Grundlegende Untersuchungen zur Abbindereaktion von Zinkphosphatzement

Zusammenfassung Das Ziel der Arbeit bestand darin, mit der Aufklärung der Abbindereaktion von Zinkphosphatzement eine Grundlage für eine gezielte Modifikation bzw. Optimierung zu schaffen, insbesondere im Hinblick auf einen Einsatz als permanenter Füllungswerkstoff. Über den Abbindechemismus war bislang lediglich bekannt, daß es sich bei den primär gebildeten Reaktionsprodukten um röntgenamorphe Phasen handelt, die sich nach Wochen bzw. Monaten in das thermodynamisch stabile Reaktionsprodukt alpha-Hopeit (alpha-Zn3(PO4)2·4H2O) umwandeln.Im Rahmen der vorliegenden Arbeit gelang durch den Einsatz der Infrarot-Reflexionsspek-troskopie (DRIFT) die Identifikation von Dizink Cyclotetraphosphat-Octahydrat (Zn2P4O12·8H2O) als röntgenamorpher Vorläuferphase. Das kondensierte Phosphat ist hydrolyseempfindlich sowie thermodynamisch instabil. Mit Hilfe der zeitaufgelösten 1H NMR-Spektroskopie konnte gezeigt werden, daß bereits nach ca. 10 Minuten eine Phasenumwandlung (topochemische Disproportionierung) in ein zunächst röntgenamorphes Orthophosphat stattfindet. Mittels 31P Doppelquanten-NMR-Spektroskopie gelang der Nachweis, daß innerhalb des röntgenamorphen Bereiches lokal nahgeordnete (nanokristalline) Bereiche auftreten, deren Ordnung sich auf einer Längenskala von 10 bis 30 Å erstreckt. Die Nanokristallite unterliegen einem Wachstumsprozeß, der schließlich zu alpha-Hopeit-Kristallen mit Ausdehnungen im Mikrometerbereich führt. Die Ursache für die primäre Ausbildung röntgenamorpher Reaktionsprodukte kann zunächst gelösten Aluminophosphatkomplexen zugeordnet werden, die im Verlauf der Abbindereaktion zu anorganischen Polymeren aggregieren und damit als Kristallisationsinhibitoren fungieren.

Bio-inspired polymer-supported nitrido molybdenum(VI) complexes for ammonia synthesis − reactivity towards protons and electrons : EPR investigations on paramagnetic molybdenum(V) and copper(II) complexes

Die biologische Stickstofffixierung durch Molybdän-haltige Nitrogenasen sowie die Erforschung des zugrundeliegenden komplexen Mechanismus (N2-Aktivierung an Metall-Zentren, 6-fache Protonierung und Reduktion, N–N Bindungsspaltung unter Bildung von Ammoniak) ist von erheblichem Interesse. Insbesondere Molybdän-Komplexe wurden bereits erfolgreich als Modellverbindungen für die Untersuchung elementarer Einzelschritte der N2-Aktivierung eingesetzt. Durch die Verwendung von Triamidoamin-Liganden ist es Schrock et al. sogar gelungen mehrere Katalysezyklen zu durchlaufen und einen Mechanismus zu formulieren. Trotz der sterisch anspruchsvollen Substituenten in den Schrock-Komplexen ist die Umsatzrate dieses homogenen Katalysators, aufgrund Komplex-Deaktivierung infolge intermolekularer Reaktionen wie Dimerisierung und Disproportionierung, limitiert. In der vorliegenden Arbeit wurden einige dieser Herausforderungen angegangen und die aktiven Spezies auf einer Festphase immobilisiert, um intermolekulare Reaktionen durch räumliche Isolierung der Komplexe zu unterdrücken.rnEin Polymer-verankertes Analogon des Schrock Nitrido-Molybdän(VI)-Komplexes wurde auf einem neuen Reaktionsweg synthetisiert. Dieser beinhaltet nur einen einzigen Reaktionsschritt, um die funktionelle Gruppe „MoN“ einzuführen. Protonierung des immobilisierten Nitrido-Molybdän(VI)-Komplexes LMoVIN (L = Polymer-verankerter Triamidoamin-Ligand) mit 2,6-Lutidinium liefert den entsprechenden Imido-Molybdän(VI)-Komplex. Durch anschließende Ein-Elektronen-Reduktion mit Cobaltocen wird der Polymer-angebundene Imido-Molybdän(V)-Komplex erhalten, bewiesen durch EPR-Spektroskopie (g1,2,3 = 1.989, 1.929, 1.902). Durch die Immobilisierung und die effektive räumliche Separation der Reaktionszentren auf der Festphase werden bimolekulare Nebenreaktionen, die oft in homogenen Systemen auftreten, unterdrückt. Dies ermöglicht zum ersten Mal die Darstellung des Imido-Molybdän(V)-Intermediates des Schrock-Zyklus.rnEPR-Spektren des als Spin-Label eingeführten immobilisierten Nitrato-Kupfer(II)-Komplexes wurden unter verschiedenen Bedingungen (Lösungsmittel, Temperatur) aufgenommen, wobei sich eine starke Abhängigkeit zwischen der Zugänglichkeit und Reaktivität der immobilisierten Reaktionszentren und der Art des Lösungsmittels zeigte. Somit wurde die Reaktivität von LMoVIN gegenüber Protonen und Elektronen, welches zur Bildung von NH3 führt, unter Verwendung verschiedener Lösungsmittel untersucht und optimiert. Innerhalb des kugelförmigen Polymers verläuft die Protonierung und Reduktion von LMoVIN stufenweise. Aktive Zentren, die sich an der „äußeren Schale“ des Polymers befinden, sind gut zugänglich und reagieren schnell nach H+/e− Zugabe. Aktive Zentren im „Inneren des Polymers“ hingegen sind schlechter zugänglich und zeigen langsame diffusions-kontrollierte Reaktionen, wobei drei H+/e− Schritte gefolgt von einer Ligandenaustausch-Reaktion erforderlich sind, um NH3 freizusetzen: LMoVIN  LMoVNH  LMoIVNH2  LMoIIINH3 und anschließender Ligandenaustausch führt zur Freisetzung von NH3.rnIn einem weiteren Projekt wurde der Bis(ddpd)-Kupfer(II)-Komplex EPR-spektroskopisch in Hinblick auf Jahn−Teller-Verzerrung und -Dynamik untersucht. Dabei wurden die EPR-Spektren bei variabler Temperatur (70−293 K) aufgenommen. Im Festkörperspektrum bei T < 100 K erscheint der Kupfer(II)-Komplex als gestreckter Oktaeder, wohingegen das EPR-Spektrum bei höheren Temperaturen g-Werte aufzeigt, die einer pseudo-gestauchten oktaedrischen Kupfer(II)-Spezies zuzuordnen sind. Diese Tatsache wird einem intramolekularen dynamischen Jahn−Teller Phänomen zugeschrieben, welcher bei 100 K eingefroren wird.

Dimerization of Poly(methyl methacrylate) Chains Using Radical Trap-Assisted Atom Transfer Radical Coupling

End-brominated poly(methyl methacrylate) (PMMABr) was prepared by atom transfer radical polymerization (ATRP) and employed in a series of atom transfer radical coupling (ATRC) and radical trap-assisted ATRC (RTA-ATRG) reactions. When coupling reactions were performed in the absence of a nitroso radical trap-traditional ATRC condition-very little coupling of the PMMA chains was observed, consistent with disproportionation as the major termination pathway for two PMMA chain-end radicals in our reactions. When 2-methyl-2-nitrosopropane (MNP) was used as the radical trap, coupling of the PMMA chains in this attempted RTA-ATRC reaction was again unsuccessful, owing to capping of the PMMA chains with a bulky nitroxide and preventing further coupling. Analogous reactions performed using nitrosobenzene (NBz) as the radical trap showed significant dimerization, as observed by gel permeation chromatography (GPC) by a shift in the apparent molecular weight compared to the PMMABr precursors. The extent of coupling was found to depend on the concentrion of NBz compared to the PMMABr chain ends, as well as the temperature and time of the coupling reaction. To a lesser extent, the concentrations of copper(I) bromide (CuBr), nitrogen ligand (N,N,N',N',N"-pentamethyldiethylenetriamine = PMDETA), and elemental copper (Cu) were also found to play a role in the success of the RTA-ATRC reaction. The highest levels of dimerization were observed when the coupling reaction was carried out at 80 degrees C for 0.5h, with ratio of 1:4:2.5:8:1 equiv of NBz: CuBr:Cu:PMDETA:PMMABr.

Extrapolated difference technique for the determination of atomization energies of Sm, Eu, and Yb bromides

An approach for the determination of atomization energies based on the extrapolated difference technique in the framework of Knudsen effusion mass spectrometry is proposed. Its essence is the use of thermodynamic data for the determination of the appearance energy of fragment ions of a reference and a special mathematical treatment of the ionization efficiency functions. The advantages of this approach are demonstrated for the cases of incongruently vaporizing lanthanide bromides that suffer from decomposition or disproportionation at high temperatures. The atomization energies for SmBr2 (7.78±0.12 eV), EuBr2 (7.51±0.11 eV), YbBr2 (7.25±0.13 eV), SmBr3 (11.09±0.10 eV), and YbBr3 (10.23±0.09 eV) molecules have been determined for the first time.

Knudsen effusion mass spectrometric determination of the complex vapor composition of samarium, europium, and ytterbium bromides

RATIONALE The vaporization of Sm, Eu, and Yb tri- and dibromides is accompanied by decomposition and disproportionation reactions. These result in complex vapor compositions whose analysis is an intricate problem for experimentalists. Approaches have been developed to interpret mass spectra and accurately determine the vapor composition of thermally unstable compounds. METHODS A sector type magnet instrument was used. A combined ion source allowed the study of both the molecular and ionic vapor compositions in the electron ionization (EI) and the thermionic emission (TE) modes. The methodological approaches were based on a joint analysis of the ionization efficiency functions, the temperature and time dependences of the ion currents, and special mathematical data evaluation. RESULTS The vaporization of SmBr3, YbBr3, SmBr2, EuBr2, and YbBr2 was studied in the temperature range of 850–1300 K. An initial stage of incongruent vaporization was observed in the case of the tribromides, SmBr2, and YbBr2. This eventually changed to a congruent vaporization stage. Various neutral (Ln, Br, Br2, LnBr, LnBr2, LnBr3, Ln2Br4, Ln2Br5, and Ln2Br6) and charged (Br–, LnBr3–, LnBr4–) species were detected at different vaporization stages. CONCLUSIONS The quantitative vapor composition of Sm, Eu, and Yb tri- and dibromides was determined. It was found that only EuBr2 was stable in the studied temperature range. The developed approaches can be useful in the case of other thermally unstable compounds.

Vapor phase composition as an indicator of thermal stability of the Sm, Eu, and Yb chlorides

The Sm, Eu, and Yb tri- and dichlorides were investigated by Knudsen effusion mass spectrometry. It was found out by the analysis of mass spectra and ionization efficiency curves that the vapor composition is complex due to the partial high temperature decomposition/disproportionation of the samples. Up to five vapor species were identified for both LnCl3 (LnCl3, LnCl2, Ln2Cl4, Ln2Cl5, and Ln2Cl6) and LnCl2 (LnCl3, LnCl2, LnCl, Ln, and Ln2Cl4). The quantitative evaluation of vapor composition was made. It indicates that the disproportionation of SmCl2 and EuCl2 is negligible in the temperature range studied whereas that of YbCl2 and the decomposition of SmCl3 and YbCl3 cannot be neglected.

The distribution of sulfur in oceanic crust with respect to oxidative diagenesis (Table 1)

A detailed geochemical-petrological examination of layer 2 basalts recovered during Leg 37 of the DSDP has revealed that the original distribution, form and abundance of igneous sulfide have been profoundly altered during low-grade oxidative diagenesis. The net result appears to have been a rather pervasive remobilization of igneous sulfide to form secondary pyrite accompanied by a bulk loss of sulfur equivalent to about 50-60% of the original igneous value, assuming initial saturation. It is suggested that during infiltration of seawater into the massive crystalline rock, igneous sulfide has experienced pervasive oxidation, under conditions of limited oxidation potential, to form a series of unstable, soluble sulfur species, primarily in the form of SO3[2-] and S2O3[2-]. Spontaneous decomposition of these intermediate compounds through disproportionation has resulted in partial reconstitution of the sulfur as secondary pyrite and the generation of SO4[2-] ion, which, due to its kinetic stability, has been lost from the basalt system and ultimately transferred to the ocean. This model not only satisfies the geochemical and petrological observations but also provides a suitable explanation for the highly variable delta34S values which characterize secondary sulfides in deep ocean floor basalts.

Sulphur and iron species in Dvurechenskii mud volcano sediments

The deep Black Sea is known to be depleted in electron-acceptors for sulphide oxidation. This study on depth distributions of sulphur species (S(II), S(0),S(n)**2-,S2O3**2-,SO3**2-,SO4**2-) in the Dvurechenskii mud volcano, a cold seep situated in the permanently anoxic eastern Black Sea basin (Sorokin Trough, 2060 m water depth), showed remarkable concentrations of sulphide oxidation products. Sulphite concentrations of up to 11 µmol L**1-, thiosulphate concentrations of up to 22 µmol L**1-, zero-valent sulphur concentrations of up to 150 µmol L**1- and up to five polysulphide species were measured in the upper 20 cm of the sediment. Electron-acceptors found to be available in the Dvurechenskii mud volcano (DMV) for the oxidation of hydrogen sulphide to sulphide oxidation intermediates are iron-minerals, and probably also reactive manganese phases. Up to 60 µmol g**1- of reactive iron-minerals and up to 170 µmol L**1- dissolved iron was present in the central summit with the highest fluid upflow and fresh mud outflow. Thus, the source for the oxidative power in the DMV are reactive iron phases extruded with the mud from an ancient source in the deeply buried sediments, leading to the formation of various sulphur intermediates in comparably high concentrations. Another possible source of sulphide oxidation intermediates in DMV sediments could be the formation of zero-valent sulphur by sulphate dependent anaerobic microbial oxidation of methane followed by disproportionation of zero-valent sulphur. Sulphide oxidation intermediates, which are produced by these processes, do not reach thermodynamic equilibrium with rhombic sulphur, especially close to the active center of the DMV due to a short equilibration time. Thus, mud volcano sediments, such as in the DMV, can provide oxidizing niches even in a highly reduced environment like the abyssal part of the Black Sea.

Pore water chemistry of ODP Sites 201-1225, 201-1226 and 201-1231

High-resolution analyses of the oxygen isotope ratio (18O/16O) of dissolved sulfate in pore waters have been made to depths of >400 meters below seafloor (mbsf) at open-ocean and upwelling sites in the eastern equatorial Pacific Ocean. d18O values of dissolved sulfate (d18O-SO4) at the organic-poor open-ocean Site 1231 gave compositions close to modern seawater (+9.5 per mil vs. Vienna-standard mean ocean water, providing no chemical or isotopic evidence for microbial sulfate reduction (MSR). In contrast, the maximum d18O values at Sites 1225 and 1226, which contain higher organic matter contents, are +20 per mil and +28 per mil, respectively. Depth-correlative trends of increasing d18O-SO4, alkalinity, and ammonium and the presence of sulfide indicate significant oxidation of sedimentary organic matter by sulfate-reducing microbial populations at these sites. Although sulfate concentration profiles at Sites 1225 and 1231 both show similarly flat trends without significant net MSR, d18O-SO4 values at Site 1225 reveal the presence of significant microbial sulfur-cycling activity, which contrasts to Site 1231. This activity may include contributions from several processes, including enzyme-catalyzed equilibration between oxygen in sulfate and water superimposed upon bacterial sulfate reduction, which would tend to shift d18O-SO4 toward higher values than MSR alone, and sulfide oxidation, possibly coupled to reduction of Fe and Mn oxides and/or bacterial disproportionation of sulfur intermediates. Large isotope enrichment factors observed at Sites 1225 and 1226 (epsilon values between 42 per mil and 79 per mil) likely reflect concurrent processes of kinetic isotope fractionation, equilibrium fractionation between sulfate and water, and sulfide oxidation at low rates of sulfate reduction. The oxygen isotope ratio of dissolved pore water sulfate is a powerful tool for tracing microbial activity and sulfur cycling by the deep biosphere of deep-sea sediments.

Mechanistic Importance of Redox Potentials and Conformational Flexibility in Electron-Transferring Flavoproteins

The mitochondrial matrix flavoproteins electron transfer flavoprotein (ETF) and electron transfer flavoprotein-ubiquinone oxidoreductase (ETF-QO) are responsible for linking fatty acid β-oxidation with the main mitochondrial respiratory chain. Electrons derived from flavoprotein dehydrogenases are transferred sequentially through ETF and ETF-QO to ubiquinone and then into the respiratory chain via complex III. In this study, the effects of changes in ETF-QO redox potentials on its activity and the conformational flexibility of ETF were investigated. ETF-QO contains one [4Fe-4S]2+,1+ and one flavin adenine dinucleotide (FAD). In the porcine protein, threonine 367 is hydrogen bonded to N1 and O2 of the flavin ring of the FAD. The analogous site in Rhodobacter sphaeroides ETF-QO is asparagine 338. Mutations N338T and N338A were introduced into the R. sphaeroides protein by site-directed mutagenesis to determine the impact of hydrogen bonding at this site on redox potentials and activity. FAD redox potentials were measured by potentiometric titration probed by electron paramagnetic resonance (EPR) spectroscopy. The N338T and N338A mutations lowered the midpoint potentials, which resulted in a decrease in the quinone reductase activity and negligible impact on disproportionation of ETF1e- catalyzed by ETF-QO. These observations indicate that the FAD is involved in electron transfer to ubiquinone, but not in electron transfer from ETF to ETF-QO. Therefore it is proposed that the iron-sulfur cluster is the immediate acceptor from ETF. It has been proposed that the αII domain of ETF is mobile, allowing promiscuous interactions with structurally different partners. Double electron-electron resonance (DEER) was used to measure the distance between spin labels at various sites and an enzymatically reduced FAD cofactor in Paracoccus denitrificans ETF. Two or three interspin distance distributions were observed for spin-labels in the αI (A43C) and βIII (A111C) domains, but only one is observed for a label in the βII (A210C) domain. This suggests that the αII domain adopts several stable conformations which may correspond to a closed/inactive conformation and an open/active conformation. An additional mutation, E162A, was introduced to increase the mobility of the αII domain. The E162A mutation doubled the activity compared to wild-type and caused the distance distributions to become wider. The DEER method has the potential to characterize conformational changes in ETF that occur when it interacts with various redox partners.