The stereochemistry of pipitzols and perezols

The stereochemistry of α-pipitzol (1a) has been established by transformation to cedrene (4) and by single crystal X-ray diffraction of α-pipitzol benzoate (1b). The stereochemistries of the related cedranolides β-pipitzol (2a), α-(1c), β-(2c) and γ-perezol (1d) are assigned from their respective ORD curves.     

Water displacement by cyanogold complexes in binuclear nickel(II) compounds based on bridging oxalate. Synthesis,structural diversity,magnetic properties,and DFT calculations

Several cyanogold complexes react with the binuclear nickel complex [(Ni(dien)(H(2)O))(2)(mu-ox)](PF(6))(2).2H(2)O to give the compounds [(Ni(dien)(H(2)O))(2)(mu-ox)]Br(2) (1), [(Ni(dien)(Au(CN)(2)))(2)(mu-ox)] (2), and [(Ni(dien))(2)(mu-ox)(mu-Au(CN)(4))](PF(6)) (3) (dien, diethilenetriamine; ox, oxalate). In the case of compounds 2 and 3, water displacement by the corresponding cyanogold complex takes place, whereas compound 1 is formed by a substitution of the anion. The crystal structures of compounds 1 and 2 present a 2D arrangement where the layers are connected by van der Waals forces (1) or N-H.Ntbd1;C hydrogen bonds (2), where each binuclear complex is hydrogen bonded to its neighbors, whereas compound 3 presents a novel structure where the tetracyanoaurate acts as a bridging ligand to give a polymeric compound. Magnetic studies of these compounds reveal an antiferromagnetic behavior. Finally, density functional theory (DFT) calculations have been performed on isolated models of compounds 2 and 3 in order to gain some insight about the different behavior of the [Au(CN)(2)](-) and [Au(CN)(4)](-) groups as ligands and proton acceptors in hydrogen bonds.     

Octahedral bipyridine and bipyrimidine dioxomolybdenum(VI) complexes: characterization,application in catalytic epoxidation,and density functional mechanistic study

Complexes of the general formula [MoO(2)X(2)L(2)] (X=Cl, Br, Me; L(2)=bipy, bpym) have been prepared and fully characterized, including X-ray crystallographic investigations of all six compounds. Additionally, the highly soluble complex [MoO(2)Cl(2)(4,4'-bis(hexyl)-2,2'-bipyridine)] has been synthesized. The reaction of the complexes with tert-butyl hydroperoxide (TBHP) is an equilibrium reaction, and leads to MoV(I) eta(1)-alkylperoxo complexes that selectively catalyze the epoxidation of olefins. Neither the Mo-X bonds nor the Mo-N bonds are cleaved during this reaction. These experimental results are supported by theoretical calculations, which show that the attack of TBHP at the Mo center through the X-O-N face is energetically favored and the TBHP hydrogen atom is transferred to a terminal oxygen of the Mo=O moiety. After the attack of the olefin on the Mo-bound peroxo oxygen atom, epoxide and tert-butyl alcohol are formed. The latter compound acts as a competitive inhibitor for the TBHP attack, and leads to a significant reduction in the catalytic activity with increasing reaction time.     

Dendritic cavitands: preparation and electrochemical properties

A new series of dendrimers containing a central cavitand core with four appended Fréchet-type dendrons, linked to the core through 4,4'-bipyridinium (viologen) subunits, have been synthesized, characterized and their electrochemical properties investigated using cyclic voltammetric measurements.     

1,3-Cyclopentanedione bis(4-methylthiosemicarbazone) monohydrochloride as a spectrophotometric reagent for the determination of lodate in acetic and perchloric acid media

1,3-Cyclopentanedione bis(4-methylthiosemicarbazone) monohydrochloride produces colored solutions with iodate ions in acid medium. The yellow color obtained has been used to proposed Spectrophotometric methods for determination of IO₃^? in the concentration range 1.0–11.0 ppm in acetic acid medium (molar absorptivity 1.08 × 10⁴ liters mol^?1 cm^?1 at a wavelength of 415 nm) and 0.5–8.0 ppm in perchloric acid medium (molar absorptivity 2.05 × 10⁴ liters mol^?1 cm^?1 at a wavelength of 400 nm).     

Application of chemometric methods to the investigation of main microcontaminant sources of endocrine disruptors in coastal and harbour waters and sediments

Identification, resolution and distribution of main microcontaminant sources of endocrine disruptors in Spanish harbours, coastal waters and sediments are investigated using chemometric methods. We investigated eighteen different endocrine disruptor chemical compounds, including non-ionic surfactants, their degradation products and linear alkylbenzene sulfonates, found in a total number of 74 samples (35 water samples and 39 sediment samples) over a period of 16 months from March 1999 to July 2000, and in 32 different geographical sites along the Spanish Mediterranean Coast (e.g. Barcelona, Tarragona, Almeria Harbour, Malaga and the Bay of Cadiz). Main environmental contamination sources of these endocrine disruptor compounds were investigated and interpreted according to their chemical composition and according to their resolved geographical distribution profiles.An erratum to this article can be found at     

A predictive vapor-pressure equation

A simple equation is presented for predicting the temperature dependence of the vapor-pressure of a pure substance along the entire (liquid + vapor) coexistence curve, from the triple point to the critical point. The proposed equation is based on the use of a dimensionless temperature reduced by using critical and triple point values, and of the Clausius–Clapeyron equation as a zeroth-order approximation. The pressure and temperature at the triple point, the normal boiling temperature, and the pressure and temperature at the critical point are required as input data. The proposed equation is verified for 53 fluids by using NIST data. These data are reproduced with an overall average deviation of 0.55%.     

Glafka 2004: Spacetime Topology from the Tomographic Histories Approach I: Non-Relativistic Case

The tomographic histories approach is presented. As an inverse problem, we recover in an operational way the effective topology of the extended configuration space of a system. This means that from a series of experiments we get a set of points corresponding to events. The difference between effective and actual topology is drawn. We deduce the topology of the extended configuration space of a non-relativistic system, using certain concepts from the consistent histories approach to Quantum Mechanics, such as the notion of a record. A few remarks about the case of a relativistic system, preparing the ground for a forthcoming paper sequel to this, are made in the end.     

Solution of Scott's Problem on the Number of Directions Determined by a Point Set in 3-Space

Let P be a set of n points in R³, not all in a common plane. We solve a problem of Scott (1970) by showing that the connecting lines of P assume at least 2n - 5 different directions if n is odd and at least 2n - 7 if n is even. The bound for odd n is sharp.     

There Are Not Too Many Magic Configurations

A finite planar point set P is called a magic configuration if there is an assignment of positive weights to the points of P such that, for every line l determined by P, the sum of the weights of all points of P on l equals 1. We prove a conjecture of Murty from 1971 and show that if a set of n points P is a magic configuration, then P is in general position, or P contains n−1 collinear points, or P is a special configuration of 7 points. The research by Rom Pinchasi was supported by a Grant from the G.I.F., the German-Israeli Foundation for Scientific Research and Development.