Extraction and DFT study on the complexation of the cesium cation with a hexaarylbenzene-based receptor

From extraction experiments and γ-activity measurements, the extraction constant corresponding to the equilibrium Cs⁺(aq) + A⁻(aq) + 1(nb) ⇆ 1·Cs⁺(nb) + A⁻ (nb) taking part in the two-phase water–nitrobenzene system (A⁻ = picrate, 1 = hexaarylbenzene-based receptor; aq = aqueous phase, nb = nitrobenzene phase) was evaluated as log K _ex (1·Cs⁺, A⁻) = 2.8 ± 0.1. Further, the stability constant of the hexaarylbenzene-based receptor·Cs⁺ complex (abbrev. 1·Cs⁺) in nitrobenzene saturated with water was calculated for a temperature of 25 °C: log β _nb (1·Cs⁺) = 4.7 ± 0.1. By using quantum mechanical DFT calculations, the most probable structure of the 1·Cs⁺ complex species was solved. In this complex having C ₃ symmetry, the cation Cs⁺ synergistically interacts with the polar ethereal oxygen fence and with the central hydrophobic benzene bottom via cation–π interaction. Finally, the calculated binding energy of the resulting complex 1·Cs⁺ is −220.0 kJ/mol, confirming relatively high stability of the considered cationic complex species.     

Experimental and theoretical study on the complexation of the cesium cation with dibenzo-30-crown-10

From extraction experiments and γ-activity measurements, the extraction constant corresponding to the equilibrium Cs⁺ (aq) + A⁻ (aq) + 1(nb) \rightleftarrows \rightleftarrows 1·Cs⁺(nb) + A⁻(nb) taking place in the two-phase water–nitrobenzene system (A⁻ = picrate, 1 = dibenzo-30-crown-10; aq = aqueous phase, nb = nitrobenzene phase) was evaluated as log K _ex (1·Cs⁺, A⁻) = 4.0 ± 0.1. Further, the stability constant of the 1·Cs⁺ complex in nitrobenzene saturated with water was calculated for a temperature of 25 °C: log β _nb (1·Cs⁺) = 5.9 ± 0.1. Finally, by using quantum–mechanical DFT calculations, the most probable structure of the resulting cationic complex species 1·Cs⁺ was derived.     

Intermediate and ion-pair formation in the outer-sphere reactions between azido-pentacyanocobaltate(III) and iron(II) polypyridyl complexes in aqueous medium

The kinetics of the reactions between azido-pentacyanocobaltate(III), Co(CN)₅N₃ ³⁻, and iron(II) polypyridyl complexes, Fe(LL)₃ ²⁺ (LL = bipy, phen), have been studied in both neutral and acidic aqueous solutions at I = 0.1 mol dm⁻³ NaCl. The reactions were carried out under pseudo-first-order conditions in which the concentration of Fe(LL)₃ ²⁺ was kept constant, and the second-order rate constants obtained for the reactions at 35 °C were within the range of 0.156–0.219 dm³ mol⁻¹ s⁻¹ for LL = bipy and 0.090–0.118 dm³ mol⁻¹ s⁻¹ for LL = phen. Activation parameters were measured for these systems. The dependence of reaction rates on acid was studied in the range [H⁺] = 0.001–0.008 mol dm⁻³. The reaction in acid medium shows interesting kinetics. Two reactive species were identified in acid medium, namely, the protonated cobalt complex and the azido-bridged binuclear complex. The electron-transfer process is proposed to go by mixed outer- and inner-sphere mechanisms in acid medium, in which electron transfer through the bridged inner-sphere complex (k ₅) is slower than through the outer-sphere path (k ₄). Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

The study of the speciation of uranyl–sulphate complexes by UV–Vis absorption spectra decomposition

Uranyl–sulphate complexes are the predominant U(VI) species present in acid solutions resulting either from underground uranium ore leaching or from the remediation of leaching sites. Thus, the study of U(VI) speciation in these solutions is of practical significance. The spectra of UO₂(NO₃)₂ + Na₂SO₄ solutions of different Φ _S = [SO₄²⁻]/[U(VI)] ratio at pH = 2 were recorded for this purpose. As the presence of uranyl-nitrate complexes should be expected under these experimental conditions, the spectra of UO₂(NO₃)₂ + NaNO₃ solutions with different Φ _N = [NO₃⁻]/[U(VI)] ratio at pH = 2 were also measured. The effects of Φ _S and Φ _N ratios value were most pronounced in wavelength interval 380–500 nm. Therefore, these parts of experimental overall spectra were used for deconvolution into the spectra of individual species by the method proposed. It enabled to calculate stability constants of anticipated species at zero ionic strength. The Specific Ion Interaction Theory (SIT) was used for this purpose. Stability constants of UO₂SO₄, UO₂(SO₄)₂²⁻, UO₂NO₃ ⁺ and UO₂(NO₃)₂ coincided well with published data, but those for UO₂(SO₄)₃⁴⁻ and UO₂(NO₃)₃⁻ were significantly lower.     

A bimetallic cyano-bridged complex based on nickel(II) and pentacyanidonitrosylferrate(II) precursors: characterization, crystal structure, and magnetic properties

A heterobimetallic cyano-bridged 1D coordination polymer of the composition [Ni(baepn)(μ-NC)Fe(CN)₃(NO)(μ-CN)]_n·3H₂O has been synthesized by the reaction of nickel(II) nitrate hexahydrate, baepn (baepn = N, N′-bis(2-aminoethyl)-1,3-propanediamine), and sodium nitroprusside dihydrate in a methanol–water mixture. The complex was characterized by physicochemical and spectroscopic methods. The crystal structure was established by single-crystal X-ray diffraction analysis. It reveals cyano-bridged heterometallic chains consisting of alternating arrays of Ni(II) and Fe(II) atoms, both being embedded in distorted octahedral environments. Low-temperature susceptibility measurements show the presence of weak antiferromagnetic exchange interactions between paramagnetic Ni(II) centers (J = −0.46 cm⁻¹) through long diamagnetic [Fe(CN)₅(NO)]²⁻ bridges. Spin state of the iron atom was established by ⁵⁷Fe M?ssbauer spectroscopy.     

Kinetics and mechanism of oxidation of ferrocyanide by N-bromosuccinimide in aqueous acidic solution

The kinetics of oxidation of ferrocyanide by N-bromosuccinimide (NBS) has been studied spectrophotometrically in aqueous acidic medium over temperature range 20–35 °C, pH = 2.8–4.3, and ionic strength = 0.10–0.50 mol dm⁻³ over a range of [Fe²⁺] and [NBS]. The reaction exhibited first order dependence on both reactants and increased with increasing pH, [NBS], and [Fe²⁺]. The rate of oxidation obeys the rate law: d[Fe³⁺]/dt = [Fe(CN)₆]^4–[HNBS⁺]/(k ₂ + k ₃/[H⁺]). An outer-sphere mechanism has been proposed for the oxidation pathway of both protonated and deprotonated ferrocyanide species. Addition of both succinimide and mercuric acetate to the reaction mixture has no effect on the reaction rate under the experimental conditions. Mercuric acetate was added to the reaction mixture to act as scavenger for any bromide formed to ensure that the oxidation is entirely due to NBS oxidation.     

Kinetics of oxidation of N,N-bis(salicylaldehyde-1,2-diaminoethane) cobalt(II) complex by periodate. Evidence for the inhibiting effect of copper(II)

A kinetic study of the oxidation of [Co(H₂L)(H₂O)₂]²⁺ (H₂L = N,N-bis (salicylaldehyde-1,2-diaminoethane) Schiff base) by periodate in aqueous solution was performed over pH (2.3–3.4) range, (0.1–0.5) mol dm⁻³ ionic strength and temperatures 20–35 °C for a range of periodate and complex concentrations. The reaction rate showed a first-order dependence on both reactants and increased with pH over the range studied. The effects of Cu(II) and Fe(II) on the reaction rate were investigated over the (1.0–9.0) × 10⁻⁵ mol dm⁻³ range. The reaction was inhibited as the concentration of Cu(II) increased, and it was independent on Fe(II) concentrations over the ranges studied. An inner-sphere mechanism is proposed for the oxidation pathways of both the protonated and deprotonated Co^II complex species.     

Electrochemical reactivity of Li–Mn–O and Li–Fe–Mn–O spinels

Electrochemical reductive dissolution of Li–Mn–O and Li–Fe–Mn–O spinels and Li⁺ extraction/insertion in these oxides were performed using voltammetry of microparticles. Both electrochemical reactions are sensitive to the Fe/(Fe+Mn) ratio, specific surface area, Li content in tetrahedral positions, and Mn valence, and can be used for electrochemical analysis of the homogeneity of the elemental and phase composition of synthetic samples. The peak potential (E _P) of the reductive dissolution of the Li–Mn–O spinel is directly proportional to the logarithm of the specific surface area. E _P of Li–Fe–Mn–O spinels is mainly controlled by the Fe/(Fe+Mn) ratio. Li⁺ insertion/extraction can be performed with Mn-rich Li–Fe–Mn–O spinels in aqueous solution under an ambient atmosphere and it is sensitive to the regularity of the spinel structure, in particularly to the amount of Li in tetrahedral positions and the Mn valence. Electronic Publication     

A study of the aerobic reaction between dopamine and Fe(III) in the presence of S<Subscript>2</Subscript>O<Subscript>3</Subscript><Superscript>2−</Superscript>

The reaction between Fe(III) and dopamine in aqueous solution in the presence of Na₂S₂O₃ was followed through UV–Vis spectroscopy, pH and oxy-reduction potential (Eh) measurements. The formation and quick disappearing of the complex [Fe(III)HL¹⁻]²⁺, HL¹⁻ = monoprotonated dopamine was observed with or without S₂O₃ ²⁻ at pH 3. An unexpected reaction occurs in presence of thiosulfate forming the stable anion complex [Fe(III)(L²⁻)₂]¹⁻, L²⁻ = dopacatecholate (λ = 580 nm) and the auto-increasing of the pH, from 3 to 7. It was proposed that H⁺ and molecular oxygen are consumed by free radical thiosulfate formed during the reaction.     

Callus induction and plant regeneration from different explants of Actinidia deliciosa

In this study, an efficient procedure was developed for callus induction and regeneration of kiwifruit (Actinidia deliciosa) using different organs of shoots developed under in vitro conditions. Effects of explants source and media (M₁, 1.0 mg l⁻¹ BA + 2.0 mg l⁻¹ 2,4-D–M₂, 1.0 mg l⁻¹ NAA + 2.0 mg l⁻¹ 2,4-D) on initiation of callus were examined in order to obtain callus for organogenesis. The best callus for plant regeneration was obtained from leaf explants on Murashige and Skoog’s medium (MS) supplemented with M₂. Formation of callus from leaf of kiwifruit (A. deliciosa) was cultured in MS medium containing different concentration of N⁶-benzylaminopurin (BA; 0.0, 1.0, 2.0, 4.0, 6.0, 8.0 mg l⁻¹) for callus proliferation and plant regeneration. Although the first shoot formation was appeared in medium containing 6.0 and 8.0 mg l⁻¹ BA, the best shoots formation was obtained in medium with 4.0 mg l⁻¹ BA.     

Fast screening immunoassay of sulfonamides in commercial fish samples

An indirect competitive enzyme-linked immunosorbent assay (ELISA) was developed in plate to detect three sulfonamide residues (sulfamerazine (SMR), sulfadimetoxine (SDM), and sulfadiazine (SDZ)) in gilthead sea bream (Sparus aurata) samples. Different extraction methodologies—using methanol/water 1:1 (v/v) + ethylene diamine tetraacetic acid (EDTA) 0.5% (m/v), acetonitrile, phosphate-buffered saline (PBS) 10 mmol L⁻¹ pH 7 and acetate buffer 100 mmol L⁻¹ pH 5—and cleanup steps, based on solid-phase extraction (C₁₈, SCX, Si) or liquid extraction with hexane, were assayed. As optimum, a fast and simple method using acetonitrile was selected to extract the sulfonamide residues from the edible muscle of fish. Due to matrix effects, a standard addition calibration curve in fish extract is necessary for quantification purposes. Sulfonamide-free samples were spiked at different concentration levels (between 30 and 90 ng g⁻¹, 5–15 ng mL⁻¹ in plate) and average recoveries (n = 8), ranging from 71% to 95%, 65% to 79%, and 72% to 95%, were obtained for SMR, SDM, and SDZ, respectively. The assay detection limits for these antibiotics were lower than 100 μg kg⁻¹ (maximum residue level established by the European Union). The accuracy was evaluated by spiking blank fish extracts at different concentrations (10–40 ng mL⁻¹, 5–20 ng mL⁻¹ in plate), and the relative errors ranged between ±20%. Finally, in order to confirm the utility of the developed ELISA as a screening methodology, fish samples from different supermarkets were analyzed, and results were compared with those obtained by a validated high-performance liquid chromatography (HPLC) method. The correlation between the results obtained by both ELISA and HPLC methods is satisfactory.      

Kinetics and mechanism of the formation of (1,8)bis(2-hydroxybenzamido)3,6-diazaoctaneiron(III) and its reactions with thiocyanate, azide, acetate, sulfur(IV) and ascorbic acid in solution, and the synthesis and characterization of a novel oxo bridged diiron(III) complex. The role of phenol–amide–amine coordination

The interaction of (1,8)bis(2-hydroxybenzamido)3,6-diazaoctane (LH₂) with iron(III) in acidic medium resulted in the formation of a mononuclear complex, Fe(LH₃)⁴⁺ which further yielded, [Fe(LH₂)]³⁺, [Fe(LH)]²⁺, and [FeL]⁺ due to protolytic equilibria. The formation of [Fe(LH₃)]⁴⁺ was investigated under varying [H⁺]_T (0.01–0.10 mol dm⁻³) and [Fe³⁺]_T (1.00 × 10⁻³–1.70 × 10⁻², [L]_T = 1.0 × 10⁻⁴ mol dm⁻³) (I = 0.3 mol dm⁻³, 10% MeOH + H₂O, 25.0 °C). The reaction was reversible and displayed monophasic kinetics; the dominant path involved Fe(OH)(OH₂) ₅ ²⁺ and LH ₄ ²⁺ . The mechanism is essentially a dissociative interchange (I _d) and the dissociation of the aqua ligand from the encounter complex, [Fe(OH₂)₅OH²⁺, H₄L²⁺] is rate limiting. The ligand binds iron(III) in a bidentate ([Fe(H₃L)]⁴⁺), tetradentate ([Fe(H₂L)]³⁺), pentadentate ([Fe(HL)]²⁺) and hexadentate fashion ([FeL]⁺) under varying pH conditions. Iron(III) promoted deprotonation of the amide and phenol moieties and chelation driven deprotonation of the sec-NH ₂ ⁺ of the trien spacer unit are in tune with the above proposition. The mixed ligand complexes, [Fe^III(LH)(X)] (X = N ₃ ⁻ , NCS⁻, ACO⁻) are also reversibly formed in solution thus indicating that there is a replaceable aqua ligand in the complex conforming to its octahedral coordination, [Fe(LH)(OH₂)]²⁺, the bound ligand is protonated at the sec-NH site. Despite the multidentate nature of the ligand the Fe^III complexes are prone to reduction by sulfur(IV) and ascorbic acid. The redox reactions of different iron(III) species, Fe^III(LH_i) which involved ternary complex formation with the reductants have been investigated kinetically as a function of pH, [S^IV]_T and [ascorbic acid]_T. The substantial pK perturbation of the bound ascorbate in [Fe(LH)(HAsc/Asc)]^+/0 (ΔpK _{{[Fe(LH)(HAsc)] − HAsc − }} > 6) is considered to be compelling evidence for chelation of HAsc⁻/Asc²⁻ leading to hepta coordination of iron(III) in the ascorbate complexes. A novel binuclear complex with composition, [Fe^III ₂C₂₀N₄H₃₅O₁₁ (NO₃)] has been synthesized and characterized by i.r., u.v.–vis, e.s.r., e.s.i.-Mass, ⁵⁷Fe Mossbauer spectroscopy and magnetic moment measurements. The complex was isolated as a mixture of two forms C ₁ and C ₂ with 75.3 and 24.7%, respectively as computed from Mossbauer data. The isomer shift (δ) (quadrupole splitting, ΔE _Q) are 0.32 mm s⁻¹ (0.75 mm s⁻¹) and 0.19 mm s⁻¹ (0.68 mm s⁻¹) for C ₁ and C ₂, respectively. The variable temperature magnetic moment measurements (10–300 K) of the sample showed that C ₁ is an oxo dimer exhibiting antiferromagnetic interaction between the iron(III) atoms (S ₁ = S ₂ = 5/2, J = − 120 cm⁻¹) while the dimer C ₂ is a high spin species (S ₁ = S ₂ = 5/2) and exhibits normal paramagnetism obeying the Curie law. The cyclic voltametry response of the sample (DMF, [TEAP] = 0.1 mol dm⁻³) displayed quasi-reversible responses at − 0.577 V and − 1.451 V (versus SCE). This is in tune with the fact that the C ₂ species reverts rapidly in solution to the relatively more stable oxo-bridged dimer (C ₁) which is reduced in two sequential steps: C₁ + e⁻ → [FeL]⁺ + Fe^II; [FeL]⁺ + e⁻ → Fe^IIL, the high labilility of the Fe^II complex is attributed to the irreversibility. The X-band e.s.r. spectrum of the polycrystalline sample at room temperature displayed a weak (unresolved) band at g = 4.2 and a strong band at g = 2.0 with hyperfine splitting due to the coordinated nitrogen (I = 1). At 77 K the band at g = 4.2 is intensified while that at g = 2 is broadened to the extent of near disappearance in agreement with the presence of the exchange coupled iron(III) centres. Electronic supplementary material Electronic supplementary material is available for this article at and accessible for authorised users. An erratum to this article is available at .     

Competition Between O2 and H2O2 in the Oxidation of Fe(II) in Natural Waters

The oxidation rates of nanomolar levels of Fe(II) in seawater (salinity S = 36.2) by mixtures of O₂ and H₂O₂ has been measured as a function of pH (5.8–8.4) and temperature (3–35∘C). A competition exists for the oxidation of Fe(II) in the presence of both O₂ (μ mol⋅L⁻¹ levels) and H₂O₂ (nmol⋅L⁻¹ levels). A kinetic model has been applied to explain the experimental results that considers the interactions of Fe(II) with the major ions in seawater. In the presence of both oxidants, the hydrolyzed Fe(II) species dominate the Fe(II) oxidation process between pH 6 and 8.5. Over pH range 6.2–7.9, the FeOH⁺ species are the most active, whereas above pH 7.9, the Fe(OH)⁰₂ species are the most active at the levels of CO²⁻₃ concentration present in seawater. The predicted Fe(II) oxidation rate at [Fe(II)]₀ = 30nmol⋅L⁻¹ and pH = 8.17 in the oxygen-saturated seawater with [H₂O₂]₀ = 50nmol⋅L⁻¹ (log ₁₀ k = −2.24s⁻¹) is in excellent agreement with the experimental value of log ₁₀ k = −2.29s⁻¹ ([H₂O₂]₀ = 55nmol⋅L⁻¹, pH = 8).     

First results on Fe solid-phase extraction from coastal seawater using anatase TiO2 nano-particles

This paper describes the application of TiO₂ nano-particles (anatase form) for the solid-phase extraction of iron from coastal seawater samples. We investigated the adsorption processes by infra-red spectroscopy. We compared in batch and on-(mini)column extraction approaches (0.1 and 0.05 g TiO₂ per sample, respectively), combined to external calibration and detection by inductively coupled plasma mass spectrometry at medium mass resolution. Globally, this titania phase was slightly more efficient with seawater than with ultra-pure water, although between pH 2 and pH 7, the Fe retention efficiency progressed more in ultra-pure water than in seawater (6.9 versus 4.8 times improvement). Different reaction schemes are proposed between Fe(III) species and the two main categories of titania sites at pH 2 (adsorption of [FeL _x ]^{(3 − x)+} via possibly the mediation of chlorides) and at pH 7 (adsorption of [Fe(OH)₂]⁺ and precipitation of [Fe(OH)₃]⁰). Under optimised conditions, the inlet system was pre-cleaned by pumping 6% HCl for ∼2 h, and the column was conditioned by aspirating ultra-pure water (1.7 g min⁻¹) and 0.05% ammonia (0.6 g min⁻¹) for 1 min. Then 3 g seawater sample was loaded at the same flow rate while being mixed on-line with 0.05% ammonia at 0.6 g min⁻¹ to adjust the pH to 7. The iron retained on the oxide powder was then eluted with 3 g 6% HCl (<0.002% residual salinity in the separated samples). The overall procedural blank was 220 ± 46 (2 s, n = 16) ng Fe kg⁻¹ (the titania was renewed in the column every 20 samples, with 2-min rinsing in between samples with 6% HCl at 1.5 g min⁻¹). The recovery estimated from the Canadian certified reference material CASS-2 was 69.5 ± 7.6% (2 s, n = 4). Typically, the relative combined uncertainty (k = 2) estimated for the measurement of ∼1 μg Fe kg⁻¹ (0.45 μm filtered and acidified to pH 1.5) of seawater was ∼12%. We applied our method to a similar sample, from the coastal region of the North Sea. The agreement well within stated uncertainties of our result with the value obtained independently by isotope dilution mass spectrometry further validated our method.     

A series of trinuclear sandwich-like cyanide-bridged iron(III)-manganese(II) complexes: synthesis,crystal structures,and magnetic properties

Four pyridinecarboxamide iron dicyanide building blocks and one Mn(III) compound have been employed to assemble cyanide-bridged heterometallic complexes, resulting in a series of trinuclear cyanide-bridged Fe^III–Mn^II complexes: {[Mn(DMF)₂ (MeOH)₂][Fe(bpb)(CN)₂]₂}·2DMF (1), {[Mn(MeOH)₄][Fe(bpmb)(CN)₂]₂}·2MeOH·2H₂O (2), {[Mn(MeOH)₄][Fe(bpdmb)(CN)₂]₂}·2MeOH·2H₂O (3) and {[Mn(MeOH)₄][Fe(bpClb)(CN)₂]₂}·4MeOH (4) (bpb²⁻ = 1,2-bis(pyridine-2-carboxamido)benzenate, bpmb²⁻ = 1,2-bis(pyridine-2-carboxamido)-4-methyl-benzenate, bpdmb²⁻ = 1,2-bis(pyridine-2-carboxamido)-4,5-dimethyl-benzenate, bpClb²⁻ = 1,2-bis(pyridine-2-carboxamido)-4-chloro-benzenate). Single-crystal X-ray diffraction analysis shows their similar sandwich-like structures, in which the two cyanide-containing building blocks act as monodentate ligands through one of their two cyanide groups to coordinate the Mn(II) center. Investigation of the magnetic properties of these complexes reveals antiferromagnetic coupling between the neighboring Fe(III) and Mn(II) centers through the bridging cyanide group. A best fit to the magnetic susceptibilities of complexes 1 and 3 gave the magnetic coupling constants J = −1.59(2) and −1.32(4) cm⁻¹, respectively.     

Modelling spur chemistry for alkaline and acidic water at high temperatures

The effect of pH and associated ionic strength on the primary yields in the radiolysis of pressurised water has been assessed by diffusion-kinetic calculations for temperatures in the range 100–300°C. Account has been taken for ionic strength I up to 0.1 mol kg⁻¹, assuming that the counter ions of H⁺ in acid solutions and of OH⁻ in base solutions have unit charge. In acid solutions, the H⁺ ions react with e⁻ _aq. The decrease in G(e⁻ _aq) and the increase in G(H) with decreasing pH becomes substantial for [H⁺] ≥ 1 × 10⁻⁴ m, but the primary yields of oxidising species are almost constant. In alkaline solutions, the OH⁻ anions affect the spur chemistry of radiation-generated protons and hydroxyl radicals for [OH⁻] ≥ 1 × 10⁻⁴ m. The scavenging of H atoms and hydrogen peroxide becomes significant for [OH⁻] ≥ 1 × 10⁻² m. The total yields G(OH) + G(O⁻) and G(H₂O₂) + G(HO₂ ⁻) are independent of base concentration below 0.01 m. In more alkaline solutions, G(OH) + G(O⁻) increases, whereas G(H₂O₂) + G(HO₂ ⁻) decreases with increasing [OH⁻]. Calculations showed the substantial yield of the reaction O⁻ + e⁻ _aq in 0.1 m base solution. Spur chemistry in alkaline hydrogenated water is not affected by the presence of H₂ if less than 0.001 m of hydrogen is added.     

Experimental evidence, stability, and the most probable structure of protonated p-tert-butylcalix[4]arenetetrakis(N,N-dimethylacetamide)

Abstract  

From extraction experiments and γ-activity measurements, the extraction constant corresponding to the equilibrium H₃O⁺(aq) + 1·Na⁺(nb) \leftrightarrows \leftrightarrows 1·H₃O⁺ (nb) + Na⁺ (aq) taking place in the two-phase water–nitrobenzene system (1 = p-tert-butylcalix[4]arenetetrakis(N,N-dimethylacetamide); aq = aqueous phase, nb = nitrobenzene phase) was evaluated as log K _ex (H₃O⁺, 1·Na⁺) = −0.1 ± 0.1. Further, the stability constant of the 1·H₃O⁺ complex in water-saturated nitrobenzene was calculated for a temperature of 25 °C as log β _nb (1·H₃O⁺) = 10.9 ± 0.2. By using quantum mechanical DFT calculations, the most probable structure of the 1·H₃O⁺ cationic complex species was derived. In this complex, the hydroxonium ion H₃O⁺ is bound partly to one phenoxy oxygen atom and partly to two carbonyl oxygens of 1 by strong hydrogen bonds and obviously by other electrostatic interactions.     

Buffer Standards for pH Measurement of N-(2-Hydroxyethyl)piperazine-N′-2-ethanesulfonic Acid (HEPES) for I=0.16 mol⋅kg−1 from 5 to 55 °C

The values of the second dissociation constant, pK ₂, of N-(2-hydroxyethyl) piperazine-N′-2-ethanesulfonic acid (HEPES) have been reported at twelve temperatures over the temperature range 5 to 55 °C, including 37 °C. This paper reports the results for the pa _H of eight isotonic saline buffer solutions with an I=0.16 mol⋅kg⁻¹ including compositions: (a) HEPES (0.01 mol⋅kg⁻¹) + NaHEPES (0.01 mol⋅kg⁻¹) + NaCl (0.15 mol⋅kg⁻¹); (b) HEPES (0.02 mol⋅kg⁻¹) + NaHEPES (0.02 mol⋅kg⁻¹) + NaCl (0.14 mol⋅kg⁻¹); (c) HEPES (0.03 mol⋅kg⁻¹) + NaHEPES (0.03 mol⋅kg⁻¹) + NaCl (0.13 mol⋅kg⁻¹); (d) HEPES (0.04 mol⋅kg⁻¹) + NaHEPES (0.04 mol⋅kg⁻¹) + NaCl (0.12 mol⋅kg⁻¹); (e) HEPES (0.05 mol⋅kg⁻¹) + NaHEPES (0.05 mol⋅kg⁻¹) + NaCl (0.11 mol⋅kg⁻¹); (f) HEPES (0.06 mol⋅kg⁻¹) + NaHEPES (0.06 mol⋅kg⁻¹) + NaCl (0.10 mol⋅kg⁻¹); (g) HEPES (0.07 mol⋅kg⁻¹) + NaHEPES (0.07 mol⋅kg⁻¹) + NaCl (0.09 mol⋅kg⁻¹); and (h) HEPES (0.08 mol⋅kg⁻¹) + NaHEPES (0.08 mol⋅kg⁻¹) + NaCl (0.08 mol⋅kg⁻¹). Conventional pa _H values, for all eight buffer solutions from 5 to 55 °C, have been calculated. The operational pH values with liquid junction corrections, at 25 and 37 °C have been determined based on the NBS/NIST standard between the physiological phosphate standard and four buffer solutions. These are recommended as pH standards for physiological fluids in the range of pH = 7.3 to 7.5 at I=0.16 mol⋅kg⁻¹.     

Enantioselective analysis of zopiclone and its metabolites in plasma by liquid chromatography/tandem mass spectrometry

A high-performance liquid chromatographic method with triple-quadrupole mass spectrometry detection (LC-MS-MS) was developed and validated for the first time for the simultaneous quantification of zopiclone and its metabolites in rat plasma samples. The analytes were isolated from rat plasma by liquid–liquid extraction and separated using a chiral stationary phase based on an amylose derivative, Chiralpak ADR-H column, and ethanol–methanol–acetonitrile (50:45:5, v/v/v) plus 0.025% diethylamine as the mobile phase, at a flow-rate of 1.0 mL min⁻¹. Moclobemide was used as the internal standard. The developed method was linear over the concentration range of 7.5–500 ng mL⁻¹. The mean absolute recoveries were 74.6 and 75.7; 61.6 and 56.9; 72.5, and 70.7 for zopiclone enantiomers, for N-desmethyl zopiclone enantiomers and for zopiclone-N-oxide enantiomers, respectively, and 75.9 for the internal standard. Precision and accuracy were within acceptable levels of confidence (<15%). The method application in a pilot study of zopiclone kinetic disposition in rats showed that the levels of (+)-(S)-zopiclone were always higher than those of (−)-R-zopiclone. Higher concentrations were also observed for (+)-(S)-N-desmethyl zopiclone and (+)-(S)-N-oxide zopiclone, confirming the stereoselective disposition of zopiclone.     

A Combined Experimental and Theoretical Study on the Complexation of Ag<Superscript>+</Superscript> with a Hexaarylbenzene-Based Receptor

From extraction experiments and γ-activity measurements, the exchange extraction constant corresponding to the equilibrium Ag⁺(aq) + 1⋅Cs⁺(nb) ⇆ 1⋅Ag⁺(nb) + Cs⁺(aq) taking part in the two-phase water–nitrobenzene system (where 1 = hexaarylbenzene-based receptor; aq = aqueous phase, nb = nitrobenzene phase) was evaluated to be log ₁₀ K _ex(Ag⁺, 1⋅Cs⁺) = −1.0±0.1. Further, the stability constant of the hexaarylbenzene-based receptor⋅Ag⁺ complex (abbreviation 1⋅Ag⁺) in nitrobenzene saturated with water, was calculated at a temperature of 25 °C: log ₁₀ β _nb(1⋅Ag⁺) = 5.5±0.2. By using quantum mechanical DFT calculations, the most probable structure of the 1⋅Ag⁺ complex species was solved. In this complex having C₃ symmetry, the cation Ag⁺ synergistically interacts with the polar ethereal oxygen fence and with the central hydrophobic benzene ring via cation–π interaction.