bluuebunny/crossref_metadata_2025 huggingface dataset:
Emulsifier‐free miniemulsion polymerization of styrene using a cationic initiator
Authors: Faridi‐Majidi Reza , Sharifi‐Sanjani Naser
Published: September 2007

Polystyrene latex particles were synthesized using a method based on emulsifier‐free miniemulsion polymerization under ultrasonic irradiation in the presence of 2,2′ azobis (2‐amidinopropane) dihydrochloride (V‐50) as a cationic ionizable water‐soluble initiator and cetyl alcohol as costabilizer. The optimized conditions were obtained by using various parameters, such as the amounts of monomer and initiator, and the time and power of ultrasonic irradiation. In optimal conditions, the latex particles appeared to be about 250 nm in diameter through scanning electron microscopy (SEM). The SEM and gel permeation chromatography (GPC) analyses and monomer conversions of emulsifier‐free miniemulsion polymerization were compared with those of conventional emulsifier‐free emulsion polymerization using V‐50 as initiator in both cases. The results showed that in the miniemulsion polymerization, the rate of polymerization (Rp) was significantly higher, and latex particles were significantly smaller than those in the conventional emulsion polymerization. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci, 2007
Synthesis of bisphenol a novolac epoxy resins for coating applications
Authors: Atta Ayman M. , Shaker Nevin O. , Nasser Notaila E.
Published: September 2007

Bisphenol A novolacs were synthesized in both melting and solution processes using p‐formaldehyde and formalin solution in presence of oxalic acid catalyst, respectively. Hydrogen nuclear magnetic resonance, 1H NMR, investigations show a high methylene bridge contents in the novolacs synthesized in a melting process. These novolacs were analyzed by gel permation chromatography (GPC) and fourier transform infrared spectroscopy (FTIR). The bisphenol A novolac was cured with 1‐(2‐amino ethyl) piprazine (AEP) as a curing agent for epoxy resins. The cured resins were evaluated as organic coating for steel. The mechanical properties of the cured epoxy resins were evaluated. The chemical resistances of the cured resins were evaluated through salt spray resistance, hot water, solvents, acid and alkali resistance measurements. The data indicate that the cured epoxy resins have excellent chemical resistances as organic coatings among other cured resins. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci, 2008
Synthesis and characterization of new dextran‐acrylamide gels
Authors: Abdurrahmanoglu Suzan , Firat Yurdun
Published: September 2007

Dextran‐acrylamide gels were synthesized in a single step reaction by using 4,4′‐azobis(4‐cynovaleric acid) as bi‐functional initiator at 60°C. Corresponding acrylamide (AaM) gels in the absence of dextran were also prepared for comparison. Several parameters such as reaction period (6, 12, and 24 h), monomer and crosslinker concentrations were varied and their effects on the properties of gels were investigated. Gels were characterized by their mechanical and swelling behaviors and in terms of structural changes using SEM. It was observed that swelling degree decreased by increasing the monomer concentration due to formation of more crosslinking points that cause tighter network structure. Mechanical measurements showed that elastic modulus of AaM gels was higher than that of dextran‐AaM gel which indicating the importance of dextran concentration on the flexibility of the network. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci, 2007
Modeling helioseismic modes in the magnetic atmosphere of the Sun
Authors: Pintér B.
Published: September 2007

The pulsation of the solar surface is caused by acoustic waves traveling in the solar interior. Thorough analyses of observational data indicate that these f and p helioseismic oscillation modes are not bounced back completely at the surface but they partially penetrate into the atmosphere. Atmospheric effects and their possible observational application are investigated in one‐dimensional magnetohydrodynamic models. It is found that f and p mode frequencies are shifted of the order of μHz due to the presence of an atmospheric magnetic field. This shift varies with the direction of the wave propagation.Resonant coupling of global helioseismic modes to local Alfvén and slow waves reduce the life time of the global modes. The resulting line width of the frequency line is of the order of nHz, and it also varies with propagation angle. These features enable us to use helioseismic observations in magnetic diagnostics of the lower atmosphere. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Absolute and convective instabilities in an inviscid compressible mixing layer
Authors: Caillol P. , Ruderman M.
Published: September 2007

We consider the stability of a compressible shear flow separating two streams of different speeds and temperatures. The velocity and temperature profiles in this mixing layer are hyperbolic tangents. The normal mode analysis of the flow stability reduces to an eigenvalue problem for the pressure perturbation. We briefly describe the numerical method that we used to solve this problem. Then, we introduce the notions of the absolute and convective instabilities and examine the effects of Mach number, and the velocity and temperature ratios of each stream on the transition between convective and absolute instabilities. Finally, we discuss the implication of the results presented in this paper for the heliopause stability. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
On the global stability of single‐planet systems
Authors: Süli Á. , Dvorak R. , Érdi B.
Published: September 2007

In this paper, we estimate the global stability properties of single‐planet systems by using a catalogue of stability maps. The data of the catalogue were used to generate probability values on the mass parameter–eccentricity plane for the occurrence of stable orbits. We showed that the probability data can be well approximated by a second order surface. Using the resulted formula the likelihood of finding Earth‐like planets in single‐planet systems can be easily estimated. As an example, we derived estimations for four known exoplanetary systems. Our formula can be useful in selecting target stars for future space missions. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Stability of fictitious Trojan planets in extrasolar systems
Authors: Schwarz R. , Dvorak R. , Süli Á. , Érdi B.
Published: September 2007

Our work deals with the dynamical possibility that in extrasolar planetary systems a terrestrial planet may have stable orbits in a 1:1 mean motion resonance with a Jovian like planet. We studied the motion of fictitious Trojans around the Lagrangian points L4/L5 and checked the stability and/or chaoticity of their motion with the aid of the Lyapunov Indicators and the maximum eccentricity. The computations were carried out using the dynamical model of the elliptic restricted three‐body problem that consists of a central star, a gas giant moving in the habitable zone, and a massless terrestrial planet. We found 3 new systems where the gas giant lies in the habitable zone, namely HD99109, HD101930, and HD33564. Additionally we investigated all known extrasolar planetary systems where the giant planet lies partly or fully in the habitable zone. The results show that the orbits around the Lagrangian points L4/L5 of all investigated systems are stable for long times (107 revolutions). (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Monte‐Carlo simulation of a possible correlation between the estimated luminosity distances and internal extinctions of type Ia supernovae
Authors: Balázs L.G. , Hetesi Zs. , Korsós M.
Published: September 2007

This paper describes a Monte Carlo simulation of type Ia supernova data. It was shown earlier that the data of SNe Ia might contain a possible correlation between the estimated luminosity distances and internal extinctions. This correlation was shown by different statistical investigations of the data. In order to remove observational biases (for example the effect of the detection limit of the observing instrument) and to test the reality of the effect found earlier we developed a simple routine which simulates extinction values, redshifts and absolute magnitudes for Ia supernovae. We pointed out that the correlation found earlier in the real data between the internal extinction and luminosity distance does not occur in the simulated sample. Furthermore, it became obvious that the detection limit of the observing devices used in supernova projects does not affect the far end of the redshift‐luminosity distance relationship of Ia supernovae. This result strengthens the earlier conclusions of the authors that SN Ia supernovae alone do not support the existence of dark energy. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
A stability study of Kuiper Belt binaries
Authors: Nagy I. , Süli Á. , Érdi B.
Published: September 2007

The dynamical structure of the orbital element space of seven Kuiper Belt binary systems is studied by numerical methods in the model of the spatial elliptic restricted three‐body problem. It is shown that three systems have an extended region of stability where additional satellites could exist. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Coronal “wave”: A signature of the mechanism making CMEs largescale in the low corona?
Authors: Attrill G.D.R. , Harra L.K. , van Driel‐Gesztelyi L. , Démoulin P. , Wülser J.‐P.
Published: September 2007

We analyse one of the first coronal waves observed by STEREO/EUVI associated with a source region just behind the limb, NOAA 10940. We apply the coronal “wave” model proposed by Attrill et al. (2007) to explain the evolution of the observed bright fronts, thereby arguing that the bright fronts and dimmings are due to magnetic reconnections between the expanding CME core and surrounding magnetic structures. We offer a discussion showing that this model provides a mechanism via which CMEs, expanding from a small source region can naturally become large‐scale in the low corona. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Numerical simulations of fast and slow coronal mass ejections
Authors: Török T. , Kliem B.
Published: September 2007

Solar coronal mass ejections (CMEs) show a large variety in their kinematic properties. CMEs originating in active regions and accompanied by strong flares are usually faster and accelerated more impulsively than CMEs associated with filament eruptions outside active regions and weak flares. It has been proposed more than two decades ago that there are two separate types of CMEs, fast (impulsive) CMEs and slow (gradual) CMEs. However, this concept may not be valid, since the large data sets acquired in recent years do not show two distinct peaks in the CME velocity distribution and reveal that both fast and slow CMEs can be accompanied by both weak and strong flares. We present numerical simulations which confirm our earlier analytical result that a flux‐rope CME model permits describing fast and slow CMEs in a unified manner. We consider a force‐free coronal magnetic flux rope embedded in the potential field of model bipolar and quadrupolar active regions. The eruption is driven by the torus instability which occurs if the field overlying the flux rope decreases sufficiently rapidly with height. The acceleration profile depends on the steepness of this field decrease, corresponding to fast CMEs for rapid decrease, as is typical of active regions, and to slow CMEs for gentle decrease, as is typical of the quiet Sun. Complex (quadrupolar) active regions lead to the fastest CMEs. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Variability and evolution in various classes of post‐AGB stars
Authors: Şahin T. , Jeffery C.S.
Published: September 2007

We aim to compare properties of early‐type post‐asymptotic giant‐branch (post‐AGB) stars, including normal first‐time B‐type post‐AGB stars, and extreme helium stars (EHes). Hipparcos photometry for 12 post‐AGB stars and 7 EHe stars has been analyzed; 5 post‐AGB stars are clearly variable. The Hipparcos data are not sufficiently sensitive to detect variability in any of the EHes. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Statistical technique to study the connection between large‐scale structures in the ISM and low‐mass star formation
Authors: Kiss Z.T.
Published: September 2007

Applying a color index selection the Point Source Catalog of the Two Micron All Sky Survey (2MASS PSC) has been searched for Classical T Tauri (CTT) stars in the 2nd and 3rd Galactic quadrant based on their apparent KS excess. The selection resulted in 3872 reliable CTT candidates. The obtained CTT sample is extended enough for statistical examination of the inhomogeneities in their distribution due to correlation with structures in the ISM, like infrared loops. A correlation was observed between the presence of dust loops and the CTT density. The latter shows an excess on loops with respect to that expected from random fluctuation in a homogeneous distribution matching with the observed overall distribution. Monte Carlo simulations were used to show the significance of the excess. The results imply that the formation of a fraction of CTTs was triggered during the loop formation. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Pulsation and orbit of AU Pegasi
Authors: Jurkovic M. , Szabados L. , Vinkó J. , Csák B.
Published: September 2007

AU Pegasi is a pulsating star in a spectroscopic binary system with an orbital period of 53.26 days. Between 1960 and 1990 an extremely rapid period increase was observed in the value of the pulsation period, but in the last 15 years the observation show that the period set in 2.411 days. Fourier analysis of photometric data obtained during the ASAS project and those taken at the Piszkéstető Mountain Station of the Konkoly Observatory during 1994–2005 indicate that AU Pegasi is pulsating in two modes simultaneously, and the ratio of the frequencies of the two modes is 0.706, a value common for double‐mode classical Cepheids. A careful analysis of other photometric observations obtained during the era of the strong period increase also revealed existence of a second mode. This may suggest that this star is not a Type II Cepheid, despite its galactic position. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
More frequencies of KUV 02464+3239
Authors: Bognár Zs. , Paparó M. , Már A. , Kerekes Gy. , Pápics P. , Molnár L. , Plachy E. , Sztankó N. , Bokor E.
Published: September 2007

Preliminary results on KUV 02464+3239, a pulsating DA white dwarf are presented. Located near the red edge of the DAV instability strip, KUV 02464+3239 shows large amplitude and long period pulsation modes. Up to now only one mode was known from a 50‐minute‐long light curve. Our more extended observations allowed the identification of three additional frequencies. The presence of previously known harmonics were confirmed and weak subharmonics are also noticeable at some parts of the light curve. This suggests the dominance of nonlinear pulsation effects from time to time. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Multidimensional indexing tools for the virtual observatory
Authors: Csabai I. , Dobos L. , Trencséni M. , Herczegh G. , Józsa P. , Purger N. , Budavári T. , Szalay A.S.
Published: September 2007

The last decade has seen a dramatic change in the way astronomy is carried out. The dawn of the the new microelectronic devices, like CCDs has dramatically extended the amount of observed data. Large, in some cases all sky surveys emerged in almost all the wavelength ranges of the observable spectrum of electromagnetic waves. This large amount of data has to be organized, published electronically and a new style of data retrieval is essential to exploit all the hidden information in the multiwavelength data. Many statistical algorithms required for these tasks run reasonably fast when using small sets of in‐memory data, but take noticeable performance hits when operating on large databases that do not fit into memory. We utilize new software technologies to develop and evaluate fast multidimensional indexing schemes that inherently follow the underlying, highly non‐uniform distribution of the data: they are layered uniform indices, hierarchical binary space partitioning, and sampled flat Voronoi tessellation of the data. These techniques can dramatically speed up operations such as finding similar objects by example, classifying objects or comparing extensive simulation sets with observations. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
J1128+592: a highly variable IDV source
Authors: Gabányi K.É. , Marchili N. , Krichbaum T.P. , Britzen S. , Fuhrmann L. , Witzel A. , Zensus J.A. , Müller P. , Liu X. , Song H.G. , Han J.L. , Sun X.H.
Published: September 2007

Short time‐scale radio variations of compact extragalactic radio quasars and blazars known as IntraDay Variability (IDV) can be explained in at least some sources as a propagation effect; the variations are interpreted as scintillation of radio waves in the turbulent interstellar medium of the Milky Way. One of the most convincing observational arguments in favor of a propagation‐induced variability scenario is the observed annual modulation in the characteristic time scale of the variation due to the Earth's orbital motion. So far there are only two sources known with a well‐constrained seasonal cycle. Annual modulation has been proposed for a few other less well‐documented objects. However, for some other IDV sources source‐intrinsic structural variations which cause drastic changes in the variability time scale were also suggested. J1128+592 is a recently discovered, highly variable IDV source. Previous, densely time‐sampled flux‐density measurements with the Effelsberg 100‐m radio telescope (Germany) and the Urumqi 25‐m radio telescope (China), strongly indicate an annual modulation of the time scale. The most recent 4 observations in 2006/7, however, do not fit well to the annual modulation model proposed before. In this paper, we investigate a possible explanation of this discrepancy. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Front‐ vs. back‐illuminated CCD cameras for photometric surveys: a noise budget analysis
Authors: Crouzet N. , Guillot T. , Fressin F. , Blazit A. , the A STEP team
Published: September 2007

Exoplanetary transit and stellar oscillation surveys require a very high precision photometry. The instrumental noise has therefore to be minimized. First, we perform a semi‐analytical model of different noise sources. We show that the noise due the CCD electrodes can be overcome using a Gaussian PSF (Point Spread Function) of full width half maximum larger than 1.6 pixels. We also find that for a PSF size of a few pixels, the photometric aperture has to be at least 2.5 times larger than the PSF full width half maximum. Then, we compare a front‐ with a back‐illuminated CCD through a Monte‐Carlo simulation. Both cameras give the same results for a PSF full width half maximum larger than 1.5 pixels. All these simulations are applied to the A STEP (Antarctica Search for Transiting Extrasolar Planets) project. As a result, we choose a front‐illuminated camera for A STEP because of its better resolution and lower price, and we will use a PSF larger than 1.6 pixels. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Extended X‐ray Absorption Fine Structure (EXAFS) Studies of Hydroxo(oxo)iron Aggregates and Minerals, and a Critique of their Use as Models for Ferritin
Authors: Heath Sarah L. , Charnock John M. , Garner C. David , Powell Annie K.
Published: September 2007

Structural models for the hydroxo(oxo)iron core of ferritin include extended mineral structures and cluster systems such as [Fe11O6(OH)6(O2CPh)15] ( = Fe11) and two clusters that crystallise in the same lattice (compound 1), namely, [Fe17(μ3‐O)4(μ3‐OH) 6(μ2‐OH)10(heidi)8(H2O)12]3+and[Fe19(μ3‐O)6(μ3‐OH)6(μ2‐OH)8(heidi)10(H2O)12]1+. The suitability of these systems as models for the structure of the core of ferritin has been tested by comparing their Fe K‐edge EXAFS and X‐ray crystallographic results with the Fe K‐edge EXAFS data on horse spleen ferritin. The interpretative procedure for the EXAFS analysis was optimised by using the X‐ray crystallographic data for compound 1 as a basis. This protocol was then used to interpret the Fe K‐edge EXAFS spectra of α‐ and γ‐Fe(O)OH and to reinterpret that previously recorded for horse spleen ferritin. The published Fe K‐edge EXAFS data on Fe11 were also considered. The Fe ⃛Fe distances provide a clear indication of the nature of the hydroxo(oxo)iron assembly. It was found that the iron–iron interactions are the most reliable guide. Clusters are shown to be more appropriate than infinite lattices as structural models for the core of ferritin.
From Quinidine to new Enantiopure Materials–Tricyclic Allylic N,O‐Acetals and a Stereospecific, Onepot Conversion of 1,2‐Secondary, Tertiary Diols into Spiroepoxides
Authors: von Riesen Cornelius , Jones Peter G. , Hoffmann H. M. R.
Published: September 2007

Hydrobromination of quinidine (1) with fuming HBr furnished diastereomeric secondary bromides 2a and 2b in 82% yield. After acetylation the resulting bromides 2a–Ac and 2b–Ac could be separated and converted stereospecifically into ethylidene rubanes Z)‐4 and (E)‐4, respectively. cis‐Dihydroxylation of (Z)‐olefin 4 with OsO4 was shown to be feasible by two catalytic variants, giving the two diastereomeric diols 5a and 5b, separable by chromatography. A simple onepot procedure was developed for converting the sterically hindered 1,2‐secondary, tertiary diols stereospecifically into spiroepoxides (5a 6a‐Ac; 5b 6b‐Ac). Our procedure involves overall inversion of configuration. The procedure complements the Kolb–Sharpless route to epoxides from 1,2‐disecondary diols with overall retention of configuration. The other two diastereomeric spiroepoxides 6c and 6d were prepared in one pot under different conditions (chloramine T, then alkali). Two unprecedented tricyclic allylic N,O‐acetals (Z)‐7 and (E)‐7 were also obtained. The structure of spiroepoxide 6c (as a CH2Cl2 monosolvate) and of tricyclic olefinic N,O‐acetal (E)‐7 was corroborated by X‐ray crystallography.
A Tricyclic Dehydrorubanone and New Isomers of the Major Quinidine Metabolite
Authors: von Riesen Cornelius , Hoffmann H. M. R.
Published: September 2007

Spiroepoxide 1 was prepared from quinidine and converted into β‐amino alcohol 3 (86% over two steps). Dihydroxylation of enantiopure oxazatricylic olefin (E)‐4 provided diastereomeric diols 5a and 5b. Stereospecific conversion of 1,2‐secondary, tertiary diol 5b into tetracyclic spiroepoxide 6 was accomplished in high yield by a one‐pot tosylation–cyclization procedure. 1,2‐Diol cleavage with NaIO4 in 80% acetic acid afforded the new tricyclic dehydrorubanone 7, containing the 4‐oxa‐7‐azatricyclo[4.3.1.03,7]‐decan‐2‐one core structure. Similarly, acetylated rubanone 9 was prepared on a 20 g scale. Reduction with NaBH4 in the presence of CeCl3 provided rubanols 10a and 10b (1:1.1). Horner–Wittig reaction of 9 with diethyl cyanomethylphosphonate was (Z)‐selective, furnishing unsaturated nitrile (Z)‐13. Conversion into the α,β‐unsaturated aldehyde (Z)‐14 and reduction afforded enartiopure allylic alcohol (Z)‐12, which is a new isomer of the key quinidine metabolite 15.
Stereoselective Transformations with Configurationally Labile α‐Phenylselenoalkyllithium Compounds
Authors: Hoffmann Reinhard W. , Klute Wolfgang
Published: September 2007

Complexation of the configurationally labile α‐phenylselenoalkyllithium compound 8 with 1,2‐bisdimethylaminocyclohexane 15 led to two diastereomeric complexes 13 and 14 in a 7:3 ratio. Owing to ligand acceleration the complexes 13 and 14 added more rapidly to benzaldehyde than the uncomplexed organolithium compound 8. Trapping of complexes 13 and 14 by benzaldehyde was shown to occur more rapidly than their equilibration. This corresponds to non‐Curtin–Hammett kinetics, in which enantiomeric enrichment in the products reflects the equilibrium ratio of the complexes 13 and 14.
Bismuth(III) Complexes of the Tripeptide Glutathione (γ‐L‐Glu–L‐Cys–Gly)
Authors: Sadler Peter J. , Sun Hongzhe , Li Hongyan
Published: September 2007

The tripeptide glutathione (γ‐L‐Glu‐L‐Cys‐Gly, GSH) is thought to play an important role in the pharmacology of bismuth drugs, but to our knowledge no chemical studies of bismuth glutathione complexes have been reported. We report here studies of interactions of the antiulcer compound ranitidine bismuth citrate (1) and [Bi(edta)]− with glutathione in aqueous solution and in intact red blood cells by NMR spectroscopy. The deprotonated thiol group is shown to be the strongest binding site for BiIII, and a complex with the stoichiometry [Bi(GS)3] is formed, as determined by 13C NMR titrations. A remarkably large lowfield shift of approximately 1.37 ppm for the β‐CH2 1H NMR resonances of GSH was observed on binding to BiIII. The complex [Bi(GS)3] is stable over the pH* range 2–10 (pH* = pH meter reading in D2O solution). A formation constant log K of 29.6±0.4 (I = 0.1 M, 298 K) for [Bi(GS)3] was determined by displacement of edta by GSH. The rate of exchange of GSH between free and bound forms is pH‐dependent, ranging from slow exchange (on the 1H NMR timescale) at low pH (ca. 3 s−1 at pH 4.0) to intermediate exchange at biological pH (ca. 1500 s−1). Such facile exchange may be important in the transport and delivery of BiIII in vivo. Spin‐echo 1H NMR showed that 1 reacts with GSH in red cells both in vivo and in vitro. A first‐order reaction of 1 with red blood cells was observed in vitro (k = 0.20±0.04 h−1, t1/2 = 3 h, 310 K), and the rate‐determining step appeared to involve the passage of BiIII through the cell membrane.
A new nickel(III) oxide family: MSr3NiO6 (M = Sc, In, Tm, Yb and Lu)
Authors: James Michael , Attfield J. Paul
Published: September 2007

The new phases MIIISr3NiIIIO6 have been prepared for M = Sc, In, Tm, Yb and Lu. Thermogravimetric analysis indicates that these phases are stoichiometric nickel(III) oxides. Rietveld refinement of their crystal structures from powder X‐ray diffraction data confirms that they adopt the rhombohedral K4CdCl6‐ type structure (space group R\documentclass{article}\pagestyle{empty}\begin{document}$ \bar 3 $\end{document}c, a = 9.6595 (2) and c = 10.8546 (3) Å for ScSr3NiO6). The M site is fully occupied for M = Sc and In, but a deficiency of scattering for M = Tm, Yb and Lu is shown to be due to Ni substitution through a simultaneous refinement of the YbSr3NiO6 structure using X‐ray and time‐of‐flight neutron diffraction data. The refined composition is (Yb0.83Ni0.17)‐Sr3NiO6. The magnetic susceptibilities of the M = Sc, In and Lu samples show Curie–Weiss behaviour down to 6K; however, ScSr3NiO6 shows a broad transition between 250 and 290 K, with Curie–Weiss behaviour above and below this anomaly. This transition is thought to be between the statically and dynamically Jahn–Teller distorted regimes of octahedrally coordinated, low‐spin Ni3+.
N‐Heterocyclic Carbenes: Generation under Mild Conditions and Formation of Group 8–10 Transition Metal Complexes Relevant to Catalysis
Authors: Herrmann Wolfgang A. , Elison Martina , Fischer Jakob , Köcher Christian , Artus Georg R. J.
Published: September 2007

Stable mono‐ and dicarbene adducts of late transition metals are readily accessible either by reaction of imidazolium salts with metal complexes bearing basic ligands, or by the cleavage of chloro‐ and acetato‐bridged dinuclear metal complexes with the free carbenes (e.g., 1,3‐dimethylimidazoline‐2‐ylidene). A general novel method for the deprotonation of N‐substituted azolium salts in liquid ammonia–the liquid ammonia route–is described. This method yields not only the known free monimeric 1,3‐dimethylimidazoline‐2‐ylidene in quantitative yield, but also otherwise hardly accessible derivatives. For example, imidazoline‐2‐ylidenes with linear, branched, cyclic, heteroatom‐substituted and even chiral hydrocarbon residues can be obtained. The nucleophilic behaviour of 1,3‐dimethyl‐imidazoline‐2‐ylidene is reported and compared with that of other donor ligands. Novel carbene complexes of RuII, RhIPdII, OsIIand IrIare presented. Reaction of the potentially chelating ligand 1,1′‐(1,2‐ethylene)‐3,3′‐dimethyldiimidazoline‐2,2′‐diylidene with [(COD)‐RhCl]2yields a dinuclear complex, in which two rhodium centres are linked by the dicarbene bridge. Four single‐crystal X‐ray diffraction structures ofnewmetalcarbene complexes (Rh, Os) are reported. N‐Heterocyclic carbene complexes of Group 8–10 transition metals are both thermally and chemically very stable. They do not show the typical reactivity of metal–carbon double bonds. For a number of reasons, these complexes must be regarded as donor adducts of the Lewis‐basicimidazoline‐2‐ylidene ligand and the Lewis‐acidicorganometallic fragment.
Di(polyprenyl) Phosphates as Models for Primitive Membrane Constituents: Synthesis and Phase Properties
Authors: Birault Véronique , Pozzi Gianluca , Plobeck Niklas , Eifler Stéphane , Schmutz Marc , Palanché Tania , Raya Jesus , Brisson Alain , Nakatani Yoichi , Ourisson Guy
Published: September 2007

Identical‐ and mixed‐chain di(polyprenyl) phosphates, derived from 3‐methyl‐2‐buten‐1‐ol, (E)‐geraniol, (E,E)‐farnesol, (E,E,E)‐geranylgeraniol, (R)‐citronellol, or (R)‐tetrahydrogeraniol have been synthesized by phosphitylation and oxidation with iodine/water. These phosphates gave organized systems in water, in particular vesicles, the physical properties of which were studied by photon correlation spectroscopy, electron microscopy, entrapment of a fluorescent dye (calcein), 31P NMR, stopped‐flow/light‐scattering studies, and fluorescence polarization with lipophilic probes. Interdigitation of mixed‐length phosphates was not found to have any favorable effect on the stability of the vesicles. The C>15 vesicles were formed despite the fact that they are composed of only one molecular species and are devoid of any reinforcer, but those with two geranyl chains appeared to be rather fragile. Phosphates containing C5 chains are soluble in water. The results obtained lend weight to the hypothesis that polyprenyl phosphates might have been primitive membrane constituents, and stimulate the search for appropriate reinforcers.
Synthesis, Crystal Structure, Electronic Structure, and Properties of Hf2In5, a Metallic Hafnide with One‐Dimensional Hf‐Hf and Two‐Dimensional In‐In Bonding
Authors: Pöttgen Rainer , Dronskowski Richard
Published: September 2007

Hf2In5—previously reported with the tentative composition “Hf3In4”—was prepared from the elements in a tantalum tube at 970 K. The X‐ray diffractional characterization by means of single‐crystal refinement reveals the presence of a tetragonal structure (a = 1024.71 (9), c = 305.66 (3) pm, P4/mbm, Z = 2) of Mn2Hg5 type. Hf2In5 is Pauli‐paramagnetic and a good metallic conductor. Quasi‐relativistic semiempirical and scalar‐relativistic ab initio band structure calculations reveal Hf2In5 to be a hafnide, composed of a two‐dimensional indium network threaded by infinite hafnium chains. The amount of In–In bonding scales counterintuitively with the interatomic distances.
The Silaguanidinium Cation and the Search for a Stable Silylium Cation in Condensed Phases
Authors: Pidun Ulrich , Stahl Martin , Frenking Gernot
Published: September 2007

Quantum mechanical calculations at the MP2/6‐31 G(d) level are reported for the silaguanidinium cation Si(NH2)(1) and derivatives thereof. The equilibrium structure1ahasD3symmetry with planar amino groups rotated out of the SiN3plane by 19.60. The Si–N bond length of1 a(1.658 Å) is intermediate between a single and a double bond. Isodesmic reactions show that the stabilization of the silylium cation1 aby the amino groups (63.5 kcal mol−1) is about 40% of the resonance stabilization of the guanidinium cation (159.3 kcal mol−1), but1 ais clearly better stabilized than alkyl‐substituted silylium cations. The electronic stabilization of1 aby the amino groups is also made obvious by the calculated complexation energy with one molecule of water. The calculated stabilization through complexation of water at HF/6‐ 31 G(d) is markedly lower for Si(NH2)3‐(H2O)+(6) (28.8 kcal mol−1) than for SiMe3(H2O)+(40.6 kcal mol−1). The tris(dimethylamino) silylium cation Si(N‐Me2)3+(8) is even more stable than1 a. The complexation energy of Si(NMe2)3‐(H2O)+(10) is only 17.3 kcal mol−1. IGLO calculations of the29Si N M R chemical shifts predict that1 aand8should not show the same extremely low shielding that is calculated for alkyl‐sub‐stituted silylium ions. The calculated29Si resonances for8are in reasonable agreement with the experimental N M R spectrum of (Me2N)3SiB(C6F5)4. AM 1 calculations predict that the substituted tripyrrolidino silylium cation12would be an even better candidate for a stable tricoordinate silylium cation in condensed phases. One of the pyrrolidine rings of12hastert‐butyl groups in the 2 and 5 positions, which serve as a steric fence around the silicon atom.
Cyclobis(Paraquat‐4,4′‐Biphenylene)–an Organic Molecular Square
Authors: Asakawa Masumi , Ashton Peter R. , Menzer Stephan , Raymo Françisco M. , Stoddart J. Fraser , White Andrew J. P. , Williams David J.
Published: September 2007

Template‐directed syntheses of cyclobis(paraquat‐4,4′‐biphenylene) (1)– a Molecular square–have been achieved by use of π‐electron‐rich macrocyclic hydroquinone‐based and acyclic ferrocene‐based templates. In particular, the use of a polyether‐disubstituted ferrocene derivative as a template permits synthesis of 1 (which is accessible only in very low yields without templates) on a preparative scale. Furthermore, the use of a macrocyclic hydroquinone‐based polyether template in corporating an ester function in one polyether chain–an (oriented) macrocycle–affords a 1 : 1 mixture of two topologically stereoisomeric [3]catenanes. Ester hydrolysis of the π‐electron‐rich macrocyclic components mechanically interlocked with 1 within the catenated structures releases the tetracationic cyclophane in quantitative yield as a result of the degradation of the [3]catenanes. The molecular square has been characterized by X‐ray crystallography, FAB mass spectrometry, 1H NMR and 13C NMR spectroscopies, and elemental analysis. The binding properties of 1 and of the smaller cyclophane cyclobis(paraquat‐p‐phenylene) toward a series of π‐electronrich guests have also been investigated with the above techniques and UV/VIS spectroscopy. The self‐assembly of the resulting supramolecular complexes in solution and in the solid state is driven mainly by π–π stacking interactions and hydrogen‐bonding interactions, as well as by edge‐to‐face T‐type interactions. In particular, the complexation of ferrocene or a ferrocene‐based derivative within the cavity of 1 suggests the possibility of constructing functioning ferrocene‐based molecular and supramolecular devices that can be controlled electrochemically in the form of catenanes, rotaxanes, and pseudorotaxanes.
Diastereoselective Zwitterionic Aza‐Claisen Rearrangement: Synthesis of Nine‐Membered Ring Lactams and Transannular Ring Contraction
Authors: Diederich Michel , Nubbemeyer Udo
Published: September 2007

The zwitterionic aza‐Claisen rearrangement of optically active 3‐pyrrolidine acryl esters and various acid chlorides to generate optically active azoninones proceeds with high simple diastereoselectivity (internal asymmetric induction) and a complete 1,3‐chirality transfer. The reaction path observed depends on the subsitution pattern of the allylic system: while the more electron‐rich alkylated allyl amine fromed predominantly von Braun type products, the α,β‐unsaturated esters could be rearranged with high yields. The azoninones thus obtained were treated with electrophiles, inducing regio‐ and diastereoselective transannular ring contractions. The resulting indolizidinones should be useful key intermediates in alkaloid synthesis.
Observing a Homogeneous Ziegler Catalyst Precursor at Work: Insertion Reactions into the Zirconium – Carbon Bond of the (Butadiene)ZrCp2/B(C6F5)3 Addition Product
Authors: Temme Bodo , Karl Jörn , Erker Gerhard
Published: September 2007

(Butadiene)zirconocene adds to B(C6F5)3 to form the metallacyclic metallocene–boron betaine system 1, which contains a C–F → Zr interaction. Addition of tert‐butylisocyanide or tert‐butyl‐cyanide leads to cleavage of the zirconium – fluorine linkage and formation of the adducts 4 and 7, respectively. With additional tert‐butylisocyanide, 4 reacts further to yield the corresponding η2‐imino‐acyl betaines 5 and 6 (2 stereoisomers), whereas 7 inserts additional tert‐butyl‐cyanide into the Zr–C bond to give a chiral metallacyclic N‐zirconaketimine complex that is probably stabilized by an internal borate → Zr interaction. Ethene and propene insertion reactions into the Zr ‐ C bond of 1 follow a similar course: the mono‐insertion products 9 and 10, respectively, generated in situ and characterized by NMR spectroscopy at – 35°C in [D8]toluene solution, are chiral and are also stabilized by internal borate → Zr coordination. Complexes 9 and 10 are likely to be intermediates in the formation of active homogeneous Ziegler polymerization catalysts.
Resonance‐Assisted O‐H ⃛ O Hydrogen Bonding: Its Role in the Crystalline Self‐Recognition of β‐Diketone Enols and its Structural and IR Characterization
Authors: Bertolasi Valerio , Gilli Paola , Ferretti Valeria , Gilli Gastone
Published: September 2007

The fact that hydrogen bonding is normally stronger than other nonbonding attractive forces can be exploited for the rational design of molecular crystals with known packing features and specific physical properties (crystal engineering). In the present paper the problem of obtaining homodromous molecular chains controlled by strong O–H ⃛ O interactions is investigated, particular attention being paid to β‐chains, that is, infinite hydrogen‐bonded chains of β‐diketone enol fragments ⃛ OCCCOH ⃛, which are linked by stronger‐than‐usual resonance‐assisted hydrogen bonds (RAHBs) and are intrinsically interesting as prototypes of a large family of switching proton bistate molecular devices. Accordingly, the crystal and molecular structures of thirteen new compounds containing the 1,3‐cyclopentanedione and 1,3‐cyclohexanedione fragment (or their heterocyclic analogues) were determined, and most of them were found to give the expected β‐chain packing pattern. Comparison with literature data makes it possible to identify seven fundamental β‐chain patterns, which can be shown to be selected by reason of the relative encumbrances of the substituents. Furthermore, a general analysis of all functional groups able to form strong OH ⃛ O bonds reveals a semiquantitative correspondence between the OH ⃛ O measurable parameters (O ⃛ O, H ⃛ O and OH distances, and ṽ(OH) IR stretching frequencies) and the hydrogen bond energy EHB, and a hierarchy of chemical functionalities that are well characterized by limited EHB ranges and that, in decreasing order of energy, can direct the crystal packing process.
Opening and Closure of the Fullerene Cage in cis‐Bisimino Adducts of C60: The Influence of the Addition Pattern and the Addend
Authors: Schick Georg , Hirsch Andreas , Mauser Harald , Clark Timothy
Published: September 2007

The synthesis, isolation, and spectroscopic characterization of the bisimino[60]fullerenes C60(NCOOR)2 (1a: R = Et, 1b. R = tBu) with a cis‐1 addition pattern as well as of their regioisomeric analogues 2–7 with different addition patterns are described. Whereas compounds 2–7 are typical fulleroaziridines and obey the rule of the minimization of [5,6] double bonds, the cis‐1 isomers 1a and 1b represent the first examples of fullerene derivatives with open transannular [6,6] bonds. Characteristic features within the fullerene framework of these valence isomers VI are the presence of a doubly bridged open 14‐membered ring with a phenanthrene perimeter as well as of an 8‐membered 1,4‐diazocine heterocycle. Moreover, it is shown that, by transforming cis‐1‐C60(NCOOtBu)2 (1b) into cis‐1‐C60(NH)2 (1c), the fullerene cage can be closed in an intraring 2π→2s̀ isomerization to valence isomer V. These are the first chemical modifications of the fullerene core that allow the synthesis of both open and closed valence isomers with the same addition pattern. Density‐functional as well as AM1 calculations corroborate the experimental findings that it depends on the addition pattern as well as on the nature of the addends whether or not the transannular [6,6] bonds are closed. Only in the cis‐1 adducts C60(NR)2 that prefer planar imino bridges (e.g., carbamates or amides) are the open forms VI more stable than the closed isomers V.
4,4‐Dimethylbicyclo[3.1.0]Hexa‐1(6),2‐Diene— A Highly Strained 1,3‐Bridged Cyclopropene
Authors: Albers Reinhard , Sander Wolfram , Ottosson Carl‐Henrik , Cremer Dieter
Published: September 2007

The photochemical rearrangement of 4,4‐dimethylcyclohexa‐2,5‐dienylidene (3) to 4,4‐dimethylbicyclo‐[3.1.0]hexa‐1(6),2‐diene (5) was investigated by means of the matrix isolation technique. Carbene 3 was generated in argon matrices at 10 K by photolysis of the corresponding diazo compound γ (>550 nm) and was characterized by IR and UV/Vis spectroscopy and its characteristic thermal reaction with triplet oxygen. Long‐wavelength irradiation (γ > 515 nm) induced an irreversible rearrangement of carbene 3 to the highly strained cyclopropene 5. This is in contrast to the previously investigated 1 H‐bicyclo[3.1.0]hexa‐3,5‐dien‐2‐one (1), which is thermally labile even at 10 K and rearranges back to 4‐oxo‐2,5‐cyclohexa‐dienylidene. The experimental findings were confirmed by ab initio calculations at the MP 2/6‐31 G(d) level. MP 2 and experimental IR frequencies are in good agreement. The strain energies of 5 and 1 are 75 and 78 kcalmol‐1, more than 20 kcal mol‐1 larger than that of cyclopropene. The somewhat greater stability of 5 than 1 is a consequence of better π delocalization in the diene unit of the bicyclic system.
X‐Ray Structural Studies of Highly Enantioselective Mn(salen) Epoxidation Catalysts
Authors: Pospisil Paul J. , Carsten Douglas H. , Jacobsen Eric N.
Published: September 2007

The relationship between catalyst structure and enantioselectivity in the asymmetric epoxidation of unfunctionalized olefins by a series of chiral Mn(sa1en) complexes (1 – 10) was examined. The X‐ray structures of 5‐coordinate complexes 5, 8, of 6‐coordinate 9 ([6,6′= ‐tBu; 4,4 = ‐tBu]+ClO4−), and 10 (6,6′= ‐tBu; 4,4′=‐Br) were determined. Catalysts 1 – 9 were derived from (R,R)‐1, 2 diaminocyclohexane and catalyst 10 from (S,S)‐1,2‐diphenylethylenediamine. Catalysts 1–9 differ in the stereoelectronic substitution of the ortho (6,6) and para (4,4) positions of the salicylidene moiety. A comparison between structures 5, 8, and 9 reveals that the ligand geometry around the metal center and the chiral diimine backbone remains remarkably constant in both five‐ and six‐coordinate cyclohexanediamine‐derived complexes; in contrast, the salicylidene regions of the complexes display a wide range of conformations. The asymmetric epoxidation of indene and 6‐cyano‐2,2‐dimethylchromene with NaOCl catalyzed by complexes 1 – 10 was effected. Systematically increasing the steric bulk on the ortho and then the para position in the order 1 (6,6′ = ‐H; 4,4′ = ‐H),2(6,6′ = ‐CH3; 4,4′ = ‐CH3),3(6,6′=‐tBu;4,4′=‐H),4(6,6′=‐tBu; 4,4′ =‐CH3), 5 (6,6′=‐tBu; 4,4′=‐tBu), and 6 (6,6′=‐tBu; 4,4′= ‐trityl), and electronically modifying the para substituents in 7 (6,6 = ‐tBu; 4,4 =‐OMe) and 8 (6.6′ = ‐tBu; 4,4′=‐OTIPS) resulted in enhanced enantioselectivities of the desired epoxides. The conformational variations observed in the solid state are likely to reflect accessible solution conformations and may help explain the high levels of stereoinduction obtained with these catalysts in the asymmetric epoxidation of unfunctionalized olefins.
Structure and Selectivity of a Small Dimeric Encapsulating Assembly
Authors: Valdés Carlos , Toledo Leticia M. , Spitz Urs , Rebek Julius
Published: September 2007

The binding selectivity of a dimeric molecular capsule in solution and its X‐ray crystal structure in the solid state are reported. The capsule is a self‐complementary bisglycoluril molecule, formed through a series of intermolecular hydrogen bonds. The dimeric host is capable of discriminating in CDCl3 solution between methane and ethane as guests, and direct evidence for the selectivity is presented from 1H NMR studies. The thermodynamic parameters for the encapsulation of methane, obtained from a VT 1H NMR (500 mHz) study, are presented.
A Straightforward, Highly Stereoselective Synthesis of Protected Isostatine Derivatives
Authors: Castejón Patricia , Moyano Albert , Pericàs Miquel A. , Riera Antoni
Published: September 2007

A novel strategy for the synthesis of isostatine derivatives has been developed. Contrary to previous approaches to isostatine, a non‐proteinogenic amino acid that is an essential component of the didemnins, the present synthesis does not require the intermediacy of the expensive amino acid D‐allo‐isoleucine, the starting material being commercially available enantiopure (S)‐2‐methyl‐1‐butanol. Steps in the sequence include catalytic asymmetric epoxidation, regioselective titanium‐promoted opening of an epoxy alcohol with an ammonia equivalent, stereospecific generation of an N‐Bocamino epoxide and its nucleophilic opening by a cyanide anion. Application of this method has permitted the enantioselective preparation of isostatine methyl ester and, for the first time, of isostatine amide, both in fully protected form.
Synthesis of a Conformationally Constrained Heparin‐like Pentasaccharide
Authors: van Boom Jacques H. , Sakairi Nobuo , van Der Marel Gijsbert A. , Basten Jan E. M. , van Boeckel Constant A. A.
Published: September 2007

An octasulfated pentasaccharide 1 having an L‐iduronic acid moiety in a fixed 1C4 conformation was synthesized by the coupling of a triosyl donor 3 with a disaccharide acceptor 4 followed by deprotection and O‐sulfation. The acceptor 4 was prepared from the fully acetylated 5‐C‐allyl‐β‐D‐glucose building block 7 by means of a TMSOTf‐promoted glycosylation, intramolecular substitution and ozonolysis of the olefinic bond as the key reactions. Compound 1 showed very low activity in an antithrombin 111‐mediated anti‐Xa assay; this reflects the importance of the presence of a flexible L‐iduronic acid moiety in heparin‐like antithrombotics.
A Novel Catalytic Cycle for the Synthesis of Epoxides Using Sulfur Ylides
Authors: Aggarwal Varinder K. , Abdel‐Rahman Hesham , Fan Li , Jones Ray V. H. , Standen Mike C. H.
Published: September 2007

A novel, neutral catalytic cycle for the synthesis of epoxides from carbonyl compounds and diazo compounds using catalytic quantities of transition metal salts and sulfides has been developed. In this catalytic cycle, the diazo compound is decomposed by the transition metal salt to give a metallocarbene, and this is picked up by the sulfide to give a sulfur ylide, which then reacts with the aldehyde to give an epoxide and returns the sulfide back into the catalytic cycle. To obtain good yields of epoxides it is necessary to maintain a low concentration of the diazo compound (by slow addition), otherwise dimerisation of the diazo compound is the dominant reaction. Factors affecting the outcome of the reaction were studied. The reactions are relatively insensitive to solvent, but are sensitive to the structure of the sulfide, the metal salt and the concentration. Unhindered sulfides give good yields of epoxides with any metal salt, but with hindered sulfides higher yields are obtained with Cu(acac)2 than with Rh2(OAc)4. The yields of epoxides are sensitive to sulfide concentration especially when using substoichiometric amounts of sulfides. Higher concentration leads to faster rates of formation and subsequent reaction of the sulfur ylides, and consequently to higher yields. This novel catalytic cycle has also been applied to base‐sensitive aldehydes. We found that our new catalytic cycle for epoxidation gives much improved yields of epoxides compared to those obtained by traditional sulfur ylide chemistry and is tolerant to a wide variety of sensitive functional groups. Ketones also participate in the catalytic cycle, although they give reduced yields of epoxides compared to aldehydes and require a slightly elevated temperature.