Copper(I) complexes with trispyrazolylmethane ligands: synthesis, characterization, and catalytic activity in cross-coupling reactions

Three novel Cu(I) complexes bearing tris(pyrazolyl)methane ligands, Tpm(x), have been prepared from reactions of equimolar amounts of CuI and the ligands Tpm, (HC(pz)(3)), Tpm*, (HC(3,5-Me(2)-pz)(3)), and Tpm(Ms), (HC(3-Ms-pz)(3)). X-ray diffraction studies have shown that the Tpm and Tpm(Ms) derivatives exhibit a 2:1 Cu:ligand ratio, whereas the Tpm* complex is a mononuclear species in nature. The latter has been employed as a precatalyst in the arylation of amides and aromatic thiols with good activity. The synthesis of a Tpm*Cu(I)-phthalimidate, a feasible intermediate in this catalytic process, has also been performed. Low temperature (1)H NMR studies in CDCl(3) have indicated that this complex exists in solution as a mixture of two, neutral and ionic forms. Conductivity measurements have reinforced this proposal, the ionic form predominating in a very polar solvent such as DMSO. The reaction of Tpm*Cu(I)-phthalimidate with iodobenzene afforded the expected C-N coupling product in 76% yield accounting for its role as an intermediate in this transformation.     

Quasi-classical rate constants for the inelastic process O2(υi≫ 1) + O2 → O2(υf) + O2

Rate constants have been calculated using quasi-classical trajectories (QCT) for the vibrational relaxation of highly excited O₂:O₂(X³Σ?,v_i) + O₂(X³Σ?,0) → O₂(X³Σ?,v_f) + O₂(X³Σ?) with 22 ≤v_i≤28. The present full-dimensional QCT results agree very well with recent quantum reduced dimensionality calculations, giving further support to the hypothesis that the observed experimental jump in depletion rates would be due partially to enhanced vibrational relaxation.     

Multisyringe flow injection analysis hyphenated with liquid core waveguides for the development of cleaner spectroscopic analytical methods: improved determination of chloride in waters

In this work, the hyphenation of the multisyringe flow injection analysis technique with a 100-cm-long pathlength liquid core waveguide has been accomplished. The Cl⁻/Hg(SCN)₂/Fe³⁺ reaction system for the spectrophotometric determination of chloride (Cl⁻) in waters was used as chemical model. As a result, this classic analytical methodology has been improved, minimizing dramatically the consumption of reagents, in particular, that of the highly biotoxic chemical Hg(SCN)₂. The proposed method features a linear dynamic range composed of two steps between (1) 0.2–2 and (2) 2–8 mg Cl⁻ L⁻¹, thus extended applicability due to on-line sample dilution (up to 400 mg Cl⁻ L⁻¹). It also presents improved limits of detection and quantification of 0.06 and 0.20 mg Cl⁻ L⁻¹, respectively. The coefficient of variation and the injection throughput were 1.3% (n = 10, 2 mg Cl⁻ L⁻¹) and 21 h⁻¹. Furthermore, a very low consumption of reagents per Cl⁻ determination of 0.2 μg Hg(II) and 28 μg Fe³⁺ has been achieved. The method was successfully applied to the determination of Cl⁻ in different types of water samples. Finally, the proposed system is critically compared from a green analytical chemistry point of view against other flow systems for the same purpose.     

Highly stereocontrolled synthesis of natural barbacenic acid,novel bisnorditerpene from Barbacenia flava

Barbacenic acid, a bisnorditerpene with five contiguous asymmetric centers (four fully substituted), has been prepared for the first time through a highly stereocontrolled route in 5.2% overall yield from a known octalone. The synthesis serves to define the absolute configuration of the natural product.     

The two-dimensional liquid-vapor coexistence line of methane adsorbed on graphite

We measured the specific heat of two-dimensional films of CH₄ adsorbed on graphite for several coverages and we observed broad peaks at the liquid-vapor transition. The critical temperature is estimated to be 70 ± 1 K, somewhat lower than reported in previous measurements. At intermediate coverages the liquid-vapor boundary is very flat indicating that it could be described by a 2D Ising model. We measured also for some coverages the triple temperature at 56.2 K and the commensurate-incommensurate transition at about 50 K, both in reasonable agreement with previous measurements.     

A desk-computer program for gran plots

The GRAN program described is designed to run on a desck computer with 32 kbyte of memory. It can plot normal titration curves (potential vs. volume) and Gran function vs. volume curves for various procedures involving selective electrodes.     

Synthesis of chiral amino epoxyaziridines: useful intermediates for the preparation of chiral trisubstituted piperidines

Chiral aminoalkyl epoxyaziridine 1 is synthesized in high yield and diastereoselectivity from L-serine. Ring opening of epoxyaziridine 1 with primary amines is carried out with total chemo- and regioselectivity, affording chiral polyfunctionalized piperidines 8. The structure of these trisubstituted piperidines is established by NMR studies.     

One-dimensional and two-dimensional quantum systems on carbon nanotube bundles

We report the first measurement of the structure of 4He atoms adsorbed on bundles of single-walled carbon nanotubes. Neutron diffraction techniques and nanotube samples closed at the end were used. At low coverage, 4He forms a 1D, single line lattice along the grooves between two nanotubes on the surface of the nanotube bundles. As coverage is increased, additional lines of 1D lattices form along the grooves. This is followed by an incommensurate, 2D monolayer covering the whole nanotube bundle surface. The lattice constants of these 1D and 2D systems are largely independent of filling once a single 1D line is formed. No occupation of the interstitial channels between nanotubes is observed in the present sample.     

CaF<Subscript>2</Subscript> thin films obtained by electrochemical processes and the effect of Tb<Superscript>3+</Superscript> doping concentration on their structural and optical properties

In this study, we analyze the effect of Tb³⁺ concentration on the structural and optical properties of CaF₂ terbium-doped thin films deposited on soda-lime glass substrates with an indium tin oxide (ITO)-sputtered coating by means of electrochemical processes. Compositional results showed that thin films have excess of F, which is believed to be the source for defect formation. XRD results showed not only shifting and widening of the CaF₂ peaks as a function of [Tb³⁺] increase in the electrolitic solution but also an additional peak when high [Tb³⁺] are used, promoting thus the formation of secondary phases such as NaTbF₄. According to photoluminescence (PL) results, when using UV excitation (λ?=?325 nm), CaF₂:Tb³⁺ thin films presented green emission (⁵D₄?→?⁷F_J, where J?=?6, 5, 4, 3, 2, 1, 0). Besides, PL studies not only allowed us to establish the quenching for Tb³⁺ concentration in the CaF₂ matrix, but they also showed that there are no changes in intensity or peak position attributed to the presence of cubic NaTbF₄ because its emission coincides with that of CaF₂ terbium-doped thin films.

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Graphical abstract Increase of [Tb³⁺] in the CaF₂ solution and the presence of Na⁺ promotes a competition effect between CaF₂:Tb³⁺ and NaTbF₄ phases under equal deposition conditions

     

Advances in methodology for the validation of methods according to the International Organization for Standardization. Application to the determination of benzoic and sorbic acids in soft drinks by high-performance liquid chromatography

Robust chemometric techniques such as least median of squares regression, H15 Huber estimator and Lenth's method are fundamental tools in the validation of analytical methods since they contribute the strategies needed to estimate efficiently parameters such as robustness, linear range, selectivity, accuracy (trueness and precision) and the capability of detection. In addition, the capability of discrimination defined as a generalisation of the capability of detection for any nominal concentration is evaluated. The new strategy proposed is applied to the validation of a chromatographic method for use in systematic analysis.