Methyl methacrylate polymerization at samarium(II)-grafted MCM-41

Sm(II)-modified periodic mesoporous silica (PMS), Sm[N(SiHMe₂)₂]₂(THF)_x@MCM-41, was used for the synthesis of Sm(II) alkyl, alkoxide, and indenyl surface species via secondary ligand exchange. The performance of the novel Sm(II)-based organometallic–inorganic hybrid materials as initiators for the graft polymerization of methyl methacrylate (MMA) is reported. All of the Sm(II) hybrid materials including the new PMMA–PMS composites were characterized via N₂ physisorption, elemental analysis, FTIR spectroscopy, and scanning electron microscopy (SEM). The organic–inorganic composites revealed complete pore blockage as well as enrichment and strong adhesion of the polymer at the exterior of the porous silica material.     

Planar and tubular perovskite-type membrane reactors for the partial oxidation of methane to syngas

Dense planar and tubular oxygen separation membranes of La_0.6Ca_0.4Fe_0.75Co_0.25O_3– were investigated as reactors for the partial oxidation (POX) of methane to syngas. Their permeation properties were measured in an air/argon pO₂ gradient as a function of temperature. At 900 °C, the oxygen flux through a 1.26-mm-thick membrane was 0.075 mol/cm²·s and through a 0.25-mm-thick tube, 0.24 mol/cm²·s.For the POX measurements, a catalyst was added to the membrane and methane was introduced on the argon side. This resulted in a gradual increase of the oxygen flux with increasing concentration of methane, reaching 2 mol/cm²·s at 900 °C with pure methane. For the planar reactor, the CO selectivity reached 99% and the CH₄ conversion 75% at 918 °C with pure methane. For the tubular reactor, the CO selectivity and CH₄ conversion were 83 and 99%, respectively, under the same conditions. After 1,400 h of operation in a tubular POX reactor, the membrane was examined revealing phase demixing and local decomposition.Presented at the OSSEP Workshop Ionic and Mixed Conductors: Methods and Processes, Aveiro, Portugal, 10–12 April 2003     

The quenching of CdSe quantum dots photoluminescence by gold nanoparticles in solution

The photoluminescence (PL) of CdSe quantum dots (QD) in aqueous media has been studied in the presence of gold nanoparticles (NP) with different shapes. The steady state PL intensity of CdSe QD (1.5-2 nm in size) is quenched in the presence of gold NP. Picosecond bleach recovery and nanosecond time-resolved luminescence measurements show a faster bleach recovery and decrease in the lifetime of the emitting states of CdSe QD in the presence of quenchers. Surfactant-capped gold nanorods (NR) with aspect ratio of 3 and surfactant-capped and citrate-capped nanospheres (NS) of 12 nm diameter were used as quenchers in order to study the effect of shape and surface charge on the quenching rates. The Stern-Volmer kinetics model is used to examine the observed quenching behavior as a function of the quencher concentration. It was found that the quenching rate of NR is more than 1000 times stronger than that of NS with the same capping material. We also found that the quenching rate decreases as the length of the NR decreases, although the overlap between the CdSe emission and the NR absorption increases. This suggests that the quenching is a result of electron transfer rather than long-range (Forster-type) energy transfer processes. The quenching was attributed to the transfer of electron with energies below the Fermi level of gold to the trap holes of CdSe QD. The observed large difference between NR and NS quenching efficiencies was attributed to the presence of the [110] facets only in the NR, which have higher surface energy.     

Platinum(II) 1,10-phenanthroline complexes of acetylides containing redox-active groups

The new diimine ligand 3,8-di-n-pentyl-4,7-di(phenylethynyl)-1,10-phenanthroline (1) was used for the synthesis of a range of Pt(II) complexes, viz.[Pt(1)Cl2], [Pt(1)(C triple bond C-Ph)2], [Pt(1)(C triple bond C-Fc)2] and [Pt(1)(C triple bond C-p-C6H4-C triple bond C-Fc)2](Fc = ferrocenyl). Crystal structure analyses were performed for [Pt(1)Cl2] and [Pt(1)(C triple bond C-Ph)2] and revealed that the di(acetylide)pi-tweezer of the latter binds a molecule of chloroform through C-H...pi hydrogen bonds. The redox and optical properties of 1 and its complexes were investigated by (spectro-)electrochemistry, UV-Vis and luminescence spectroscopy, and an energy level diagram was derived for [Pt(1)(C triple bond C-Fc)2] and related compounds on the basis of the data collected. The ferrocenyl-substituted Pt(II) complexes are donor-sensitiser assemblies. Intramolecular quenching of the photoexcited Pt(II) diimine unit leads to very short luminescence lifetimes for [Pt(1)(C triple bond C-p-C(6)H(4)-C triple bond C-Fc)2](2 ns) and [Pt(1)(C triple bond C-Fc)2](0.3 ns), as opposed to [Pt(1)(C triple bond C-Ph)2](0.7 micros). Excimer formation has been observed for [Pt(1)(C triple bond C-Ph)(2)] at room temperature in dichloromethane and at low temperatures in frozen glassy dichloromethane and 2-methyltetrahydrofuran solution, but not in the solid state.     

4-Aminobicyclo[2.2.2]octanone Derivatives with Antiplasmodial and Antitrypanosomal Activities

4-Aminobicyclo[2.2.2]octanones were converted to their N-oxides and to 4-aminobicyclo[2.2.2]octanes. Furthermore, the 6,7-bis-(4-methoxyphenyl) analogues were synthesized. All products were screened for their activities against Trypanosoma b. rhodesiense and Plasmodium falciparum. The pharmacological results were compared with those of formerly tested bicyclo[2.2.2]octanones and bicyclo[2.2.2]octanols. Structure-activity relationships are discussed.     

Herstellung enantiomerenreiner cis- oder trans-konfigurierter 2-(tert-Butyl)-3-methylimidazolidin-4-one aus den Aminosäuren (S)-Alanin, (S)-Phenylalanin, (R)-Phenylglycin, (S)-Methionin und (S)-Valin

Preparation of the Enantiomerically Pure cis- and trans-Configurated 2-(tert-Butyl)-3-methylimidazolidin-4-ones from the Amino Acids (S)-Alanine, (S)-Phenylalanine, (R)-Phenylglycine, (S)-Methionine, and (S)-Valine In contrast to α-hydroxy and α-mercapto carboxylic acids, simple α-amino acids do not form acetal-type derivatives ( 2 , X = NH) with pivalaldehyde. For the generation of amino-acid-derived chiral, nonracemic enolates (cf. 3 ), and hence, for the α-alkylation of amino acids without racemization and without an external chiral auxiliary, the imidazolidinones 12–14 were prepared diastereoselectively. To this end, the methyl or ethyl esters of amino-acid hydrochlorides were first converted to N-methylamides of amino acids which in turn were condensed with pivalaldehyde to give (neopentylidenamino)amides ( 11 ). These Schiff bases could be cyclized either to trans-or to cis-imidazolidinones ( 12, 14 and 13 , respectively), which were obtained in enantiomerically pure form after recrystallization. The enantiomeric purities were confirmed by HPLC with chiral stationary phases or by ¹H-NMR spectroscopy in the presence of chiral shift reagents. The configurations (cis, trans) were assigned by NOE measurements on 300- or 360-MHz ¹H-NMR spectrometers.     

Copper oxide nanocrystals

It is well-known that inorganic nanocrystals are a benchmark model for nanotechnology, given that the tunability of optical properties and the stabilization of specific phases are uniquely possible at the nanoscale. Copper (I) oxide (Cu(2)O) is a metal oxide semiconductor with promising applications in solar energy conversion and catalysis. To understand the Cu/Cu(2)O/CuO system at the nanoscale, we have developed a method for preparing highly uniform monodisperse nanocrystals of Cu(2)O. The procedure also serves to demonstrate our development of a generalized method for the synthesis of transition metal oxide nanocrystals. Cu nanocrystals are initially formed and subsequently oxidized to form highly crystalline Cu(2)O. The volume change during phase transformation can induce crystal twinning. Absorption in the visible region of the spectrum gave evidence for the presence of a thin, epitaxial layer of CuO, which is blue-shifted, and appears to increase in energy as a function of decreasing particle size. XPS confirmed the thin layer of CuO, calculated to have a thickness of approximately 5 A. We note that the copper (I) oxide phase is surprisingly well-stabilized at this length scale.     

Application of atom-photon coincidence techniques to the determination of the differential cross section for excitation ofK(42 P) in low-energy K-N2, CO collisions

The differential cross section for the processK(4² S)+N₂, CO→K(4² P)+ N₂, CO is presented for reduced scattering angles τ<2·10⁴ eV deg. The inelastic process is identified by determining the time-correlation between the inelastically scattered potassium atom and the emittedK(4² P→4² S)photon. The performance of different coincidence techniques is compared. From the differential cross sections values for the position of the curve crossing responsible for the excitation process are derived (R _c=2.77, 2.48 Å andV(R _c )=0.55, 1.00 eV for K-N₂, CO respectively). The values indicate that the lowest ionic intermediate state of the form K⁺-N ₂ ^? , CO^? is responsible for the excitation process.     

Expanding the hydride chemistry: antiperovskites A3MO4H (A = Rb,Cs; M = Mo,W) introducing the transition oxometalate hydrides

The four compounds A₃MO₄H (A = Rb, Cs; M = Mo, W) are introduced as the first members of the new material class of the transition oxometalate hydrides. The compounds are accessible via a thermal synthesis route with carefully controlled conditions. Their crystal structures were solved by neutron diffraction of the deuterated analogues. Rb₃MoO₄D, Cs₃MoO₄D and Cs₃WO₄D crystallize in the antiperovskite-like K₃SO₄F-structure type, while Rb₃WO₄D adopts a different orthorhombic structure. ²H MAS NMR, Raman spectroscopy and elemental analysis prove the abundance of hydride ions next to oxometalate ions and experimental findings are supported by quantum chemical calculations. The tetragonal phases are direct and wide band gap semiconductors arising from hydride states, whereas Rb₃WO₄H shows a unique, peculiar valence band structure dominated by hydride states.

The synthesis, structures and electronic properties of the first four heteroanionic compounds containing both hydride and transition oxometalate ions are reported.