No-D NMR spectroscopy as a convenient method for titering organolithium (RLi), RMgX, and LDA solutions

[reaction: see text] The concentration of organolithium, organomagnesium halide, and lithium diisopropylamide solutions can be reliably determined using No-D NMR spectroscopy by integration against the added internal standard 1,5-cyclooctadiene (COD) (or cyclooctene). In addition, common impurities and degradation products can be assessed.     

Spectrophotometric determination of EDTA

Both iron(III) and its EDTA complex in 0-1N H(2)SO(4) medium absorb in the ultraviolet region of the spectrum, with absorption maxima at 305 and 258 nm respectively. EDTA can thus be determined by addition of excess of Fe(III) and measurement of the absorbance at the two wavelengths. An EDTA concentration as low as 10(-6)M can be determined with a fair degree of accuracy. The method is free from interferences from most common metal ions, and is simple and rapid.     

Determination of the Concentrations of Metal Cations in Aqueous Solutions Using Proton NMR Spectral Area Integration of the EDTA Complexes

This paper introduces an alternative method to determine the concentrations of metal cations in aqueous solutions, using area integration of proton NMR spectra of EDTA (the disodium salt of ethylenediaminetetraacetic acid) metal complexes. Using this method, a certain volume of unknown metal cation solution is added to an excess amount of standard EDTA solution, and the ratio of the remaining free EDTA and the EDTA–metal complex in the solution can be calculated from the integration areas of the proton NMR spectra. Unlike the conventional complexometric titration, where indicators need to be used to determine the endpoint, this NMR method aims to provide a simple method to eliminate the need for using indicators. This method, when EDTA is used as the standard, is valid for determining the concentrations of nonparamagnetic transition and rare earth metal cations, and for the concentrations of group IIIA and IVA metal cations.     

Limits of the molar-ratio method

The limiting factors (one constant molar-ratio of metal to ligand, constant excess of masking reagent and buffer, and high enough effective stability) of the molar ratio method are discussed. The lowest values of the effective stability constant for the true metal/ligand ratio to be determined for the complex and for a metal determination to be relatively independent of ligand concentration are calculated.     

No-D NMR (no-deuterium proton NMR) spectroscopy: a simple yet powerful method for analyzing reaction and reagent solutions

[reaction: see text] The title technique is a convenient and powerful method for directly monitoring or assaying any reaction mixture or reagent solution. Examples of some common processes (Fischer esterification, lithiation, butyllithium/THF compatibility, olefin metathesis, and a quantification assay), each interrogated in its native solvent, are presented. The spectral data are easy to acquire, and the information content makes a compelling case for routine use of No-D NMR spectroscopy.     

Voltammetric determination of mercury(II) at a carbon paste electrode in aqueous solutions containing tetraphenylborate ion

The determination of mercury(II) ions can be achieved by monitoring the decrease in the oxidation peak of the tetraphenylborate ion in the presence of this metal ion at a carbon paste electrode. The reaction between mercury(II) and the tetraphenylborate ion results in the formation of diphenylmercury, thus providing the method with good selectivity over other metal ions. Using anodic stripping voltammetry in a neutral electrolyte, a linear dependence of the decrease of peak height was observed on increasing the mercury(II) concentration in the range 1 x 10(-6)-8 x 10(-9)M mercury(II). Zinc(II), cadmium(II), lead(II), nickel(II), cobalt(II), tin(II), potassium(I) and ammonium(I) ions did not interfere at a 1000-fold concentration excess. Iron(III) and chromium(III) did not interfere at a 250-fold and 50-fold concentration excess, respectively. Following masking procedures, copper(II), bismuth(III) and silver(I) did not interfere at a 100-fold concentration excess. The method can be used to determine the concentration of mercury(II) in natural waters contaminated by this metal.     

Preconcentration and determination of calcium and magnesium in brine with flow-injection systems

On-line preconcentration on a chelating resin (Dowex A-1) and elution with 0.1 M hydorchloric acid is followed by spectrophotometry based on the metal complexes formed with 1- (2-hydroxy-4-diethylamino-1-phenylazo)-2-hydroxynaphthalene-3,6-disulfonic acid. The total concentration of calcium and magnesium is determined; in a second sample, calcium is masked with a ligand buffer containing excess of barium(II) and EGTA, and magnesium is determined. The calcium concentration is measured by difference. Magnesium (1–30 μg l^?1 and calcium (8– 10 μg l^?1) in 2.5 M sodium chloride can be determined. Calcium and magnesium in analytical reagent-grade sodium chloride and potassium chloride and primary standard sodium chloride are aslo determined. The method based on the exchange between calcium ions and Mg(EDTA) is proposed to enchance the sensitivity for calcium.     

Simple pulse-acquire NMR methods for the quantitative analysis of calcium, magnesium and sodium in human serum

Simple pulse-acquire NMR methods are presented for accurate quantitation of calcium (Ca(2+)), magnesium (Mg(2+)) and sodium (Na(+)) in human serum. Ca(2+) and Mg(2+) can be determined simultaneously by (1)H NMR via their EDTA (ethylenediaminetetraacetic acid) complexes. Spectra are acquired before and after addition of EDTA, and the difference spectrum is used for integration of the signals from the complexes relative to an internal reference. Serum sodium can also be determined through pulse-acquire (23)Na NMR by integration of the free sodium signal relative to a reference signal (Na(+)+EDTA in a coaxial capillary tube). The method shows excellent accuracy and precision for all three metal ions. Slow chemical exchange between complexed and free EDTA at the natural pH of serum does not limit the accuracy of the determination of Ca(2+) and Mg(2+), and the large errors associated with spin-echo spectral editing methods are avoided.     

Magnetic Resonance of Paramagnetic Complexes

The line width of the ESR and NMR signals of paramagnetic transition metal complexes is determined mainly by the electron spin-lattice relaxation time τ_e. Values of τ_e greater than 10^?9 lead to ESR spectra that are readily resolved, while values smaller than 10^?11 give NMR spectra having small line widths. Since fast relaxation processes are effective in nearly all transition metal complexes with several unpaired electrons and in all complexes having an orbitally degenerate ground state, the NMR method has a wider scope. The sign and magnitude of the electron-nucleus coupling can be determined with great sensitivity from the NMR spectra, whereas only the magnitude of this interaction can be determined from the ESR spectra. Free spin densities can be found very accurately from the NMR shifts, and the method can therefore be advantageously applied to kinetic measurements, e.g. on short-lived contact complexes.     

Complexation of zinc octaalkylporphyrin with mono-, di-, and triethylenediamines in toluene

The complexation of zinc octaalkylporphyrin with mono-, di-, and triethylenediamines in toluene was studied by spectrophotometry using the molar ratio method and by ¹H NMR. The effect of the structure of the two-center organic base on the formation of 1 : 1 or 2 : 1 metal porphyrin-ligand complex was established. The stability constants of the resulting complexes were calculated and the concentration ranges of their existence were determined.     

No-D NMR study of the pathway for n-BuLi "oxidation" of 1,5-cyclooctadiene to dilithium cyclooctatetraene dianion [Li2COT2-

[Reaction: see text] The transformation of 1,5-cyclooctadiene (1) into cyclooctatetraene (3) by way of dianion 2 is an interesting reaction of considerable preparative value. The mechanism was initially suggested to involve lithium hydride loss from 4a/4b, followed by two deprotonations to produce 2. This No-D 1H NMR study indicates the long-term stability of 4a/4b (in the absence of additional n-BuLi) and suggests a different mechanistic sequence, in which 4a/4b is deprotonated a second time prior to LiH ejection.     

Qualitative and quantitative 1H NMR spectroscopy for determination of divalent metal cation concentration in model salt solutions,food supplements,and pharmaceutical products by using EDTA as chelating agent

This paper introduces an ¹H NMR method to identify individual divalent metal cations Be²⁺, Mg²⁺, Ca²⁺, Sr²⁺, Zn²⁺, Cd²⁺, Hg²⁺, Sn²⁺, and Pb²⁺ in aqueous salt solutions through their unique signal shift and coupling after complexation with the salt of ethylenediaminetetraacetic acid (EDTA). Furthermore, quantitative determination applied for the divalent metal cations Ca²⁺, Mg²⁺, Hg²⁺, Sn²⁺, Pb²⁺, and Zn²⁺ (limit of quantification: 5–22 μg/ml) can be achieved using an excess of EDTA with aqueous model salt solutions. An internal standard is not required because a known excess of EDTA is added and the remaining free EDTA can be used to recalculate the quantity of chelated metal cations. The utility of the method is demonstrated for the analysis of divalent cations in some food supplements and in pharmaceutical products.     

The determination of enantiomeric excess of valine by ODESSA solid-state NMR experiment

The enantiomeric excess (ee) can be determined by many methods; one among them is nuclear magnetic resonance in solid-state (SS NMR). In this study we used the SS NMR ODESSA experiment for determination of the ee of valine.     

Complexometric determination of citric acid with copper

A method is presented in which citrate is determined by complexation with excess of copper(II), followed by back-titration with EDTA. Small amounts of citrate can be determined by this method in the presence of large amounts of most organic acids present in natural products. Interfering reductants are removed by treatment with permanganate, amino-acids with nitrous acid, and metal ions with a cation-exchange resin. The indicator used for end-point detection is Szechromotrope, 4,4'-sulphonyl-bis(azobenzene) dichromotropic acid.     

From sulfur-amine solutions to metal sulfide nanocrystals: peering into the oleylamine-sulfur black box

In this Article, we present our findings on the formation of metal sulfide nanocrystals from sulfur-alkylamine solutions. By pulsed field gradient diffusion NMR along with the standard toolbox of 1D and 2D NMR, we determined that sulfur-amine solutions used as a sulfur precursor exist as alkylammonium polysulfides at low temperatures. Upon heating to temperatures used in nanocrystal synthesis, the polysulfide ions react with excess amine to generate H(2)S, which combines with the metal precursor to form metal sulfide. Four different reaction pathways were found, each of which produced H(2)S and the byproducts identified in this Article. Thioamides were identified as an intermediate and were shown to exhibit much more rapid kinetics than sulfur-alkylamine solutions at low temperatures in the synthesis of metal sulfide nanocrystals.     

Coulometrische argentometrie : Quantitative bestimmung von chlor-, brom- und jodionen

By electrolytic oxidation of various metal anodes, metal ions can be quantitatively introduced into solution. This process can be used for coulometric determination of anions forming insoluble salts with metal ions.Semimicro and micro determinations of chloride, bromide and iodide ions with electrolytically generated silver ions have been performed at constant current and by a simple apparatus. The end-points have been determined with adsorption indicators, i.e. with dichloro(R)fluorescein for chlorides and eosin for bromides and iodides. The lower concentration limit of applicability of this method depends on the sensitivity of the end-point determination method used.The developed method of coulometric argentometry presents a new possibility for coulometric standardization of volumetric solutions, in conformity with our earlier proposals, as well as a further support for the introduction of the coulomb as a primary standard in all analytical chemistry.     

含穴醚N8O3配体的金属大环化合物的NMR研究

在溶液中,室温下对含有穴醚Ｎ８Ｏ３配体（Ｈ３Ｌ）的金属大环化合物［Ｎａ（Ｈ３Ｌ）］（ＣｌＯ４）３Ｈ２Ｏ（２）和［Ｌａ（Ｈ３Ｌ）（ＯＨ）２］（ＣｌＯ４）３（３）,作了１ＨＮＭＲ谱的测定,在化合物（２）－乙腈溶液中加入过量Ｌａ（ＣｌＯ４）３后的（２＋Ｌａ）和化合物（３）还作了１３Ｃ和１Ｈ－１ＨＣＯＳＹ（氢－氢相关二维谱）、１Ｈ－１３ＣＨＭＱＣ（氢核检测的异核多量子相干相关谱）及１Ｈ－１３ＣＨＭＢＣ（氢核检测的异核多键相关谱）的ＮＭＲ谱测定,归属了所有１Ｈ,１３Ｃ的谱线。对其配位行为通过１ＨＮＭＲ试验作了简单的讨论。     

Histidine-rich branched peptides as Cu(II) and Zn(II) chelators with potential therapeutic application in Alzheimer's disease

Two histidine-rich branched peptides with one lysine as a branching unit have been designed and synthesized by solid-phase peptide synthesis. Their complex formation with Cu(II) and Zn(II) as well as their ability to attenuate the metal-ion induced amyloid aggregation has been characterized. Both peptides can keep Cu(II) and Zn(II) in complexed forms at pH 7.4 and can bind two equivalents of metal ions in solutions with excess metal. The stoichiometry, stability and structure of the complexes formed have been determined by pH potentiometry, UV-Vis spectrophotometry, circular dichroism, EPR and NMR spectroscopy and ESI-MS. Both mono- and bimetallic species have been detected over the whole pH range studied. The basic binding mode is either a tridentate {N(amino), N(amide), N(im)} or a histamine-type of coordination which is complemented by the binding of far imidazole or amino groups leading to macrochelate formation. The peptides were able to prevent Cu(II)-induced Aβ(1-40) aggregation but could not effectively compete for Zn(II) in vitro. Our results suggest that branched peptides containing potential metal-binding sites may be suitable metal chelators for reducing the risk of amyloid plaque formation in Alzheimer's disease.     

A precise spectrophotometric determination of a major constituent in pure materials: Application to pure magnesium metal

A spectrophotometric method is proposed for the direct assay of pure materials. The bulk of the metal to be determined is reacted with an accurately measured amount of a chelating agent, and the small excess of metal ion is determined spectrophotometrically with a chromogenic reagent. The optimal conditions can be predicted by drawing the theoretical spectrophotometric titration curves near the equivalent point of a supposed titration with the chelating agent as titrant and the chromogenic reagent as indicator. The method was applied to pure magnesium metal with EDTA as the chelating agent and calmagite as the chromogenic reagent, and the relative standard deviation was found to be 0.014%. The accuracy and precision appear to be much better than those attainable by titrimetry or differential spectrophotometry.     

Experimental (FT-IR, FT-Raman, H, C NMR) and theoretical study of alkali metal 2-aminonicotinates

The influence of lithium, sodium, potassium, rubidium and cesium on the electronic system of 2-aminonicotinic acid (2-ANA) was studied by the methods of molecular spectroscopy. The vibrational (FT-IR, FT-Raman) and NMR (¹H and ¹³C) spectra of 2-aminonicotinic acid and its alkali metal salts were recorded. Characteristic shifts and changes in intensities of bands along the metal series were observed. The changes of chemical shifts of protons (¹H NMR) and carbons (¹³C NMR) in the series of studied alkali metal 2-aminonicotinates (2-AN) were observed too.Optimized geometrical structures of the studied compounds were calculated by the B3LYP method using the 6-311++G** basis set. Aromaticity indices, atomic charges, dipole moments and energies were also calculated. The theoretical chemical shifts in ¹H and ¹³C NMR spectra and theoretical wavenumbers and intensities of IR and Raman spectra were determined. The calculated parameters were compared to the experimental characteristics of the studied compounds.