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Glass Matrix (glass + matrix)
Selected AbstractsEnhanced Blue Emission from Transparent Oxyfluoride Glass,Ceramics Containing Pr3+:BaF2 NanocrystalsJOURNAL OF THE AMERICAN CERAMIC SOCIETY, Issue 4 2010Kaushik Biswas Transparent glass,ceramics containing Pr3+:BaF2 nanocrystals in the chemical composition of SiO2,BaF2,K2CO3,La2O3,Sb2O3 oxyfluoride glass systems have been prepared from melt quenching and with a subsequent heat-treatment method. The luminescence and structural properties of these materials have been evaluated and the results are reported. Rietveld analysis of X-ray diffraction patterns and investigation of transmission electron microscopy confirmed the presence of BaF2 nanocrystals dispersed in the heat-treated glass matrices. Measured UV-Vis-NIR absorption spectra exhibited nine bands of the transitions 3H4,3P2, (1I6, 3P1), 3P0, 1D2, 1G4, 3F3, 3F2, 3H6, and 3H5 from all the samples with nondegenerated 1I6 and 3P1 levels in the glass,ceramics. The photoluminescence spectra show an enhancement in the intensities upon ceramization, indicating the incorporation of Pr3+ ions into BaF2 nanocrystals that possess a low phonon energy (346 cm,1). This has further been corroborated from the observation of a significant threefold increase in the relative intensity ratio of blue (3P0,3H4) to red (1D2,3H4, 3P0,3H6) emissions from glass,ceramics compared with the glass. This is due to a significant decrease of multiphonon nonradiative relaxation from the 3P0 to the 1D2 level of Pr3+ in glass,ceramics. Time-resolved spectra exhibit 3P0 -level decays faster than the 1D2 level. [source] Sol,Gel Synthesis of PZT,Glass Nanocomposites Using a Simple System and CharacterizationINTERNATIONAL JOURNAL OF APPLIED CERAMIC TECHNOLOGY, Issue 5 2006V. Kumar A simple inorganic system has been developed for the sol,gel synthesis of piezoelectric PZT,glass nanocomposites. Nanocrystalline PZT are nucleated from the amorphous xerogel through controlled heat treatment at 600°C. The formation of the crystalline phase, particle size, morphology, kinetics, and mechanism of crystallization of PZT in the glass matrix have been studied using X-ray diffraction, scanning electron microscopy, and differential thermal analysis. The piezoelectric characteristics of the PZT,glass compositions have been determined, and are correlated with the microstructure and the crystalline phase of PZT. [source] Small-angle X-ray scattering measurements of helium-bubble formation in borosilicate glassJOURNAL OF APPLIED CRYSTALLOGRAPHY, Issue 5 2006Alexander Y. Terekhov Small-angle X-ray scattering (SAXS) measurements have been performed to study helium-bubble formation in borosilicate glass. Helium was introduced by He+ implantation over an energy range of 1 to 2,MeV to give a uniform distribution over ,1,µm depth. The implanted dose was varied from 9 × 1013 to 2.8 × 1016,ions,cm,2, corresponding to a local concentration range of 40 to 11200 atomic parts per million (a.p.p.m.) averaged over the implantation depth. The SAXS response was fit with the Percus,Yevick hard-sphere interaction potential to account for interparticle interference. The fits yield helium-bubble radii and helium-bubble volume fractions that vary from 5 to 15,Å and from 10,3 to 10,1, respectively, as the dose increased from 9 × 1013 to 2.8 × 1016,cm,2. The SAXS data are also consistent with maximum helium solubility with respect to bubble formation between 40 and 200 a.p.p.m. in the borosilicate glass matrix. [source] In situ SAXS studies of the morphological changes of an alumina,zirconia,silicate ceramic during its formationJOURNAL OF APPLIED CRYSTALLOGRAPHY, Issue 4 2006Rudolf Winter Small-angle X-ray scattering is used at two energies, one either side of the zirconium K -edge, to probe the in situ formation of an alumina,zirconia,silicate ceramic. The use of energies either side of the edge allows the decomposition of information regarding the scattering from the zirconia particles from that of the glass matrix. Porod slope data show how the nanoparticles progress from being relatively isolated particles to becoming agglomerates as the pore network in the glass collapses. The shape of the agglomerates resembles the pore network of the glass at low temperature. The Guinier radii of the particles show the growth of the agglomerates past the Littleton softening point, whilst still resolving the primary particles. [source] Melatonin modulates the action of near infrared radiation on cell adhesionJOURNAL OF PINEAL RESEARCH, Issue 3 2003Tiina I. Karu Abstract: The adhesion of human cervical cancer (HeLa) cells to a glass matrix is evaluated following their irradiation in a suspension with a pulsed near-infrared (IR) light-emitting diode (wavelength 820 nm, pulse repetition frequency 10 Hz, irradiation dose 16,120 J/m2) when melatonin (4 × 10,11 to 4 × 10,5 m) is added to cell suspension immediately before or after the irradiation. Also, the dependence of visible-to-near-IR radiation (600,840 nm, 52 J/m2) on cell adhesion (action spectrum) is recorded in absence and presence of melatonin (4 × 10,6 m). It is found that melatonin in pharmacological concentrations (but not in physiological range) inhibited cell adherence. Irradiation of cells before or after melatonin treatment normalizes cell adhesion to control level. Melatonin in pharmacological concentrations eliminates stimulation of cell attachment induced by irradiation. Pre-treatment (but not post-treatment) with melatonin in the physiological concentration eliminates cell adhesion stimulation induced by irradiation. Melatonin modifies the light action spectrum significantly in near IR region (760,840 nm only). Thus, the peak at 820,830 nm characteristic for the light action spectrum is fully reduced. [source] Biaxial Flexural Strength and Microstructure Changes of Two Recycled Pressable Glass CeramicsJOURNAL OF PROSTHODONTICS, Issue 3 2004Mohammad Albakry BSc Purpose: This study evaluated the biaxial flexural strength and identified the crystalline phases and the microstructural features of pressed and repressed materials of the glass ceramics, Empress 1 and Empress 2. Materials and Methods: Twenty pressed and 20 repressed disc specimens measuring 14 mm × 1 mm per material were prepared following the manufacturers' recommendations. Biaxial flexure (piston on 3-ball method) was used to assess strength. X-ray diffraction was performed to identify the crystalline phases, and a scanning electron microscope was used to disclose microstructural features. Results: Biaxial flexural strength, for the pressed and repressed specimens, respectively, were E1 [148 (SD 18) and 149 (SD 35)] and E2 [340 (SD 40), 325 (SD 60)] MPa. There was no significant difference in strength between the pressed and the repressed groups of either material, Empress 1 and Empress 2 (p > 0.05). Weibull modulus values results were E1: (8, 4.7) and E2: (9, 5.8) for the same groups, respectively. X-ray diffraction revealed that leucite was the main crystalline phase for Empress 1 groups, and lithium disilicate for Empress 2 groups. No further peaks were observed in the X-ray diffraction patterns of either material after repressing. Dispersed leucite crystals and cracks within the leucite crystals and glass matrix were features observed in Empress 1 for pressed and repressed samples. Similar microstructure features,dense lithium disilicate crystals within a glass matrix,were observed in Empress 2 pressed and repressed materials. However, the repressed material showed larger lithium disilicate crystals than the singly pressed material. Conclusions: Second pressing had no significant effect on the biaxial flexural strength of Empress 1 or Empress 2; however, higher strength variations among the repressed samples of the materials may indicate less reliability of these materials after second pressing. [source] On-site analysis of Chinese Cloisonné enamels from fifteenth to nineteenth centuriesJOURNAL OF RAMAN SPECTROSCOPY, Issue 7 2010Burcu K Abstract A selection of 22 rare Chinese cloisonné enamels, from fifteenth century to nineteenth century A.D., has been studied on-site in the storage rooms of the musée des Arts décoratifs in Paris. The Raman signatures of the transparent and/or opacified glass matrix are discussed and compared with those that were previously recorded on glazed pottery, enameled and stained glasses. Enamels mostly belong to lead-based potash-lime glasses. Three different compositions, lead-potash-lime (fifteenth, sixteenth, eighteenth and nineteenth century), soda-rich (sixteenth,seventeenth century) and soda-lime (seventeenth century) are identified according to the wavenumber maxima of the SiO stretching and bending multiplets. Most of the pigment signatures are similar to those recorded on ceramic glazes and glass enamels, which proves the link between the technologies but a specific opacifier is observed: fluorite (CaF2). Naples Yellow pigment variations give characteristic signatures. Additionally, a comparison is made with Limoges enamels (sixteenth,nineteenth century A.D.). Copyright © 2009 John Wiley & Sons, Ltd. [source] Raman scattering of acoustical modes of silicon nanoparticles embedded in silica matrix,JOURNAL OF RAMAN SPECTROSCOPY, Issue 1-3 2006M. Ivanda Abstract The Raman scattering from acoustical phonons of silicon quantum dots in glass matrix was investigated. Two peaks that correspond to symmetric and quadrupolar spheroidal vibrations were found. A model calculation for in- and off-resonance scattering conditions was used, which considered the homogeneous broadening due to interaction with matrix and the inhomogeneous broadening due to particle size distribution. A strong dependence of the light-to-vibration coupling coefficient on the particles size was needed for fitting the Raman data. This result suggests that resonance with electronic transitions of the silicon nanoparticles is important for excitation at 514.5 nm. The size distribution obtained from the Raman data is in agreement with the results of high-resolution transmission electron microscopy. Copyright © 2006 John Wiley & Sons, Ltd. [source] In situ Raman scattering studies of high-pressure stability and transformations in the matrix of a nanostructured glass,ceramic compositeJOURNAL OF RAMAN SPECTROSCOPY, Issue 10 2005Kristina E. Lipinska-Kalita Abstract High-pressure Raman scattering studies have been performed on a glass-based composite consisting of nanometer-sized gallium oxide aggregates embedded in a potassium-silicate host glass using the diamond anvil cell technique. The Raman spectra of this heterophase nanocomposite showed a range of pressure-induced structural transformations occurring in the glass matrix. Compression from ambient pressure up to 10.8 GPa indicated a progressive reduction in the width of the intertetrahedral SiOSi angle distribution, which was completely reversible on decompression to ambient pressure. At higher pressures, the Raman spectra demonstrated a breakdown of the intermediate-range order in the glass matrix of the nanocomposite. The enhancement of scattering intensity in the region of the D-defect band at 565 cm,1 together with the blue shift of the main SiOSi symmetric stretching wavenumber are evidence of a permanent reduction in SiO4 ring statistics toward smaller-than-six-ring configurations in the three-dimensional glass network. Starting from 13 GPa, the Raman spectra displayed a remarkable decrease in the scattering intensity of the SiOSi symmetric stretching that has been related to a coordination change of the silicon atom. The Raman spectrum of the composite quenched from 23 GPa to ambient conditions illustrated the pressure-driven, permanent reconstructive modification of the glass matrix in the nanocomposite. The pressure-induced evolution of the Raman peaks assigned to the gallium oxide phase indicated a progressive densification of the nanocrystalline phase, reversible on decompression to ambient pressure. Copyright © 2005 John Wiley & Sons, Ltd. [source] Preparation and Optical Property of Glass Ceramics Containing Ruby CrystalsJOURNAL OF THE AMERICAN CERAMIC SOCIETY, Issue 10 2010Jumpei Ueda We have developed the glass ceramics containing ruby crystals by heat treatment of a Cr3+ -doped SiO2,Al2O3,Y2O3 glass at 1550°C. The shape of precipitated crystals in the glass matrix was rod type whose average width and length were around 10 and 200 ,m, respectively. From an X-ray diffraction measurement and an electron probe X-ray microanalysis, it was confirmed that the rod crystals were ,-Al2O3. Intense emission bands of Cr3+ at 694 nm, called the R -lines, were observed in the glass ceramics heat treated above 1300°C. In addition, the intense red luminescence was observed only in the ,-Al2O3 rod crystals by a photoluminescence mapping image. [source] Positron Lifetime Study of the Crystal Evolution and Defect Formation Processes in a Scintillating GlassJOURNAL OF THE AMERICAN CERAMIC SOCIETY, Issue 10 2009Jiaxiang Nie The crystal evolution and defect formation in scintillating glasses as a consequence of thermal annealing were studied by annihilation lifetime spectroscopy and UV-Vis absorption spectroscopy. The annihilation lifetime spectra and UV-Vis spectra were recorded on glass 50SiO2,45ZnO,5BaF2 before and after annealing at 580°C for 16, 32, and 48 h, respectively. The results show that the three lifetime components (,1, ,2, and ,3) and the corresponding intensities (I1, I2, and I3) change systematically with increasing annealing time. This reflects the crystal evolution and defect formation in the glass matrix. The continued crystal evolution was also revealed by the UV-Vis spectra, as the absorption edge of the material shifted to a lower energy with prolonged annealing. [source] Role of Sulfate in Structural Modifications of Sodium Barium Borosilicate Glasses Developed for Nuclear Waste ImmobilizationJOURNAL OF THE AMERICAN CERAMIC SOCIETY, Issue 12 2008Raman K. Mishra A sodium barium borosilicate glass matrix with a higher solubility of sulfate has been developed recently at Bhabha Atomic Research Centre for vitrification of sulfate bearing high-level nuclear waste. We report here the studies carried out to understand the influence of sulfate ion on the three-dimensional borosilicate network. Experiments were carried out with sodium barium borosilicate base glass samples loaded with varying amounts of SO42, (0,5 mol%). Phase separation studies on the samples revealed that as much as 3 mol% of SO42, can be loaded within the base glass without any phase separation, however, beyond this limit BaSO4 (barite) crystallizes within the matrix. Thermal analyses of the samples indicated a shift in glass transition temperature from 534° (0 mol% SO42,) to 495°C (3 mol% SO42,) and it remained more or less unaltered afterwards even with high SO42, loading. A similar observation of structure stabilization was obtained from 29Si MAS,NMR studies also, which showed that with 2 mol% of SO42, loading, the Q2:Q3 ratio changed from 59:41 (for samples with 0 mol% SO42, loading) to 62:38 and it remained almost the same afterwards even with higher SO42, loading. 11B MAS NMR patterns of the glass samples, however, remained unchanged with SO42, loading ([BO4]:[BO3]=38:62). Based on 29Si and 11B MAS NMR studies, the authors propose two different ways of interaction of SO42, ions with the borosilicate network: (i) the network modifying action of SO42, ions with -Si,O,Si- linkages, at low SO42, ion concentration (<2 mol%) and (ii) the preferential interaction of SO42, with the Ba2+ ions at high SO42, concentration (>2 mol%). [source] Laser fluence and shot number dependence of laser-induced optical properties modification of transparent materialsLASER PHYSICS LETTERS, Issue 12 2006K. Jamshidi-Ghaleh Abstract Micro-fabrication in transparent materials by femtosecond laser-induced modification of the optical properties inside transparent materials, preferentially optical glasses, has received much attention. Ultra short laser pulses are capable of inducing these modifications without cracking or even melting the glasses. This kind of modification, which appears as darkening inside glass relates to the trapped electrons or holes at defect sites in the glass matrix. The femtosecond laser technology allows controlling and modifying the optical properties of glasses on a sub-micrometer scale. Laser fluence and laser shot number are important parameters for femtosecond laser-induced modification of glasses. This paper presents the 800 nm femtosecond laser-induced optical modification inside soda lime glasses at different laser fluence levels and laser shot numbers. Modification thresholds at different applied laser fluences and shot numbers are measured. Pulse energy accumulation model is proposed to predict the behaviour of darkened area spots with applied laser fluence and shot numbers. Our experimental results and the pulse energy accumulation model are in good agreement. (© 2006 by Astro, Ltd. Published exclusively by WILEY-VCH Verlag GmbH & Co. KGaA) [source] Micrometeorites from the northern ice cap of the Novaya Zemlya archipelago, Russia: The first occurrenceMETEORITICS & PLANETARY SCIENCE, Issue 3 2003Dmitry D. BADJUKOV The 1 Kyr old glacier has decreased in volume and coverage during the last 40 years, leaving the spherules contained in the ice at the margins of the glacier where they can be easily collected. The spherules are similar in their appearance, texture, and mineralogy to cosmic spherules found in deep-sea sediments in Greenland and Antarctica. Silicate spherules have typical bar-like textures (75%) or porphyritic textures (15%), while other spherules are glassy (7%). The spherules from Novaya Zemlya are altered only slightly. There are spherules consisting of iron oxides, metal cores with iron oxide rims, a continuous network of iron oxide dendrites in a glass matrix, and particles rich in chromite (3%). Some spherules contain metal droplets and relict forsterite and low-Ca pyroxene. Silicate spherule compositions match compositions of other cosmic spherules. Both Nova Zemlya and other cosmic spherules are close to carbonaceous chondrite matrices in patterns of variations for Ca, Mg, Si, and Al, which might suggest that their predecessor was similar to carbonaceous chondrite matrices. Unmelted micrometeorites are generally depleted in Ca and Mg and enriched in Al relative to cosmic spherules. The depletion of the micrometeorites in Ca and Mg can be connected with their terrestrial alteration (Kurat et al. 1994), while the Al enrichment seems to be primary. [source] Effect of heat treatment on various physical properties in Zr80Ni20 metallic glassPHYSICA STATUS SOLIDI (A) APPLICATIONS AND MATERIALS SCIENCE, Issue 8 2006I. Kokanovi Abstract The effect of heat treatment on various physical properties in Zr80Ni20 metallic glass has been investigated by means of differential scanning calorimetry and electrical resistivity measurements. Experimental values for the crystallisation temperature and activation energy of the crystallisation processes were derived by studying these processes at different heating rates. The electrical resistivity of the annealed samples up to a temperature slightly above the first crystallization exotherm decreases with decreasing heating rates and increasing annealing temperatures. The temperature dependence of the electrical resistivity of the samples has been interpreted in terms of the weak localization of electrons and contribution by electron,phonon scattering. The contribution by electron,phonon scattering increases with increasing the fraction of crystalline phases in the glass matrix. The superconducting transition temperature, Tc, of the Zr80Ni20 metallic glass annealed at heating rates of 60 K/min and 10 K/min up to annealing temperatures below its second crystallisation exotherm decreases with decreasing heating rates and increasing annealing temperatures. The homogeneity of the annealed Zr80Ni20 metallic glass is discussed by using the superconducting transition width as a criterion. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Surface phonons in CdS1,xSex nanoparticles embedded in a dielectric mediumPHYSICA STATUS SOLIDI (C) - CURRENT TOPICS IN SOLID STATE PHYSICS, Issue 9 2009Yu. M. Azhniuk Abstract Raman scattering by surface phonons in CdS1,xSex nanocrystals grown in a borosilicate glass matrix, is studied. CdSe-like and CdS-like surface phonon frequencies are shown to increase with the relevant component content in the nanocrystal composition. No clear size dependence of the surface phonon frequencies has been observed. On the other hand, surface phonons in CdS1,xSex nanocrystals exhibit resonant dependence on the Raman excitation wavelength. It is emphasized that measurements of Raman scattering by surface phonons in CdS1,xSex nanocrystals give no reason to judge upon the nanocrystal shape. (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Shallow- and deep-luminescence centers in AgI-based superionic conductor glassPHYSICA STATUS SOLIDI (C) - CURRENT TOPICS IN SOLID STATE PHYSICS, Issue 2 2003Shosuke Mochizuki Abstract The photoluminescence (PL), time-resolved photoluminescence (TRPL) and PL-excitation (PLE) spectra of superionic conductor glass (AgI)0.85(Ag2WO4)0.15 have been measured at different temperatures between 8 K and room temperature. The PL intensity peak is observed at 426 nm which corresponds to free exciton wavelength of pristine AgI. There are also different PL bands well related to the internal and interface states of mesoscopic AgI particles in Ag2WO4 glass matrix. This may give important information about the origins of superionic conduction. With increasing excitation light intensity, the PL efficiencies of several emission bands become saturated, except for the free exciton band. [source] THE APPLICATION OF TIME-OF-FLIGHT SECONDARY ION MASS SPECTROMETRY (ToF-SIMS) TO THE CHARACTERIZATION OF OPAQUE ANCIENT GLASSES*ARCHAEOMETRY, Issue 6 2009F. J. M. RUTTEN Time-of-flight secondary ion mass spectrometry (ToF-SIMS) has been used, for the first time, for the characterization of opaque ancient glasses. Isotope-specific chemical imaging with sub-micron resolution enabled the separate analysis of opacifiying inclusions and the surrounding glass matrix. Phase identification has been demonstrated and quantification of the matrix composition has been investigated by use of Corning Glass Standard B as a model. Trace element detection limits are typically in the range 0.5,5.0 ppm atomic,in favourable cases down to 0.01 ppm. For the analysis of inclusions in particular, this has the potential to provide new information of use in establishing provenance and trade routes by ,fingerprinting' as well as the investigation of manufacturing techniques, as demonstrated by comparisons between glasses and with EDX data from the same samples. [source] ANTIMONATE OPAQUE GLAZE COLOURS FROM THE FAIENCE MANUFACTURE OF LE BOIS D'ÉPENSE (19TH CENTURY, NORTHEASTERN FRANCE)*ARCHAEOMETRY, Issue 5 2009M. MAGGETTI Three types of antimony-based, opaque ceramic colours were used in the faience workshop of Le Bois d'Épense during the first decades of the 19th century; that is, yellow, tawny and green. Yellow is generated by lead antimonate crystals (Naples Yellow), which are incorporated into an uncoloured glass matrix. According to SEM,EDS measurements, these pigments contain iron. The tawny colour is the optical result of the combined presence of similar yellow, iron-bearing lead antimonate particles in a Fe-rich, brownish glass matrix. The green opaque colour is produced by the combination of a blue cobalt glass and yellow Pb,Sn,Fe-antimonate crystals. Cores of zoned pigments lighten the recipes, according to which the pigments were produced. First, they were synthesized by calcination, ground and then mixed with a colourless, brown or blue glass powder. The resulting powder mixture was added to a liquid agent and used as high-temperature ceramic colour. [source] Enantioselective [4+2]-Cycloaddition Reaction of a Photochemically Generated o -Quinodimethane: Mechanistic Details, Association Studies, and Pressure EffectsCHEMISTRY - A EUROPEAN JOURNAL, Issue 9 2004Benjamin Grosch Dipl. Abstract 1,2,3,4-Tetrahydro-2-oxoquinoline-5-aldehyde (2) was prepared from m -aminobenzoic acid and 3-ethoxyacryloyl chloride (4) in 19,% overall yield. Compound 2 underwent a photochemically induced [4+2]-cycloaddition reaction with various dienophiles upon irradiation in toluene solution. The exo product 10,a was obtained with acrylonitrile (9,a) as the dienophile, whereas methyl acrylate (9,b) and dimethyl fumarate (9,c) furnished the endo products 11,b and 11,c (69,77,% yield). The reactions proceeded at ,60,°C in the presence of the chiral complexing agent 1 (1.2 equiv) with excellent enantioselectivity (91,94,% ee). The enantiomeric excess increases in the course of the photocycloaddition as a result of the lower product association to 1. The intermediate (E)-dienol 8 was spectroscopically detected at ,196,°C in an EPA (diethyl ether/isopentane/ethanol) glass matrix. The association of the substrate 2 to the complexing agent 1 was studied by circular dichroism (CD) titration. The measured association constant (KA) was 589,M,1 at room temperature (25,°C) and normal pressure (0.1 MPa). An increase in pressure led to an increased association. At 400 MPa the measured value of KA was 703,M,1. Despite the stronger association the enantioselectivity of the reaction decreased with increasing pressure. At 25,°C the enantiomeric excess for the enantioselective reaction 2 + 9,a,10,a decreased from 68,% ee at 0.1 MPa to 58,% ee at 350 MPa. This surprising behavior is explained by different activation volumes for the diastereomeric transition states leading to 10,a and ent - 10,a. 1,2,3,4-Tetrahydro-2-oxochinolin-5-aldehyd (2) wurde ausgehend von m -Aminobenzoesäure und 3-Ethoxyacryloylchlorid (4) in fünf Schritten und einer Gesamtausbeute von 19,% hergestellt. Die Verbindung ließ sich in Toluol als Lösungsmittel mit verschiedenen Dienophilen in einer photochemisch induzierten [4+2]-Cycloaddition umsetzen (69,77,% Ausbeute), wobei als Hauptprodukt mit Acrylnitril (9,a) das exo -Produkt 10,a entstand. Methylacrylat (9,b) und Dimethylfumarat (9,c) lieferten die endo -Produkte 11,b and 11,c. In Gegenwart des chiralen Komplexierungsreagenz, 1 (1.2 Äquiv.) verliefen die Reaktionen mit exzellenter Enantioselektivität (91,94,% ee). Der Enantiomerenüberschuß nahm im Verlauf der photochemischen Umsetzung zu, was man auf die relativ zum Substrat 2 niedrigere Assoziation des Produkts zurückführen kann. Das intermediär gebildete (E)-Dienol 8 wurde spektroskopisch in einer EPA (Ether/i -Pentan/Ethanol) Glasmatrix bei ,196,°C nachgewiesen. Die Assoziation des Substrats 2 an das Komplexierungsreagenz 1 wurde durch CD-Titration genauer untersucht. Die Assoziationskonstante (KA) wurde bei Zimmertemperatur (25,°C) und Normaldruck (0.1 MPa) zu 589,M,1bestimmt. Bei höherem Druck beobachtete man eine verstärkte Assoziation und bei 400 MPa wurde eine Assoziationskonstante von KA=703,M,1bestimmt. Trotz der stärkeren Assoziaion nahm die Enantioselektivität mit wachsendem Druck ab. Bei 25,°C sank der Enantiomerenüberschuß der enantioselektiven Reation 2 + 9,a,10,a von 68,% ee bei 0.1 MPa auf 58,% ee bei 350 MPa. Dieses überraschende Verhalten läßt sich möglicherweise durch die unterschiedlichen Aktivierungsvolumina für die Übergangszustände erklären, die zu 10,a und ent - 10,a führen. [source] | ||||||||||||||||