Mechanism of Reduction of Diphenylacetylene by Metallic Lithium

A zero-order reaction converts cis-dilithiostilbene ( 1 ), formed upon reduction of diphenylacetylene by lithium, into the trans isomer 2 . The cis-monolithiated adduct acts as a catalyst.     

Zur Existenz tetraedrischer Polykationen [In5]7+ in der Verbindung „Na23In5O15”︁ (=Na24In5O15?) einem ungewöhnlichen Oxidationsprodukt der intermetallischen Phase NaIn

Tetrahedral Polycations [In₅]⁷⁺ in ‘‘Na₂₃In₅O₁₅”︁”︁ (=Na₂₄In₅O₁₅?) an Unusual Oxidation Product of the Alloy NaIn The already published, red-transparent, air sensitive, cubic compound Na₂₄In₅O₁₅ (a = 1107.7 pm, Z = 2, I43m) is an intermediate oxidation product of the intermetallic phase NaIn. It is reinvestigated with respect to the question whether it contains the tetrahedral polycation [InIn₄]⁶⁺ or [InIn₄]⁷⁺. In the latter case the chemical composition would be Na₂₃In₅O₁₅ instead of Na₂₄In₅O₁₅ and the polycation would be in accordance with the Zintl-Klemm concept. Na₂₄In₅O₁₅ is prepared in single phased samples for the first time and investigated by means of the Rietveld refinement technique of X-ray powder data. Based on the results of the Rietveld refinements and new single crystal data in comparison to the published values we note anomalies in the occupancies of the Na positions and a pronounced anisotropy in the thermal parameters of one oxygen position. These investigations together with results obtained by analytical scanning electron microscopy, Extended Hückel and MAPLE calculations give strong evidence for the assumption of [InIn₄]⁷⁺ instead of [InIn₄]⁶⁺ and the associated correct composition Na₂₃In₅O₁₅. The problems of a reliable characterization of this unique compound are discussed.     

Nature and structure of aluminum surface sites grafted on silica from a combination of high-field aluminum-27 solid-state NMR spectroscopy and first-principles calculations

The determination of the nature and structure of surface sites after chemical modification of large surface area oxides such as silica is a key point for many applications and challenging from a spectroscopic point of view. This has been, for instance, a long-standing problem for silica reacted with alkylaluminum compounds, a system typically studied as a model for a supported methylaluminoxane and aluminum cocatalyst. While (27)Al solid-state NMR spectroscopy would be a method of choice, it has been difficult to apply this technique because of large quadrupolar broadenings. Here, from a combined use of the highest stable field NMR instruments (17.6, 20.0, and 23.5 T) and ultrafast magic angle spinning (>60 kHz), high-quality spectra were obtained, allowing isotropic chemical shifts, quadrupolar couplings, and asymmetric parameters to be extracted. Combined with first-principles calculations, these NMR signatures were then assigned to actual structures of surface aluminum sites. For silica (here SBA-15) reacted with triethylaluminum, the surface sites are in fact mainly dinuclear Al species, grafted on the silica surface via either two terminal or two bridging siloxy ligands. Tetrahedral sites, resulting from the incorporation of Al inside the silica matrix, are also seen as minor species. No evidence for putative tri-coordinated Al atoms has been found.     

Deconstructing Approximate Offsets

We consider the offset-deconstruction problem: Given a polygonal shape?Q with n vertices, can it be expressed, up to a tolerance??? in Hausdorff distance, as the Minkowski sum of another polygonal shape P with a disk of fixed radius? If it does, we also seek a preferably simple-looking solution?P; then, P??s offset constitutes an accurate, vertex-reduced, and smoothened approximation of Q. We give an O(nlogn)-time exact decision algorithm that handles any polygonal shape, assuming the real-RAM model of computation. A variant of the algorithm, which we have implemented using the cgal library, is based on rational arithmetic and answers the same deconstruction problem up to an uncertainty parameter ??; its running time additionally depends on ??. If the input shape is found to be approximable, this algorithm also computes an approximate solution for the problem. It also allows us to solve parameter-optimization problems induced by the offset-deconstruction problem. For convex shapes, the complexity of the exact decision algorithm drops to O(n), which is also the time required to compute a solution?P with at most one more vertex than a vertex-minimal one.     

Electron density measurements of argon surface-wave discharges

The electron density of microwave-generated surface-wave discharges in argon have been measured using Stark broadening and calculated from the measured wavelengths of the standing surface wave. Results obtained from these two techniques compare well. The electron density varies from 10¹³ to 10¹⁴/cm³ for pressures ranging from 50 to 800 torr.     

On the mechanism of bibenzyl formation by deoxidation/reduction of aromatic species with lithium 4,4′-di-t-butylbiphenyl radical anion - a correction

In the reaction of benzil with lithium in the presence of 4,4′-dit-butylbiphenyl (DBB)trans-stilbene4 and dilithiobibenzyl5 are intermediates instead of the reported tetralithiobibenzyl2.