An EPR and 1H NMR active mixed-valence manganese (III/II/III) trinuclear compound

A mixed-valence Mn(III)-Mn(II)-Mn(III) trinuclear complex of stoichiometry MnIIIMnIIMnIII(Hsaladhp)2(Sal)4.2CH3CN (1), where H3saladhp is a tridentate Schiff-base ligand, has been structurally characterized with X-ray crystallography. The Mn(III)Mn(II)Mn(III) angles are strictly 180 degrees as required by crystallographic inversion symmetry. The complex is valence-trapped with two terminal Mn(III) ions in a distorted square pyramidal geometry. The Mn(III)...Mn(II) separation is 3.495 A. The trinuclear complex shows small antiferromagnetic exchange J coupling. The magnetic parameters obtained from the fitting procedure in the temperature range 10-300 K are J1 = -5.7 cm-1, g = 2.02, zJ = -0.19 cm-1, and R = 0.004. The EPR spectrum was obtained at 4 K in CHCl3 and in tetrahydrofuran glasses. The low-field EPR signal is a superposition of two signals, one centered around g = 3.6 and the other, for which hyperfine structure is observed, centered around g = 4.1 indicating an S = 3/2 state. In addition, there is a 19-line signal at g = 2.0. The multiline signal compares well with that observed for the S2 or S0* states of the oxygen-evolving complex. 1H NMR data reveal that the trinuclear compound keeps its integrity into the CHCl3 solution. Crystal data for complex 1: [C54H52N4O18Mn3], M = 1209.82, triclinic, space group P1, a = 10.367(6) A, b = 11.369(6) A, c = 13.967(8) A; alpha = 112.56(1) degree, beta = 93.42(2) degrees, gamma = 115.43(1) degree, Z = 1.     

Copper inverse-9-metallacrown-3 compounds showing antisymmetric magnetic behaviour

The use of phenyl 2-pyridyl ketoxime (PhPyCNO)/X(-)"blends" (X(-) = OH(-), Cl(-), ClO(4)(-)) in copper chemistry has yielded trinuclear clusters that have been characterized as inverse-9-metallacrown-3 accommodating one or two anions. The magnetic behaviour has shown a large antiferromagnetic interaction. The discrepancy between the Brillouin curve and the experiment has been assigned to the influence of the antisymmetric interaction. By introducing in the magnetization formula the antisymmetric terms derived from the fitting of the susceptibility data the simulated curve become almost superimposable to the experimental one. From the EPR findings it has been shown that the compound [Cu(3)(PhPyCNO)(3)(OCH(3))(Cl)(ClO(4))] () having isosceles magnetic symmetry or lower (delta not equal 0), the antisymmetric exchange is important (G not equal 0) and DeltaE > hv. The structures of the two complexes have been determined by single-crystal X-ray crystallography.     

An analytical and experimental comparison of optimal actuator and error sensor location for vibration attenuation

Feedforward active control of the flexural waves in a single and L-shaped plate has been analytically and experimentally investigated. The plates are simply supported along two parallel edges, and free at the other two ends. Point forces were used to generate the primary and secondary plate excitations. The plate flexural displacement is described by a combination of a travelling wave solution and a modal expansion. The flexural wave coefficients were determined using the boundary conditions, continuity equations at the driving force locations, and continuity equations at the corner junction for the L-shaped plate. The control actuator and error sensor are optimally located in order to achieve the best control performance.     

Scale size of magnetic turbulence in tokamaks probed with 30-MeV electrons

Measurements of synchrotron radiation emitted by 30-MeV runaway electrons in the TEXTOR-94 tokamak show that the runaway population decays after switching on neutral beam injection (NBI). The decay starts only with a significant delay, which decreases with increasing NBI heating power. This delay provides direct evidence of the energy dependence of runaway confinement, which is expected if magnetic modes govern the loss of runaways. Application of the theory by Mynick and Strachan [Phys. Fluids 24, 695 (1981)] yields estimates for the "mode width" (delta) of magnetic perturbations: delta<0.5 cm in Ohmic discharges, increasing to delta = 4.4 cm for 0. 6 MW NBI.     

A cationic 24-MC-8 manganese cluster with ring metals possessing three oxidation states [Mn(II)4Mn(III)6Mn(IV)2(mu4-O)2(mu3-O)4(mu3-OH)4(mu3-OCH3)2(pko)12](OH)(ClO4)3

Reaction of Mn(ClO4)2 with di-pyridyl ketone oxime, (2-py)2C=NOH, gives the novel cluster [Mn(II)4Mn(III)6Mn(IV)2(mu4-O)2(mu3-O)4(mu3-OH)4(mu3-OCH3)2(pko)12](OH)(ClO4)3 1. It is the only example of a 24-MC-8, and the first metallacrown with ring metal ions in three different oxidation states. Magnetic measurements show antiferromagnetic behavior.     

Numerical prediction of turbulent boundary layer noise from a sharp-edged flat plate

An efficient hybrid uncorrelated wall plane waves–boundary element method (UWPW-BEM) technique is proposed to predict the flow-induced noise from a structure in low Mach number turbulent flow. Reynolds-averaged Navier-Stokes equations are used to estimate the turbulent boundary layer parameters such as convective velocity, boundary layer thickness, and wall shear stress over the surface of the structure. The spectrum of the wall pressure fluctuations is evaluated from the turbulent boundary layer parameters and by using semi-empirical models from literature. The wall pressure field underneath the turbulent boundary layer is synthesized by realizations of uncorrelated wall plane waves (UWPW). An acoustic BEM solver is then employed to compute the acoustic pressure scattered by the structure from the synthesized wall pressure field. Finally, the acoustic response of the structure in turbulent flow is obtained as an ensemble average of the acoustic pressures due to all realizations of uncorrelated plane waves. To demonstrate the hybrid UWPW-BEM approach, the self-noise generated by a flat plate in turbulent flow with Reynolds number based on chord Re_c = 4.9 × 10⁵ is predicted. The results are compared with those obtained from a large eddy simulation (LES)-BEM technique as well as with experimental data from literature.     

Acoustic signature of a submarine hull under harmonic excitation

The structural and acoustic responses of a submarine under harmonic force excitation are presented. The submarine hull is modelled as a cylindrical shell with internal bulkheads and ring stiffeners. The cylindrical shell is closed by truncated conical shells, which in turn are closed at each end using circular plates. The entire structure is submerged in a heavy fluid medium. The structural responses of the submerged vessel are calculated by solving the cylindrical shell equations of motion using a wave approach and the conical shell equations with a power series solution. The far-field radiated sound pressure is then calculated by means of the Helmholtz integral. The contribution of the conical end closures on the radiated sound pressure for the lowest circumferential mode numbers is clearly observed. Results from the analytical model are compared with computational results from a fully coupled finite element/boundary element model.     

Free vibrational characteristics of isotropic coupled cylindrical-conical shells

This paper presents the free vibrational characteristics of isotropic coupled conical-cylindrical shells. The equations of motion for the cylindrical and conical shells are solved using two different methods. A wave solution is used to describe the displacements of the cylindrical shell, while the displacements of the conical sections are solved using a power series solution. Both Donnell-Mushtari and Flügge equations of motion are used and the limitations associated with each thin shell theory are discussed. Natural frequencies are presented for different boundary conditions. The effect of the boundary conditions and the influence of the semi-vertex cone angle are described. The results from the theoretical model presented here are compared with those obtained by previous researchers and from a finite element model.