Film flow over strongly undulated bottom profiles at low Reynolds numbers – Experimental study

We study gravity driven films of a Newtonian fluid flowing down sinusoidal bottom profiles. We give experimental evidence of vortices in the film flow under creeping flow conditions down to Reynolds numbers of the order of 10^–5. The vortices are created in the valleys of the undulated bottom profile. They are visualized employing a particle image velocimeter with fluorescent tracers. It turns out that the vortices are generated beyond a critical film thickness.     

Thermally induced Lengthening of weak Dative Bonds in Donor Functionalized Organoaluminum Alkoxides observed by X‐ray Single Crystal Diffraction

The length of weak dative bonds in donor functionalized organo aluminum alkoxides [R₂Al—O—R'—Y]₂ (Y = NR₂, OR, SR) show a temperature dependence in the solid state that is distinctively different from the common effect of libration. This observation is interpreted as being caused by thermal excitation. The possible general relevance of this phenomenon for the study of very weak bonds in molecular systems is discussed.     

Chemische und cyclovoltammetrische Untersuchung der Redoxreaktionen der Decahalogendecaborate closo‐[B10X10]2– und hypercloso‐[B10X10]· – (X = Cl,Br). Kristallstrukturanalyse von Cs2[B10Br10] · 2 H2O

Chemical and Cyclovoltammetric Investigation of the Redoxreactions of the Decahalodecaborates closo ‐[B₁₀X₁₀]^2– and hypercloso ‐[B₁₀X₁₀]^{· –} (X = Cl, Br)¹⁾. Crystal Structure Analysis of Cs₂[B₁₀Br₁₀] · 2 H₂O The oxidation of the decachloro‐closo‐decaborates(2–) Cs₂[B₁₀Cl₁₀] or [Me₄N]₂[B₁₀Cl₁₀] with Tl(CF₃COO)₃ leads to the corresponding radical monoanion hypercloso‐[B₁₀Cl₁₀] ^{· –}, which was characterized by ESR and UV/Vis spectroscopy. [B₁₀Cl₁₀] ^{· –} does not dimerize like [B₁₀H₁₀] ^{· –} but it is reduced by acetonitrile to the dianion [B₁₀Cl₁₀]^2–. Cs₂[B₁₀Cl₁₀] reacts with stronger oxidation agents like CoF₃ (in dichloromethane) or XeF₂ (in perfluorhexane), respectively, to yield B₉Cl₉ and, in traces, B₈Cl₈. In opposite to this, the decabromoderivative Cs₂[B₁₀Br₁₀] does not show any reaction with Tl(CF₃COO)₃ in acetonitrile or with CoF₃ in CH₂Cl₂. The oxidation of the dianions [B₁₀X₁₀]^2– (X = Cl, Br) was studied by electroanalytical methods (cyclic voltammetry, chronoamperometry, chronocoulometry). Formal potentials were determined for the two steps of the reaction, which do not seem to be affected by structural rearrangements. The crystal structure of Cs₂[B₁₀Br₁₀] · 2 H₂O was analyzed by single‐crystal X‐ray diffraction. Cs₂[B₁₀Br₁₀] · 2 H₂O crystallizes monoclinic (space group I2/a, (no. 15), Z = 8, a = 1361.54(9) pm, b = 1215.89(5) pm, c = 3108.4(2) pm, α = 90°, β = 97.916(8)°, γ = 90°). The closo‐cluster B₁₀Br₁₀^2– has a bicapped square antiprismatic structure with idealized D_4d symmetry.     

Synthese und Struktur von Mo2NCl7

Synthesis and Structure of Mo₂NCl₇ The reaction of VN with MoCl₅ at 175 °C in a sealed glass ampoule yields the molybdenum(V) nitride chloride Mo₂NCl₇ in form of air sensitive black crystals with the triclinic space group P1¯ and a = 905.7(8); b = 975.4((6); c = 1283.4(8) pm, α = 103.13(4)°; β = 109.83(5)° und γ = 98.58(5)°. The crystal structure is built up from dinuclear units [Mo₂N₂Cl₇]^3— and [Mo₂Cl₇]³⁺, which are connected by asymmetric nitrido bridges to form endless chains. Within both dinuclear units the Mo atoms are bridged by three Cl atoms resulting in a Mo‐Mo distance of 349.2(3) pm in the unit [Mo₂N₂Cl₇]^3—. In case of [Mo₂Cl₇]³⁺, however, a shorter Mo‐Mo distance of 289.4(3) pm is observed, which can be interpreted by a single bond. Correspondingly a reduced magnetic moment of 0.95 B.M. per Mo atom is observed.     

Neubestimmung der Kristallstruktur von NbOCl3

NbOCl₃ was obtained from a reaction of NbCl₅ and Nb₂O₅ at 260?C. Contrary to the literature data, NbOCl₃ crystallizes in the non‐centrosymmetric space group P&4macr;2₁m as determined by single‐crystal and powder X‐ray diffraction data (crystal: a = b = 1089.59(6) pm, c = 394.79(2) pm, Z = 4, R₁ = 0.0229, wR₂ = 0.0459, powder: a = b = 1086.36(6) pm, c = 393.65(2) pm). The niobium atoms are surrounded by distorted octahedra built of four chlorine atoms and two oxygen atoms in trans positions. Two such octahedra are edge‐bridged through shared chlorine atoms, forming dimers. These units are linked to each other by apical oxygen atoms forming one‐dimensional Nb₂Cl₆O₂ chains parallel [001]. Contrary to the literature data two different Nb‐O distances are obtained.