An unusual effect of fast neutron irradiation of lanthanide /III/ chlorides

When anhydrous EuCl₃, GdCl₃, DyCl₃ or Eu₂O₃ are irradiated with fast neutrons and then dissolved in dilute hydrochloric acid, small amounts of reducing species, believed to be divalent lanthanide cations, are revealed. The amount produced increases with the thermodynamic stability of the Ln²⁺ species /Eu>Dy>Gd>/.     

Atmospheric pressure photoionization mass spectrometry as a tool for the investigation of the hydrolysis reaction mechanisms of phosphite antioxidants

The hydrolysis reaction mechanism of phosphite antioxidants is investigated by liquid chromatography-mass spectrometry (LC/MS). The phosphites were chosen because they differed in chemical structure and phosphorus content. Dopant assisted-atmospheric pressure photoionization (DA-APPI) is chosen as the ion source for the ionization of the compounds. In our previous work, DA-APPI was shown to offer an attractive alternative to atmospheric pressure chemical ionization (APCI) since it provided background-ion free mass spectra and higher sensitivity [M. Papanastasiou, et al., Polymer Degradation and Stability 91 (11) (2006) 2675–2682]. In positive ion mode, the molecules are generally detected in their protonated form. In negative ion mode, the phosphites are unstable and only fragment ions are observed; these however, are characteristic of each phosphite and may be used for the identification of the analytes in complex mixtures.The analytes under investigation are exposed to accelerated humid ageing conditions and their hydrolytic pathway and stability is investigated. Different substituents around the phosphorus atom are shown to have a significant effect on the stability of the phosphites, with phenol substituents producing very hydrolytically stable structures. Alkanox P24 and PEP-36 follow a similar hydrolytic pathway via the scission of the first and then the second PO_phenol bonds, eventually leading to the formation of phenol, phosphorous acid and pentaerythritol as end products. HP-10 exhibits a rather different structure and the products detected suggest scission of either the PO_hydrocarbon or one of the PO_phenol bonds. A phenomenon similar to that of autocatalysis is observed for all phosphites and is attributed to the formation of dialkyl phosphites as intermediate products.     

Formal synthesis of leustroducsin B via Reformatsky/Claisen condensation of silyl glyoxylates

A formal synthesis of leustroducsin B has been completed. The synthesis relies upon a recently developed Reformatsky/Claisen condensation of silyl glyoxylates and enantioenriched β-lactones that establishes two of the molecule's three core stereocenters and permits further elaboration to an intermediate in Imanishi's synthesis via reliable chemistry (Prasad reduction, asymmetric pentenylation, Mitsunobu inversion).     

Accurate O-H bond dissociation energy differences of hydroxylamines determined by EPR spectroscopy: computational insight into stereoelectronic effects on BDEs and EPR spectral parameters

Differences in O-H bond dissociation enthalpies (ΔBDEs) between the hydroxylamine of (15)N-labeled TEMPONE and 10 N,N-di-tert-alkyl hydroxylamines were determined by EPR. These ΔBDEs, together with the g and a(N) values of the derived nitroxide radicals, are discussed in relation to various geometric, intramolecular dipole/dipole, and steric effects and in relation to the results from DFT calculations. We find that dipole/dipole interactions are the dominant factors in dictating a(N) values and O-H BDEs in all of these structurally similar nitroxides and hydroxylamines, respectively. The importance of including the Boltzmann distribution of conformations for each nitroxide in the a(N) calculations is emphasized.     

Molybdenum site structure of Escherichia coli YedY, a novel bacterial oxidoreductase

We report a structural characterization using X-ray absorption spectroscopy of the molybdenum site of Escherichia coli YedY, a novel oxidoreductase related to be the sulfite oxidase family of molybdenum enzymes. We find that the enzyme can exist in Mo(V) and Mo(IV) oxidation states but cannot be readily oxidized to the Mo(VI) form. Mo(V) YedY has molybdenum coordination similar to that of sulfite oxidase, with one Mo═O at 1.71 ?, three Mo-S at 2.39 ?, and one Mo-OH at 2.09 ?, which elongates to 2.20 ? upon reduction to Mo(IV), indicating Mo-OH(2) coordination. The Mo(V) enzyme also possesses a long Mo-O coordination at 2.64 ?, which may be due to oxygen coordination by Asn-45 O(δ), with Mo-O(δ) approximately trans to the Mo═O group. A comparison with sulfite oxidase indicates that YedY possesses a much more uniform Mo-S coordination, with a maximum permitted deviation of less than 0.05 ?. Our results indicate that the YedY active site shows considerable similarity to but also important differences from that of reduced forms of sulfite oxidase.     

A Surrogate for Debye-Waller Factors from Dynamic Stokes Shifts

We show that the short-time behavior of time-resolved fluorescence Stokes shifts (TRSS) are similar to that of the intermediate scattering function obtained from neutron scattering at q near the peak in the static structure factor for glycerol. This allows us to extract a Debye-Waller (DW) factor analog from TRSS data at times as short as 1 ps in a relatively simple way. Using the time-domain relaxation data obtained by this method we show that DW factors evaluated at times ≥ 40 ps can be directly influenced by α relaxation and thus should be used with caution when evaluating relationships between fast and slow dynamics in glassforming systems.     

Analytic calculation of nucleation rates from a kinetic Monte Carlo simulation of flow induced crystallization in polymers

We present an analytically derived model for flow induced crystallization (FIC), based upon the recent Graham–Olmsted simulation. We use combinatorial techniques to calculate nucleation energy landscapes, which correctly predict simulation data. Applying both the analytic calculation and the simulation, we put forward a simple expression relating nucleation rate to polymer chain stretch. We also investigate bimodal blends, an important step to understanding polydisperse systems and eventually modeling industrial polymer melts.     

Multi-functionality of fluorescent nanocrystals in glass ceramics

Thermal processing of as-made fluorozirconate glasses, which were additionally doped with rare-earth and chlorine ions, results in the formation of fluorescent nanocrystals therein. For medical applications, the glasses were doped with divalent europium ions as the fluorescent rare-earth ion, while trivalent neodymium was used to develop up-conversion systems. The samples were annealed up to 290 °C to initiate the growth of hexagonal or orthorhombic phase BaCl₂ nanocrystals therein. Upon annealing some of the rare-earth ions were incorporated into the BaCl₂ nanocrystals leading to enhanced fluorescence properties. The particle diameters were in the range of a few nanometers to several tens of nanometers.