Tuning the Nigerian slit gong

An experimental and theoretical investigation of the Nigerian slit gong is reported. It is shown that in tuning the gong the artisan ensures that the frequencies of the two lowest mechanical resonances are nearly coincident with the frequencies of two of the acoustic resonances of the internal cavity. Four possible tuning parameters are identified and the effects of changing these parameters are discussed.     

Organofluorosilanes as Model Compounds for 18F‐Labeled Silicon‐Based PET Tracers and their Hydrolytic Stability: Experimental Data and Theoretical Calculations (PET=Positron Emission Tomography)

Water stable! Radiochemists have recently discovered silicon chemistry as a tool for the introduction of ¹⁸F into biomolecules for positron emission tomography (PET) imaging. ¹⁸F‐labeled PET tracers must be stable towards defluorination under physiological conditions. Here, a theoretical model of organofluorosilane hydrolysis is developed that correlates with the experimentally determined hydrolytic half‐lives and allows estimation of the stability of newly designed compounds (see scheme).

     

Gold nanoparticle decoration of DNA on silicon

Electrostatic assembly of cationic nanoparticles onto the negatively charged backbone of double-stranded DNA has been shown to produce one-dimensional chains with potential use as nanoelectronic components. In this paper, micron long DNA templates stretched on aminosilane- and hexamethyldisilazane-modified silicon surfaces are used to assemble 3.5 nm gold nanoparticles passivated with cationic thiocholine. Atomic force microscopy is used to analyze the density and defects along the approximately 5 nm high structures, with comparison between positively charged and neutral surfaces. Low background adsorption of nanoparticles is facilitated by both these surface chemistries, while the neutral surface yields a more densely packed assembly.     

Dynamics of the tyrosine triplet state from magnetic resonance-saturated phosphorescence decay measurements

Analysis of the phosphorescence decays measured during magnetic resonance saturation of sublevel populations has been carried out on tyrosine and tyrosinate triplet states at 1.17°K in zero field. The individual sublevel decay constants and spin-lattice relaxation rate constants are derived. Relative intersystem crossing rates to the sublevels are obtained from flash excitation microwave-induced delayed phosphorescence measurements. Intersystem crossing is not spin-selective in tyrosine, but becomes so upon ionization near pH = 12. The spin-selective intersystem crossing mechanism in tyrosinate is discussed in terms of a model proposed by Bersohn.     

Is the enthalpy of fusion of tris(acetylacetonato)metal(III) complexes affected by their potential energy in the crystal state?

In this Article we present enthalpies of fusion and melting points obtained from new thermochemical measurements of tris(acetylacetonato)metal(III), M(acac)(3), complexes (M = Fe, Al, Cr, Mn, Co) using differential scanning calorimetry (DSC) and evaluate them in relation to their different values found in the literature. An enthalpy of fusion of 27.67 kJ mol(-)(1) was derived for Mn(acac)(3) from a symmetrical DSC thermogram captured for the first time. The enthalpy value was indirectly confirmed with the solubility measurements of Mn(acac)(3) in acetylacetone. A hypothesis has been stated that the enthalpy of fusion and the potential energy of M(acac)(3) in the crystal state may be related. To calculate molecular in-crystal potential energy, in this Article we proposed a molecular mechanics model for the M(acac)(3) class of compounds. Nine X-ray crystal structures of M(acac)(3) complexes (M = Fe, Al, V, Mn, Co, Cr, Sc) were included in the modeling. The conformational potential energy was minimized for a molecule surrounded by other molecules in the crystal lattice. The partial charges from two schemes, the electrostatic potential (ESP) fit and the natural population analysis (NPA), were used to construct two types of force fields to examine which force field type would yield a better fit with the experimental thermal properties. The final force fields were named FF-ESP and FF-NPA. Both force field sets reproduced well the experimental crystal data of nine M(acac)(3) complexes as well as of tris(3-methyl-2,4-pentanedionato-O,O')cobalt(III). Only in-crystal potential energies derived by FF-NPA yielded a significant correlation (correlation coefficient R = -0.71) with the measured enthalpies of fusion. The enthalpy of fusion for Co(acac)(3) could not be determined experimentally because of simultaneous decomposition and fusion, and it is predicted to be 33.2 kJ mol(-)(1) from the correlation regression line.     

NMR.-spektroskopische Methoden als Reinheitskriterien isomerer Weinsäuren und Weinsäureester

NMR.-spectroscopic Methods as Criteria of the Purity of Isomeric Tartaric Acids and Their Esters meso-Tartaric acid ( 2 ) can be distinguished either from the natural (+)-(2R, 3R)-tartaric acid ( 1 ) or the ‘unnatural’ (?)-(2S, 3S)-tartaric acid ( 1 ′) or their racemic mixture, by ¹H-NMR.-spectral resolution using europium chloride in aqueous solution. Diastereomeric esters have been prepared from different esters of tartaric acid 3 and the Mosher reagent 4 and the purities of the enantiomers of 3 have been checked by ¹H-NMR. spectroscopy.     

Synthese von optisch aktiven,natürlichen Carotinoiden und strukturell verwandten Naturprodukten. IV. Synthese von (3R, 3′R, 6′R)-Lutein. Vorläufige Mitteilung

Synthesis of optically active natural carotenoids and structurally related compounds. IV. Synthesis of (3R, 3′R, 6′R)-lutein The synthesis of (3R, 3′R, 6′R)-lutein ( 19 ) according to the building principle C₂₅+C₁₅?C₄₀ is reported utilizing (R)-4-hydroxy-2,6,6-trimethyl-2-cyclohexen-1-one ( 4 ) as a readily available key intermediate.