Defect‐Controlled Electronic Properties in AZn2Sb2 Zintl Phases

Experimentally, AZn₂Sb₂ samples (A=Ca, Sr, Eu, Yb) are found to have large charge carrier concentrations that increase with increasing electronegativity of A. Using density functional theory (DFT) calculations, we show that this trend can be explained by stable cation vacancies and the corresponding finite phase width in A_1?xZn₂Sb₂ compounds.     

Oliveridepsidones A–D,antioxidant depsidones from Garcinia oliveri

Four new prenylated depsidones, oliveridepsidones A–D, were isolated from the bark of Garcinia oliveri collected in Vietnam. Their structures were elucidated using mainly NMR techniques (¹H and ¹³C NMR, HMQC, HMBC and NOE experiments). Copyright © 2012 John Wiley & Sons, Ltd.     

Using Gas-Phase Guest–Host Chemistry to Probe the Structures of b Ions of Peptides

Middle-sized b _n (n????5) fragments of protonated peptides undergo selective complex formation with ammonia under experimental conditions typically used to probe hydrogen?Cdeuterium exchange in Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR-MS). Other usual peptide fragments like y, a, a*, etc., and small b _n (n????4) fragments do not form stable ammonia adducts. We propose that complex formation of b _n ions with ammonia is characteristic to macrocyclic isomers of these fragments. Experiments on a protonated cyclic peptide and N-terminal acetylated peptides fully support this hypothesis; the protonated cyclic peptide does form ammonia adducts while linear b _n ions of acetylated peptides do not undergo complexation. Density functional theory (DFT) calculations on the proton-bound dimers of all-Ala b ₄ , b ₅ , and b ₇ ions and ammonia indicate that the ionizing proton initially located on the peptide fragment transfers to ammonia upon adduct formation. The ammonium ion is then solvated by N⁺-H??O H-bonds; this stabilization is much stronger for macrocyclic b _n isomers due to the stable cage-like structure formed and entropy effects. The present study demonstrates that gas-phase guest?Chost chemistry can be used to selectively probe structural features (i.e., macrocyclic or linear) of fragments of protonated peptides. Stable ammonia adducts of b ₉ , b ₉ -A, and b ₉ -2A of A₈YA, and b ₁₃ of A₂₀YVFL are observed indicating that even these large b-type ions form macrocyclic structures.     

Application of In-Line Fiber-Optic Raman Spectroscopy to Monitoring Emulsion Polymerization Reactions

INTRODUCTION

Emulsion polymerization has a large number of applications in the chemical and process industry. It is one of the predominant processes for the commercial manufacture of polymeric materials used in paints, adhesives, and synthetic rubbers etc. Raman spectroscopy has been identified as a promising tool for monitoring the aqueous emulsion polymerization processes as it provides direct information about the changes in the double bond content from the monomer as the polymerization reaction progresses.