Control of molecular architecture by steric and electronic factors: dinuclear side-by-side vs. tetranuclear [2 x 2] grid-type silver(I) complexes

Silver(I) complexes of the two closely related linear bis-bidentate pyridazine-based Schiff-base ligands 5 and 6 exhibit very different solid state molecular architectures, easily deformed grid vs. side-by-side, due to steric and electronic factors.     

On nonlinear preconditioners in Newton–Krylov methods for unsteady flows

The application of nonlinear schemes like dual time stepping as preconditioners in matrix‐free Newton–Krylov‐solvers is considered and analyzed, with a special emphasis on unsteady viscous flows. We provide a novel formulation of the left preconditioned operator that says it is in fact linear in the matrix‐free sense, but changes the Newton scheme. This allows to get some insight in the convergence properties of these schemes, which is demonstrated through numerical results. Copyright © 2009 John Wiley & Sons, Ltd.     

A note on terminology in multigrid methods

We compare terminology used in the literature on multigrid methods for compressible computational fluid dynamics to that used in linear multigrid theory. Several popular iterative and direct smoothers are presented side-by-side using the same terminology. We argue for greater analysis of these methods in order to place them into a more rigorous framework and to identify the most promising candidates for future development. (© 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim)     

Two‐dimensional implicit time‐dependent calculations on adaptive unstructured meshes with time evolving boundaries

An implicit multigrid‐driven algorithm for two‐dimensional incompressible laminar viscous flows has been coupled with a solution adaptation method and a mesh movement method for boundary movement. Time‐dependent calculations are performed implicitly by regarding each time step as a steady‐state problem in pseudo‐time. The method of artificial compressibility is used to solve the flow equations. The solution mesh adaptation method performs local mesh refinement using an incremental Delaunay algorithm and mesh coarsening by means of edge collapse. Mesh movement is achieved by modeling the computational domain as an elastic solid and solving the equilibrium equations for the stress field. The solution adaptation method has been validated by comparison with experimental results and other computational results for low Reynolds number flow over a shedding circular cylinder. Preliminary validation of the mesh movement method has been demonstrated by a comparison with experimental results of an oscillating airfoil and with computational results for an oscillating cylinder. Copyright © 2005 John Wiley & Sons, Ltd.     

Uranium-sulfilimine chemistry. Hydrolysis of Cp*2UCl2 with HNSPh2 · H2O and the crystal structure of Cp*2UCl(OH)(HNSPh2), a metallocene terminal hydroxy complex of tetravalent uranium

The reaction of Cp*₂UCl₂(HNSPh₂) with HNSPh₂ · H₂O in 1:1 stoichiometry, produces Cp*₂UCl(OH)(HNSPh₂) in good yield. This is the first structurally characterized metallocene f-element complex containing a terminal hydroxy ligand. Cp*₂UCl(OH)(HNSPh₂) is an intermediate in the formation of a tetra uranium-oxo-cluster [Cp^*(Cl)(HNSPh₂)U(μ₃-O)(μ₂-O)₂U(Cl)(HNSPh₂)₂]₂, which forms by hydrolysis of Cp*₂UCl₂ with excess HNSPh₂ · H₂O.     

Two dinuclear iron(II) complexes K[Fe2(L1)(SCN)4]·2(C3H8O) and [Fe2(L1)(SeCN)3(C5H5N)]·H2O are stabilised in the '[HS-LS]' state by a bis-tetradentate pyrazolate-based ligand

Two air-sensitive dinuclear iron(II) complexes, K[Fe(II)(2)(L(1))(SCN)(4)]·2(C(3)H(8)O) (1) and [Fe(II)(2)(L(1))(SeCN)(3)(C(5)H(5)N)]·H(2)O (2), of 3,5-bis[N,N-bis(2-pyridylmethyl)aminomethyl]-1H-pyrazolate [(L(1))(-)] have been prepared. Interestingly, complex 1 is anionic, featuring four coordinated SCN(-) anions and a potassium counterion whereas complex 2 is neutral, containing a coordinated pyridine molecule and only three coordinated SeCN(-) anions. These are the first iron complexes reported for this type of ligand. Magnetic measurements and M?ssbauer spectra show that both 1 and 2 are in a '[HS-LS]' mixed spin state between 300 and 2 K.     

Sequence-specific B-DNA flexibility modulates Z-DNA formation

Conversion of right-handed B-DNA into left-handed Z-DNA is one of the largest structural transitions in biology that plays fundamental roles in gene expression and regulation. Z-DNA segments must form within genomes surrounded by a sea of B-DNA and require creation of energetically costly B/Z junctions. Here, we show using a combination of natural abundance NMR R(1ρ) carbon relaxation measurements and CD spectroscopy that sequence-specific B-DNA flexibility modulates the thermodynamic propensity to form Z-DNA and the location of B/Z junctions. We observe sequence-specific flexibility in B-DNA spanning fast (ps-ns) and slow (μs-ms) time scales localized at the site of B/Z junction formation. Further, our studies show that CG-repeats play an active role tuning this intrinsic B-DNA flexibility. Taken together, our results suggest that sequence-specific B-DNA flexibility may provide a mechanism for defining the length and location of Z-DNA in genomes.     

Is there a spatial correlation in the distribution of adsorbed atoms in the cages of a zeolite?

Monte Carlo simulations of the cage-to-cage jumps of Xe atoms in a crystalline zeolite using the experimentally observed molecular rate constants for cage-to-cage jumps were carried out to determine if there is a systematic spatial correlation of the distributions of Xe atoms among the cages at equilibrium. The neighbours of cages having an Xe occupancy that is less than the average occupancy are found to have distributions that are skewed toward higher occupancy compared with the overall distribution. On the other hand, the neighbours of cages having an Xe occupancy that is greater than the average occupancy are found to have distributions that are skewed toward lower occupancy than the overall distribution.     

Congruences for Broken k-Diamond Partitions

We prove two conjectures of Paule and Radu from their recent paper on broken k-diamond partitions.     

Density dependence of 129Xe N.M.R. chemical shifts in O2 and NO

For neutral species it is possible to choose a gauge in which the molecular hamiltonian is translationally invariant. Here it is shown that for charged species this is not entirely possible, but that, if molecular coordinates are employed, the lack of invariance may be confined to the translational part of the hamiltonian.