Organically modified Pd-silica catalysts applied in Heck coupling

Pd-silica catalysts prepared by depositing Pd onto silica precursors modified by surface methyl or phenyl groups through the reduction of Pd2+ ions with surface Si-H functions exhibit high activity and selectivity in Heck coupling.     

The kinetics of homogeneous nucleation of silver nanoparticles stabilized by polymers

The formation of Ag nanoparticles synthesized by homogeneous nucleation, stabilized by polymers (PVA and PVP) was monitored by UV–Vis spectrophotometry and transmission electron microscopy. Our aim was to differentiate between the two main phases of particle formation, i.e. nucleation and growth and to characterize their rates with the help of appropriate kinetic equations. Time resolved spectrophotometric measurements revealed that particle formation is an autocatalytic process: a slow, continuous nucleation phase (3–5 min) is followed by a rapid, autocatalytic growth phase where the maximal particle size is 5–7 nm. By freezing the reaction mixture, the process of particle growth can be followed from 5 to 40 min on TEM pictures. The first order rate constants were calculated and they are strongly depend on the polymer concentration. If the growing particles are attached by PEI to the surface of a solid support, the formation of silver nanoparticles can also be followed by atomic force microscopy (AFM) and we can control the particle growth on mica surface. The cross section analysis of the pictures show, that the particle growing process can be also monitored at solid–liquid interface.     

The derivation of explicit solutions to Wannier–Stark resonances for electrons on coupled chains

Electrons on infinite coupled chains with nearest neighbour couplings under uniform electric and magnetic fields can be expressed as conditionally solvable systems, which concerns both discrete coordinate and wavenumber representations. We then have to account for multiparameter extra-conditions relying on the single chain phase of the system, which amounts to perform a suitable selection of parameters. The implicit plots provided by such conditions exhibit both regular and irregular patterns. This results in the onset of a finite number of Wannier–Stark resonances, now by performing rescalings needed. However, this time the resonance width is sensitive to the quantum number characterizing the Stark-ladder. Bound-state limits, rescalings and approximations proceeding irrespective of the wavenumber have also been presented.     

Atomic force microscopy investigation of electrochemically produced carbon nanotubes

Carbon nanostructures have been synthesized in NaCl-MgCl₂ and in NaCl-CaCl₂ salt melts and the extracted material was investigated by tapping-mode atomic force microscopy (TM-AFM) and scanning electron microscopy. Some interesting new nanostructures were found and investigated as torus-shaped carbon structures with a ring diameter of 300-400 nm and 10-15 nm height. These tori are closely related to the wrapped SWNT rings described recently. They are probably formed during the electrolysis. A chain-like structure was also revealed.     

Overtone spectroscopy of some benzaldehyde derivatives

Overtone spectrum of o, m and p-nitrobenzaldehydes and p-chlorobenzaldehyde has been studied in 2000–12000 cm⁻¹ region. Vibrational frequencies and anharmonicity constants for aryl as well as alkyl CH stretch vibrations have been determined. We have also determined the internuclear distances for the aryl CH bond in the different molecules. The small variation observed in these distances is an indication of the substitution effect. It is observed that in the case of p-disubstituted benzens, the shift in aryl CH bond is proportional to sum of the Hammet σ of the substituents. However in the case of o-disubstituted benzenes it is only 80% of the para-substituted shift.     

Coupled chains in electric and magnetic fields revisited

Coupled chains in electric and magnetic fields are discussed in terms of interplays between periodicity conditions and the factorization of the wavefunction in the wavenumber representation. Proceeding in this manner yields a quickly tractable matching condition providing the quantization rule to the alternative derivation of complex energy bands. Besides achieving a deeper understanding of Wannier-Stark ladders in terms of complex resonance energies, we now have the opportunity to establish related energy levels by resorting to a suitable conversion technique. This amounts to perform an immediate integration of such energy bands over the Brillouin-zone. Such energy levels exhibit, this time, a safe dependence on the chain parameter. Dynamic localization effects, conserved currents and stripe conductivity formulae have also been discussed.     

Formation of gold nanoparticles in diblock copolymer micelles with various reducing agents

Gold nanoparticles (Au NPs) were prepared by the reduction of HAuCl₄ acid incorporated into the polar core of poly(styrene)-block-poly(2-vinylpyridine) (PS-b-P2VP) copolymer micelles dissolved in toluene. The formation of Au NPs was controlled using three reducing agents with different strengths: hydrazine (HA), triethylsilane (TES), and potassium triethylborohydride (PTB). The formation of Au NPs was followed by transmission electron microscopy, UV–Vis spectroscopy, isothermal titration calorimetry (ITC), and dynamic light scattering (DLS). It was found that the strength of the reducing agent determined both the size and the rate of formation of the Au NPs. The average diameters of the Au NPs prepared by reduction with HA, TES, and PTB were 1.7, 2.6, and 8 nm, respectively. The reduction of Au(III) was rapid with HA and PTB. TES proved to be a mild reducing agent for the synthesis of Au NPs. DLS measurements demonstrated swelling of the PS-b-P2VP micelles due to the incorporation of HAuCl₄ and the reducing agents. The original micellar structure rearranged during the reduction with PTB. ITC measurements revealed that some chemical reactions besides Au NPs formation also occurred in the course of the reduction process. The enthalpy of formation of Au NPs in PS-b-P2VP micelles reduced by HA was determined.