Analytical Investigations of Bacterial Cellulose

The cultivation of the bacterium Acetobacter xylinus AX 5 was carried out in the common Hestrin-Schramm medium containing D -glucose as C-source and citric acid as buffer component. HPLC studies proved to be convenient methods to investigate the stability and interactions of these constituents in the starting culture liquid. Within the initial sterilization step and limited by the citric acid, up to 6% of the D -glucose was partially isomerized to D-fructose and degraded to dark-yellow products. In static culture, A. xylinus AX 5 produces cellulose extracellularly on the surface of this medium. Solid-state NMR spectroscopy represents a suitable analytical method to characterize the supramolecular structure of the bacterial cellulose in never-dried, air-dried, and freeze-dried states. It could be demonstrated that the drying process reduces the degree of crystallinity in the range of about 12% without changes in the Iα/β ratio of these cellulose modifications.     

Manipulation of Cu adatoms on anisotropic Cu surfaces using scanning tunneling microscopy

The physical origin of tip-induced motion of Cu adatoms on anisotropic Cu surfaces is investigated by means of total energy calculations which are based on three different semi-empirical potentials. The calculations show that for certain tip–adatom distances the activation barrier for the adatom to move towards the tip disappears completely, whereas the barrier in the opposite direction increases and the adatom experiences an attractive force towards the tip. The general trends do not depend on the shape and chemical nature of the tip, but quantitatively there appear differences.     

Aluminiumorganyle mit pentakoordiniertem Aluminium-Atom: Synthese und Molekülstrukturen von [AlX2{2,6-(NEt2CH2)2C6H3}] (X = Cl,Et, H)

Aluminium Organyls with Pentacoordinate Aluminium: Syntheses and Molecular Structures of [AlX₂{2,6-(NEt₂CH₂)₂C₆H₃}] (X = Cl, Et, H) The reaction of [Li{2,6-(NEt₂CH₂)₂C₆H₃}]₂ with AlCl₃ or Et₂AlCl gives [AlX₂{2,6-(NEt₂CH₂)₂C₆H₃}] [X = Cl ( 1 ), Et ( 2 )] in good yield. 1 reacts with NaH in toluene to give [AlH₂{2,6-(NEt₂CH₂)₂C₆H₃}] ( 3 ). 1–3 were characterised spectroscopically (¹H, ¹³C, ²⁷Al n.m.r., i.r., mass spectroscopy). In solution at room temperature 1–3 exhibit dynamic behaviour. For 1 and 3 this can be frozen out below 278 K (¹H n.m.r.), indicating the presence of monomeric molecules with pentacoordinate Al at low temperature. Such species are also observed in the solid state as shown by an X-ray structure determination on 1 (monoclinic space group P2₁/n, a = 9.7325(14), b = 13.552(5), c = 28.858(7) Å, β = 99.57(2)°, V = 3753(2) ų, Z = 8, at 223(2) K) and 2 (monoclinic space group C2/c, a = 15.0045(12), b = 9.2986(8), c = 14.9955(12) Å, β =99.512(1)°, Z = 4, at 223(2) K).     

Phosphaniminato‐Cluster von Eisen. Die Kristallstrukturen von [FeCl(NPEt3)]4, [Fe(C=C–SiMe3)(NPEt3)]4 und [Fe3Cl4{NP(NMe2)3}3]

Phosphoraneiminato Cluster of Iron. The Crystal Structures of [FeCl(NPEt₃)]₄, [Fe(C=C–SiMe₃)(NPEt₃)]₄, and [Fe₃Cl₄{NP(NMe₂)₃}₃] The reaction of iron dichloride with the silylated phosphaneimine Me₃SiNPEt₃ in the presence of potassium fluoride at 165 ?C leads to the phosphoraneiminato complex [FeCl(NPEt₃)]₄ ( 1 ). Compound 1 forms black, moisture and oxygen sensitive crystals. According to the crystal structure analysis 1 has a heterocubane structure, in which the iron and the nitrogen atoms of the NPEt₃^– groups occupy the corners of a distorted cube and form Fe–N–Fe bond angles of 83.1? and N–Fe–N angles of 96.5?. This results in significantly short Fe…Fe contacts of 272.9 pm. The results of magnetic susceptibility measurements in the range of temperatures from 1.8 to 293 K and the ⁵⁷Fe‐Mössbauer spectra in the range of temperatures from 2 to 300 K are reported. Compound 1 reacts with the lithiated acetylenes LiC=C–CMe₃ and LiC=C–SiMe₃ in n‐hexane to form the iron‐organic derivatives [Fe(C=C–R)(NPEt₃)]₄ [R = CMe₃ ( 2 a ), R = SiMe₃ ( 2 b )] keeping the heterocubane structure. Compounds 2 a and 2 b form crystals which are very reactive and also black. According to the crystal structure analysis 2 b has a Fe₄N₄ heterocubane structure which is less distorted than that in 1 with bond angles Fe–N–Fe of 85.5? and N–Fe–N of 94.2?. This leads to the longer Fe…Fe contacts of 281.4 pm. With the dimethylamido derivative Me₃SiNP(NMe₂)₃ iron dichloride reacts under conditions similar to those in the synthesis of 1 to form the dark green mixed‐valenced Fe^II/Fe^III cluster [Fe₃Cl₄{NP(NMe₂)₃}₃] ( 3 ). According to the crystal structure analysis the three iron atoms in 3 are connected via one μ₃‐N atom of a NP(NMe₂)₃^– ligand, via two μ‐N atoms of the two remaining phosphoraneiminato ligands, and via one μ‐Cl atom to form an incomplete heterocubane skeleton.     

Phosphoraneiminato Complexes of Manganese and Cobalt with Heterocubane Structure

The phosphoraneiminato complexes [MnBr(NPEt₃)]₄ ( 1 ) and [M₄Br₅{NP(NMe₂)₃}₃] with M = Mn ( 2 ) and M = Co ( 3 ) are prepared by melting reactions from the anhydrous metal dibromides with the silylated phosphaneimines Me₃SiNPR₃ (R = Et, NMe₂) in the presence of potassium fluoride. All complexes are characterized by crystal structure analyses. 1 forms an only slightly distorted Mn₄N₄ heterocubane skeleton with an approximate T_d symmetry and short Mn…Mn distances of average 295.7 pm. In the structures of 2 and 3 one μ₃-NPEt₃ ligand of 1 is replaced by one bromine atom with μ₃-function. This leads to the novel heterocubane type M₄N₃Br with approximate C₃ symmetry. The deformation of the cubic skeleton leads to metal-metal distances of different lengths, i. e. 292.7 and 323.6 pm in 2 and 274.4 and 306.2 pm in 3 . Temperature dependent magnetic susceptibility measurements between 300 and 5 K on 1 have shown that strong antiferromagnetic coupling exists between the Mn(II) ions with S = 5/2, with a large negative Weiss constant of Θ = –694 K.     

Formaldehyde Formation on Vanadium Oxide Surfaces V2O3(0001) and V2O5(001): How does the Stable Methoxy Intermediate Form?

Hydroxy‐mediated methoxy formation or stabilization is probably an important process in many methanol adsorption systems. Hydrogen atoms originating from the scission of the methanol O? H bond react with the substrate and form water. This process may result 1) in the production of additional surface defects as reactive centers for methoxy formation and 2) in the stabilization of methoxy groups by suppression of methanol formation.

     

Dirichlet forms for singular one-dimensional operators and on graphs

We treat the time evolution of states on a finite directed graph, with singular diffusion on the edges of the graph and glueing conditions at the vertices. The operator driving the evolution is obtained by the method of quadratic forms on a suitable Hilbert space. Using the Beurling–Deny criteria we describe glueing conditions leading to positive and to submarkovian semigroups, respectively.