472 research outputs found
Implications of a temperature-dependent magnetic anisotropy for superparamagnetic switching
The macroscopic magnetic moment of a superparamagnetic system has to overcome
an energy barrier in order to switch its direction. This barrier is formed by
magnetic anisotropies in the material and may be surmounted typically after
10^9 to 10^12 attempts per second by thermal fluctuations. In a first step, the
associated switching rate may be described by a Neel-Brown-Arrhenius law, in
which the energy barrier is assumed as constant or a given temperature. Yet,
magnetic anisotropies in general depend on temperature themselves which is
known to modify the Neel-Brown-Arrhenius law. We illustrate quantitatively the
implications of a temperature-dependent anisotropy on the switching rate and in
particular for the interpretation of the prefactor as an attempt frequency. In
particular, we show that realistic numbers for the attempt frequency are
obtained when the temperature dependence of the anisotropy is taken into
account.Comment: 15 pages, 5 figure
A new approach for adipose tissue regeneration based on human mesenchymal stem cells in contact to hydrogels: an in vitro study
FUNCTIONALIZED SILICON-COMPOUNDS WITH OMEGA-TETRAMETHYL AND OMEGA-PENTAMETHYLCYCLOPENTADIENYLALKYL LIGANDS - MOLECULAR-COMPONENTS FOR THE PREPARATION OF METALLIC POLYMERS
Jutzi P, Heidemann T, Neumann B, Stammler H-G. Funktionalisierte Siliciumverbindungen mit [omega]-Tetramethyl- und [omega]-Pentamethylcyclopentadienylalkyl-Liganden: Molekulare Bausteine zur Darstellung von Metall-haltigen Polymeren. Journal of Organometallic Chemistry. 1994;472(1-2):27-38.Peralkylierte Cyclopentadien-Systeme des Typs Me5C5(CH2)3Si(Me)mY3-m, die ĂŒber eine Alkyliden-Spacergruppe mit einer funktionalisierten Silan-Einheit verknĂŒpft sind, sind auf zwei verschiedenen Wegen synthetisiert worden. Als Beispiel fĂŒr den ersten Weg wird die Synthese des Disiloxans [Me5C5CH2)3Si(Me)2]2O (2) beschrieben, welches ausgehend von der Iodverbindung [I(CH2)3Si(Me)2]2O (1) durch Umsetzung mit Me5C5K dargestellt werden kann. Das Trichlorsilan Me5C5(CH2)3SiCl3 (4), als Synthesebeispiel fĂŒr den zweiten Weg, ist ĂŒber Hydrosilylierung von 1-Prop-2-enyl-1,2,3,4,5-pentamethylcyclopenta-2,4-dien (3) zugĂ€nglich. Beide Synthesewege sind auch zur Darstellung der teilweise alkylierten Cyclopentadiensysteme des Typs Me4HC5(CH2)3Si(Me)mY3-m geeignet. So fĂŒhren die Umsetzungen der [omega]-Iodalkyl-triethoxysilane I(CH2)nSi(OEt)3 (n = 1, 2, 3) mit Me4HC5K zu den entsprechenden [omega]-(Tetramethylcyclopentadienyl)alkyl-triethoxysilanen 5â7. Verbindungen 5â7 liegen als Isomerengemische vor; das VerhĂ€ltnis zwischen den Isomeren, die ein allylstĂ€ndiges oder ein vinylstĂ€ndiges Wasserstoffatom am Cyclopentadienring aufweisen, ist abhĂ€ngig von der SpacerlĂ€nge. Isomerengemische der 4-(Tetramethylcyclopentadienyl)butyl-silane des Typs Me4HC5(CH2)4Si(Me)mCl3-m (m = 1, 2, 3) (8â10) mit ausschlieĂlich allyistĂ€ndigem Wasserstoffatom am Cyclopentadienring können durch Hydrositylierung von 1-But-3-enyl-2,3,4,5-tetramethyleyclopentadien dargestellt werden. Die am Siliciumatom funktionalisierten Verbindungen 2 und 4â10 können weiter derivatisiert werden. An ausgewĂ€hlten Beispielen wird die Hydrolyse, die Alkoholyse, die reduktive Kupplung, die Heterogenisierung und die Polykondensationsreaktion nĂ€her untersucht. So erhĂ€lt man das Silanol 12 und das Disiloxan 13, die Alkoxysilane 14â17, das Disilan 18 und die funktionalisierten Kieselgele 19â20. Verbindungen 2, 4â10 und 12â20 dienen als âPoolâ zur Darstellung von Ăbergangsmetall-Derivaten. Beispielsweise wird die DicarbonylâCobalt-Verbindung 21 durch Umsetzung von 8 mit Co2(CO)8 gebildet. Reaktion von 16 mit K und FeCl2 liefert das Ferrocen-Derivat 22. Die [eta]4-gebundenen Pt(II)- und Pd(II)-Komplexe 23â27 können durch Umsetzung der funktionalisierten Si---O-Verbindungen 2, 13, und 19â20 mit [PtCl2(Ethylen)]2 oder PdCl2(PhCN)2 erhalten werden.Peralkylated cyclopentadiene systems of the type Me5C5(CH2)3Si(Me)mY3-m, which possess cyclopentadiene units connected with a functionalized silane fragment by an alkylidene spacer group, are prepared via two routes. As an example of the first route, the disiloxane [Me5C5(CH2)3Si(Me)2]2O (2) has been synthesized from the corresponding iodo compound [I(CH2)3Si(Me)2]2O (1) by reaction with Me5C5K. The trichlorosilane Me5C5(CH2)3SiCl3 (4), as an example of a compound prepared via the second route, has been isolated after hydrosilylation of 1-prop-2-enyl-1,2,3,4,5-pentamethylcyclopenta-2,4-diene (3) with HSiCl3. Both synthetic methods are also suitable for the preparation of partly alkylated cyclopentadiene systems of the type Me4HC5(CH2)nSi(Me)mY3-m. Thus the omega-iodoalkyltriethoxysilanes I(CH2)nSi(OEt)3 (n = 1, 2, 3) react with Me4HC5K to give the corresponding omega-(tetramethyl-cyclopentadienyl)alkyl-triethoxysilanes 5-7. Compounds 5-7 consist of a mixture of isomers; the ratio between isomers having an allylic or vinylic hydrogen atom at the cyclopentadiene ring depends on the spacer length. Isomeric mixtures of the 4-(tetramethyl-cyclopentadienyl)butyl-silanes of the type Me4HC5(CH2)4Si(Me)mCl3-m (M = 1, 2, 3) (8-10) with an allylic hydrogen atom at the cyclopentadiene ring have been prepared by hydrosilylation of 1-but-3-enyl-2,3,4,5-tetramethylcyclopentadiene. The silane fragment in the alkylated cyclopentadiene systems 2 and 4-10 can be modified further. As examples hydrolysis, alcoholysis, reductive coupling, heterogenisation and polycondensation reactions are described. Following theses procedures the corresponding silanol 12 and disiloxane 13 have been prepared as well as the alkoxysilanes 14-17, the disilane 18 and the functionalized silicagels 19 and 20. Compounds 2, 4-10 and 12-20 serve as a ''pool'' for the preparation of transition metal complexes. The dicarbonyl cobalt compound 21 has been synthesized by reaction of 8 with CO2(CO)8. Reaction of 16 with K and FeCl2 gives the corresponding ferrocene derivative 22. The eta4-bound Pt(II) and Pd(II)-complexes 23-27 have been prepared by reaction of the functionalized Si-O-compounds 2, 13, and 19-20 with [PtCl2(Ethylen)]2 and PdCl2(PhCN)2, respectively
Quantum mechanics of an electron in a homogeneous magnetic field and a singular magnetic flux tube
The eigenvalue problem of the Hamiltonian of an electron confined to a plane
and subjected to a perpendicular time-independent magnetic field which is the
sum of a homogeneous field and an additional field contributed by a singular
flux tube, i.e. of zero width, is investigated. Since both a direct approach
based on distribution-valued operators and a limit process starting from a
non-singular flux tube, i.e. of finite size, fail, an alternative method is
applied leading to consistent results. An essential feature is quantum
mechanical supersymmetry at g=2 which imposes, by proper representation, the
correct choice of "boundary conditions". The corresponding representation of
the Hilbert space in coordinate space differs from the usual space of
square-integrable 2-spinors, entailing other unusual properties. The analysis
is extended to so that supersymmetry is explicitly broken. Finally,
the singular Aharonov-Bohm system with the same amount of singular flux is
analysed by making use of the fact that the Hilbert space must be the same.Comment: 23 pages, LaTeX, minor change
PENTAMETHYLDISILANYL-SUBSTITUTED CYCLOPENTADIENES - SYNTHESIS, STRUCTURE AND DYNAMIC BEHAVIOR
Jutzi P, Kleimeier J, Krallmann R, Stammler H-G, Neumann B. Pentamethyldisilanyl-substituierte Cyclopentadiene: Synthese, Struktur und dynamisches Verhalten. Journal of Organometallic Chemistry. 1993;462(1-2):57-67.The pentamethyldisilanyl-substituted cyclopentadienes Me(n)C(5)H(6-n-m)(Si(2)Me(5))(m) (for n = 0: 1 (m = 1), 2 (m = 2), 3 (m = 3), 4 (m = 4); for n = 1: 5 (m = 1), 7 (m = 2), 9 (m = 3); for n = 3: 13 (m = 1), 14 (m = 2); for n = 4: 15 (m = I)) are accessible in good yields by treatment of the corresponding cyclopentadienyllithium compounds with Me(5)Si(2)Cl. The mono-Me(5)Si(2)-substituted species 1 and 5 are present only to a small extend in form of vinylic isomers and to a greater extend as isomers with the Me(5)Si(2)-group in allylic position; the latter possess a dynamic structure due to sigmatropic rearrangements. In the twice-Me(5)Si(2)-substituted cyclopentadienes 2 and 7, the 5,5 and 2,5 isomers are observed, which can be interconverted by silatropic shifts; in addition, the presence of two vinylic isomers can be proved in the case of 2. In the cyclopentadiene species 3 and 9 with three Me(5)Si(2) groups, only the 2,5,5 isomers can be detected by NMR spectroscopy. Compound 3 possesses a fluxional structure and can thus be deprotonated. On the other hand, 9 does not show a fluxional behaviour and thus cannot be deprotonated. The cyclopentadiene 4 with four Me(5)Si(2) substituents possesses a static structure and cannot be deprotonated. The 2,3,5,5 position of the substituents is proved by an X-ray crystal structure analysis. Only two Me(5)Si(2) groups can be incorporated in the carbon skeleton of 1,2,4-trimethylcyclopentadiene, whereby compounds of the type 1,2,4-Me(3)C(5)H(3-n)(Si(2)Me(5))(n) (13: n = 1; 14: n = 2) are formed. Surprisingly, 14 cannot be deprotonated with (n)BuLi and KH, respectively. The reaction of Me(4)C(5)HLi with Me(5)Si(2)Cl leads to the cyclopentadiene Me(4)C(5)HSi(2)Me(5) (15). Though compound 15 can be deprotonated, further reaction of the resulting anion with Me(5)Si(2)Cl does not lead to the expected cyclopentadiene Me(4)C(5)(Si(2)Me(5))(2) (16). On the other hand, 16 can be prepared by metallation of 14 with C8K and further reaction with CH3I. In contrast to 14, compound 4 cannot be deprotonated with C8K; the reaction of 4 with C8K and CH3I leads to 9 via Si-C bond splitting. The pentamethyldisilanyl-substituted pentamethylcyclopentadiene Me(5)C(5)Si(2)Me(5) (17) is obtained by reaction of Me(5)C(5)K with Me(5)Si(2)Cl; compound 17 shows dynamic behaviour; the migration of the Me(5)Si(2) group is slower than that of the Me(3)Si group in Me(5)C(5)SiMe(3). Three ElMe(3) groups can be introduced stepwise into the 1,2,4-Me(3)C(5)H(3) molecule, as demonstrated by the exemplary synthesis of the cyclopentadienes 1,2,4-Me(3)C(5)H(3-n)(SiMe(3))(n) (10: n = 1; 11: n = 2) and 1,2,4-Me(3)C(5)(SiMe(3))(2)SnMe(3) (12).Die Si2Me5-substituierten Cyclopentadiene MenC5H6-n-m(Si2Me5)m (fĂŒr n = 0: 1 (m = 1), 2 (m = 2), 3 (m = 3), 4 (m = 4); fĂŒr n = 1: 5 (m = 1), 7 (m = 2), 9 (m = 3); fĂŒr n = 3: 13 (m = 1), 14 (m = 2); fĂŒr n = 4: 15 (m = 1)) sind durch Umsetzung der entsprechenden Cyclopentadienyl-Lithium-Verbindung mit Me5Si2Cl in guten Ausbeuten zugĂ€nglich. In den einfach Si2Me5-substituierten Systemen 1 und 5 findet man zu einem geringen Anteil Isomere mit vinylstĂ€ndiger Si2Me5-Gruppe und zu einem ĂŒberwiegenden Anteil das Isomer mit allylstĂ€ndiger Si2Me5-Gruppe, welches aufgrund von sigmatropen Umlagerungen eine dynamische Struktur besitzt. In den zweifach Si2Me5- substituierten Cyclopentadienen 2 und 7 beobachtet man die jeweiligen 5,5- und 2,5-Isomere, welche durch Silatropie miteinander im Gleichgewicht stehen; zusĂ€tzlich lassen sich in 2 noch zwei Isomere mit ausschlieĂlich vinylstĂ€ndigen Substituenten nachweisen. In den dreifach Si2Me5-substituierten Systemen 3 und 9 ist nur das 2,5,5-Isomere nachweisbar. 3 besitzt eine dynamische Struktur und ist deshalb deprotonierbar. 9 hingegen ist nicht dynamisch und aufgrund des Fehlens einer Allyl-H-Funktion nicht deprotonierbar. Auch das vierfach Si2Me5-substituierte Cyclopentadien 4 zeigt keine MolekĂŒldynamik und kann nicht deprotoniert werden; die 2,3,5,5-Anordnung der Substituenten in 4 wird anhand einer Röntgenstrukturanalyse belegt. Im 1,2,4-Trimethylcyclopentadien gelingt jedoch nur die EinfĂŒhrung von zwei Si2Me5-Gruppen, wobei die Verbindungen des Typs 1,2,4-Me3C5H3-n(Si2Me5)n (13: n = 1; 14: n = 2) entstehen. Ăberraschenderweise ist 14 mit nBuLi oder KH nicht deprotonierbar. Die Umsetzung von Me4C5HLi mit Me5Si2Cl fĂŒhrtzum Cyclopentadien Me4C5HSi2Me5 (15). Obwohl 15 deprotonierbar ist, gelingt durch Umsetzung des Anions mit Me5Si2Cl die Synthese von Me4C5(Si2Me5)2 (16) nicht. Verbindung 16 lĂ€Ăt sich allerdings durch Metallierung von 14 mit C8K und anschlieĂende Umsetzung mit CH3I darstellen. Im Gegensatz dazu kann 4 mit C8K nicht deprotoniert werden; die Umsetzung mit C8K und CH3I lĂ€uft ĂŒber Si-C-Bindungsspaltung zu 9. Das Cyclopentadienyldisilan Me5C5Si2Me5 (17) erhĂ€lt man durch Umsetzung von Me5C5K mit Me5Si2Cl; 17 zeigt dynamisches Verhalten, die Wanderungsgeschwindigkeit der Si2Me5-Gruppe ist geringer als die der SiMe3-Gruppe im Cyclopentadienylsilan Me5C5SiMe3. Im Cyclopentadien 1,2,4-Me3C5H3 lassen sich sukzessiv drei ElMe3-Gruppen (El = Si, Sn) einfĂŒhren, wie durch die beispielhafte Synthese von 1,2,4-Me3C5H3-n(SiMe3)n (10: n = 1, 11: n = 2) und 1,2,4-Me3 C5(SiMe3)2SnMe3 (12) gezeigt wird
Pair decay width of the Hoyle state and carbon production in stars
The pair decay width of the first excited 0âș state in ÂčÂČC (the Hoyle state) is deduced from a novel analysis of the world data on inelastic electron scattering covering a wide momentum transfer range, thereby resolving previous discrepancies. The extracted value ÎÏ = (62.3 ± 2.0) ÎŒeV is independently confirmed by new data at low momentum transfers measured at the S-DALINAC and reduces the uncertainty of the literature values by more than a factor of three. A precise knowledge of ÎÏ is mandatory for quantitative studies of some key issues in the modeling of supernovae and of asymptotic giant branch stars, the most likely site of the slow-neutron nucleosynthesis process
The Code of Protest. Images of Peace in the West German Peace Movements, 1945-1990
The article examines posters produced by the peace movements in the Federal Republic of
Germany during the ColdWar, with an analytical focus on the transformation of the iconography
of peace in modernity. Was it possible to develop an independent, positive depiction of peace
in the context of protests for peace and disarmament? Despite its name, the pictorial selfrepresentation
of the campaign âFight against Nuclear Deathâ in the late 1950s did not draw
on the theme of pending nuclear mass death. The large-scale protest movement in the 1980s
against NATOâs 1979 âdouble-trackâ decision contrasted female peacefulness with masculine
aggression in an emotionally charged pictorial symbolism. At the same time this symbolism
marked a break with the pacifist iconographic tradition that had focused on the victims of war.
Instead, the movement presented itself with images of demonstrating crowds, as an anticipation
of its peaceful ends. Drawing on the concept of asymmetrical communicative âcodesâ that has
been developed in sociological systems theory, the article argues that the iconography of peace in
peace movement posters could not develop a genuinely positive vision of peace, since the code of
protest can articulate the designation value âpeaceâ only in conjunction with the rejection value
âwarâ
Gelatin-based hydrogels promote chondrogenic differentiation of human adipose tissue-derived mesenchymal stem cells in vitro
Due to the weak regeneration potential of cartilage, there is a high clinical incidence of articular joint disease, leading to a strong demand for cartilaginous tissue surrogates. The aim of this study was to evaluate a gelatin-based hydrogel for its suitability to support chondrogenic differentiation of human mesenchymal stem cells. Gelatin-based hydrogels are biodegradable, show high biocompatibility, and offer possibilities to introduce functional groups and/or ligands. In order to prove their chondrogenesis-supporting potential, a hydrogel film was developed and compared with standard cell culture polystyrene regarding the differentiation behavior of human mesenchymal stem cells. Cellular basis for this study were human adipose tissue-derived mesenchymal stem cells, which exhibit differentiation potential along the adipogenic, osteogenic and chondrogenic lineage. The results obtained show a promotive effect of gelatin-based hydrogels on chondrogenic differentiation of mesenchymal stem cells in vitro and therefore encourage subsequent in vivo studies
Frieden schaffen ohne Waffen: FĂŒr nichtmilitĂ€rische Konfliktlösungen in und durch Europa: BeitrĂ€ge zum FĂŒnften Dresdner Friedenssymposium am 15. Februar 1997
Das FĂŒnfte Dresdner Friedenssymposium zum Thema âFrieden schaffen ohne Waffen. FĂŒr nichtmilitĂ€rische Konfliktlösungen in und durch Europaâ.:Verzeichnis des Inhalts und der Autoren.
SymposiumbeitrÀge:
- Wolfgang Scheler, Die Friedenskonzeption in verÀnderter Sicherheitslage. Konsistenz und Modifikation.
- Gerda Zellentin, Gesellschaftliche Gruppen und nichtmilitÀrische Konfliktbearbeitung - Chancen und Risiken.
- Hans-Jochen Vogel, Schalom und adé? Kirchliche Friedensbewegung und Konziliarer Prozess in der DDR wirken weiter. Mit einem Zeitzeugnis aus dem Jahre 1984.
Aus der Diskussion:
- Peter Strutinsky, Frieden durch Demokratie?
- Wilfried Neumann, Unsere Aufgabe: Erziehung zu Frieden und Versöhnung.
- Lorenz Knorr, Kriegsursachen und friedliche Konfliktregelung.
- Peter Gerlinghoff, FrĂŒhzeitige Konfliktanalyse - eine Vorbedingung ziviler KonfliktbewĂ€ltigung.
- Gerd Hommel, FĂŒr nichtmilitĂ€rische Konfliktlösungen und Ăberwindung von Ausgrenzung aus Glaubens-, ethnischen und politischen GrĂŒnden.
- Ingeborg Philipp, Kosmisches Zeitalter.
- Jan Ć umavsky, Die Beziehungen zwischen Deutschen und Tschechen und die 'Deklaration'
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