A Robust, Environmentally Benign Catalyst for Highly Selective Hydroformylation

By a sol-gel process a rhodium complex containing a diphosphane with a large natural P-Rh-P bite angle is covalently anchored in a silica matrix (see picture). The immobilized catalyst is a very selective hydroformylation catalyst that is completely and conveniently separated from the product and can be reused in numerous cycles.     

Refinement of local and long-range structural order in theophylline-binding RNA using (13)C-(1)H residual dipolar couplings and restrained molecular dynamics.

13C-(1)H residual dipolar couplings (RDC) have been measured for the bases and sugars in the theophylline-binding RNA aptamer, dissolved in filamentous phage medium, and used to investigate the long-range structural and dynamic behavior of the molecule in the solution state. The orientation dependent RDC provide additional restraints to further refine the overall structure of the RNA-theophylline complex, whose long-range order was poorly defined in the NOE-based structural ensemble. Structure refinement using RDC normally assumes that molecular alignment can be characterized by a single tensor and that the molecule is essentially rigid. To address the validity of this assumption for the complex of interest, we have analyzed distinct domains of the RNA molecule separately, so that local structure and alignment tensors experienced by each region are independently determined. Alignment tensors for the stem regions of the molecule were allowed to float freely during a restrained molecular dynamics structure refinement protocol and found to converge to similar magnitudes. During the second stage of the calculation, a single alignment tensor was thus applied for the whole molecule and an average molecular conformation satisfying all experimental data was determined. Semirigid-body molecular dynamics calculations were used to reorient the refined helical regions to a relative orientation consistent with this alignment tensor, allowing determination of the global conformation of the molecule. Simultaneously, the local structure of the theophylline-binding core of the molecule was refined under the influence of this common tensor. The final ensemble has an average pairwise root mean square deviation of 1.50 +/- 0.19 A taken over all heavy atoms, compared to 3.5 +/- 1.1 A for the ensemble determined without residual dipolar coupling. This study illustrates the importance of considering both the local and long-range nature of RDC when applying these restraints to structure refinements of nucleic acids.     

A High-Field EPR Study of the Accelerated Dynamics of the Amorphous Fraction of Semicrystalline Poly(dimethylsiloxane) at the Melting Point

The reorientation of a small paramagnetic tracer in poly(dimethylsiloxane) (PDMS) has been investigated by high-field electron paramagnetic resonance spectroscopy at a Larmor frequency of 285 GHz. The tracer is confined in the disordered phase of the semicrystalline PDMS. A sudden change of the rotational dynamics is observed close to the melting point (213 K) of the crystallites. This points to strong coupling between the crystalline and the disordered fractions of PDMS. Below the glass transition ( \(T_\mathrm{g} \sim 150 \mathrm{K}\) ), the tracer reorientation occurs via small angle jumps, with no apparent distribution of the correlation times. Above \(T_\mathrm{g}\) , a power-law distribution of correlation times is evidenced.     

An updated search of steady TeV γ-ray point sources in northern hemisphere using the Tibet air shower array

Using the data taken from Tibet Ⅱ High Density (HD) Array (1997 February--1999 September) and Tibet-Ⅲ array (1999 November--2005 November), our previous northern sky survey for TeV γ-ray point sources has now been updated by a factor of 2.8 improved statistics. From 0.0° to 60.0° in declination (Dec) range, no new TeV γ-ray point sources with sufficiently high significance were identified while the well-known Crab Nebula and Mrk421 remain to be the brightest TeV T-ray sources within the field of view of the Tibet air shower array. Based on the currently available data and at the 90% confidence level (C.L.), the flux upper limits for different power law index assumption are re-derived, which are approximately improved by 1.7 times as compared with our previous reported limits.     

Multifrequency EPR spectra of molecular oxygen in solid Air

Multifrequency EPR spectra in the 94 to 550 GHz range were performed on solid air samples condensed at 5 K in the waveguide of a single pass probe. The spectra of molecular oxygen were observed and interpreted in the frame of the spin Hamiltonian model as axial S = 1 spectra with a zero field splitting parameter D = 3.572(3) cm⁻¹. The result of this study is relevant in the field of high field–high frequency EPR application in which solid air O₂ is a common paramagnetic impurity.     

One-step synthesis of gold and silver hydrosols using poly(N-vinyl-2-pyrrolidone) as a reducing agent

Synthesis of gold and silver hydrosols was carried out in a one-step process by reduction of aqueous solutions of metal salts using poly(N-vinyl-2-pyrrolidone) (PVP). Both kinds of metal nanoparticles were obtained without the addition of any other reducing agent, at low temperatures and using water as the synthesis solvent. Shape, size, and optical properties of the particles could be tuned by changing the employed PVP/metal salt ratio. It is proposed that PVP acts as the reducing agent suffering a partial degradation during the nanoparticles synthesis. Two possible mechanisms are proposed to explain the reduction step: direct hydrogen abstraction induced by the metal ion and/or reducing action of macroradicals formed during degradation of the polymer. Initial formation of the macroradicals might be associated with the metal-accelerated decomposition of low amounts of peroxides present in the commercial polymer.     

Amplitude modulated transversal and longitudinal EPR spectroscopy

The amplitude modulated EPR spectroscopy is analyzed both in the time and the frequency domain. The results of numerical calculations and analytical approximate treatments indicate that the signal lineshape is differently affected by relaxation mechanisms when transversal or longitudinal detection is used in spectroscopies with variable frequencies of modulation. Measurements of longitudinally detected electron-spin double resonance obtained in dependence on the frequency of modulation confirm the lineshape expected by the theoretical analysis.