On the electronic structure of the hydrogenase H-cluster

X-ray absorption spectroscopic measurements and density functional calculations suggest that the hydrogenase H-cluster is best described as an electronically inseparable 6Fe-cluster due to extensive delocalization of frontier molecular orbitals of the iron centres, sulfide and the non-innocent dithiolate ligands.     

Half-metallic digital ferromagnetic heterostructure composed of a delta-doped layer of Mn in Si

We propose and investigate the properties of a digital ferromagnetic heterostructure consisting of a delta-doped layer of Mn in Si, using ab initio electronic-structure methods. We find that (i) ferromagnetic order of the Mn layer is energetically favorable relative to antiferromagnetic, and (ii) the heterostructure is a two-dimensional half-metallic system. The metallic behavior is contributed by three majority-spin bands originating from hybridized Mn-d and nearest-neighbor Si-p states, and the corresponding carriers are responsible for the ferromagnetic order in the Mn layer. The minority-spin channel has a calculated semiconducting gap of 0.25 eV. The band lineup is found to be favorable for retaining the half-metal character to near the Curie temperature. This kind of heterostructure may be of special interest for integration into mature Si technologies for spintronic applications.     

Decay of pure quantum turbulence in superfluid 3He-B

We describe measurements of the decay of pure superfluid turbulence in superfluid 3He-B, in the low temperature regime where the normal fluid density is negligible. We follow the decay of the turbulence generated by a vibrating grid as detected by vibrating wire resonators. Despite the absence of any classical normal fluid dissipation processes, the decay is consistent with turbulence having the classical Kolmogorov energy spectrum and is remarkably similar to that measured in superfluid 4He at relatively high temperatures. Further, our results strongly suggest that the decay is governed by the superfluid circulation quantum rather than kinematic viscosity.     

Hydrocarbon chemistry : by G.A. Olah (C. Eaborn). J. Organomet. Chem., 334 (1987) C49-C50.

Sequential displacement of both N₂ ligands from cis-[Mo(N₂)₂(PMe₂Ph)₄] by CNMe occurs by a dissociative (I_d) mechanism (k₂/k₁～5,k₁ 0.020 min^?1 at 273 K) via the intermediate cis-[Mo(N₂)(CNMe)(PMe)₂Ph)₄] For t-BuNC substitution, the only detected intermediate appears to be [Mo(CNBu-t)(PMe₂Ph)₄] and no intermediate was detected in reactions of trans-[Mo(N₂)₂(Ph₂PCH₂CH₂PPh₂)₂] with CNMe. N₂ appears to be labilised by cis-ligands in the order t-BuNC > CNMe > N₂ > NCR.     

Sensitive method for the analysis of phospholipid subclasses and molecular species as 1-anthroyl derivatives of their diglycerides

A sensitive high-performance liquid chromatographic (HPLC) method for the separation and quantitation of phospholipid subclasses and molecular species has been developed. Phospholipids for analysis are hydrolyzed to the diradyl glycerols (DGs) with phospholipase C and the resulting DGs reacted with a molar excess of 1-anthroyl nitrile in the presence of quinuclidine or 4-dimethylaminopyridine to form a stable adduct. The anthroyl-DGs were separated into alkenylacyl, alkylacyl, and diacyl subclasses either by using normal-phase HPLC or by thin-layer chromatography on silica gel G plates. Molecular species within alkenylacyl, alkylacyl, and diacyl subclasses were separated using reversed-phase HPLC. Separation of the individual subclasses was achieved for ethanolamine phosphoglycerides from bovine brain, as well as choline and ethanolamine phosphoglycerides from human neutrophils. Separation and quantitation of individual molecular species were carried out for alkenylacyl, alkylacyl, and diacyl subclasses of bovine brain ethanolamine phosphoglycerides by their absorbance at 254 nm with correction for recoveries as normalized to the internal standard 1,2-dipentadecanoyl-3-phosphatidylcholine added before the hydrolysis of phospholipids with phospholipase C or 1,2-dipentadecanoyl-3-anthroyl glycerol added after complete derivatization. The extinction coefficient of the 1-anthroyl derivatives were greater than 68,000 permitting the generation of concentration-dependent determinations which were linear to less than 1 pmol when monitored at 254 nm. Thus, this procedure provides a new and very sensitive method for the quantitation of picomole quantities of phospholipids or DGs by HPLC techniques.     

Electrochemical cleavage of N=N bonds at a Mo2(mu-SMe)3 site relevant to the biological reduction of dinitrogen at a bimetallic sulfur centre

The reduction of diazene complexes [Mo(2)Cp(2)(mu-SMe)(3)(mu-eta(2)-H-N=N-R)](+) (R=Ph (3 a); Me (3 b)) and of the hydrazido(2-) derivative [Mo(2)Cp(2)(mu-SMe)(3)[mu-eta(1)-N=N(Me)H]](+) (1 b) has been studied by cyclic voltammetry, controlled-potential electrolysis, and coulometry in THF. The electrochemical reduction of 3 a in the presence of acid leads to cleavage of the N=N bond and produces aniline and either the amido complex [Mo(2)Cp(2)(mu-SMe)(3)(mu-NH(2))] 4 or the ammine complex [Mo(2)Cp(2)(mu-SMe)(3)(NH(3))(X)] 5, depending on the initial concentration of acid (HX=HTsO or CF(3)CO(2)H). The N=N bond of the methyldiazene analogue 3 b is not cleaved under the same conditions. The ability of 3 a but not 3 b to undergo reductive cleavage of the N=N bond is attributed to electronic control of the strength of the Mo-N(R) bond by the R group. The electrochemical reduction of the methylhydrazido(2-) compound 1 b in the presence of HX also results in cleavage of the N=N bond, with formation of methylamine, 4 (or 5) and the methyldiazenido complex [Mo(2)Cp(2)(mu-SMe)(3)(mu-eta(1)-N=N-Me)]. Formation of the last of these complexes indicates that two mechanisms (N=N bond cleavage and possibly H(2) production) are operative. A pathway for the reduction of N(2) at a dinuclear site of FeMoco is proposed on the basis of these results.     

Equilibrium behavior of confined triblock copolymer films

Using a two-dimensional self-consistent field calculation, we determine the equilibrium morphology of thin films of ABC triblock copolymers confined between hard, smooth plates. The B segment is chosen to be the central block and all the blocks are incompatible. The chains microphase-segregate into a lamellar phase, with the stripes either perpendicular or parallel to the walls. When all the monomer-surface interactions are identical, the perpendicular orientation has the lowest free energy. When a repulsion is introduced between the surface and the A and C monomers. The surface interactions further stabilize the perpendicular orientation. At strong surface interactions, the morphology of the perpendicular structure is controlled by the overall thickness of the molten layer. In comparing diblocks to triblocks as candidates for forming laterally patterned films, our work indicates that triblocks possess distinct advantages over diblocks. First, no special effort needs to be taken to establish neutral surfaces. Second, the film does not have to be confined between two substrates. Thus, triblocks can be used to fabricate patterned polymer surfaces, which can be used for novel optical or electronic applications.