Beiträge zur Chemie der Pyrrolpigmente, 25. Mitt. Die diastereomeren (geometrisch isomeren) Biliverdindimethylester—Struktur,Konfiguration und Konformation

Photoisomerization of the symmetrically substituted bilatrienes-abc1 and2 adsorbed on aluminum oxide yields the photoisomers1 and2, whereas the biliverdindimethylester (3) gives rise to two isomers3 and3. By X-ray photoelectron spectrometry of the N_1s-level all these diastereomers were shown to have a bislactam structure. From¹H-NMR experiments (chemical shifts, Lanthanide induced shifts, double resonance experiments) the configurations (Z,Z,Z) at the methin positions were assigned to the educts—the photoisomers1 and2 having the configurations (Z,Z,E). The configurations (Z,Z,E) and (E,Z,Z) could be assigned to the diastereomers3 and3. The conformational aspects within these series are discussed and on the basis of several studies asyn-syn-syn-helical arrangement is deduced for the systems of configuration (Z,Z,Z). For the (Z,Z,E) isomers in the region of (Z)-configuration a conformation resembling the (Z,Z,Z)-systems is reasonable, but in the region of configuration (E) a stronger twisting at the methin single bond due to steric crowding should be favoured.

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24. Mitt.:H. Falk undG. Höllbacher, Mh. Chem.109, 1429 (1978).     

Convexity and concavity of eigenvalue sums

It is well known that(H), the sum of the negative eigenvalues of a Hermitian matrixH, is a concave and increasing function ofH. In contrast to this, we prove that forA nonsingular Hermitian andP positive definite, the functionP(AP)=(P ^1/2 AP ^1/2) is convex and decreasing. Several other results of this nature are also proved.     

Dynamics of the finite SK spin-glass

The dynamical behavior of a Sherrington-Kirkpatrick spin-glass model consisting of a large but finite number of Ising spins with a time evolution given by Glauber dynamics is investigated. Starting from the resummation of a diagrammatic expansion we derive a differential equation for the response function which allows us to handle nonperturbative effects. This enables us to find explicit expressions for the dynamical behavior of response and correlation function on time scales related to those free energy barriers which diverge with system sizeN. For the largest of these barriers we find a behavior proportional toN with =1/3.     

Diazabutadiene derivatives of ytterbocenes. Syntheses,properties, and crystal structures of the (C5Me5)2Yb(ButNCHCHNBut) and [CpYb(μ2-OC13H8—C13H8O)(THF)]2 complexes

Oxidation of (C₅Me₅)₂Yb(THF)₂ with diazabutadiene Bu^tN=CHCH=NBu^t (DAD) afforded the (C₅Me₅)₂Yb(Bu^tNCHCHNBu^t) complex (1). The magnetic measurements and X-ray diffraction study confirmed the trivalent state of the ytterbium atom and the radical nature of the DAD ligand in complex 1. The oxidation state of ytterbium in the (C₅Me₅)₂YbDAD—solvent system depends on the coordinating properties of the solvent, whereas the ytterbium atom in the Cp₂YbDAD complex (2) remains trivalent regardless of the solvent nature. In complex 2, the redox replacement of DAD^·– with 9-fluorenone accompanied by the pinacol dimerization of 9-fluorenone and detachment of one Cp ligand from the ytterbium atom gave rise to the dimeric [CpYb(²-OC₁₃H₈-C₁₃H₈O)(THF)]₂ complex (3). The structure of complex 3 was established by X-ray diffraction analysis.     

On the chemistry of pyrrole pigments,LXXXVIII: Nonlinear optical properties of linear oligopyrroles

Summary The second order polarizabilities, _xxx, of six linear oligopyrroles of the arylalkylidene-dipyrrinone type (the synthesis of two of them is reported) were estimated from their solvatochromic shifts. A dimethylaminophenylmethylene and a benzodithioleylidenemethylene derivative (2,6) were found to exhibit rather high _xxx values.

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Zur Chemie von Pyrrolpigmenten, 88. Mitt.: Nicht-lineare optische Eigenschaften von linearen Oligopyrrolen

Zusammenfassung Die Polarisierbarkeiten zweiter Ordnung, _xxx, von sechs linearen Oligopyrrolen des Arylalkyliden-pyrrolinon-Typs (die Synthese zweier solcher Derivate wird mitgeteilt) wurden aus ihrem solvatochromen Verhalten abgeschätzt. Ein Dimethylaminophenylmethylen-und ein Benzodithiolylidenmethylen-Derivat (2,6) zeigen bemerkenswert hohe _xxx-Werte.

     

Bimetallic Ag‐Pt Sub‐nanometer Supported Clusters as Highly Efficient and Robust Oxidation Catalysts

A combined experimental and theoretical investigation of Ag‐Pt sub‐nanometer clusters as heterogeneous catalysts in the CO→CO₂ reaction (COox) is presented. Ag₉Pt₂ and Ag₉Pt₃ clusters are size‐selected in the gas phase, deposited on an ultrathin amorphous alumina support, and tested as catalysts experimentally under realistic conditions and by first‐principles simulations at realistic coverage. In situ GISAXS/TPRx demonstrates that the clusters do not sinter or deactivate even after prolonged exposure to reactants at high temperature, and present comparable, extremely high COox catalytic efficiency. Such high activity and stability are ascribed to a synergic role of Ag and Pt in ultranano‐aggregates, in which Pt anchors the clusters to the support and binds and activates two CO molecules, while Ag binds and activates O₂, and Ag/Pt surface proximity disfavors poisoning by CO or oxidized species.