Stereoselective migration of sterically hindered organoboranes in cyclic and acyclic systems. A stereoselective allylic C-H activation reaction

The thermal migration of cyclic and acyclic organoboranes were studied. In most cases, a stereoselective 1,2-dyotropic migration was observed, allowing the stereocontrol of three contiguous chiral centers. Scope and limitations of this thermal migration are presented.     

A supramolecular hydrogen‐bonded complex between 1,3,5‐tris(diisobutylhydroxysilyl)benzene and trans‐bis(4‐pyridyl)ethylene

The trisilanol 1,3,5‐(HOi‐Bu₂Si)₃C₆H₃ ( 7 ), prepared in three steps from 1,3,5‐tribromobenzene via the intermediates 1,3,5‐(Hi‐Bu₂Si)₃C₆H₃ ( 8 ) and 1,3,5‐(Cli‐Bu₂Si)₃C₆H₃ ( 9 ) forms an equimolar complex with trans‐bis(4‐pyridyl)ethylene (bpe), 7 ·bpe, whose structure was investigated by X‐ray crystallography. The hydrogen‐bonded network features a number of SiO? H(H)Si and SiO? H hydrogen bridges. Evidence was found for cooperative strengthening within the sequential hydrogen bonds. Copyright © 2007 John Wiley & Sons, Ltd.     

Vibrational analysis of phenol/(methanol)1

Ab initio calculations at the Hartree-Fock 4-31G* level were performed in order to calculate binding energies and vibrational frequencies of the phenol/CH₃OH-cluster and two deuterated isotopomers (d-phenol/CH₃OD,d-phenol-CD₃OD). The minimum energy structure is trans-linear, as for the phenol/H₂O-cluster. The calculated frequencies of phenol and methanol as well as the intramolecular frequencies of the phenol/CH₃OH-cluster are assigned to experimental values. The calculated intermolecular frequencies of the phenol/CH₃OH-cluster are compared with the available experimental frequencies of theS ₀ (andS ₁)-state of the phenol/methanol-cluster and the similarp-cresol/methanol-cluster. Assignments are suggested for the σ andp ₁-mode. In order to clarify the assignment of the low frequency vibration at 22 cm^?1 anharmonic corrections for the β₂-mode of the phenol/CH₃OH-cluster are calculated. These calculations show only slight anharmonicity compared with the β₂-mode calculations carried out for the phenol/H₂O-cluster.     

Methods and Results of Optical Investigations on Semiconductor Interfaces

Measurement of the state of polarization of reflected light (ellipsometry) permits the determination of the thickness and refractive index of thin layers formed on a surface. The IR absorption spectra of such thin layers, which can be measured by means of internal reflection spectroscopy (IRS), provide information about their chemical composition. These methods have been used to study adsorption processes and the formation of reaction layers at semiconductor interfaces, and may also be used for measurements of free charge carriers in the space-charge region and in surface states. Results of such investigations are given in this article.     

Thermoreversible gelation of polyacrylonitrile/dimethylformamide-solution

The thermoreversible gelation of semi-diluted atactic polyacrylnitrile (PAN)/dimethylformamide (DMF)-solutions has been studied. The structural features of PAN/DMF-gels, formed by supercooling have been investigated by DSC-, x-ray- and swelling measurements. A new structural model has been introduced to describe the morphology of the junction zones of PAN/DMF-gels and to also explain the gelation behavior of PAN/DMF-solutions as the structural features of PAN/DMF-gels. The junction zones of a PAN/DMF-gel have been defined as ordered junction zones.A gelation enthalpy of about H=–6 kJ/mol supports the idea that an ordered junction zone is formed by intermolecularly neighboring stereoregular parts of atactic PAN chains due to a nucleation process in the solution. It can be defined as a strongly distributed fringed micelle.     

Interaction of Pd-overlayers with SnO2: comparative XPS, SIMS, and SNMS studies

Summary We report about interaction processes between palladium (Pd) and tin dioxide (SnO₂) studied with various surface spectroscopic techniques. Total sputter yields necessary for absolute depth calibration in SIMS are determined for SnO₂. Clustering of palladium occurs at low temperatures. Small changes in the XPS relative core level intensities of Pd and Sn allow to determine cluster sizes. Oxidation of Pd in the presence of oxygen at T470 K is a prerequisite for diffusion of Pd²⁺ ions into SnO₂ layers. The latter process is confirmed and described quantitatively by evaluating the SIMS and SNMS measurements.     

Upper limit for thermal direct photon production in heavy-ion collisions at 60 and 200A·GeV

The production of direct photons has been investigated in reactions ofp and¹⁶O projectiles at 60 and 200A·GeV with C and Au nuclei. Photon and ⁰ spectra have been measured in the pseudorapidity range 1.52.1 for the transverse momentum region 0.4 GeV/cp _T 2.8 GeV/c employing the lead-glass spectrometer SAPHIR. An upper limit of 15% at the 90% confidence level for the direct photon signal relative to the neutral pion production is obtained from the comparison of measured photon spectra with Monte Carlo simulations of the hadronic background based on the reconstructed yield of ⁰ and mesons. Consequences for a possible phase transition to a quark-gluon plasma are discussed.     

Reverse micelles dissolved in supercritical xenon: an NMR spectroscopic study

Reverse micelles currently gain increasing interest in chemical technology. They also become important in biomolecular NMR due to their ability to host biomolecules such as proteins. In the present paper, a procedure for the preparation of high-pressure NMR samples containing reverse micelles dissolved in supercritical xenon is presented. These reverse micelles are formed by sodium bis(2-ethylhexyl) sulfosuccinate (AOT). For the first time, NMR spectroscopy could be applied to reverse micelles in supercritical xenon. The AOT/H(2)O/Xe system was studied as a function of experimental parameters such as xenon pressure, water content, and salt concentration. Optimum conditions for reverse micelle formation in supercritical xenon could be determined. It is, furthermore, demonstrated that biomolecules such as amino acids and proteins can be incorporated into the reverse micelles dissolved in supercritical xenon.     

GC-MS Studies on Derivatization of Creatinine and Creatine by BSTFA and Their Measurement in Human Urine

In consideration of its relatively constant urinary excretion rate, creatinine (2-amino-1-methyl-5H-imidazol-4-one, MW 113.1) in urine is a useful endogenous biochemical parameter to correct the urinary excretion rate of numerous endogenous and exogenous substances. Reliable measurement of creatinine by gas chromatography (GC)-based methods requires derivatization of its amine and keto groups. Creatinine exists in equilibrium with its open form creatine (methylguanidoacetic acid, MW 131.1), which has a guanidine and a carboxylic group. Trimethylsilylation and trifluoroacetylation of creatinine and creatine are the oldest reported derivatization methods for their GC-mass spectrometry (MS) analysis in human serum using flame- or electron-ionization. We performed GC-MS studies on the derivatization of creatinine (d₀-creatinine), [methylo-²H₃]creatinine (d₃-creatinine, internal standard) and creatine (d₀-creatine) with N,O-bis(trimethylsilyl)trifluoroacetamide (BSTFA) using standard derivatization conditions (60 min, 60 °C), yet in the absence of any base. Reaction products were characterized both in the negative-ion chemical ionization (NICI) and in the positive-ion chemical ionization (PICI) mode. Creatinine and creatine reacted with BSTFA to form several derivatives. Their early eluting N,N,O-tris(trimethylsilyl) derivatives (8.9 min) were found to be useful for the precise and accurate measurement of the sum of creatinine and creatine in human urine (10 µL, up to 20 mM) by selected-ion monitoring (SIM) of m/z 271 (d₀-creatinine/d₀-creatine) and m/z 274 (d₃-creatinine) in the NICI mode. In the PICI mode, SIM of m/z 256, m/z 259, m/z 272 and m/z 275 was performed. BSTFA derivatization of d₀-creatine from a freshly prepared solution in distilled water resulted in formation of two lMate-eluting derivatives (14.08 min, 14.72 min), presumably creatinyl-creatinine, with the creatininyl residue existing in its enol form (14.08 min) and keto form (14.72 min). Our results suggest that BSTFA derivatization does not allow specific analysis of creatine and creatinine by GC-MS. Preliminary analyses suggest that pentafluoropropionic anhydride (PFPA) is also not useful for the measurement of creatinine in the presence of creatine. Both BSTFA and PFPA facilitate the conversion of creatine to creatinine. Specific measurement of creatinine in urine is possible by using pentafluorobenzyl bromide in aqueous acetone.