Polarography in hexamethylphosphoramide: Effect of supporting electrolyte cations

Polarographic reductions of various metal ions such as the silver, cupric, zinc, cobaltous, nickel, ferric, ferrous ions and hydrogen ion in hexamethylphosphoramide (HMPA), have been investigated in the supporting electrolytes with various perchlorates. The reduction of most of these ions is strongly influenced by the cation of the supporting electrolyte. In the presence of the tetraethylammonium ion, when the size of the cation of the supporting electrolyte is small and easily adsorbed on the negatively charged electrode surface, the reductions of metal ions are controlled by some preceding processes and are naturally irreversible. The rate of reduction becomes more rapid with the increase of the size of the cation. Thus, in Hex₄NClO₄ or LiClO solutions, the reduction of these various metal ions takes place almost totally under diffusion control, although the waves of most of metal ions show a maximum. These effects of the cation of the supporting electrolytes on reduction can be explained as a phenomenon occurring on the electrode surface. This phenomenon has been reported in previous papers [1] on the reductions of the alkali and alkaline earth metal ions. The difference in the electrocapillary curves in these solutions is rarely shown at the potential around the electrocapillary maximum, but it is very obviously shown at more negative potential. The difference in the effect of the size of the cation of the supporting electrolyte on reduction of metal ion coincides well with the difference in the electrocapillary curves in these solutions: the effect of the size of the supporting electrolyte cation on the polarographic reduction is rarely shown at the potential around the electrocapillary maximum, but it is very obviously shown at more negative potential; therefore this effect is due to the electrode double-layer difference.     

Equilibrium Study on Ion-Pair Formation in Water and Distribution Between Water and m-Xylene of Tetraalkylammonium Picrates

The 1:1 ion-pair formation constants (K _IP) of tetraalkylammonium (Me₄N⁺, Et₄N⁺, Pr₄N⁺, Bu₄N⁺, and Bu₃MeN⁺) picrates in water were determined by capillary electrophoresis at 25°C. The ion-pair extraction constants (K _ex,ip) of the picrates from water to m-xylene were determined by a batch-extraction method at 25°C, and the distribution constants (K _D) of the neutral ion-pairs were calculated from the relationship K _D = K_ex,ip/K _IP. The tetraalkylammonium ion having more methylene groups generally forms a slightly more stable ion-pair with the picrate ion in water, which is attributed to the lower hydration of the cation. For Me₄N⁺, Et₄N⁺, Pr₄N⁺, and Bu₄N⁺, the distribution of the ion pair into m-xylene increases in that order, and a linear relationship was found between log K _D and the number of methylene groups in the cation. This is consistently explained by the regular solution theory. It was also revealed that the ion pairs have a strong specific interaction with water. The ion pair of Bu₃MeN⁺ has a higher distribution constant than that expected from the relationship between log K _D and the number of methylene groups for the symmetrical tetraalkylammonium ions. The cation dependence of the ion pair extractability is mostly governed by that of the distribution of the ion pair.     

Adsorption phenomena on a mercury electrode in aqueous cryptand[2.2.2] solutions by the background of tetraalkylammonium salts

The electrochemical behavior of cryptand[2.2.2] (Cry) is studied on a mercury electrode in aqueous solutions of tetraalkylammonium tetrafluoroborates (Me₄N⁺, Et₄N⁺, and Bu₄N⁺). Cryptand [2.2.2] is shown to exhibit high surface activity in Me₄ NBF₄ nd Et₄NBF₄ solutions. Based on the model of two parallel capacitors supplemented by the Frumkin adsorption isotherm, the adsorption parameters of Cry by the background of Me₄BNF₄ were calculated using the regression analysis methods. The calculated dependences of the differential capacitance on the potential adequately agree with experimental curves. The adsorption characteristics of Cry in the studied solutions are compared with those in MgSO₄ solutions. By the background of Bu₄NBF₄, Cry molecules and Bu₄N⁺ cations exhibit very close surface activity and form a mixed adsorption layer.     

Thermal properties of <Emphasis Type="Italic">n</Emphasis>-R<Subscript>4</Subscript>NBr solutions in binary solvents containing formamide

The enthalpies of solution of tetraethyl- and tetra-n-hexylammonium bromides have been measured in mixtures of formamide with ethylene glycol at 298.15 and 313.15 K in the whole mole fraction range by the calorimetric method. The standard enthalpies of solution in binary mixtures have been calculated with Redlich–Rosenfeld–Meyer type equation. The enthalpy and heat capacity parameters of pair interaction of organic electrolytes with EG in FA and with FA in EG have been computed and discussed. The enthalpy interaction parameters of single ions with EG in FA medium have been evaluated and compared with those for ion–water and ion–MeOH interaction in FA. The standard heat capacities of solution have been evaluated. The excess enthalpies of solution, Δ_sol H ^E, of Et₄NBr, Bu₄NBr, and Hex₄NBr have been determined. The Δ_sol H ^E values are positive for Et₄NBr and negative for Bu₄NBr and Hex₄NBr and become more negative from Bu₄NBr to Hex₄NBr.     

Viscosities of Some Tetraalkylammonium and Alkali-Metal Salts in N,N-Dimethylacetamide at 25°C

The viscosities of solutions of tetrapropylammonium bromide (Pr₄NBr), tetrabutylammonium bromide (Bu₄NBr), tetrapentylammonium bromide (Pen₄NBr), tetrahexylammonium bromide (Hex₄NBr), tetraheptylammonium bromide (Hep₄NBr), tetraoctylammonium bromide (Oct₄NBr), tetrabutylammonium tetraphenylborate (Bu₄NBPh₄), sodium tetraphenylborate (NaBPh₄), and potassium tetraphenylborate (KBPh₄) in N,N-dimethylacetamide are reported at 25^°C. The viscosity data havebeen analyzed by the Jones-Dole equation for associated electrolytes to evaluate the viscosity B coefficients of the electrolytes. These data have also been analyzed by the transition-state theory to obtain the contribution of the solutes to the free energy of activation for viscous flow of the solution. The ionic contribution to the viscosity B coefficient and the free energy of activation for viscous flow have been estimated using of the reference electrolyte Bu₄NBPh₄. The bromide, tetraphenylborate, and tetraalkylammonium ions are found to be weakly solvated in N,N-dimethylacetamide, whereas significant solvation has been detected for sodium and potassium ions. The viscosity of the solvent is greatly modified by the presence of all the ions investigated here with the exception of the bromide ion.     

Tetraalkylammonium Picrates in the Dichloromethane–Water System: Ion-Pair Formation and Liquid–Liquid Distribution of Free Ions and Ion Pairs

Equilibria concerning picrates of tetraalkylammonium ions (Me₄N⁺, Et₄N⁺, Pr₄N⁺, Bu₄N⁺, Bu₃MeN⁺) in a dichloromethane−water system have been investigated at 25 ^∘C. The 1:1 ion-pair formation constants (K _IP,o ^o) in dichloromethane at infinite dilution were conductometrically determined. The distribution constants (K _D ^o) of the ion pairs and the free cations between the solvents were determined by a batch-extraction method. The K _IP,o ^o value varies in the cation sequence, Bu₄N⁺ ≈ Pr₄N⁺ ≈ Et₄N⁺ < Bu₃MeN⁺ < < Me₄N⁺; this trend is explained by the electrostatic cation−anion interaction taking into account the structures of the ion pairs determined by density functional theory calculations. For the ion pairs of the symmetric R₄N⁺ cations, there is a linear positive relationship between log₁₀ K _D ^o and the number of methylene groups in the cation (N _{CH 2}). The ion pair of asymmetric Bu₃MeN⁺ has a higher distribution constant than that expected from the above log₁₀ K _D ^o versus N _{CH 2} relationship. These cation dependencies of log₁₀ K _D ^o for the ion pairs are explained theoretically by using the Hildebrand-Scatchard equation. For all the cations, the log₁₀ K _D ^o value of the free cation increases linearly with N _{CH 2}; the variation of log₁₀ K _D ^o is discussed by decomposing the distribution constant into the Born-type electrostatic contribution and the non-Born one, and attributed to the latter that is governed by the differences in the molar volumes of the cations. The cation dependencies of the ion-pair extractability and ion pairing in water are also discussed. An erratum to this article can be found at     

Freezing points,osmotic coefficients,and activity coefficients of some salts in ethylene carbonate: A high dielectric constant solvent without hydrogen bonding

The freezing points, conductivities, and densities of NaI, KI, CsI, Bu₄NCl, Bu₄NBr, Bu₄NI, Et₄NBr, and Pr₄NBr (where Et = ethyl, Pr = propyl, and Bu =n-butyl) in ethylene carbonate have been measured. Osmotic and activity coefficients were calculated from the results. All of the salts studied are strong electrolytes. The trends in the osmotic coefficients of the alkali metal iodides are NaI>KI>CsI, showing that Na⁺ is more solvated by ethylene carbonate than Cs⁺. For the tetraalkylammonium halides, the order of osmotic coefficients are Et₄NBrPr₄NBrBu₄NCl>Bu₄NBr>Bu₄NI. This is the same order as observed in two other high-dielectric-constant solvents, water andN-methylacetamide. The results indicate that the smaller anions are more solvated than the larger anions in ethylene carbonate in contrast to the usual behavior of dipolar aprotic (basic) solvents, such as dimethyl sulfoxide.     

The reaction ofC-phenylcalix[4]resorcinarene-based polymer with quaternary ammonium and potassium cations

The resol polycondensation ofC-phenylcalix[4]resorcinarene with formaldehyde affords a corss-linked polymer possessing ion-exchange ability. The ion-exchange capacity of the polymer with respect to NH₄ ⁺, Me₄N⁺, Et₄N⁺, Bu₄N⁺, and K⁺ cations was determined. The equilibrium in the systemsC-phenylcalix[4]resorcinarene-based polymer—binary or ternary aqueous solutions of electrolytes was studied by potentiometric titration and quantumchemical MNDO/PM3 calculation Published inIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 8, pp. 1475–1477, August, 2000.     

Studies on the effect of the cation nature on the kinetics of the isotopic exchange reaction between chloride ion and O,O-diphenylphosphorochloridothionate in acetonitrile

Rate constants and activation parameters for the isotopic exchange reactions between (PhO)₂PSCl and M³⁶Cl (M = Me₄N⁺, Et₄N⁺, n-Bu₄N⁺, Et₃HN⁺, EtH₃N⁺, Li⁺) in acetonitrile were measured in order to find the effect of the cation nature onthe kinetics of the reaction. The rate constants measured for a range of concentrations of Et₃HN³⁶Cl, EtH₃N³⁶Cl, and Li³⁶Cl were analyzed using the Acree equation. The equivalent conductance of LiCl in acetonitrile was determined. The nature of the cation has no effect on the mechanism of the reaction. The cation changes only the experimental rate constant proportionally to the dissociation degree of the salt. Smaller values of the rate constant and smaller activation parameters ΔH? and ΔS? for the reaction with Li³⁶Cl indicate the existenceof the intermolecular interaction between lithium ions and O,O-diphenylphosphorochloridothionate.     

II. Ionic association and mobility in propylene carbonate

Conductance data are reported for Ph₄AsPic, Ph₄PPic, Ph₄SbPic, Hex₄NPic, Bu₄PPic, Et₄NSbCl₆ in propylene carbonate at 25°C in the concentration range 1×10^–4 to 15×10^–4 M. The data were analyzed by the Justice modification of the Fuoss-Hsia equation and all salts studied were found to be associated and to form solvent separated ion pairs. Application of the Barthel-Bjerrum model of ion association permitted calculation of the non coulombic portion of the potentials of mean force, W_±. Ionic limiting equivalent conductances of six ions were calculated using known values of R₄N⁺, and Pic^– ions. Walden products of ions in propylene carbonate were examined in the light of modern ion mobility theories, including Boyd-Zwanzig, Hubbard-Onsager, and Hubbard-Kayser models of ion solvent interactions.     

Thermal properties of tetraalkylammonium bromides in several solvents

The enthalpies of solution of tetraethyl- and tetra-n-hexylammonium bromides have been measured in mixtures of formamide with ethylene glycol at 298.15 and 313.15 K in the whole mole fraction range by the calorimetric method. The standard enthalpies of solution in binary mixtures have been calculated with Redlich–Rosenfeld–Meyer type equation. The enthalpy and heat capacity parameters of pair interaction of organic electrolytes with EG in FA and with FA in EG have been computed and discussed. The enthalpy interaction parameters of single ions with EG in FA medium have been evaluated and compared with those for ion–water and ion–MeOH interaction in FA. The standard heat capacities of solution have been evaluated. The excess enthalpies of solution, Δ_sol H ^E, of Et₄NBr, Bu₄NBr, and Hex₄NBr have been determined. The Δ_sol H ^E values are positive for Et₄NBr and negative for Bu₄NBr and Hex₄NBr and become more negative from Bu₄NBr to Hex₄NBr.     

Protonation of carbonate in aqueous tetraalkylammonium salts at 25 degrees C

Protonation constants of carbonate were determined in tetramethylammonium chloride (Me₄NCl_aq 0.1 ≤ I/mol kg⁻¹ ≤ 4) and tetraethylammonium iodide (Et₄NI_aq 0.1 ≤ I/mol kg⁻¹ ≤ 1) by potentiometric ([H⁺]-glass electrode) measurements. Dependence of protonation constants on ionic strength was taken into account by modified specific ion interaction theory (SIT) and by Pitzer models. Literature data on the protonation of carbonate in NaCl_aq (0.1 ≤ I/mol kg⁻¹ ≤ 6) were also critically analysed. Both protonation constants of carbonate follow the trend Et₄NI > Me₄NCl > NaCl. An ion pair formation model designed to take into account the different protonation behaviours of carbonate in different supporting electrolytes was also evaluated.     

Conductance of 1,1-electrolytes in acetonitrile solutions from −40° to 35°C

Conductance data for perchlorates of Li⁺, K⁺, Me₄N⁺, Et₄N⁺, Pr₄N⁺, Bu₄N⁺, iodides of K⁺, Me₄N⁺, i-Am₃BuN⁺, and tetraphenylborates of Na⁺, Bu₄N⁺ and i-Am₃BuN⁺ in acetonitrile solution in the temperature range −40° to 35°C are reported. Λ_° (limiting molar conductance) and K_A (association constant) are evaluated for several temperatures using a conductance equation based on the chemical model of electrolyte solutions including short range forces. Limiting molar ion conductances, λ _ΰ ⁱ , at −35°, −25°, −15°, −5°, 5°, 15° and 25°C are evaluated from temperature dependent limiting transference numbers. Enthalpies and entropies of association, obtained from the temperature dependence of the association constants, are also presented. Dedicated to the memory of Professor Raymond M. Fuoss.     

Raman spectra of selectively deuterated tetramethylammonium ions—II. Fermi resonance in the methyl stretching region

The Raman spectra of six selectively deuterated Me₄N⁺ ions in the methyl stretching regions are reported and the Fermi resonance multiplets caused by interactions with methyl bending overtones are discussed. For the rather simple cases of a single CH₃ group in the d₉ ion and a single CD₃ group in Me₄N⁺-d₃ three independent methods for the evaluation of the unperturbed symmetric stretching frequency from the observed Fermi resonance triplet are compared, and the matrix elements W₁₂ and W₁₃, describing this interaction, are determined from the perturbed frequencies and intensities. It is shown that a simple procedure, which determines ν⁰_s from the centre of gravity of an observed Fermi resonance multiplet, can also be applied to more complex cases, e.g. the polarized multiplets of the isotopically pure ions Me₄N⁺ and Me₄N⁺-d₁₂. The origin of the complex Fermi resonance pattern in the latter species is discussed in relation to the increased complexity in the methyl bending region, that has been previously analysed.     

Structural Characterization of the Solvate Complexes of the Lithium Salts of Diorganophosphides and Phosphinideneborates; A Pathway to Phosphorus-Boron Double Bonds

Abstract

The structures of several solvated lithium diorganophosphides are described. These may take a variety of structures including chain-like polymers with alternating Li⁺ and PR₂ ^? groups, dimeric species with PR₂ ^? groups bridging two Li⁺ ions or mononuclear species having terminal ^?PR₂ groups which have pyramidal geometries at phosphorus. The Li⁺ ions in all structures are solvated by either THF or Et₂O bases. Separation of the Li⁺ can be effected using 12-crown-4 to coordinate Li⁺ as [Li(12-crown-4)₂]⁺ affording free [PR₂]^? counterions. An extension of these techniques has led to the synthesis of the first compounds which have B-P double bonds. These are the compounds [Li(Et₂O)₂PRBMes₂] and [Li(12-crown-4)₂][PRBMes₂](R=Ph, C₆H₁₁,Mes) which have B-P bond lengths of 1.82 – 1.83Å.     

Electrochemical studies of sulfonates in non-aqueous solvents: Part I. Polarographic reductions of alkali metal ions with sulfonate supporting electrolyte in N,N-dimethylformamide and acetonitrile

The effect of the supporting electrolytes tetraethylammonium p-toluenesulfonate, methanesulfonate and trifluoromethanesulfonate on the polarographic reduction of alkali metal ions in N,N-dimethylformamide and acetonitrile has been investigated. In tetraethylammonium trifluoromethanesulfonate supporting electrolyte, the polarographic reductions of alkali metal ions are almost the same as those in perchlorate in both solvents. In tetraethylammonium p-toluenesulfonate and methanesulfonate, the half-wave potentials shift to more negative potentials, especially in acetonitrile. These shifts can be explained in terms of ion-pair formation between the alkali metal ion and the supporting electrolyte anion.     

Electrochemical carboxylation of fluorocontaining imines with preparation of fluorinated N-phenylphenylglycines

A possibility of obtaining fluorine-containing N-phenylphenylglycine derivatives at yields of up to 85% via the electrochemical carboxylation of corresponding benzalanilines was shown. The influence of imine's electron structure, the nature of supporting electrolyte and cathodic material on such processes is examined. It was found, that increasing electron accepting ability of the substituents in benzylidene and aniline fragments of the imine molecule lead to decrease of amino acid yields. The dependence of the N-phenyl-p-fluorophenylglycine yield on the cathode material (Zn, GC, Cu, Ag, Pt) and on the nature of the supporting electrolytes (Bu₄NBr, Et₄NBr, Et₄NClO₄, PhCH₂Me₃NClO₄, LiBF₄, LiClO₄, NaBF₄ and KBF₄) was investigated. The highest amino acid yields were obtained at cathodes (GC and Zn) that do not exhibit specific adsorption of fluorine-containing imines, as well as in the presence of background salts (Alk₄NBr) whose cations do not show tendency to strong ion pairing with anion radicals formed by the electrochemical activation of the imines.     

Polarographic waves of tri-n-butyl phosphate and polarographic determination of uranium in the presence of tri-n-butyl phosphate

The results of a study on the polarographic behaviour of TBP and its influence on the determination of uranyl ions is presented. The half-wave potential of the adsorption wave of TBP depends on the concentration of TBP, type of supporting elec trolyte and its concentration. In the presence of TBP the polarographic wave of U(VI) ion is changed. Below 7·10^?5 M TBP the polarographic wave of U(VI) is not affected, between 7·10^?5 and 2·10^?4 M TBP the shape, height and half-wave potential of U(VI) waves are changed and above 2·10^?4 M, up to saturated solution of TBP, the waves of U(VI) do, not change further. The bes supporting electrolytes for the determination of U(VI) are KNO₃ or NaClO₄ in concentrations of 0.1 to 0.5 M, pH 1–2 and TBP concentrations from 3·10^?4 to 1.2·10^?3 M.     

The cooperative effect in ion-pair recognition by a ditopic hemicryptophane host

The heteroditopic hemicryptophane 1 , which bears a tripodal anion binding site and a cation recognition site in the molecular cavity, proved to be an efficient ion‐pair receptor. The hemicryptophane host binds anions selectively depending on shape and hydrogen‐bond‐accepting ability. It forms an inclusion complex with the Me₄N⁺ ion, which can simultaneously bind anionic species to provide anion@[ 1? Me₄N⁺] complexes. The increased affinity of [ 1? Me₄N⁺] for anionic species is attributed to a strong cooperative effect that arises from the properly positioned binding sites in the hemicryptophane cavity, thus allowing the formation of the contact ion pair. Density functional theory calculations were performed to analyze the Coulomb interactions of the ion pairs, which compete with the ion‐dipole ones, that originate in the ion–hemicryptophane contacts.     

Tetraalkylammonium Ions in Aqueous and Non-aqueous Solutions

Property data for tetraalkylammonium cations, [H(CH₂) _n ]₄N⁺, are reviewed. They pertain to the isolated cations and their transfer from the gas phase into aqueous solutions. Various properties of these cations in aqueous and non-aqueous solutions and data for their transfer between these are also reviewed. Emphasis is placed on the dependence of data on the length n of the alkyl chains rather than on the absolute values. Most of the data are available only for the first four members of the series. The properties of the isolated ions increase linearly with the chain length. Molar enthalpies of formation of the gaseous and aqueous cations, and absolute standard molar enthalpies of hydration, are derived. Standard molar entropies of dissolution of several salts in water are obtained from their solubilities and enthalpies of solution. The molar entropies of the crystalline iodides of the first four members of the series then give the standard partial molar entropies of the aqueous cations and their molar entropies of hydration. The standard partial molar volumes in aqueous and non-aqueous solutions are quite linear with n and in non-aqueous solutions the molar volume hardly depends on the nature of the solvent. On transfer from water to non-aqueous solvents the volume of Me₄N⁺ suffers some shrinkage, that of Et₄N⁺ appears to be unaffected, but from Pr₄N⁺ onwards an increasing expansion takes place. This unexpected result is tentatively explained by hydrophobic intra-molecular association of pairs of alkyl chains in aqueous solutions, resulting in a tightening of the structure. The transfer of the R₄N⁺ cations from water into non-aqueous solvents is governed by a large positive entropy change, outweighing the smaller positive enthalpy change. The transport properties of the aqueous R₄N⁺ cations are non-linear with n. A major impediment to movement is thus the sticking of the water molecules to the ice-like hydrophobic hydration sheaths of the larger cations. The number of water molecules affected by the hydrophobic cations is open to widely differing estimates resulting from various approaches, and constitute an open issue.