Synthesis, Structure, and Reactivity of Disiloxa[3]tetrelocenophanes

Tetramethyldisiloxa[3]metallocenophanes of the heavy group 14 elements germanium, 2a, tin, 2b, and lead, 2c, (tetrelocenophanes) have been synthesized by the reaction of dilithiated ligand, 1, with the corresponding element(II) chloride. The plumbocenophane, 2c, forms one-dimensional coordination polymers in the solid state, while the germanocenophane, 2a, and the stannocenophane, 2b, are monomeric. Furthermore, the reactivity of the stannocenophane, 2b, and the plumbocenophane, 2c, toward N-heterocyclic carbenes was explored. Although the coordination of carbene is reversible in solution at room temperature, the corresponding carbene complexes, 3a,b, could be structurally characterized, illustrating the Lewis acidity of the central atom in these metallocenophanes

Magnesocenophane-Catalyzed Amine Borane Dehydrocoupling

The Lewis acidities of a series of [n]magnesocenophanes (1 a-d) have been investigated computationally and found to be a function of the tilt of the cyclopentadienyl moieties. Their catalytic abilities in amine borane dehydrogenation/dehydrocoupling reactions have been probed, and C[1]magnesocenophane (1 a) has been shown to effectively catalyze the dehydrogenation/dehydrocoupling of dimethylamine borane (2 a) and diisopropylamine borane (2 b) under ambient conditions. Furthermore, the mechanism of the reaction with 2 a has been investigated experimentally and computationally, and the results imply a ligand-assisted mechanism involving stepwise proton and hydride transfer, with dimethylaminoborane as the key intermediate

Coherent Control of Stationary Light Pulses

We present a detailed analysis of the recently demonstrated technique to generate quasi-stationary pulses of light [M. Bajcsy {\it et al.}, Nature (London) \textbf{426}, 638 (2003)] based on electromagnetically induced transparency. We show that the use of counter-propagating control fields to retrieve a light pulse, previously stored in a collective atomic Raman excitation, leads to quasi-stationary light field that undergoes a slow diffusive spread. The underlying physics of this process is identified as pulse matching of probe and control fields. We then show that spatially modulated control-field amplitudes allow us to coherently manipulate and compress the spatial shape of the stationary light pulse. These techniques can provide valuable tools for quantum nonlinear optics and quantum information processing.Comment: 27 pages, 10 figure

Educação escolar e transformação social: leituras de Manacorda e Mészáros

O presente texto é resultado de nossa pesquisa em nível de mestrado em educação, cujo objetivo é analisar, nos escritos de Manacorda e Mészáros, o papel da educação escolar no processo de transformação social. Trata-se de uma pesquisa teórica que analisa as obras dos autores sobre educação. Organizamos o artigo em três partes: na primeira abordaremos aspectos biográficos dos pensadores, na segunda tratamos sobre educação escolar e transformação social; finalizamos com as considerações finais. Como resultado, destacamos que para os pensadores o papel da educação escolar deve ser o de elevar consciência crítica dos trabalhadores, além disso, deve se articular a uma educação não escolar, que defenda a necessidade de transformação social estrutural em todas as esferas da vida humana, que altere a subordinação do trabalho ao capital e contribua com a superação da sociedade de classes

Numerical study of the laminar premixed flame stabilization on a slot burner : comparison between detailed and FGM models

It is still a challenging task to numerically solve flames using detailed chemical kinetics in multidimensional geometries of practical applications. To overcome this difculty, many eforts have been done to develop chemical kinetics reducing techniques, such as ILDM, FPV, FPI, and FGM. Although these techniques are widely discussed in the literature, their implementation is not straightforward. In the present work, the FGM technique is implemented to solve a two-dimensional laminar premixed flame of CH4∕air stabilized by heat losses to the burner rim. This confguration is explored to test the FGM prediction capabilities for some stable conditions, one of them close to the blow-of limit. Temperature, mass fraction of selected species, and the burning velocity variation along the flame surface are presented, and limitations of the technique were identifed by comparing FGM results against the direct integration of the full set of conservation equation. In general, the FGM technique has shown a good quantitative agreement when compared with the direct integration

Diphosphanylmetallocenes of Main-Group Elements

Several 1,1’-diphosphanyl-substituted metallo cenes of magnesium (magnesocenes) were synthesized, structurally characterized, and their reactivity and coordina tion chemistry were investigated. Transmetalation of these magnesocenes gives access to group 14 metallocenes (tetre locenes), as well as to group 15 stibonocenes. These s- and p-block metallocenes represent a novel class of bis(phos phanyl) ligands, exhibiting Lewis-amphiphilic character. Their coordination chemistry towards different transition-metal and main-group fragments was investigated and different complexes are presented

Bis(di-tert-butylindenyl)tetrelocenes

The synthesis and characterization of bis(di-tert-butylindenyl) germanium(II), tin(II) and lead(II) complexes are reported, which includes the first structurally authenticated example of a bis(indenyl)germanocene. The species were studied in detail in solution and in the solid, which includes single crystal X-ray diffraction and NMR spectroscopy, as well as Mössbauer spectroscopy of the tin compound

Influence of wavelength and accumulated fluence at picosecond laser-induced surface roughening of copper on secondary electron yield

Ultrashort-pulse laser processing of copper is performed in air to reduce the secondary electron yield (SEY). By UV (355 nm), green (532 nm), and IR (1064 nm) laser-light induced surface modification, this study investigates the influence of the most relevant experimental parameters, such as laser power, scanning speed, and scanning line distance (represented as accumulated fluence) on the ablation depth, surface oxidation, topography, and ultimately on the SEY. Increasing the accumulated laser fluence results in a gradual change from a Cu 2 O to a CuO-dominated surface with deeper micrometer trenches, higher density of redeposited surface particles from the plasma phase, and a reduced SEY. While the surface modifications are less pronounced for IR radiation at low accumulated fluence (,1000 J/cm2 ), analogous results are obtained for all wavelengths when reaching the nonlinear absorption regime, for which the SEY maximum converges to 0.7. Furthermore, independent of the extent of the structural transformations, an electron-induced surface conditioning at 250 eV allows a reduction of the SEY maximum below unity at doses of 5×10 -4 C/mm2 . Consequently, optimization of processing parameters for application in particle accelerators can be obtained for a sufficiently low SEY at controlled ablation depth and surface particle density, which are factors that limit the surface impedance and the applicability of the material processing for ultrahigh vacuum systems. The relations between pro- cessing parameters and surface features will provide guidance in treating the surface of vacuum components, especially beam screens of selected magnets of the Large Hadron Collider or of future colliders