Carbon-13 NMR spectra of rifamycins

The natural abundance ¹³C Fourier transform magnetic resonance spectra of rifamycin S and some of its derivatives have been studied. A combination of five different approaches has made unambiguous assignments for most of the resonances possible: (1) comparative study of the non- decoupled and noise-decoupled spectra; (2) ¹³C spectral characteristics; (3) spectral comparison between derivatives; (4) selective proton decoupling; (5) biogenetic evidence. Pulse and Fourier transform ¹³C NMR spectroscopy provides a more complete picture of these complex molecules than was previously obtained by ¹H NMR spectroscopy.     

Flame emission studies of several metal chlorides with vapor-phase sampling

A method is described for the production of spectra of aluminum chloride, iron(III) chloride, silicon tetrachloride and titanium tetrachloride vapors by flame emission spectrometry. The aluminum and iron(III) chloride vapors were prepared by heating solid samples in reaction flasks; silicon and titanium tetrachlorides have sufficient vapor pressures at ambient temperatures to produce vapor-phase samples. Techniques have been developed to introduce the sample into the flame as a vapor, and to accommodate a large concentration of sample while requiring minimum preparation. Spectra were obtained individually and as a mixture over 240–600 nm. The analytical wavelength was chosen for each element, and 10-s integrations were made by utilizing a microcomputer to slew rapidly to the line of interest, hold for 10 s on the emission line, move off wavelength, hold for a 10-s background measurement and slew rapidly to the next line of interest. The microcomputer was also used to digitize and display the number of photons counted.     

Intramolecular cyclization of styrene–p-divinylbenzene copolymers

Styrene has been copolymerized at low conversion with minor quantities of p-divinyl-benzene (p-DVB) in (10–15%) solution in toluene and cyclohexane. Under these conditions the molecular weight of the polystyrene formed in the absence of p-DVB was controlled by chain transfer, and the copolymerization coefficients of the styrene and the p-DVB agreed with previous work. Polymer molecular weights were studied as a function of conversion. At very low conversions the number-average (2.2 × 10⁵) and the weight-average (4.4 × 10⁵) molecular weights were unaffected by substituting some of the styrene by p-DVB, but as the reaction continued M?_n increased slowly and M?_w much faster. On the other hand, even at the lowest conversions the intrinsic viscosity was drastically reduced by the introduction of p-DVB, and the radius of gyration, as measured by light scattering, fell. Infrared studies on the polymer show that the concentration of pendent double bonds in low-conversion copolymers is about half of the doubly substituted phenyl groups. It is concluded that the first polymer chains formed are extensively cyclized with the formation of a relatively large number of small rings.     

Influence of the Native Oxide Layer on the Silicon Surface During Initial Stages of Nitridation

 Thin SiO₂ layers were produced by thermal oxidation of Si wafer material. To study the effect of nitridation on the oxide layers, the specimens were nitrided in a furnace at high temperature. Non-destructive ion beam analysis was performed to determine changes in the elemental concentrations and depth profiles of the major components. In particular, N and O concentrations were measured using the non-resonant nuclear reactions ¹⁴N(d, α)¹²C and ¹⁶O(d, p)¹⁷O, respectively. To obtain depth profiles of the as-prepared and nitrided specimens, the samples were measured with RBS and heavy ion elastic recoil detection analysis. The ion beam analyses revealed an increase in thickness of the SiO₂ layers with temperature. The specimens nitrided at 1200 °C were almost free of N. Surface topology investigations with scanning electron microscopy revealed concentric annular artificial patterns at the surfaces. In the centre of the pattern, only silicon was measured. Additionally, a band consisting of Si, O, and N surrounding the pattern was discovered. The findings are in agreement with specimens prepared at higher temperatures. Received June 19, 2000. Revision December 9, 2000.     

Metal Ion Recognition. Interaction of New Oxygen-Nitrogen Donor Macrocycles with Selected Transition and Post-Transition Metal Ions

As part of an ongoing investigation of the factors influencing metal ionrecognition, we have investigated structure/function relationships involvingthe metal-ion binding by three new N-benzyl-substituted, 15- and 16-membered,macrocyclic ligands incorporating N₂O ₃- and N ₃O ₃-donor sets (withthe N ₃O ₃-system consisting of a N ₂O ₃-macrocyclic ring with an attachedCH ₂CH ₂NCH ₂C ₆H ₅ pendant arm). Selected solid complexes of thelatter ligand were isolated and the X-ray structures of individual Ni(II) and Ag(I) complexeswere obtained.Where solubility permitted, potentiometric titration studies in 95% methanolwere employed to investigate the binding affinities of all three ligand derivativestowards Co(II), Ni(II), Cu(II), Zn(II), Cd(II), Ag(I) and Pb(II). The 15-memberedN ₂O ₃-ring was found to be selective for Ag(I) over the other six metalsinvestigated, including Cu(II). However, the presence of a further nitrogen donorin the form of the pendant benzylamine functionality in the N ₃O ₃-donorsystem results in an increase in its binding affinity for Ag(I) but an even greaterincrease occurs for Cu(II). As a consequence, the latter ion is now more stronglybound than Ag(I). The factors influencing these respective selectivities are discussed.     

Fast Exfoliation and Functionalisation of Two‐Dimensional Crystalline Carbon Nitride by Framework Charging

Two‐dimensional (2D) layered graphitic carbon nitride (gCN) nanosheets offer intriguing electronic and chemical properties. However, the exfoliation and functionalisation of gCN for specific applications remain challenging. We report a scalable one‐pot reductive method to produce solutions of single‐ and few‐layer 2D gCN nanosheets with excellent stability in a high mass yield (35 %) from polytriazine imide. High‐resolution imaging confirmed the intact crystalline structure and identified an AB stacking for gCN layers. The charge allows deliberate organic functionalisation of dissolved gCN, providing a general route to adjust their properties.     

New tetranorlabdanoic acid from aerial parts of Salvia aethiopis

Salvia aethiopis is a perennial plant native to Eurasia and known by the common names Mediterranean sage or African sage. This plant has been used in Iranian medicine as a carminative and tonic. The ethanolic extract of aerial part of S. aethiopis exhibited moderate to weak activity towards delta and kappa opioid receptors (46.3 and 45.3% displacement, respectively). Chromatographic purification of the ethanolic extract on silica gel column led to isolation of one new: 3α-hydroxy-8α-acetoxy-13,14,15,16-tetranorlabdan-12-oic acid (I) and three known compounds: sesquiterpene spathulenol (II), β-sitosterol (III) and β-sitosterol-3-O-glucoside (IV). The structures of all isolated compounds were elucidated by their NMR (1D and 2D) and MS spectral data. All the fractions and isolated compounds were tested for cannabinoid and opioid receptor binding assays.     

Forces between a Rigid Probe Particle and a Liquid Interface

The effect of disjoining pressure between a rigid spherical probe particle (attached to an AFM cantilever) and a liquid interface (e.g., oil/water or air/water) is treated in an analytic manner to describe the total force F exerted on the probe as a function of the distance X of the probe from the rigid substrate (AFM stage) on which the liquid interface resides. Two cases (i) a flat interface under gravity and (ii) a drop whose size is sufficiently small that gravity can be neglected have been examined. A simple numerical algorithm is given for computing F(X) (the AFM observable) from a given form for the disjoining pressure. Numerical results are displayed for electrostatic probe/interface interactions which reveal the linear compliance regime experimentally observed in AFM experiments on these systems. The slope of the linear compliance regime is shown to be a function of the properties of the interface (capillary length, particle radius, drop size, contact angle of drop on rigid substrate etc.). Copyright 2001 Academic Press.     

Simple syntheses, structural diversity, and Tishchenko reaction catalysis of neutral homoleptic rare earth(II or III) 3,5-di-tert-butylpyrazolates--the structures of

The homoleptic rare-earth pyrazolate complexes [Sc(tBu2pz)3], [Ln2(tBu2pz)6] (Ln = La, Nd, Sm, Lu), [Eu4(tBu2pz)8] and the mixed oxidation state species [Yb2(tBu2pz)5] (tBu2pz = 3,5-di-tert-butylpyrazolate) have been prepared by a simple reaction between the corresponding rare-earth metal and 3,5-di-tert-butylpyrazole, in the presence of mercury, at elevated temperatures. In addition, [Yb2(tBu2pz)6] was prepared by redox transmetallation/ligand exchange between ytterbium, diphenylmercury(II) and tBu2pzH in toluene, whilst the same reactants in toluene under different conditions or in diethyl ether gave [Yb2(tBu2pz)5]. The complexes of the trivalent lanthanoids display dimeric structures [Ln2(tBu2pz)6] (Ln = La, Nd, Yb, Lu) with chelating eta2-terminal and eta2:eta2-bridging pyrazolate coordination. The considerably smaller Sc3+ ion forms monomeric [Sc(tBu2pz)3] of putative D3h molecular symmetry, with pyrazolate ligands solely eta2-bonded. [Eu4(tBu2pz)8] is a structurally remarkable tetranuclear EuII complex with two types of europium centres in a linear array. The outer two are bonded to one terminal and two bridging pyrazolates, and the inner two are coordinated by four bridging ligands. Unprecedented mu-eta5:eta2 pyrazolate ligation is observed, with each outer Eu2+ sandwiched between two eta5-bonded pyrazolate groups, which are also eta2-linked to an inner Eu2+. The two inner Eu2+ ions are linked together by two equally occupied components of each of two symmetry related, disordered pyrazolate groups with one component eta4:eta2 bridging and one eta3:eta2 bridging. [La2(tBu2pz)6] has also been shown to be a Tishchenko reaction catalyst with several organic substrates.