaction d'acétyléniques perfluorés avec (η5C5H5)2NbH3: Mise en evídence de la coupure de liaison C-F,de la formation de liaison Nb-F et de la reduction du niobium. Structure cristalline d'un dérivé niobiacyclopentadiène

Hexafluorobutyne (CF₃CCCF₃ = Hfb) reacts with Cp₂NbH₃ (Cp = η⁵-C₅H₅) affording five new complexes I–V, whose structures have been studied by mass,

Ir, ESR and NMR spectroscopy. The X-ray study of II shows the presence of a planar NbC₄ ring Nb—C(1): 2.25(5); C(1)—C(2): 1.31(7); C(2)—C(3): 1.53(8); C(3)—(4): 1.34(7); C(4)—Nb: 2.23(5) Å. With trifluoropropyne (CF₃CCH = tfp), only the analog of III, i.e. Cp₂NbFf(tfpH), has been isolated.