Edison revisited: Electro mechanical effects in wet porous materials

Edison discovered that the coefficient of friction between a metallic plate and a porous material moistened with a dilute electrolyte could be modulated by an electric field. In experiments on the same kind of contacts (clays or chalks on carbon or metals) but without continuous tangential relative motion we measure two electro-mechanical effects at frequencies of the order of 10 kHz. An alternating field induces an alternating normal force between the porous material and the conducting base. The force is lagging by versus the field. A forced normal relative motion induces through the contact a current nearly in phase with the motion. For an explanation we start from Helmholtz theory of stationary electrophoretic phenomena. We present a model in which liquid motions obey the Helmholtz laws. It explains decently the phase relations between causes and effects, and approximately the values of the effects. In optical experiments on contacts between a wet clay and the transparent conducting coating of a glass plate we measure in the frequency range 1-100 kHz a modulation of reflecting power induced by an alternating potential. The decrease of reflecting power is lagging by an angle close to behind the field. We think the modulation observed is induced by a modulation of the amount of liquid in the film present between glass and clay. In friction experiments this alternating liquid lubrication acting exclusively at the very places where friction occurs may have significant effect. Received: 22 July 1997 / Revised: 21 October 1997 / Accepted: 21 November 1997     

Dynamic surface behaviour of VPO catalysts under reactive and non-reactive gas compositions: an in situ XAS study

The surface of an activated vanadium phosphorus oxide (VPO) catalyst was investigated by means of in situ X-ray absorption spectroscopy in the total electron yield mode. We observed significant changes of the V L₃-near edge X-ray absorption fine structure (NEXAFS) when the material was transferred from room temperature to working conditions at 400 °C in the reaction atmosphere. We studied the same VPO material under different gas compositions comprising the reaction mixture of n-butane and oxygen, pure oxygen and vacuum to elucidate the influence of the gas–surface interaction and the effect of the temperature. The results of this extensive study indicate a dynamic response of the catalyst surface to the applied conditions.     

Tris‐cyclometalated Iridium(III) Complexes with Three Different Ligands: a New Example with 2‐(2,4‐Difluorophenyl)pyridine‐Based Complex

An iridium(III) complex comprising three different cyclometalated phenylpyridine‐based ligands was designed and synthesized. Interestingly, mixed‐ligand complexes could be obtained by using a simple and straightforward procedure. A tris(heteroleptic) Ir^III complex was obtained as a mixture of stereoisomers that could not be separated. Photophysical properties of the tris(heteroleptic) complex was investigated by UV/VIS absorption and luminescence spectroscopy, and compared with those of the parent homoleptic complexes. Modelling by time‐dependent density functional theory (TD‐DFT) was also performed to elucidate the nature and the location of the excited state, and to support the experimental results.     

Dissecting Ubiquitin Signaling with Linkage‐Defined and Protease Resistant Ubiquitin Chains

Ubiquitylation is a complex posttranslational protein modification and deregulation of this pathway has been associated with different human disorders. Ubiquitylation comes in different flavors: Besides mono‐ubiquitylation, ubiquitin chains of various topologies are formed on substrate proteins. The fate of ubiquitylated proteins is determined by the linkage‐type of the attached ubiquitin chains, however, the underlying mechanism is poorly characterized. Herein, we describe a new method based on codon expansion and click‐chemistry‐based polymerization to generate linkage‐defined ubiquitin chains that are resistant to ubiquitin‐specific proteases and adopt native‐like functions. The potential of these artificial chains for analyzing ubiquitin signaling is demonstrated by linkage‐specific effects on cell‐cycle progression.     

Modeling Fouling Factors for Microscale Heat Exchangers

This article presents an approach to the modeling of CaCO₃ fouling and cleaning in a microscale cross-flow heat exchanger. The fouling progress was detected by thermal, fluid dynamical, and optical measures. In general, the observed fouling phenomena at microscale were comparable to those at macroscale. The detected thermal fouling resistance was between 10^? 6 and 10^? 3 m^? 2 K^? 1. As expected, crystallization fouling strongly depends on the surface temperature. In addition, the surface coverage can be a useful measure for both processes—fouling and cleaning. An extended approach to derive a reasonable fouling allowance was formulated through a fractional fouling resistance.     

Incorporation of an Amino Function in a (1S,2S,3R)-3-Hydroxy-2-methoxy-1-cyclohexane Carboxylic Acid

(1S,2S,3R)-3-Hydroxy-2-methoxy-1-cyclohexanecarboxylic acid was synthesized for the construction of an amino acid to be used in a constrained ring didemnin B analog ( 2 ). The amine functionality was introduced into the cyclohexane ring by reductive amination using sodium triacetoxyborohydride or by oxime formation followed by reduction.     

Crystal Structures of Mono-, Di-, and Triaminoguanidinium Sulfate,as Well as Azidoformamidinium Sulfate: Important Precursors for Syntheses of Nitrogen Rich Ionic Compounds

Bis(1-aminoguanidinium) sulfate monohydrate (AG₂SO₄ … H₂O, 1), bis(1,3-diamino-guanidinium sulfate (DAG₂SO₄, 2), bis(1,3,5-triaminoguanidinium) sulfate dihydrate (TAG₂SO₄ … 2 H₂O, 3) and bis(azidoformamidinium) sulfate (AF₂SO₄, 5) were synthesized and characterized by multinuclear NMR, IR, and Raman spectroscopy and elemental analysis. In the synthesis of 3, double protonated triaminoguanidinium sulfate (HTAGSO₄, 4) was obtained as a byproduct. The molecular structures of 1–5 in the crystalline state were determined by low-temperature single crystal X-ray diffraction. 1: orthorhombic, Pnma, a = 6.7222 (8) Å, b = 14.153 (2) Å, c = 11.637 (1) Å, V = 1107.1(2) ų, Z = 4, ρ_calc.= 1.586 g cm^?3 R₁ = 0.0442, wR₂ = 0.1007 (all data). 2: hexagonal, P6₁22, a,b = 6.6907 (1) Å, c = 43.4600 (8) Å, γ= 120°, V = 1684.86 (5) ų, Z = 6, ρ_calc.= 1.634 g cm^?3, R₁ = 0.0321, wR² = 0.0714 (all data). 3: monoclinic, C2/c, a = 9.6174 (8) Å, b = 22.858 (1) Å, c = 6.7746 (5) Å, β= 109.49 (1), V = 1404.0 (4) ų, Z = 4, ρ_calc.= 1.620 g cm_?3, R₁ = 0.0292, wR₂ = 0.0781 (all data). 4: monoclini c, P2₁/c, a = 8.9998 (9), b = 6.3953 (6), c = 13.3148(12) Å, β= 99.679 (8), V = 755.44 (13) ų, Z = 4, ρ_calc.= 1.778 g cm^?3, R₁ = 0.0305, wR₂ = 0.0809 (all data); 5: orthorhombic, Pbca, a = 11.3855 (9), b = 7.1032 (6), c = 12.807 (1) Å, V = 1035.74 (14) ų, Z = 4, ρ_calc.= 1.720 g cm^?3, R₁ = 0.0389, wR₂ = 0.0862 (all data).     

Concatenating Suzuki Arylation and Buchwald–Hartwig Amination by A Sequentially Pd-Catalyzed One-Pot Process—Consecutive Three-Component Synthesis of C,N-Diarylated Heterocycles

The concatenation of Suzuki coupling and Buchwald-Hartwig amination in a consecutive multicomponent reaction opens a concise, modular and efficient one-pot approach to diversely functionalized heterocycles, as exemplified for 3,10-diaryl 10H-phenothiazines, 3,9-diaryl 9H-carbazoles, and 1,5-diaryl 1H-indoles, in high yields starting from simple staring materials. Moreover, this one-pot reaction is a sequentially palladium-catalyzed process that does not require additional catalyst loading after the first coupling step.     

New cleavable isocyanides for the combinatorial synthesis of α-amino acid analogue tetrazoles

3-Substituted 3-isocyano propionates as new cleavable isocyanides in combinatorial tetrazole synthesis via Ugi-reaction are introduced. The obtained 5-substituted tetrazoles are transformed into carboxylic acid isosteric 5-substituted 1H-tetrazoles under basic cleavage conditions.