In‐plane structure of ferecrystalline compounds

This work provides a review on the in‐plane structure of a recently established subgroup of misfit layer compounds, namely ferecrystals. While misfit layer compounds can be described as layered intergrowths of two distinct substructures in which only one axis of the subunits remains incommensurate, for ferecrystals both constituents retain independent in‐plane crystalline structures. This is accompanied by extensive rotational disorder, a key feature of ferecrystals. Our comparison of the in‐plane structures of misfit layer compounds and ferecrystals suggests that the weaker registration between the subunits in ferecrystals allows the synthesis of a wider variety of compounds, varying both the range of constituents, their mismatch and their thicknesses. The in‐plane structures of the individual constituents in ferecrystalline compounds exhibit interesting features such as 2D‐symmetry, size induced structural transformation, misfit parameters that depend on the component thicknesses, and pairing distortions of constituent layers.     

Matrix-assisted laser desorption/ionization sample preparation optimization for structural characterization of poly(styrene-co-pentafluorostyrene) copolymers

The influence of the sample preparation parameters (the choice of the matrix, matrix:analyte ratio, salt:analyte ratio) was investigated and optimal conditions were established for the MALDI time-of-flight mass spectrometry analysis of the poly(styrene-co-pentafluorostyrene) copolymers. These were synthesized by atom transfer radical polymerization. Use of 2,5-dihydroxybenzoic acid as matrix resulted in spectra with consistently high ion yields for all matrix:analyte:salt ratios tested. The optimized MALDI procedure was successfully applied to the characterization of three copolymers obtained by varying the conditions of polymerization reaction. It was possible to establish the nature of the end groups, calculate molecular weight distributions, and determine the individual length distributions for styrene and pentafluorostyrene monomers, contained in the resulting copolymers. Based on the data obtained, it was concluded that individual styrene chain length distributions are more sensitive to the change in the composition of the catalyst (the addition of small amount of CuBr₂) than is the pentafluorostyrene component distribution.     

Synthesis and characterization of indium oxide-hematite magnetic ceramic solid solution

Indium oxide-doped hematite xIn₂O_3*(1-x)??-Fe₂O₃ (molar concentration x = 0.1?C0.7) solid solutions were synthesized using mechanochemical activation by ball milling. XRD patterns yield the dependence of lattice parameters and grain size as function of milling time. After 12 h of milling, the completion of In^3?+? substitution of Fe^3?+? in hematite lattice occurs for x = 0.1. For x = 0.3, 0.5 and 0.7, the substitutions between In^3?+? and Fe^3?+? into hematite and respectively, In₂O₃ lattices occur simultaneously. The lattice parameters of ??-Fe₂O₃ (a and c) and In₂O₃ (a) vary with milling time. For x = 0.1, Mössbauer spectra were fitted with one, two, or three sextets versus milling time, corresponding to gradual substitution of In^3?+? for Fe^3?+? in hematite lattice. For x = 0.3, Mössbauer spectra after milling were fitted with three sextets and two quadrupole-split doublets, representing In^3?+? substitution of Fe^3?+? in hematite lattice and Fe^3?+? substitution of In^3?+? in two different sites of In₂O₃ lattice. For x = 0.5 and 0.7, Mössbauer spectra fitting required two sextets and one quadrupole-split doublet, representing coexistence of In^3?+? substitution of Fe^3?+? in hematite lattice and Fe^3?+? substitution of In^3?+? in indium oxide lattice. The recoilless fraction studied versus milling time for each molar concentration exhibited low values, consistent with the occurrence of nanoparticles in the system. SEM/EDS measurements revealed that the mechanochemical activation by ball milling produced xIn₂O_3*(1-x)??-Fe₂O₃ solid solution system with a wide range of particle size distribution, from nanometer to micrometer, but with a uniform distribution of Fe, In, and O elements.     

Xe nuclear magnetic resonance line shapes in channels decorated with paramagnetic centers

To make predictions of the Xe NMR line shapes for Xe in channels decorated with paramagnetic centers, we consider a model system using the O(2) molecule as the paramagnetic center. The previously calculated quantum mechanical Xe@O(2) hyperfine tensor for various configurations of Xe in the presence of O(2) provides a model for the hyperfine response of Xe atom to the presence of a paramagnetic center. The averaging is carried out using the same grand canonical Monte Carlo methodology as for calculating NMR line shapes for Xe in diamagnetic channels, modified to include the effects of the hyperfine tensor response. We explore the temperature dependence of the Xe line shapes, the dependence on the concentration, and the symmetry of distribution of embedded paramagnetic centers, on the orientation of the paramagnetic center axis with respect to the channel axis, and on the radial distance of the paramagnetic center from the axis of the channel. We predict Xe line shape signatures of the presence and orientation of paramagnetic centers and deduce which tensor elements provide measures of concentration and radial distance of paramagnetic centers from the channel axis.     

Free resolutions in multivariable operator theory

Let be the complex polynomial ring in d variables. A contractive -module is Hilbert space equipped with an action such that for any ,

||z₁ξ₁+z₂ξ++z_dξ_d||²||ξ₁||²+||ξ₂||²++||ξ_d||².

Such objects have been shown to be useful for modeling d-tuples of mutually commuting operators acting on a Hilbert space. There is a subclass of the category of contractive modules whose members play the role of free objects. Given a contractive -module, one can construct a free resolution, i.e. an exact sequence of partial isometries of the following form:

(*)

where is a free module for each i0. The notion of a localization of a free resolution will be defined, in which for each λB_d there is a vector space complex of linear maps derived from (*):

We shall show that the homology of this complex is isomorphic to the homology of the Koszul complex of the d-tuple (¹,²,…,^d), of where ⁱ is the ith coordinate function of a Möbius transform on B_d such that (λ)=0.     

A shape-dependent hydrophobic effect for tetrazoles

An adaptive tetrazole-derived host provides insight into tetrazolate-biomolecule interactions, and is the first member of a new family of receptors that function in pure water.     

Preparation of fused polycyclic vinylcyclopropanes via radical cascade reactions

Radical cascades employing (dichloromethyl)dimethylsilyl ethers as both a point of radical initiation and termination, allow efficient entry to fused polycyclic cyclopropanes, and are also suitable for the design of other radical processes terminated by beta-elimination of chloride.     

Dehydrogenation of Amine–Boranes Using p-Block Compounds

Amine–boranes have gained a lot of attention due to their potential as hydrogen storage materials and their capacity to act as precursors for transfer hydrogenation. Therefore, a lot of effort has gone into the development of suitable transition- and main-group metal catalysts for the dehydrogenation of amine–boranes. During the past decade, new systems started to emerge solely based on p-block elements that promote the dehydrogenation of amine–boranes through hydrogen-transfer reactions, polymerization initiation, and main-group catalysis. In this review, we highlight the development of these p-block based systems for stoichiometric and catalytic amine–borane dehydrogenation and discuss the underlying mechanisms.