Structure, ordering, and bonding of half antiperovskites: PbNi3/2S and BiPd3/2S

In a model study the crystal structures of shandite (Pb₂Ni₃S₂), parkerite (Bi₂Ni₃S₂) and their Pd homologues are investigated in terms of ordered half antiperovskites AM_3/2S (A = Pb, Bi; M = Ni, Pd). This addresses fundamental questions on the structural relations, ordering and chemical bonding. From crystal structure investigations a new cubic parkerite variant is presented for Bi₂Pd₃S₂ that fits in an ordering model equivalently to shandite and parkerite. Type–antitype relations to ordered oxygen deficit perovskites are presented. With the relation to the superconductor Ni₃MgC a model is deduced that provides the complete crystal structure and symmetry in terms of the Ni and Pd ordering in antiperovskite superstructures. Therein a systematic ab initio investigation on the relative stability of shandite and parkerite structures is carried out for the first time. From the DFT modelling results the preferences of the ordering variants and the distinct differences in the atomic coordination spheres are discussed. The bonding in the systems is investigated by site projected density of states and covalent bond energy calculations.     

Die «Zip»-Reaktion: Eine neue Ringerweiterungsreaktion. Synthese von 17-, 21- und 25-gliedrigen Polyaminolactamen. 4. Mitteilung über Umamidierungsreaktionen

The Zip-reaction: A New Method for the Synthesis of Macrocyclic Polyaminolactams The 21- and 25-membered aminolactams 11 and 25 were synthesized from the 13-membered lactam 4 . To introduce the ring enlargement unit (a propylamino group) 4 was N-alkylated using acrylonitrile and the resulting product hydrogenated. Repetition of this reaction sequence gave 3 , which was converted in the presence of base in 90% yield to the ring-enlarged macrocyclic base 11 (Scheme 2). In a similar but stepwise synthesis consisting of two separate ring-enlargement reactions 4 was transformed to 11 via 13 (Scheme 4). Introducing three ringenlargement units into 4 the 25-membered aminolactam 25 was synthesized in 84% yield (Scheme 5). The mechanism of the ring-enlargement reaction is given in Scheme 3. In comparison to a zip-fastener or zipper this reaction is called “zipreaction”.     

Chelation or Non-Chelation Control in Addition Reactions of Chiral α- and β- Alkoxy Carbonyl Compounds [New Synthetic Methods (44)]

The addition of C-nucleophiles such as Grignard reagents or enolates to chiral α- or β-alkoxy aldehydes or ketones creates a new center of chirality and is therefore diastereogenic. In order to control stereoselectivity, two strategies have been developed: (1) Use of Lewisacidic reagents which form intermediate chelates, these being attacked stereoselectively from the less hindered side (chelation control); (2) use of reagents incapable of chelation, stereoselective attack being governed by electronic and/or steric factors (non-chelation control). Generally, the two methods lead to the opposite sense of diastereoselectivity. It is possible to predict the outcome by careful choice of organometallic reagents containing elements such as Li, Mg, B, Si, Sn, Cu, Zn, or Ti. For corrigendum see DOI: 10.1002/anie.198407461     

EXCITED STATE BEHAVIOR OF PHYTOCHROME Pr

The primary excited state processes of phytochrome, the plant chromoprotein responsible for most photomorphogenetic responses, are investigated by picosecond intensity-dependent transmission measurements. At sufficient high excitation intensity (starting at photon flux densities of about 1′10²⁵ cm^?2 s^?l) a moderate bleaching of the absorption is observed. For the quantitative analysis of the experimental data a rate equation system approach for the energy levels responsible for the ultrafast transformation P_r→ lumi-R and the photon transport through the sample was utilized. The following was found for the excited state parameters of the phytochrome P_r: (1) an excited state absorption of the same order of magnitude as the ground state absorption (cross section: S?_exc= 4′10^?16 cm^?2 at 650 nm), (2) a fast relaxation of the higher excited singlet state on the femtosecond time scale.     

Remarks on the correlation function implied by the Bjerrum theory of ion pairing

The Friedman theory of correlation functions implied by the Bjerrum theory is generalized by taking into account additional terms and by improving the basic expression for the free energy. By combining the mean spherical approximation (MSA) and the mass action law (MAL) good agreement with HNC and MC data is reached.     

Cycloadditions of Aryl‐Substituted 1,2,4‐Triazines with 2‐Cyclopropylidene‐1,3‐dimethylimidazolidine – Zwitterions as Discrete Intermediates

Cycloadditions of 2‐cyclopropylidene‐1,3‐dimethylimidazolidine ( 1 ), a strong, electron‐rich C‐nucleophile, with a variety of aryl‐substituted 1,2,4‐triazines occur at temperatures between ?100 and +100°, depending on the substitution pattern. At low temperatures, zwitterions, formed by nucleophilic attack of 1 on the triazines, could be detected spectroscopically and, in some cases, isolated. Two types of zwitterions were found: 1) those where the new bond was linked to C(5) of the triazine and which were formed in a reversible dead‐end equilibrium, and 2) those where the new bond was linked either to C(3) or C(6). The latter exhibited the same regiochemistry as the final cycloadducts, and might be intermediates of a two‐step Diels–Alder reaction. Energies and structural characteristics for stationary points in the reaction of monosubstituted triazines with 1 in the gas phase and in CH₂Cl₂ solution were calculated at the Becke3LYP/6‐311+G(d,p)//Becke3LYP/6‐31G(d) level of theory. Different reaction mechanisms are discussed on the basis of steric, electronic, and solvent effects.     

Investigation of redox properties of different PtSn/Al2O3 catalysts.

Alumina-supported Sn and PtSn particles are studied by 119Sn M?ssbauer spectroscopy after oxidation and reduction under various conditions. The observed species of Sn(IV), Sn(II) and Sn(0) are grouped in several categories, each being characterised by distinct structural properties. Tin phases in contact with the support or with platinum are identified. The results are used to establish a model describing the phase transformations occurring in PtSn particles under oxidising or reducing conditions. Particular attention is paid to the reduction/reoxidation mechanisms governing H2/O2 double titrations. An increased reactivity of tin towards oxygen, induced by the contact with platinum, is demonstrated. It is shown that tin contributes to the oxygen uptake VO1 of a first titration cycle by platinum-catalysed transformation of Sn(II) into an oxometallic phase Pt(x)Sn(O). The oxygen titre VO2 of a second cycle is due to O2 chemisorption on platinum only.     

Detector quantum efficiency: An important parameter for FT-IR spectroscopy

Spectral detectivities (D ^*()) are commonly used to characterize the sensitivity of infrared detectors, but for most spectroscopic purposes they are very misleading.For photon detectorsspectral detector quantum efficiencies ( _q ()) are needed in order to calculate the (S/N) ratio for the photon-noise limited situation. An equation to calculate _q(#x03C3;) fromD ^*() values is presented. The results for some common IR-detectors are surprising.As an application for the use of _q () values theS/N ratio of an FT-IR spectrum from an optimized FTS instrument under photon-limited conditions is given.     

Synthesis and crystal structure of tetraethylammonium-μ-trichloro-heptachloro-diaqua-trirhenate(III)-dihydrate,NEt4[Re3Cl10(H2O)2] · 2 H2O

NEt₄[Re₃Cl₁₀(H₂O)₂] · 2 H₂O ( 1 ) was obtained from hydrochloric acid solutions of ReCl₃ and tetraethylammonium chloride, NEt₄Cl, by isothermal evaporation as dark red crystals. 1 crystallizes in the orthorhombic crystal system, space group Pnma, Z = 4, with a = 1838,7(2), b = 1456.9(1), c = 972.08(7) pm, V_m = 391.81(6) cm³ · mol^?l. The crystal structure consists of [Re₃Cl₁₀(H₂O)₂]^? anions that are arranged in the fashion of a hexagonal closest-packing of spheres. These are held together by partially disordered NEt₄⁺ cations and are bound into a hydrogen bonding system with the crystal water.     

The influence of additives on the X-ray induced aggregation of malate synthase monitoring of the aggregation processin situ by time-resolved small-angle X-ray scattering

The X-ray induced aggregation of the sulfhydryl enzyme malate synthase in aqueous solution was monitoredin situ by time-resolved small-angle X-ray scattering. Experiments were performed in the absence/presence of various additives: formate, superoxide dismutase, catalase, NaCl, acetyl-CoA, glyoxylate, malate, pyruvate, -ketobutyrate, oxaloacetate, glycollate, lactate. The scattering curves were measured as a function of the time of irradiation and were analysed in terms of radii of gyration, degrees of aggregation, distance distribution functions, and parameters derived therefrom. Irradiation in the absence of additives resulted in a strong aggregation of enzyme particles. Each of the additives impeded aggregation, however to a different extent. The OH scavenger formate reduced aggregation efficiently; less pronounced effects were registered for superoxide dismutase and/or catalase (the scavengers for and H₂O₂), and for NaCl. Very pronounced diminutions of the aggregation phenomena were provided by substrates or analogues; the efficiency of these substances as radioprotectors may be explained by their action as both scavengers and specific ligands. Based on these results some implications for the performance of conventional small-angle X-ray scattering experiments on biopolymers are derived.

---

Der Einfluß von Zusätzen auf die röntgeninduzierte Aggregation der Malatsynthase. Registrierung des Aggregationsvorganges in situ mittels zeitaufgelöster Röntgenkleinwinkelstreuung

Zusammenfassung Die röntgeninduzierte Aggregation des Sulfhydrylenzyms Malatsynthase in wäßriger Lösung wurdein situ mittels zeitaufgelöster Röntgenkleinwinkelstreuung messend verfolgt. Streuexperimente wurden in An- bzw. Abwesenheit verschiedener Zusätze durchgeführt: Formiat, Superoxiddismutase, Catalase, NaCl, Acetyl-CoA, Glyoxylat, Malat, Pyruvat, -Ketobutyrat, Oxalacetat, Glycolat, Lactat. Die Streukurven wurden als Funktion der Bestrahlungsdauer registriert. Die Auswertung lieferte Streumassenradien, Aggregationsgrade, Abstandsverteilungsfunktionen, und daraus abgeleitete Parameter. Die Bestrahlung in Abwesenheit von Zusätzen verursachte eine starke Aggregation der Enzymteilchen. Jeder der Zusätze verminderte die Aggregation, wenn auch in unterschiedlichem Ausmaß. Der OH Fänger Formiat verringerte die Aggregation wirksam; weniger stark ausgeprägte Effekte ergaben sich für die bzw. H₂O₂ Fänger Superoxiddismutase bzw. Catalase und für NaCl. Substrate und Substratanaloge reduzierten das Ausmaß der Aggregation besonders wirkungsvoll; der Schutzeffekt dieser Substanzen kann durch ihre zweifache Wirkung als Fänger bzw. spezifische Liganden erklärt werden. Ausgehend von diesen Ergebnissen werden einige Schlußfolgerungen für die Durchführung üblicher Röntgenkleinwinkelexperimente an Biopolymeren abgeleitet.