Bromide Ion Pair Research Articles

Constrained Molecular Dynamic Simulation of the Potential Mean Force of Lithium Bromide Ion Pairs in Acetonitrile

Molecular dynamic simulations of Li+, and Br− ions in acetonitrile were carried out. The simulated structural properties were compared to experimental data. The solvent potentials of Li+-Br−, Li+-Li+, and Br−-Br− were evaluated using constrained molecular dynamics (CMD) simulations, to determine the solvent contribution to the total force acting on the solute and estimate the liquid arrangements according to the potential of mean force (PMF) values. The PMF of friction kernels and the passage across the Li+-Br− barrier was studied using the Grote–Hynes theory. The union-separation development happens in the polarization confining system.

Stable Ligand Coordination at the Surface of Colloidal CsPbBr3 Nanocrystals.

Ruling over the surface chemistry of metal halide perovskite nanocrystals (NCs) is crucial to access reliable luminophores. Here, we provide an atomic-level description of the surface of colloidal CsPbBr3 NCs, achieving an effective passivation strategy that leads to near-unity photoluminescence quantum yield. To this end, we used two different types of CsPbBr3 NCs, which had been synthesized with an outer shell of either oleylammonium bromide ion pairs or Cs-oleate complexes. We perturbed the dynamic equilibria at the NCs' surface with ligands from a comprehensive library, including amines (and their conjugated acids) with different basicities, chain lengths, and steric encumbrances. We demonstrate that control of both ligand binding affinity and ligand-to-NC molar ratio is essential to attain thermodynamically stable coordination of the NC surface. We thus present a reliable protocol for managing the surface chemistry of colloidal CsPbBr3 NCs and for selectively addressing their ligand-induced morphological (and structural) transformations.

Competition between substitution and elimination reaction channels from fragmentation ratios of diquaternary ammonium bromide and formate ion pairs

Fragmentation by collisionally induced dissociation in triple quadrupole mass spectrometer of pairs of six series of quaternary diamines (gemini surfactants) with two anions: formate and bromide has been studied. Amines included alkanediyl-α,ω-bis-(N,N-dimethyl-N-dodecyl ammonium bromide), alkanediyl-α,ω-bis-(N-hydroxyethyl-N-methyl-N-dodecyl ammonium bromide), oligo(oxa)ethyl-α,ω-bis-(N,N-dimethyl-N-dodecyl ammonium bromide) and three series of hexadecyl homologues. Range of atoms forming spacer varied from 2 to 23. It was confirmed that fragmentation occurs via either nucleophilic substitution or elimination reactions. All possible α-carbon atoms can be attacked in the substitution reaction, so can be all possible hydrogens of β-carbon in the elimination reaction. Additionally transferred to anion can be hydroxy group hydrogen. It leads together to eight different reaction paths. Depending on the spacer size and the kind of anion the ratio of fragmentation channels varies dramatically. For short spacers length ion pairs with formic anion dissociate through elimination, while bromide through substitution at spacer carbon. When spacer length is long enough to accommodate central anion between two quaternary nitrogens the substitution channel becomes the dominating one for both anions with exception when one of the substituents is hydroxyethyl. Then elimination is the only channel for ion pairs with formate anions, but for bromide anion division between substitution and elimination channels is not much affected. From the trends in channel ration and additional experiment with asymmetric geminis reveal it was concluded that the transition state is probably substantially different for long and short spacer geminis.

Analytical speciation of chromium in in-vitro cultures of chromate-resistant filamentous fungi

In this work, different analytical speciation schemes have been used to study the reduction of Cr(VI) by a chromate-resistant strain of filamentous fungi Ed8 (Aspergillus sp), indigenous to contaminated industrial wastes. As demonstrated previously, this strain has the capability to reduce chromate present in the growth medium without its accumulation in the biomass, yet the reduced chromium end-products have not been characterized. Liquid growth medium, initially containing 50 mg L(-1) Cr(VI), was analyzed for Cr(III)/Cr(VI) and for total Cr at different time intervals (0-24 h) after inoculation with fungi. Three hyphenated procedures, based on the Cr(III)-EDTA formation and species separation by anion-exchange or ion-pairing reversed-phase chromatography with ICP-MS or DAD detection were used. The results obtained for Cr(VI) in each case were consistent, demonstrating efficient reduction of chromate during 24 h of Ed8 growth. However, pre-column complexation with EDTA did not ensure complete recovery of the reduced forms of chromium in the above procedures. An alternative speciation scheme, based on extraction of Cr(VI)-benzyltributylammonium bromide (BTAB) ion pairs into chloroform and subsequent determination of residual chromium by ICP-MS has provided evidence on the effective conversion of chromate into reduced chromium species in the growth medium. The results indicate the feasibility of using Ed8 strain for chromate bioremediation purposes. Analytically it can be concluded that speciation of chromium in biological systems should not be limited to its two most common oxidation states, because the actual reduced chromium species are not converted quantitatively to Cr(III)-EDTA.

Uranyl ion-selective optical test strip

A novel, optical sensor, test strip has been developed for the spectrophotometric determination of trace amounts of uranyl ions, UO 2 2+, based on immobilization of C.I. Mordant Blue 29 (Chromazurol S)/cetyl N, N, N-trimethyl ammonium bromide ion pair on a triacetyl cellulose membrane. Optimization of the sensor for the detection of low levels of uranyl ion is described. The test strip responded linearly to uranyl ions between 3.0 × 10 −7 and 6.0 × 10 −5 mol L −1; the reproducibility of the sensor at a medium level of UO 2 2+ activity was ±0.55%. The optical sensor can be regenerated using 0.01 mol L −1 HCl or 0.01 mol L −1 NaF solution after 10 min. The developed test strip was used in the determination of UO 2 2+ in ground water samples.

Ordered array and motion of water molecules and ions at the interface between KBr(c)/NaCl(c) and water

Abstract The atomic-resolution images at the terrace of KBr(c)/NaCl(c) surface covered with water have been observed using an atomic-force microscope. The direct observation of the vertical motion of cations as well as the water-molecules ordered array at the interface was for the first time demonstrated in the atomic-resolution AFM image. The observed water molecules bridge each bromide ion-pair and are sandwiched between the potassium ions at the higher level right on the underlying bromide ions.

The influence of perchlorates on the fluorescence quenching of 9,10-dichloroanthracene by bromide salts in acetone

Fluorescence quenching of 9,10-dichloroanthracene by lithium bromide and tetra- n-butylammonium bromide in acetone has been investigated in the presence of perchlorate salts. In the presence of LiBr, the Stern–Volmer (S–V) plots exhibit downward curvatures indicating that two species are responsible for the quenching process, namely free bromide anions and lithium bromide ion pairs. The addition of a perchlorate salt modifies the S–V dependencies due to the influence of perchlorates on the degree of lithium bromide dissociation. The association constant of lithium bromide has been determined by conductivity measurements and it agrees well with the estimates made from the fluorescence quenching measurements. The mechanism of fluorescence quenching by lithium bromide is discussed on the basis of electron transfer and the heavy-atom effect.

ChemInform Abstract: Formation of Bromocarbenium Bromide Ion Pairs in the Electrophilic Bromination of Highly Reactive Olefins in Chlorinated Aprotic Solvents.

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Formation of Bromocarbenium Bromide Ion Pairs in the Electrophilic Bromination of Highly Reactive Olefins in Chlorinated Aprotic Solvents.

The kinetics and the products of bromination of several substituted stilbenes with tetrabutylammonium tribromide (TBAT) have been investigated in aprotic solvents at different temperatures. Stilbenes bearing electron-withdrawing or moderately electron-donating substituents gave stereospecifically the anti addition products. The reactions followed a second-order rate law, and an inverse kinetic isotope effect (KIE), k(H)/k(D) = 0.85(0.05), was found for the bromination of cis-stilbene. The reactions of cis- and trans-4,4-dimethoxystilbenes yielded mixtures of meso and d,l dibromides both in chloroform and 1,2-dichloroethane. The rate constants (k(Br)()3()-) measured for the latter olefins deviated considerably from the Hammett correlations, and added bromide had a significant effect on the rates. The reactions of these activated stilbenes with molecular Br(2), carried out at low Br(2) concentration, followed a mixed second/third-order rate law. The kinetic and product distribution data for the reaction, with TBAT, of stilbenes bearing electron-withdrawing or moderately electron-donating substituents are interpreted on the basis of the known mechanism involving a product- and rate-determining nucleophilic attack by bromide on the olefin-Br(2) pi-complex. The data related to the bromination of the more activated methoxystilbenes are rationalized considering that, for these olefins, even in aprotic solvents, the ionization of the initially formed 1:1 pi-complex to a bromocarbenium bromide ion pair can compete both with the formation of a bromonium-tribromide ion pair and with the nucleophilic attack by Br(-). For this second-order process (first order in Br(2)), the kinetic constants and the activation parameters have been measured in chloroform and 1,2-dichloroethane and the activation parameters have been compared with those related to the third-order Br(2) addition and to the reaction with TBAT.

Ultrafast experiments on intermolecular electron transfer in the benzene - bromine atom charge transfer complex

Ultrafast pump - probe measurements have been made on the benzene - bromine atom charge transfer (CT) complex in CCl 4 and cyclohexane solutions. Ultrafast optical excitation of the CT band of the complex yields an ion pair, which is comprised of a benzene cation and a bromide anion. The rate of charge recombination between the bromide and the benzene cation in the ion pair has been observed to be much faster than the rate of diffusion apart. The charge recombination rate is accelerated at high benzene concentrations as a result of the formation of a benzene dimer cation - bromide ion pair which undergoes much faster charge recombination than the benzene cation - bromide ion pair.

Determination of dissolved copper, nickel and cadmium in natural waters by high-performance liquid chromatography

A novel technique has been developed to determine dissolved copper, nickel and cadmium in natural waters, and in particular estuarine and sea-waters, by using high-performance liquid chromatography (HPLC). The main performance criteria were that the method was sensitive, reproducible and interference free. This has been achieved by developing a system whereby the trace metals were initially preconcentrated on a bonded-silica (C2) guard column coated with an ammonium pyrrolidin-1-yldithioformate (ammonium pyrrolidine dithiocarbamate)–cetyltrimethylammonium bromide ion pair. This selectively removed the trace metals from the sample matrix, thereby excluding potential interferents. After preconcentration the metal–dithiocarbamate complexes were flushed with a modified organic mobile phase from the pre-column onto a C18 reversed-phase analytical column for separation. The complexes were detected by means of a variable-wavelength ultraviolet detector. Performance data were obtained, yielding limits of detecton of better than 0.5 µg l–1 for copper, cadmium and nickel, for 10 ml sample volumes. The precision was better than 5% relative standard deviation for concentrations in the range 0.5–10 µg l–1. Once optimized the technique was evaluated by analysing estuarine samples with a variety of salinities and comparing the data with those obtained by an accepted reliable method: chelation–solvent extraction–atomic absorption spectrometry. Results showed that there was no significant difference in the data obtained by the two methods. A single calibration graph could be used for samples of all salinities by using the HPLC method.

Cis-trans isomerization of an α,α-difluoroolefin catalyzed by bromine

The cis-trans isomerization of 1 by Br 2 in CCl 4 is described. A bridged bromonium cation α to CF 2 is destabilized resulting in a carbocation bromide ion pair at the β-carbon capable of free rotation and loss of bromine to form 2 . A CF 2 group appears to be more effective in promoting isomerization than aryl in the case of stilbenes.

Studies in ion-pair chromatography : II. Retention of positive and negative ions and neutral solutes in tetrabutylammonium bromide-containing methanol—water eluents on lichrosorb rp-18

The retention of positively and negatively charged ions, as well as of neutral solutes, was studied in 0–70% methanol in 25 m M aqueous phosphate buffer (pH 2.1–3.4) eluents as a function of the tetrabutylammonium bromide ion pair reagent concentration. The log k′ vs. mobile phase pairing ion concentration ( P m ) plots (which showed P m - and [methanol]-dependent maxima) could be transformed into log k′ vs. stationary phase pairing ion concentration ( P s ) curves by using the simultaneously determined adsorption isotherms of tetrabutylammonium bromide. While the shape of the log k′ vs. P m curves of oppositely charged solutes changed considerably with the concentration of methanol in the eluent, the log k′ vs. P s curves were more similar, and their maxima are in the 80–120 μmole/g range. The log k′ vs. P m curves of similarly charged solutes were more alike. The methanol concentration of the eluent influences the retention of oppositely charged solutes more sensitively than that of the similarly charged solutes. At constant P m the log k′ vs. [methanol] curves of oppositely charged solutes were S-shaped, while the log k′ vs. [methanol] curves at constant P s values ran similar to the log k′ vs [methanol] curves obtained in the absence of ion-pairing reagents.

Determination of Procaine and Related Local Anesthetics. I. Partition Chromatographic Separation and Assay of Mixtures of Procaine with Tetracaine and with Propoxycaine

Determination of ionization and extraction constants for procaine, tetracaine, and propoxycaine led to selection of a simple partition chromatographic system for separation and assay of mixtures of these anesthetics. A 65% solution of chloroform in isooctane elutes tetracaine or propoxycaine from a pH 4:sodium bromide column; procaine is retained and subsequently eluted by chloroform as the bromide ion-pair. The anesthetics are then determined spectrophotometrically. Results of assay of standard and commercial formulations are presented.