Resolution of discrepancy between backward angle cross-section data for neutron-deuteron elastic scattering

High-accuracy differential cross-section data for neutron-deuteron elastic scattering at centre-of-mass backward angles in the range from 140° to 178° have been measured to accuracies better than ±3% for incident neutron energies of 8.0, 10.0, and 14.0 MeV. The measurements were made by detection of the recoil deuterons and protons from a mixed CD₂-CH₂ foil. The new data are compared to existing data and to three-nucleon calculations with the Bonn-B nucleon-nucleon potential. We conclude that the present measurements unambiguously resolve the discrepancies among previously reported data of the same type.     

Covalent attachment of organic monolayers to silicon carbide surfaces

This work presents the first alkyl monolayers covalently bound on HF-treated silicon carbide surfaces (SiC) through thermal reaction with 1-alkenes. Treatment of SiC with diluted aqueous HF solutions removes the native oxide layer (SiO2) and provides a reactive hydroxyl-covered surface. Very hydrophobic methyl-terminated surfaces (water contact angle theta = 107 degrees ) are obtained on flat SiC, whereas attachment of omega-functionalized 1-alkenes also yields well-defined functionalized surfaces. Infrared reflection absorption spectroscopy, ellipsometry, and X-ray photoelectron spectroscopy measurements are used to characterize the monolayers and show their covalent attachment. The resulting surfaces are shown to be extremely stable under harsh acidic conditions (e.g., no change in theta after 4 h in 2 M HCl at 90 degrees C), while their stability in alkaline conditions (pH = 11, 60 degrees C) also supersedes that of analogous monolayers such as those on Au, Si, and SiO2. These results are very promising for applications involving functionalized silicon carbide.     

Electroless gold island thin films: photoluminescence and thermal transformation to nanoparticle ensembles

Electroless gold island thin films are formed by galvanic replacement of silver reduced onto a tin-sensitized silica surface. A novel approach to create nanoparticle ensembles with tunable particle dimensions, densities, and distributions by thermal transformation of these electroless gold island thin films is presented. Deposition time is adjusted to produce monomodal ensembles of nanoparticles from 9.5 +/- 4.0 to 266 +/- 22 nm at densities from 2.6 x 1011 to 4.3 x 108 particles cm-2. Scanning electron microscopy and atomic force microscopy reveal electroless gold island film structures as well as nanoparticle dimensions, densities, and distributions obtained by watershed analysis. Transmission UV-vis spectroscopy reveals photoluminescent features that suggest ultrathin EL films may be smoother than sputtered Au films. X-ray diffraction shows Au films have predominantly (111) orientation.     

Methylthiolate on Au(111): adsorption and desorption kinetics

Low energy electron diffraction, Auger electron spectroscopy, X-ray photoelectron spectroscopy and line of sight mass spectrometry have been used to study the adsorption and desorption of dimethyldisulfide (DMDS) on Au(111). At 300 K adsorption is dissociative, forming a chemisorbed adlayer of methylthiolate with a 1/3 ML, (sq rt 3 x sq rt 3)R30 degrees, structure. At 100 K adsorption is molecular, with dissociation to form the 1/3 ML (sq rt 3 x sq rt 3)R30 degrees methylthiolate structure occurring at 138-160 K. A physisorbed DMDS layer, with a coverage of 1/6 ML of DMDS, forms on top of the (sq rt 3 x sq rt 3)R30 degrees chemisorbed MT surface for T < or = 180 K, with multilayers forming for T < or = 150 K. In temperature programmed desorption, multilayers of DMDS desorbed with zero order kinetics and an activation energy of 41 kJ mol(-1); the physisorbed layer desorbed with first order kinetics, exhibiting repulsive lateral interactions with an activation energy which varied from 63 kJ mol(-1) (theta = 0) to 51 kJ mol(-1) (theta = 1); the chemisorbed methylthiolate layer desorbed associatively as DMDS via the physisorbed layer, the activation energy for the reaction, 2 methylthiolate --> physisorbed DMDS, exhibiting repulsive lateral interactions with an activation energy which varied from 65 kJ mol(-1) (theta = 0) to 61 kJ mol(-1) (theta = 1). The physisorbed disulfide layer explains the pre-cursor state adsorption kinetics observed in sticking probability measurement, while its relatively facile formation provides a mechanism by which thiolate self-assembled monolayers can become mobile at room temperature.     

Simultaneous capillary electrophoresis competitive immunoassay for insulin, glucagon, and islet amyloid polypeptide secretion from mouse islets of Langerhans

A capillary electrophoresis competitive immunoassay was developed for the simultaneous quantitation of insulin, glucagon, and islet amyloid polypeptide (IAPP) secretion from islets of Langerhans. Separation buffers and conditions were optimized for the resolution of fluorescein isothiocyanate (FITC)-labeled glucagon and IAPP immunoassay reagents, which were excited with the 488 nm line of an Ar(+) laser and detected at 520 nm with a photomultiplier tube (PMT). Cy5-labeled insulin immunoassay reagents were excited by a 635 nm laser diode module and detected at 700 nm with a separate PMT. Optimum resolution was achieved with a 20mM carbonate separation buffer at pH 9.0 using a 20 cm effective separation length with an electric field of 500 V/cm. Limits of detection for insulin, glucagon, and IAPP were 2, 3, and 3 nM, respectively. This method was used to monitor the simultaneous secretion of these peptides from as few as 14 islets after incubation in 4, 11, and 20 mM glucose for 6h. For insulin and IAPP, a statistically significant increase in secretion levels was observed, while glucagon levels were significantly reduced in the 4 and 11 mM glucose conditions. To further demonstrate the utility of the assay, the Ca(2+)-dependent secretion of these peptides was demonstrated which agreed with published reports. The ability to examine the secretion of multiple peptides may allow for the determination of regulation of secretory processes within islets of Langerhans.     

Efficient synthesis of an adenosine A2a agonist: glycosylation of 2-haloadenines and an N2-alkyl-6-chloroguanine

A convergent synthesis of adenosine A2a agonist 1 in the form of its maleate salt 2 was achieved. The key step in this approach was the highly selective 9beta-glycosylation reaction between 2-haloadenines or an N(2)-alkyl-6-chloroguanine and a D-ribose derivative containing a 2-ethyltetrazolyl moiety. Glycosylations of other purine derivatives were also examined, and the methods developed provide efficient access to a variety of adenosine analogues such as 2-alkylaminoadenosines, an attractive class of compounds with antiinflammatory activity.     

Reactions of the dichloroboryl complex of osmium, Os(BCl2)Cl(CO)(PPh3)2, with water, alcohols, and amines: Crystal structures of Os[B(OH)2]Cl(CO)(PPh3)2, Os[B(OEt)2]Cl(CO)(PPh3)2, and

The iridium(I) complex [Ir(CO₂Me)(CO)₂(PPh₃)₂] undergoes a transesterification reaction with the alcohols CH₂C(R)CH₂OH (R = H, Me), MeCCCH₂CH₂OH, and HOCH₂CH₂OH to afford the complexes

[Ir(CO₂CH₂CH₂CMe)(CO)₂(PPh₃)₂] and [Ir(CO₂CH₂CH₂OH)(CO)₂(PPh₃)₂], respectively. In contrast the acetylenic alcohol HCCCH₂CH₂OH gives [Ir(CCCH₂CH₂OH)(CO)PPh₃)₂]. Some reactions of the new complexes are described.     

Some transesterification reactions of [Ir(CO2Me)(CO)2(PPh3)2]

The iridium(I) complex [Ir(CO₂Me)(CO)₂(PPh₃)₂] undergoes a transesterification reaction with the alcohols CH₂C(R)CH₂OH (R = H, Me), MeCCCH₂CH₂OH, and HOCH₂CH₂OH to afford the complexes

[Ir(CO₂CH₂CH₂CMe)(CO)₂(PPh₃)₂] and [Ir(CO₂CH₂CH₂OH)(CO)₂(PPh₃)₂], respectively. In contrast the acetylenic alcohol HCCCH₂CH₂OH gives [Ir(CCCH₂CH₂OH)(CO)PPh₃)₂]. Some reactions of the new complexes are described.