Hydrogen bonding: Part 19. IR and NMR study of the lower hydrates of choline fluoride and acetylcholine chloride

Acetylcholine chloride, like choline chloride, forms a liquid salt dihydrate, and a crystalline monohydrate that only exists at reduced pressure; at atmospheric pressure the monohydrate disproportionates into liquid dihydrate and anhydrous acetylcholine chloride. Both choline and acetylcholine chlorides give endothermic dissolution in water. In contrast, choline fluoride gives exothermic dissclution in water, and forms an extra-ordinarily stable monohydrate in which choline cation hydroxyls form strong hydrogen bonds to an H₄O₂F^2?₂ cluster anion. Since the hydration behavior of choline fluoride is like that of unsubstituted tetraalkylammonium fluorides, the unusual hydration behavior of choline and acetyline chlorides results from the presence of chloride ion, and is not an intrinsic property of cholinergic cations.     

Hydrogen bonding : Part 17. IR and NMR study of the lower hydrates of choline chloride

Choline chloride forms two lower hydrates — a dihydrate and a monohydrate — with quite unusual properties. The dihydrate is a highly structured liquid salt; the IR spectrum is similar to that of a crystalline framework clathrate hydrate, and there are separate ¹H-NMR signals for the cation hydroxyl and water protons. The dihydrate is a crystalline solid at reduced pressure. The crystalline monohydrate only exists at reduced pressure; at atmospheric pressure it disproportionates to liquid dihydrate and anhydrous choline chloride. The anhydrous choline chloride thus formed is a previously unreported crystal modification of choline chloride.     

Anion binding behavior of heterocycle-strapped calix[4]pyrroles

A comparative study of the halide and benzoate anion binding properties of a series of phenyl, pyrrole, and furan-strapped calix[4]pyrroles has been carried out. These receptors, which have previously been shown to bind the chloride anion (Yoon et al., Angew. Chem., Int. Ed. 47(27):5038–5042, 2008), were found to bind bromide and benzoate anion (studied as the corresponding tetrabutylammonium salts) with near equal affinity in acetonitrile, albeit less well than chloride, as determined from ITC measurements or NMR spectroscopic titrations. This stands in marked contrast to the parent octamethylcalix[4]pyrrole, where the carboxylate anion affinities are substantially higher than those for bromide anion under identical conditions. This finding is rationalized in terms of tighter binding cavity present in the strapped systems. For all three anions for which quantitative data could be obtained (i.e., Cl^?, Br^?, PhCO₂ ^?), the pyrrole-strapped system displayed the highest affinity, although the relative enhancement was found to depend on the anion in question. In the specific case of fluoride anion binding to the pyrrole-strapped receptor, two modes of interaction are inferred, with the first consisting of binding to the calix[4]pyrrole via NH-anion hydrogen bonds, followed by a process that involves deprotonation of the strapped pyrrolic NH proton. A single crystal X-ray diffraction analysis provides support for the first of these modes and further reveals the presence of a methanol molecule bound to the fluoride anion.     

Hydration of choline salts studied by NMR relaxation

The concentration dependence of T₁ for protons and deuterons in H₂O and D₂O solutions of three choline salts is anomalously high in dilute solutions. We suggest that this is due to formation of clathrate hydrate shells. Acetylcholine chloride does not show anomalous behaviour.     

Complex formation of cucurbit[6]uril with amines in the presence of different salts

The complex formation of cucurbit[6]uril with cyclohexylmethylamine hydrochloride and hexylamine hydrochloride has been studied in aqueous solution using potentiometric titrations. The influence of salts like NaCl, KCl, RbCl, CsCl, BaCl₂ and tetramethylammonium chloride and tetraethylammonium perchlorate on the complex formation with amines has been studied at different concentrations of these salts. With increasing salt concentration the stability constants for the complex formation with cyclohexylamine hydrochloride and hexylamine hydrochloride decreases. However, no specific influence is observable. These results show that 2:1 complexes between these salts and cucurbituril are not formed or that their concentration in solution is rather low.     

Effects of ion pairing on the capillary electrophoretic separation and ultraviolet‐absorption detection of quaternary ammonium cations using meso‐octamethylcalix[4]pyrrole as the background electrolyte additive

Five quaternary ammonium cations, including tetramethylammonium, tetraethylammonium, hexadecyltrimethylammonium, benzyltrimethylammonium, and 1‐butyl‐3‐methylimidazolium, have been separated by capillary electrophoresis. A direct ultraviolet method has been achieved when tetrabutylammonium fluoride was the background electrolyte and meso‐octamethylcalix[4]pyrrole was the background electrolyte additive. The ultraviolet spectra of meso‐octamethylcalix[4]pyrrole and cation mixtures showed that redshifts can be attributed to the size of cations, and the maximum absorption wavelength shifted from 218 to 230 nm when tetrabutylammonium cation was substituted with tetramethylammonium cation or tetraethylammonium cation. Conductivity measurements were performed to evaluate the ion‐pairing effect of tetrabutylammonium fluoride in a mixture of acetonitrile/ethanol (80:20, v/v), and the ion‐pairing formation constant, K_ip, was calculated (K_ip = 14.8 ± 0.3 L/mol) using the Fuoss extended model. Ion pairing also occurs between cations of the analytes and counterion, a fluoride complex of meso‐octamethylcalix[4]pyrrole. The tetramethylammonium cations associate more strongly with this counterion than the tetraethylammonium cation that contributes to the change of selectivity in capillary electrophoresis separation. The effective mobilities of the cations with trimethyl groups, such as tetramethylammonium cation, benzyltrimethylammonium cation, and hexadecyltrimethylammonium cation, decreased faster than others with the increase of meso‐octamethylcalix[4]pyrrole concentration, highlighting the fact that the ion‐pairing effect played an important role in this method.     

Higher hydrates of lithium chloride,lithium bromide and lithium iodide

For lithium halides, LiX (X = Cl, Br and I), hydrates with a water content of 1, 2, 3 and 5 moles of water per formula unit are known as phases in aqueous solid–liquid equilibria. The crystal structures of the monohydrates of LiCl and LiBr are known, but no crystal structures have been reported so far for the higher hydrates, apart from LiI·3H₂O. In this study, the crystal structures of the di‐ and trihydrates of lithium chloride, lithium bromide and lithium iodide, and the pentahydrates of lithium chloride and lithium bromide have been determined. In each hydrate, the lithium cation is coordinated octahedrally. The dihydrates crystallize in the NaCl·2H₂O or NaI·2H₂O type structure. Surprisingly, in the tri‐ and pentahydrates of LiCl and LiBr, one water molecule per Li⁺ ion remains uncoordinated. For LiI·3H₂O, the LiClO₄·3H₂O structure type was confirmed and the H‐atom positions have been fixed. The hydrogen‐bond networks in the various structures are discussed in detail. Contrary to the monohydrates, the structures of the higher hydrates show no disorder.     

测定溶解度组装阴离子手性离子液体的研究

本文利用无机盐和有机盐在有机溶剂中的溶解度的差异,设计组装手性羧酸阴离子离子液体.测定氯化钠、氯化钾、溴化钠和溴化钾和手性有机羧酸盐((R)-2-羟基-4-苯丁酸钠或钾)在甲醇、乙醇或丙酮中的溶解度,一定温度下计算溶解度比值,理论推测该条件下阴离子交换比例.研究发现,当无机盐与有机盐的溶解度比值小于0.05时,阴离子交...     

Polarographic behaviour of DL aspartic acid at the D.M.E.

dl Aspartic acid is reduced at the d.m.e. in 0.1M tetramethylammonium bromide, tetraethylammonium bromide, sodium chloride, potassium chloride, potassium nitrate, sodium perchlorate and lithium sulphate; in aqueous media. The waves are irreversible, diffusion controlled involving one electron transfer process determined by millicoulometry. The values of the kinetic parameters, transfer coefficient () and formal rate constant (k° _{f, h}) have been calculated byKoutecky's method and are 0.479 and 15.9×10^–16 respectively.With 1 Figure     

Spectroscopic, structural, and theoretical studies of halide complexes with a urea-based tripodal receptor

A urea-based tripodal receptor L substituted with p-cyanophenyl groups has been studied for halide anions using (1)H NMR spectroscopy, density functional theory (DFT) calculations, and X-ray crystallography. The (1)H NMR titration studies suggest that the receptor forms a 1:1 complex with an anion, showing a binding trend in the order of fluoride > chloride > bromide > iodide. The interaction of a fluoride anion with the receptor was further confirmed by 2D NOESY and (19)F NMR spectroscopy in DMSO-d(6). DFT calculations indicate that the internal halide anion is held by six NH···X interactions with L, showing the highest binding energy for the fluoride complex. Structural characterization of the chloride, bromide, and silicon hexafluoride complexes of [LH(+)] reveals that the anion is externally located via hydrogen bonding interactions. For the bromide or chloride complex, two anions are bridged with two receptors to form a centrosymmetric dimer, while for the silicon hexafluoride complex, the anion is located within a cage formed by six ligands and two water molecules.     

Clathrate Hydrates of Tetrabutylammonium and Tetraisoamylammonium Halides

Clathrate formation was considered for two series of systems: (C₄H₉)₄NG–H₂O and iC₅H₁₁)₄NG–H₂O G = F^-, Cl^-, Br^-, I^-). Clathrate hydrates of tetraisoamylammonium halides were shown to melt at higher temperatures than those of the butyl series. In passing from fluoride to bromide, the stability of compounds of the butyl series falls significantly and tetrabutylammonium iodide does not produce polyhydrates. In the isoamyl series, the melting points of polyhydrates vary insignificantly for different halides. In addition, the highest melting hydrate of tetraisoamylammonium bromide melts at a slightly higher temperature than chloride hydrates, indicating not only a hydrophilic effect of the anion on clathrate formation.     

Hydrogen bonding: Part IX. Low temperature infrared spectra of H4O2F22− and H6O42− cluster anions in tetramethylammonium fluoride and hyd

At 10–20 K the broad room temperature IR bands of the hydrogen bonded cluster anions in tetramethylammonium fluoride and hydroxide monohydrates and monohydrates-d_n are resolved into a number of components. The band patterns are consonant with distortion of formally tetrahedral H₄F₂O₂^2? and H₆O₄^2? ions of T_d point group to C2v and S₄, respectively in a C_4h⁵ unit cell isomorphous with tetramethylammonium hexafluorosilicate. A lattice mode has been identified for each monohydrate.     

Enthalpies of solution of some tetraalkylammonium bromides in binary mixtures of water and some amides

Enthalpies of solution of tetraethylammonium bromide in mixtures of N,N-dimethylformamide (DMF) and water and of tetramethylammonium bromide in mixtures of DMF and water, N-methylformamide (NMF) and water, formamide (F) and water, and of NMF and F have been measured calorimetrically at 25°C in the whole mole-fraction range. The enthalpies of solution of tetraethylammonium bromide in DMF-water reach a maximum value while the enthalpies of solution of tetraethylammonium bromide in this mixture show a flat, broad maximum and a minimum. A similar behavior is found in NMF-water and F-water mixtures. In the NMF-F system the enthalpies of solution display a nearly ideal behavior. The results are compared with those of tetra-n-butylammonium bromide and of tetra-n-propylammonium bromide.     

Influence of additives on the clouding behavior of amphiphilic drug solutions

The present work focuses on the clouding phenomenon in an amphiphilic drug [amitriptyline (AMT), which is a tricyclic antidepressant] solution. A 50-mM AMT solution prepared in 10 mM of sodium phosphate (SP) buffer was taken where the cloud point (CP) was found to decrease with increasing pH. The same CP decreasing trend (with pH increase) followed in the presence of a fixed concentration (50 mM) of added salts [NaBr, and tetra-n-butylammonium bromide (TBuAB)]. The addition of increasing amounts of quaternary bromides (tetramethylammonium bromide, tetraethylammonium bromide, tetra-n-propylammonium bromide, TBuAB, and tetra-n-pentylammonium bromide) to 50 mM of AMT solution (prepared in 10 mM of SP buffer) caused continuous increase in CP, which was found to be dependent upon the alkyl chain length of that particular salt. The similar type of CP increase was also observed in the presence of conventional (cetyltrimethylammonium bromide and tetradecyltrimethylammonium bromide) and gemini surfactants [bis(hexadecyldimethylammonium)hexane, bis(hexadecyldimethylammonium)pentane, and bis(hexadecyldimethylammonium)butane]. The overall behavior was discussed in terms of electrostatic interactions, micellar growth, and mixed micelle formation.     

Calix[4]pyrrole‐Based Heteroditopic Ion‐Pair Receptor That Displays Anion‐Modulated,Cation‐Binding Behavior

A new ditopic ion‐pair receptor 1 was designed, synthesized, and characterized. Detailed binding studies served to confirm that this receptor binds fluoride and chloride ions (studied as their tetraalkylammonium salts) and forms stable 1:1 complexes in CDCl₃. Treatment of the halide‐ion complexes of 1 with Group I and II metal ions (Li⁺, Na⁺, K⁺, Cs⁺, Mg²⁺, and Ca²⁺; studied as their perchlorate salts in CD₃CN) revealed unique interactions that were found to depend on both the choice of the added cation and the precomplexed anion. In the case of the fluoride complex [ 1? F]^? (preformed as the tetrabutylammonium (TBA⁺) complex), little evidence of interaction with the K⁺ ion was seen. In contrast, when this same complex (i.e., [ 1? F]^? as the TBA⁺ salt) was treated with the Li⁺ or Na⁺ ions, complete decomplexation of the receptor‐bound fluoride ion was observed. In sharp contrast to what was seen with Li⁺, Na⁺, and K⁺, treating complex [ 1? F]^? with the Cs⁺ ion gave rise to a stable, receptor‐bound ion‐pair complex [Cs ?1? F] that contains the Cs⁺ ion complexed within the cup‐like cavity of the calix[4]pyrrole, which in turn was stabilized in its cone conformation. Different complexation behavior was observed in the case of the chloride complex [ 1? Cl]^?. In this case, no appreciable interaction was observed with Na⁺ or K⁺. In addition, treating [ 1? Cl]^? with Li⁺ produces a tightly hydrated dimeric ion‐pair complex [ 1? LiCl(H₂O)]₂ in which two Li⁺ ions are bound to the crown moiety of the two receptors. In analogy to what was seen in the case of [ 1? F]^?, exposure of [ 1? Cl]^? to the Cs⁺ ion gives rise to an ion‐pair complex [Cs ?1? Cl] in which the cation is bound within the cup of the calix[4]pyrrole. Different complexation modes were also observed when the binding of the fluoride ion was studied by using the tetramethylammonium and tetraethylammonium salts.     

Partial molal heat capacities of tetraalkylammonium bromides in methanol from 10 to 50°C

Enthalpies of solution habe beenmeasured for tetraethyl-, tetra-n-propyl-, and tetra-n-butylammonium bromides in anhydrous methanol at several temperatures ranging from 5 to 55°C. The data were extrapolated to infinite dilution by an extended Debye-Hückel equation to obtain standard enthalpies of solution ΔH ₃ ⁰ , and the integral heat method was employed to obtain partial molal heat capacities , of the corresponding salts from 10 to 50°C. These data, along with similar data for tetramethylammonium bromide previously reported, were used to assign an, absolute heat capacity to the bromide ion in anhydrous methanol. Comparison of the absolute heat capacities of the bromide and tetraalkylammonium ions in water and methanol suggests that the processes occurring in the two solvents are quite dissimilar.     

Enthalpies of Transfer of Tetraalkylammonium Bromides and CsBr from water to Aqueous DMF at 298.15 K

Enthalpies of transfer of tetraalkylammonium bromides and CsBr from water to aqueous DMF mixtures are reported and analyzed in terms of a new solvation theory. It was found that a previous equation could not reproduce these data over the whole range of solvent compositions. Using a new solvation theory to model the enthalpies of transfer shows excellent agreement between experimental and calculated values over the entire range of solvent compositions. The analyses show that tetrapropylammonium bromide, Pr₄NBr, and tetrapentylammonium bromide, Pen₄NBr, are preferentially solvated by water; in contrast tetrabutylammonium bromide, Bu₄NBr, is preferentially solvated by DMF. The solvation of tetramethylammonium bromide, Me₄NBr, and cesium bromide, CsBr, is random. The extent to which the tetraalkylammonium bromides disrupt solvent–solvent bonds increases systematically in going from Me₄NBr to Pen₄NBr.     

Effects of ion-pairing and hydration on the SNAr reaction of the F- with p-chlorobenzonitrile in aprotic solvents

Theoretical ab initio calculations including liquid phase optimizations were used to investigate the S(N)Ar reaction of the fluoride ion with p-chlorobenzonitrile in dimethyl sulfoxide solution. The effect of the counter ion and hydration of the fluoride ion with one water molecule was analyzed. The calculations indicate that the gas-phase S(N)Ar reaction is more favorable than the corresponding S(N)2 reactions involving fluoride ion and 2-chlorobutane. However, the substantially higher solvent effect on the S(N)Ar reaction makes the nucleophilic substitution on the aromatic ring less favorable than the aliphatic reaction in the liquid phase. For the anhydrous tetrabutylammonium fluoride system, the theoretical free energy barrier of 22.1 kcal mol(-1) is close to the experimental one of 24.4 kcal mol(-1). The smaller tetramethylammonium cation strongly associates with the fluoride ion and increases the barrier by 5 kcal mol(-1). Similarly, just one water molecule hydrating the fluoride ion has the same effect. An analysis of the reaction involving the ion pair and the free anion in different polarity media predicts an unexpected behavior and indicates there is an ideal solvent polarity for each counter ion.     

Regioselective ring opening of 2-methylaziridine derivatives with F- and F-fluoride

Regioselectivity of the nucleophilic ring opening of N-benzoyl (Bz) and N-benzyloxycarbonyl (Cbz) activated 2-methylaziridines with anhydrous tetramethylammonium fluoride, anhydrous hydrogen fluoride, and ¹⁹F or [¹⁸F]-labelled potassium cryptand fluoride ([K₂₂₂][^18/19F]) were investigated. Whereas all reactions with rigorously anhydrous N(CH₃)₄F did not ring-open the aziridines, reactions with anhydrous HF exclusively yielded the 2-fluoropropanamine derivatives. Reactions of Bz-protected and Cbz-protected 2-methylaziridine with [K₂₂₂][^18/19F] yielded the 2-fluoropropanamine and 1-fluoro-2-propanamine derivatives as the major products, respectively, and represents the first example of regiocontrol during ring opening of aziridines with [¹⁸F]-fluoride.