Activation and protonation of dinitrogen at the FeMo cofactor of nitrogenase

The protonation of N2 bound to the active center of nitrogenase has been investigated using state-of-the-art density-functional theory calculations. Dinitrogen in the bridging mode is activated by forming two bonds to Fe sites, which results in a reduction of the energy for the first hydrogen transfer by 123 kJ/mol. The axial binding mode with open sulfur bridge is less reactive by 30 kJ/mol and the energetic ordering of the axial and bridged binding modes is reversed in favor of the bridging dinitrogen during the first protonation. Protonation of the central ligand is thermodynamically favorable but kinetically hindered. If the central ligand is protonated, the proton is transferred to dinitrogen following the second protonation. Protonation of dinitrogen at the Mo site does not lead to low-energy intermediates.     

Crystal and molecular structures of alkali oxalates: first proof of a staggered oxalate anion in the solid state

The molecular and crystal structures of solvent-free potassium, rubidium, and cesium oxalates have been determined ab initio from high-resolution synchrotron and X-ray laboratory powder patterns. In the case of potassium oxalate K(2)C(2)O(4) (a = 10.91176(7) A, b = 6.11592(4) A, c = 3.44003(2) A, orthorhombic, Pbam, Z = 2), the oxalate anion is planar, whereas in cesium oxalate Cs(2)C(2)O(4) (a = 6.62146(5) A, b = 11.00379(9) A, c = 8.61253(7) A, beta = 97.1388(4) degrees, monoclinic, P2(1)/c, Z = 4) it exhibits a staggered conformation. For rubidium oxalate at room temperature, two polymorphs exist, one (beta-Rb(2)C(2)O(4)) isotypic to potassium oxalate (a = 11.28797(7) A, b = 6.29475(4) A, c = 3.62210(2) A, orthorhombic, Pbam, Z = 2) and the other (alpha-Rb(2)C(2)O(4)) isotypic to cesium oxalate (a = 6.3276(1) A, b = 10.4548(2) A, c = 8.2174(2) A, beta = 98.016(1) degrees, monoclinic, P2(1)/c, Z = 4). The potassium oxalate structure can be deduced from the AlB(2) type, and the cesium oxalate structure from the Hg(99)As type, respectively. The relation between the two types of crystal structures and the reason for the different conformations of the oxalate anion are discussed.     

Homo-polymerization of alpha-olefins and co-polymerization of higher alpha-olefins with ethylene in the presence of CpTiCl2(OC6H4X-p)/MAO catalysts (X = CH3, Cl)

Cyclopentadienyl-titanium complexes containing -OC6H4X ligands (X = Cl,CH3) activated with methylaluminoxane (MAO) were used in the homo-polymerization of ethylene, propylene, 1-butene, 1-pentene, 1-butene, and 1-hexene, and also in co-polymerization of ethylene with the alpha-olefins mentioned. The -X substituents exhibit different electron donor-acceptor properties, which is described by Hammett's factor (sigma).The chlorine atom is electron acceptor, while the methyl group is electron donor. These catalysts allow the preparation of polyethylene in a good yield. Propylene in the presence of the catalysts mentioned dimerizes and oligomerizes to trimers and tetramers at 25 degrees C under normal pressure. If the propylene pressure was increased to 7 atmospheres,CpTiCl2(OC6H4CH3)/MAO catalyst at 25 degrees gave mixtures with different contents of propylene dimers, trimers and tetramers. At 70 degrees C we obtained only propylene trimer.Using the catalysts with a -OC(6)H(4)Cl ligand we obtained atactic polymers with M(w) 182,000 g/mol (at 25 degrees C) and 100,000 g/mol (at 70 degrees C). The superior activity of the CpTiCl2(OC6H4Cl)/MAO catalyst used in polymerization of propylene prompted us to check its activity in polymerization of higher alpha-olefins (1-butene, 1-pentene, 1-hexene)and in co-polymerization of these olefins with ethylene. However, when homo-polymerization was carried out in the presence of this catalyst no polymers were obtained. Gas chromatography analysis revealed the presence of dimers. The activity of the CpTiCl2(OC6H4Cl)/MAO catalyst in the co-polymerization of ethylene with higher alpha-olefins is limited by the length of the co-monomer carbon chain. Hence, the highest catalyst activities were observed in co-polymerization of ethylene with propylene (here a lower pressure of the reagents and shorter reaction time were applied to obtain catalytic activity similar to that for other co-monomers). For other co-monomers the activity of the catalyst decreases as follows: propylene >1-butene > 1-pentene > 1-hexene. In the case of co-polymerization of ethylene with propylene, besides an increase in catalytic activity, an increase in the average molecular weight M(w) of the polymer was observed. Other co- monomers used in this study caused a decrease of molecular weight. A significant increase in molecular weight distribution (M(w)/M(n)) evidences a great variety of polymer chains formed during the reaction.     

Reaction of 5,5-diphenyl-2-thiohydantoin with 1,3-dibromopropane : Crystal and molecular structure of 2,3,4,5-tetrahydro-6,6-diphenylimidazo [2, 1-b]-thiazine-7 (6H)-one

The reaction between the potassium salt of 5,5-diphenyl-2-thiohydantoin (1) and 1,3-dibromopropane carried out in DME under anhydrous conditions has been found to give two isomeric diphenylimidazothiazines 2 and 3. When the reaction of 1 with 1,3-dibromopropane was performed in protic solvents (EtOH, HOH, NaOH) 2 and 3-(3-mercaptopropyl) - 5,5 - diphenylthiohydantoin (4) were formed. The latter is the product of hydrolysis of 3 taking place under the reaction conditions. 2,3,4,5 - Tetrahydro - 6,6 - diphenylimidazo [2,1-b] - thiazine - 7 (6H) - one (2) crystallises in space group P2₁/n with a =10.812(3), b =14.905(7), c =9.885(4) Å, β = 104.91(2)°. The 5-membered ring in 2 is planar whereas the 6-membered thiazine ring adopts the sofa conformation.     

Hollow nanoparticles via stepwise complexation and selective decomplexation of poly(ethylene imine)

The preparation of hollow nanoparticles with amino groups on the inner side via the stepwise complexation and selective decomplexation of poly(ethylene imine) is presented.     

Optische Aktivität und Magnetrotation als Beiträge zur chemischen Bindung und zur Elektronenkonfiguration der 1. Periode des Systems der Elemente (1). Teil II

Zusammenfassung Es wird über die Synthesen der L,L-, D,D- und D,L-Formen des 1,4-Cyclo-Phe-digly-phe-digly, des D,D-1,3-Cyclo-D-Phe-gly-D-phe-trigly sowie der Dimethyl- und Dibenzylester der 1,4-L,L-Cyclo-glu-digly-glu-digly berichtet und die Reversibilit?t der zum Drehungsbetrage Null für den sichtbaren Bereich der optischen Aktivit?t beim übergang derα-trans-trans-Formen in die ebenen polarisierten elektronenisomerenβ-Formen besprochen, eine Beobachtung, die bisher an allen 1,4 symmetrisch substituierten L-L, und D,D-Formen gemacht wurde (vgl. insbesondere die diesbezüglichen Tyrosin-Typen). Das Drehungsverhalten wird mit dem übergang der tetraedrischen Asymmetriezentren inga-1,4-Stellung in die durch entgegengesetzte Spinanordnungen ausgezeichneten Proton-stabilisierten ebenen Biradikale an Cα ₁ und Cα ₄, begründet. — Die Beobachtungen an Cyclopeptiden in Verbindung mit früheren Befunden über dieα- undβ-Elektronenleitung lieferten Beitr?ge zur Elektronenanordnung im Diamant- und Graphitgitter, letztere haben als Prototypen aliphatischer bzw. aromatischer Verbindungen zu neuen Vorstellungen über die Verknüpfung der Elektronenkonfiguration mit der Raumstruktur, über Radikale und Onium-Verbindungen, über die C-C(Einfach)- und C=C(Doppel)-Bindung geführt. — Das Studium von SH/SS-Redox-systemen unter Beteiligung eines³⁵-markierten cyclischen Hexapeptiddisulfids lieferte einen wichtigenBeitrag zur Frage des Peptidkettenauf- und umbaus, wie auch von Cyclopeptiden z. B. vom Typ des Cyclo-L-his-digly-L-tyr-digly; durch Polarisation erzeugte metastabile, angeregte Zust?nde konnten nachgewiesen werden. – Schlie?lich werden die Halbleitereffekte von Polypeptiden, Fermentmodellen etc. besprochen und dieβ-überg?nge unter und oberhalb der Energieschwelle mit den Vorg?ngen in Gittern verknüpft. — Schlie?lich wird über dieα- undgb-Formen von Polarkomplexverbindungen sowie über polare Heterocyclen mit Dreipunktstruktur berichtet. Mit 10 Abbildungen in 34 Einzeldarstellungen Herrn Prof. Dr. F. Micheel zum 60. Geburtstag gewidmet.     

Separation of aminoalkanephosphonic acid enantiomers by indirect UV detection capillary electrophoresis with application of cyclodextrins

Indirect UV detection capillary electrophoresis (CE) was used for the separation of aminoalkanephosphonic acid (AP) enantiomers by applying commercially available cyclodextrins as chiral discriminators. The results show that the separation of the enantiomers depends on pH of the background electrolyte, the molar ratio of cyclodextrin to aminophosphonic acid, and on the type of the applied chiral selector. Optimization of process conditions allowed enantiomeric baseline separation or partial separation of 12 out of 14 alpha-aminophosphonic acids studied. This type of CE might therefore be successfully used for routine determination of enantiomeric purity of aminophosphonic acids.     

On spin dependent potentials in supersymmetric QCD

Starting from the supersymmetric extension of the nonabelian path dependent phase factor we try to get an understanding of the physical role of its components. We extend to SUSY-QCD (broken or unbroken) the technique of expanding the quark propagator in gauge field background in terms of inverse powers of quark mass which has been used earlier by Eichten and Feinberg to study spin dependent quark potentials in QCD. The contribution to the \(Q\bar Q\) potential induced by gluino exchange is expressed in terms of gluino insertions into the Wilson loop. A rough quantitative estimate is given on the basis of perturbation theory. Finally the extension to the \(S\bar Q\) and 3Q system is discussed.