Antimony-121 Mössbauer spectral study of alpha-Zn4Sb3

The M?ssbauer spectra of alpha-Zn4Sb3, a compound that is best formulated as alpha-Zn13Sb10 or (Zn2+)13(Sb3-)6(Sb24-)2, have been measured between 5 and 120 K. The resulting six spectra have been simultaneously fit with two components in the ratio of 3:2 corresponding to the Sb3- and Sb2- ions identified in this valence semiconductor. The fits yield temperature independent isomer shifts of -8.17(2) and -9.73(2) mm/s and quadrupole interactions of -4.9(2) and 0 mm/s for the Sb3- and Sb2- ions, respectively; the corresponding M?ssbauer temperatures are 197(5) and 207(5) K, temperatures that are lower than the Debye temperature of beta-Zn4Sb3. The isomer shifts correspond to electronic configurations between 5s25p6 and 5s1.755p4.01 for the Sb3- ions and between 5s25p5 and 5s1.805p3.38 for the Sb2- ions, configurations that are in good agreement with the expected configurations for this valence semiconductor and with the results of band structure calculations.     

The Picard group of the forms of the affine line and of the additive group

We obtain an explicit upper bound on the torsion of the Picard group of the forms of ${\mathbb{A}}_k^1$ and their regular completions. We also obtain a sufficient condition for the Picard group of the forms of ${\mathbb{A}}_k^1$ to be nontrivial and we give examples of nontrivial forms of ${\mathbb{A}}_k^1$ with trivial Picard groups.     

Hyperfine coupling constants of β-phosphorylated nitroxides: Subtle interplay between steric strain,hyperconjugation, and dipole-dipole interactions

Solvent effects in β-phosphorylated nitroxides show that nitrogen and phosphorus hyperfine coupling constants a_N and a_P, increase and decrease with increasing polarity, polarizity, and Hydrogen Bond Donor effects of solvents, respectively. In a series of articles, it was shown that the driving interaction controlling the change in a_P is the maximization of the N^+?—O^– … ???⁺PO^– dipole – dipole interaction. In this work, we show that the steric strain in spiro β-phosphorylated nitroxides affords the opposite trend for a_P, that is, a_P increases with increasing solvent properties features.     

Free‐Radical Dispersion Polymerization of Methyl Methacrylate in Supercritical Carbon Dioxide: A Parametric Analysis with Reaction Calorimetry

In an attempt to reduce the emission of hazardous VOCs, SCFs have been intensively investigated as alternative solvents during the last two decades. In this study, the application of reaction calorimetry, an efficient tool to obtain kinetic and safety data, is presented on the free radical dispersion polymerization of MMA in scCO₂. The effect of three major parameters with respect to the reaction evolution are studied, namely of stirrer type and stirring speed, the monomer and CO₂ concentration and the stabilizer concentration. The effective formation of the dispersion as well as the balance between the polymer formed in the continuous phase or inside the particles are key features of this analysis.

     

Existence and Stability of the log–log Blow-up Dynamics for the <Emphasis Type="Italic">L</Emphasis><Superscript>2</Superscript>-Critical Nonlinear Schrödinger Equation in a Domain

Let be the L ²-critical nonlinear Schr?dinger equation, in a domain with initial data in (Dirichlet boundary condition) and . We prove existence and stability of finite time blow-up dynamics with the log-log blow-up speed Moreover, for a suitable class of finite time blow-up solutions, we derive global rigidity properties which turn out to be modeled after the ones. Submitted: October 14, 2005. Revised: October 26, 2006. Accepted: February 2, 2007.     

Similarity Results for Operators of Class <Emphasis Type="Italic">C</Emphasis><Subscript>0</Subscript>

If T is a multiplicity-free contraction of class C ₀ with minimal function m _T , then it is quasisimilar to the Jordan block S(m _T ). In case m _T is a Blaschke product with simple roots forming a Carleson sequence, we show that the relation between T and S(m _T ) can be strengthened to similarity. Under the additional assumption that u(T) has closed range for every inner divisor \({u\in H^\infty}\) of m _T , the result also holds in the more general setting where the roots have bounded multiplicities.     

trans-Methylpyridine cyclen versus cross-bridged trans-methylpyridine cyclen. Synthesis, acid-base and metal complexation studies (metal = Co2+, Cu2+, and Zn2+)

The synthesis of the cross-bridged cyclen CRpy(2) {4,10-bis((pyridin-2-yl)methyl)-1,4,7,10-tetraazabicyclo[5.5.2]tetradecane}, a constrained analogue of the previously described trans-methylpyridine cyclen Cpy(2) is reported. The additional ethylene bridge confers to CRpy(2) proton-sponge type behaviour which was explored by NMR and potentiometric studies. Transition metal complexes have been synthesized (by complexation of both ligands with Co(2+), Cu(2+) and Zn(2+)) and characterized in solution and in the solid state. The single crystal X-ray structures of [CoCpy(2)](2+), [CuCpy(2)](2+) and [ZnCpy(2)](2+) complexes were determined. Stability constants of the complexes, including those of the cross-bridged derivative, were determined using potentiometric titration data and the kinetic inertness of the [CuCRpy(2)](2+) complex in an acidic medium (half-life values) was evaluated by spectrophotometry. The pre-organized structure of the cross-bridged ligand imposes an additional strain for the complexation leading to complexes with smaller thermodynamic stability in comparison with the related non-bridged ligand. The electrochemical study involving cyclic voltammetry underlines the importance of the ethylene cross-bridge on the redox properties of the transition metal complexes.     

A convergent approach to polycyclic aromatic hydrocarbons

A new concise route to Polycyclic Aromatic Hydrocarbons (PAHs) through radical addition and cyclisation of xanthates is described.     

exo-Diastereoisomer of 10-aryl-1,4,7-triazabicyclo[5.2.1]decane as intermediary in specific derivatisation of triazacyclononane

Reaction of triazacyclononane (tacn) and aromatic aldehydes leads to aminal adducts, which exhibit only the exo configuration. In these aminal compounds, secondary amine function possesses a higher reactivity towards electrophilic reactants than the two nitrogen atoms linked to aminal carbon, giving rise to the specific derivatisation of tacn by different functionalised groups. Study of this behaviour also permits the access to a ditopic tacn–cyclam bicyclic polyamine.     

Stable Blowup Dynamics for the 1‐Corotational Energy Critical Harmonic Heat Flow

We exhibit a stable finite time blowup regime for the 1‐corotational energy critical harmonic heat flow from ?² into a smooth compact revolution surface of ?³ that reduces to the semilinear parabolic problem for a suitable class of functions f. The corresponding initial data can be chosen smooth, well localized, and arbitrarily close to the ground state harmonic map in the energy‐critical topology. We give sharp asymptotics on the corresponding singularity formation that occurs through the concentration of a universal bubble of energy at the speed predicted by van den Berg, Hulshof, and King. Our approach lies in the continuation of the study of the 1‐equivariant energy critical wave map and Schrödinger map with ??² target by Merle, Raphaël, and Rodnianski. © 2012 Wiley Periodicals, Inc.