Axisymmetric cellular structures revisited

For an axisymmetric cellular structure of wavenumberq ₀, the amplitude of the first linearly unstable mode is proportional to the Bessel functionJ ₀(q ₀r). If the radius of the structure is much larger thanq ₀ ^–1 , the first mode has a large relative maximum near the center. The nonlinear saturation of this instability gives an amplitude of order ^1/2, being the small growth rate of the linear instability. The saturation amplitude near the center is of order ^1/4. This prediction is confirmed by numerical analysis and numbers are given to make possible a comparison with the Rayleigh-Bénard instability.

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Resumé Pour une structure cellulaire axisymétrique de nombre d'ondeq ₀, l'amplitude du premier mode instable est proportioneile á la fonction de BesselJ ₀(q ₀r). Si le rayon de la structure est très supérieur àq ₀ ^–1 ce premier mode a un grand maximum relatif au centre. La saturation non linéaire de cette instabilité conduit à une amplitude en ^1/2 ( taux de croissance-petit-de l'instabilité linéaire). L'amplitude de saturation au centre est d'ordre ^1/4. Cette prédiction est confirmée par l'analyse numérique, et les nombres nécessaires à la comparaison avec l'instabilité de Rayleigh-Bénard sont donnés.

     

Phase separation in a three-dimensional,two-phase,hydrodynamic lattice gas

The dynamics of phase separation is explored using an immiscible 3D lattice-gas model. Scaling laws for the growth rate and power spectraS(k) of the growth patterns are computed. For small wavenumbersS(k) shows a crossover fromk ² tok ⁴ behavior. The theoretical prediction for the asymptotic domain growthRt ^2/3 is supported by our results. We discuss the possibility to observe an intermediatet scaling. We show the influence of hydrodynamic forces in symmetric and asymmetric mixtures by comparing simulations with and without momentum conservation. The structure functionS(k) is not significantly modified by hydrodynamics, but the growth rate changes clearly. As a general result, it is shown that, in spite of the unusual thermodynamics of this model, many characteristics of the growth dynamics are surprisingly in agreement with the classical theoretical and experimental results.     

Characterization of the Ni(III) intermediate in the reaction of (1,4,8,11-tetraazacyclotetradecane)nickel(II) perchlorate with KHSO5: implications to the mechanism of oxidative DNA modification

We report the detection and characterization of the Ni(III) intermediates generated by reaction of (1,4,8,11-tetraazacyclotetradecane)nickel(II) perchlorate with KHSO5. Four Ni(III) intermediates can be trapped or detected through variation in Cl- or KHSO5 concentrations. Upon oxidation of [Ni(cyclam)]2+ by 2.5 equiv of KHSO5, deprotonation of the cyclam ligand generates two red Ni(III) species with lambda max = 530 nm and g perpendicular = 2.20 and g parallel = 2.02 or g perpendicular = 2.16 and g parallel = 2.01 for the axial 4-coordinate or 6-coordinate dichloride species, respectively. These forms decay to Ni(II) products via complex ligand oxidation mechanisms. The Ni(III) dichloride species can be reprotonated and subsequently binds to DNA via an outer-sphere interaction as evidenced by the inverted sign of the CD signal near 400 nm. Cumulatively, the results indicate that the Ni(III) center is coordinately saturated under excess chloride conditions but is still able to interact with DNA substrates. This suggests alternative mechanistic pathways for DNA modification by reaction of [Ni(cyclam)]2+ with KHSO5 and possibly other Ni(II) complexes as well.     

Spectroscopic and electronic structure studies of protocatechuate 3,4-dioxygenase: nature of tyrosinate-Fe(III) bonds and their contribution to reactivity.

The geometric and electronic structure of the high-spin ferric active site of protocatechuate 3,4-dioxygenase (3,4-PCD) has been examined by absorption (Abs), circular dichroism (CD), magnetic CD (MCD), and variable-temperature-variable-field (VTVH) MCD spectroscopies. Density functional (DFT) and INDO/S-CI molecular orbital calculations provide complementary insight into the electronic structure of 3,4-PCD and allow an experimentally calibrated bonding scheme to be developed. Abs, CD, and MCD indicate that there are at least seven transitions below 35 000 cm(-1) which arise from tyrosinate ligand-to-metal-charge transfer (LMCT) transitions. VTVH MCD spectroscopy gives the polarizations of these LMCT bands in the principal axis system of the D-tensor, which is oriented relative to the molecular structure from the INDO/S-CI calculations. Three transitions are associated with the equatorial tyrosinate and four with the axial tyrosinate. This large number of transitions per tyrosinate is due to the pi and importantly the sigma overlap of the two tyrosinate valence orbitals with the metal d orbitals and is governed by the Fe-O-C angle and the Fe-O-C-C dihedral angles. The previously reported crystal structure indicates that the Fe-O-C angles are 133 degrees and 148 degrees for the equatorial and axial tyrosinate, respectively. Each tyrosinate has transitions at different energies with different intensities, which correlate with differences in geometry that reflect pseudo-sigma bonding to the Fe(III) and relate to reactivity. These factors reflect the metal-ligand bond strength and indicate that the axial tyrosinate-Fe(III) bond is weaker than the equatorial tyrosinate-Fe(III) bond. Furthermore, it is found that the differences in geometry, and hence electronic structure, are imposed by the protein. The consequences to catalysis are significant because the axial tyrosinate has been shown to dissociate upon substrate binding and the equatorial tyrosinate in the enzyme-substrate complex is thought to influence asymmetric binding of the chelated substrate moiety via a strong trans influence which activates the substrate for reaction with O2.     

The dielectric function of LnSF rare-earth fluorosulfides (Ln=La, Ce): experiment and theory

The absorption properties of LaSF and CeSF in the UV-Visible range have been investigated on the basis of first principles density functional theory (DFT) calculations and from electron energy-loss spectroscopy (EELS) measurements. The extinction coefficient k, as well as the refractive index n, determined experimentally from the loss function Im(−1/ε), were compared with the corresponding factors extracted from the calculated dielectric tensor ε₂. The k and n values for the two compounds were expected to be very close to each other, owing to the chemical similarity of La and Ce. However, it was found that the nature of the electronic transitions in LaSF and CeSF strongly influences the k and n values with the result that the refractive index n and the extinction coefficient k are substantially larger for LaSF than for CeSF.     

The Quest for Single-Source Precursors for BaTiO3 and SrTiO3

The reactions between titanium alkoxides Ti(OR)₄ (R = Et,i Pr) and strontium -diketonates Sr(-dik)₂ (-dik = thd, acac) were investigated. The various Sr-Ti species, Sr₂Ti₂(-dik)₄ (OR)₈, have a 1:1 Sr:Ti stoichiometry and were characterized by elemental analysis, FT-IR and by single-crystal X-ray diffraction for Sr₂Ti₂(₃-OiPr)₂ (-OiPr)₄ (OiPr)₂(thd)₄ (1). The hydrolysis-polycondensation reactions of the various species were investigated and the resulting powders analyzed by light scattering and XRD. While acetone was found to have little influence on the hydrolysis reactions of the Sr-Ti species, polycondensation of Ti(OiPr)₄ in neat acetone offers a trinuclear enolate Ti(₃-O)₂(OCMe=CH₂)₃ (OiPr)₅(iPrOH) (4). Comparisons between the Ba-Ti and Sr-Ti systems are given.     

Synthesis and structural characterization of some titanium butoxides modified with chloroacetic acids

Reactions of Ti(OBu^s)₄ and Ti(OBuⁿ)₄ with chloroacetic acids (mono-, di-, and tri-) were carried out in toluene in 1:1 and 1:2 molar ratios at room temperature to generate new precursors of titania. These modified alkoxides/oxo-alkoxides were characterized by spectroscopic methods. A solution of Ti(OBuⁿ)₂(OOCCCl₃)₂ in toluene left for crystallization at ?30 °C yielded single crystals of the product Ti₆(μ₂-O)₂(μ₃-O)₂(μ₂-OC₄H₉)₂(OC₄H₉)₆(OOCCCl₃)₈. Single-crystal X-ray diffraction revealed the molecular structure to be composed of a hexanuclear unit in which two Ti₂O₁₀ units (made by two edge-sharing octahedra) are linked with two corner-sharing octahedra.     

Spontaneous surface magnetisation of single crystal MnF2 in the antiferromagnetic state

Spontaneous magnetisation of (100) and (010) surfaces of single crystal MnF₂ in the antiferromagnetic state has been discovered. The sign of the surface magnetisation is determined by the difference in dielectric constants of MnF₂ and ambient matter: magnetisation is directed to the substance with smaller . Received 28 August 1998 and Received in final form 15 December 1998