High-pressure synthesis and study of the crystal and magnetic structure of the distorted SeNiO3 and SeMnO3 perovskites

We describe the preparation of SeMO(3) (M = Ni, Mn) under high pressure conditions (3.5 GPa), starting from reactive H(2)SeO(3) and MO mixtures, contained in sealed gold capsules under the reaction conditions 850 degrees C for 1 h. The polycrystalline samples have been studied by neutron powder diffraction (NPD) data and magnetization measurements. SeMO(3) (M = Ni, Mn) are orthorhombically distorted perovskites (space group Pnma). Below T(N) approximately 104 K (M = Ni) and T(N) approximately 53.5 K (M = Mn) these oxides experience an antiferromagnetic ordering, as demonstrated by susceptibility and NPD measurements. The magnetic reflections observed in the neutron patterns can be indexed with a propagation vector k = 0. Both compounds present the same magnetic structure, which is given by the basis vector (0, 0, A(z)). It can be described as antiferromagnetic (010) layers of magnetic moments lying along the c direction, which are antiferromagnetically coupled along the b direction. For the Ni(2+) ions, the ordered magnetic moment at T = 2.3 K is 2.11(3) micro(B), whereas for Mn(2+) at T = 2.6 K, |m| = 4.64(2) micro(B), consistent with the electronic configurations te (Ni(2+)) and te (Mn(2+)).     

Theoretical study of bimetallic magnetic nanostructures: Co<Subscript>n</Subscript>Pd<Subscript>N-n</Subscript>, n=0,1,...N,N=3,5,7,13

The geometrical structure, atomic ordering, electronic and magnetic properties of closed shell Co_n Pd_N-n for n=0, 1, ...,N, and N=3, 5, 7 and 13, have been studied. The whole set of values for N and n, as well as most of the isomers were analyzed. The electronic, magnetic, and structural characteristics were calculated within the density functional theory by means of the SIESTA code using the generalized gradient approximation of Perdew-Burke-Ernzerhof for the exchange and correlation potential. The lowest energy geometries obtained were the triangle, hexahedron, decahedron, and icosahedron. We also performed calculations assuming noncollinear magnetic moment arrangements, but the lowest energy configurations correspond to the collinear geometry. Finally we analyze in detail the chemical order between the two components in the cluster and its influence in the magnetic order.     

Reactions of TiX4 and VOCl3 with P=S ligands

Summary The reactions of TiX₄ (X=Cl or Br) with the bidentate ligands, L, (Figure 1), yield hexacoordinate complexes TiX₄·L, whereas similar reactions with VOCl₃ lead to reduction of vanadium and give rise to vanadium(IV) pentacoordinate complexes, VOCl₂·L. All the compounds have been characterized by elemental analyses, i.r., visible and e.p.r. spectra. The occupation of the sixth coordination position in pentacoordinate complexes, VOCl₄·L, by different donor solvents, has been studied by means of visible and e.p.r. spectra.     

Alkyl and cyclic substituted thiuram disulphides as ligands. titanium(iv), vanadium(iv), and oxovanadium(iv) complexes

Summary Titanium(IV) hexacoordinate thiuram disulphide complexes of the type [TiX₄R₂NC(S)SSC(S)NR₂}] (X=Cl or Br; R=Me, Et, piperidinyl or morpholinyl) have been prepared by reaction between TiX₄ (X=Cl or Br) and the thiuram disulphide. Similar reactions with VOCl₃ lead to reduction of vanadium(V) and give rise to the oxovanadium(IV) pentacoordinate complexes [VOCl₂{R₂NC(S)SSC(S)NR₂}]. However, the reactions of these same thiuram disulphide ligands with [VCl₂(THF)₂] (THF=tetrahydrofuran) cause oxidation of vanadium and to the reduction of the disulphide to the corresponding dithiocarbamate [R₂NCS₂]^–, resulting in new dichlorobis (dithiocarbamate)vanadium(IV) complexes [VCl₂-(R₂NCS₂)₂]. All the compounds have been characterized by elemental analyses, i.r., visible and e.p.r. spectra. Both thiuram disulphides and dithiocarbamate ligands exhibit bidentate     

Energetics for the new forms of carbon-clusters

The growth of closedC _n -structures like fullerenes, shelled fullerenes, tubules and capped tubules controlled by the interplay of surface- and bending-energy is studied. Tubules are less stable than corresponding fullerenessC _n , and these are forn>n _c less stable than shelled fullerenes. Growth of fullerenes from graphite sheets requires bond breaking and bond rearrangement to form pentagons and finite temperatures to overcome energy barriers. Thermodynamical arguments are used to discuss the temperature- and size-dependence of the formation of the new forms of carbon. We argue that trapping of foreign atoms or molecules inside the closed structure may be achieved most efficiently by mixing these with carbon clusters before caging occurs.     

Automated determination of sugar in molasses

The importance of determining sugars in molasses is emphasized, and a short critical review of the methods available is presented. The difficulties and sources of errors in determining total sugar levels in molasses and sugar mixtures are discussed. A continuous automated procedure based on the hexacyanoferrate(III)/(II) redox reaction has been optimized by reducing the errors and interferences to a minimum. With this technique, sugar concentrations of 0–10 gl^-1 can be determined directly.     

QM/MM modelling of oleamide hydrolysis in fatty acid amide hydrolase (FAAH) reveals a new mechanism of nucleophile activation

Fatty acid amide hydrolase (FAAH), a promising target for the treatment of several central and peripheral nervous system disorders, such as anxiety, pain and hypertension, has an unusual catalytic site, and its mechanism has been uncertain; hybrid quantum mechanics/molecular mechanics (QM/MM) calculations reveal a new mechanism of nucleophile activation (involving a Lys-Ser-Ser catalytic triad), with potentially crucial insights for the design of potent and selective inhibitors.     

Noise-controlled self-replicating patterns

We present novel numerical evidence of spot self-replication controlled by noise in a simple autocatalytic reaction-diffusion system. The system dynamics exhibits a noise controlled transition from stripe growth to spot replication. The growth kinetics is also controlled by noise, and there is an optimal noise intensity for which the multiplication rate of spots is maximal. For larger noise intensities, the spots become unstable and the system is attracted by the trivial steady state. Some of the effects are reminiscent of both polymer chain and cell colony formation in random environments.     

Raman modes of index-identified freestanding single-walled carbon nanotubes

Using electron diffraction on freestanding single-walled carbon nanotubes, we have determined the structural indices (n,m) of tubes in the diameter range from 1.4 to 3 nm. On the same freestanding tubes, we have recorded Raman spectra of the tangential modes and the radial breathing mode. For the smaller diameters (1.4-1.7 nm), these measurements confirm previously established radial breathing mode frequency versus diameter relations and would be consistent with the theoretically predicted proportionality to the inverse diameter. However, for extending the relation to larger diameters, either a yet unexplained environmental constant has to be assumed, or the linear relation has to be abandoned.