Efficient simultaneous reverse Monte Carlo modeling of pair-distribution functions and extended x-ray-absorption fine structure spectra of crystalline disordered materials

An efficient implementation of simultaneous reverse Monte Carlo (RMC) modeling of pair distribution function (PDF) and EXAFS spectra is reported. This implementation is an extension of the technique established by Krayzman et al. [J. Appl. Cryst. 42, 867 (2009)] in the sense that it enables simultaneous real-space fitting of x-ray PDF with accurate treatment of Q-dependence of the scattering cross-sections and EXAFS with multiple photoelectron scattering included. The extension also allows for atom swaps during EXAFS fits thereby enabling modeling the effects of chemical disorder, such as migrating atoms and vacancies. Significant acceleration of EXAFS computation is achieved via discretization of effective path lengths and subsequent reduction of operation counts. The validity and accuracy of the approach is illustrated on small atomic clusters and on 5500-9000 atom models of bcc-Fe and α-Fe(2)O(3). The accuracy gains of combined simultaneous EXAFS and PDF fits are pointed out against PDF-only and EXAFS-only RMC fits. Our modeling approach may be widely used in PDF and EXAFS based investigations of disordered materials.     

A Hybrid Ring‐Opening Metathesis Polymerization Catalyst Based on an Engineered Variant of the β‐Barrel Protein FhuA

A β‐barrel protein hybrid catalyst was prepared by covalently anchoring a Grubbs–Hoveyda type olefin metathesis catalyst at a single accessible cysteine amino acid in the barrel interior of a variant of β‐barrel transmembrane protein ferric hydroxamate uptake protein component A (FhuA). Activity of this hybrid catalyst type was demonstrated by ring‐opening metathesis polymerization of a 7‐oxanorbornene derivative in aqueous solution.     

Comparison between Hydrogen and Halogen Bonds in Complexes of 6-OX-Fulvene with Pnicogen and Chalcogen Electron Donors

Quantum chemical calculations are applied to complexes of 6-OX-fulvene (X=H, Cl, Br, I) with ZH₃/H₂Y (Z=N, P, As, Sb; Y=O, S, Se, Te) to study the competition between the hydrogen bond and the halogen bond. The H-bond weakens as the base atom grows in size and the associated negative electrostatic potential on the Lewis base atom diminishes. The pattern for the halogen bonds is more complicated. In most cases, the halogen bond is stronger for the heavier halogen atom, and pnicogen electron donors are more strongly bound than chalcogen. Halogen bonds to chalcogen atoms strengthen in the order O<S<Se<Te, whereas the pattern is murkier for the pnicogen donors. In terms of competition, most halogen bonds to pnicogen donors are stronger than their H-bond analogues, but there is no clear pattern with respect to chalcogen donors. O prefers a H-bond, while halogen bonds are favored by Te. For S and Se, I-bonds are strongest, followed Br, H, and Cl-bonds in that order.     

Aerobic Epoxidation of Alkenes Catalysed by Cobalt(II) 1,1,1,5,5,5-Hexafluoroacetylacetonate or Cobalt(II) Benzoylacetonate

The aerobic epoxidation of terminal or electron deficient alkenes with an aldehyde does not proceed with cobalt(II) acetylacetonate but goes to completion with the cobalt(II) benzoylacetonate and cobalt(II) 1,1,1,5,5,5-hexafluoroacetylacetonate complexes.     

Detailed comparison of the pnicogen bond with chalcogen,halogen, and hydrogen bonds

The characteristics of the pnicogen bond are explored using a variety of quantum chemical techniques. In particular, this interaction is compared with its halogen and chalcogen bond cousins, as well as with the more common H‐bond. In general, these bonds are all of comparable strength. More specifically, they are strengthened by the presence of an electronegative substituent on the electron‐acceptor atom, and each gains strength as one moves down the appropriate column of the periodic table, for example, from N to P to As. These noncovalent bonds owe their stability to a mixture in nearly equal parts of electrostatic attraction and charge transfer, along with a smaller dispersion component. The charge transfer arises from the overlap between the lone pair of the electron donor and a σ* antibond of the acceptor. The angular characteristics of the equilibrium geometry result primarily from a compromise between electrostatic and induction forces. Angular distortions of the H‐bond are typically less energetically demanding than comparable bends of the other noncovalent bonds. © 2012 Wiley Periodicals, Inc.     

Preparation and structural determination of siloxane-modified sulfone-containing epoxy resins

Siloxane-modified sulfone-containing epoxy resins (ESBS) were prepared by polycondensation of PMPS and/or PDMS siloxane oligomers with EBS, the sulfone-containing epoxy resin. Structures were analyzed by IR, ¹H-, and ¹³C-NMR. The siloxane content in the copolymers was determined by ¹H-NMR with an integration technique. Epoxy equivalent weight (EEW) determination indicated that the oxirane ring of EBS was intact with this hot-melt procedure. The GPC measurement of these ESBS copolymers showed that molecular weight (MW) increased with increasing siloxane content in PMPS-modified copolymers. Evidence of siloxane incorporation in the copolymer was discussed. © 1996 John Wiley & Sons, Inc.     

RCM of tripeptide dienes containing a chiral vinylcyclopropane moiety: impact of different Ru-based catalysts on the stereochemical integrity of the macrocyclic products

[structures: see text] Tripeptide dienes containing an (1R,2S)-vinyl aminocyclopropylcarboxylate residue were cyclized to beta-strand scaffolds under ring-closing metathesis (RCM). Conformational factors, ligand effects, and reaction conditions were evaluated. A protocol was developed for the efficient synthesis of 15-membered ring peptides in high diastereomeric purity. These peptides are key synthetic precursors to antiviral agents that target the hepatitis C virus and represent the first class of clinically validated pharmaceutical agents that are synthesized in large scale using RCM.     

Ozonolysis of tetramethylethylene: characterization of cyclic and open-chain oligoperoxidic products

The ozonolysis of tetramethylethylene (TME) in solution to high conversion in nonparticipating solvents at -60 degrees C yields predominantly oligoperoxides. For the first time, these products have been characterized using electrospray ionization mass spectrometry (ESI-MS) under soft ionization conditions. The predominant structure formed in reactions carried out in pentane (up to 2.0 M TME) is shown to be the cyclic hexamer of acetone carbonyl oxide (oligocarbonyl oxide with degree of polymerization, n = 6), but cyclic structures with n up to 19 are observed. A small proportion of the oligoperoxides formed are open-chain compounds with end groups that suggest that chain termination of oligocarbonyl oxides can occur through reaction with either water or hydrogen peroxide. Ozonolysis in dried butyl acetate similarly produces mainly cyclic oligoperoxides. However, ozonolyses carried out in undried butyl acetate yield mainly open-chain oligoperoxides, confirming that propagating carbonyl oxide chains are readily terminated by water. Relative amounts of the open-chain oligomers so-formed suggest that undried butyl acetate contains ca. 0.1% w/w water. The ozonolysis of TME in the participating solvent, methanol, at -60 degrees C yields 2-methoxyprop-2-yl hydroperoxide via reaction of acetone carbonyl oxide with methanol; no oligoperoxidic products are formed in this case.     

The Effect of Preparation Method and Calcination Temperature on the Crystallite Size and Surface Area of Perovskite-Type SrFeO x

We have investigated the synthesis of perovskite-type SrFeO_x (2.5 x 3.0) using three preparative methods: sol-gel, mechanochemical processing and solid state reactions at high temperature of the corresponding oxides. The sample obtained after calcination of the gel from sol-gel method, contained the least amount of strontium carbonate impurity. The amount of strontium carbonate impurity decreased with the increase in calcination temperature. Perovskites obtained have been characterized by X-ray diffraction (XRD) and nitrogen adsorption isotherms. Samples obtained from three methods have been compared with respect to calcination temperature, crystallite size and specific surface area.Issued as NRCC No. 46479.