Structure and stability of ion pairs A−·K+·H2O with a strong anion

Characteristics of the ion pairs HCOO^–·Na⁺·H₂O, HCOO^–·K⁺·H₂O, and also Na⁺·H₂O and K⁺·H₂O were calculated by the nonempirical Hartree—Fock—Roothan linear-combination-of-atomic-orbitals self-consistent-field (SCF) molecular-orbital method in a two-exponent Dunning basis using an extended set of Huzinaga—Dunning Gaussian functions. The basis was supplemented by polarization functions ofd type for the oxygen atom andp type for the H atom and also by diffusion functions ofp type for the oxygen atom. Characteristics of the ion pairs HCOO^–·Li⁺ and HCOO^–·Na⁺ were calculated taking into account the electronic correlation according to the Möller — Plesset second-order perturbation theory. Significant quantitative difference was observed in the hydration of ionogens and free cations. The stability of the ionogens HCOOMe in aqueous solutions, increasing from Li⁺ to Cs⁺, is not explained by the difference between the energies of complexation and the energies of hydration of the cations. The better solubility of the salt molecule with a cation of smaller radius is due to the higher degree of hydration of that ionogen.N. N. Semenov Institute of Chemical Physics, Russian Academy of Sciences, 117977 Moscow. Translated fromIzvestiya Akademii Nauk, Seriya Khimicheskaya, No. 12, pp. 2700–2707, December, 1992.     

Monohydration selectivity mechanism of alkali cations in the potassium channel of excitable biomembranes

Nonempirical quantum chemical method Hartree–Fock–Roothan LCAO SCF MO in a two-exponent Dunning basis with the use of an extended set of Gaussian functions by Huzinaga–Dunning with consideration of electron correlation according to the Meller–Plesset theory of excitations of the second order was used to study monohydrates of Li⁺, Na⁺, K⁺, and HCOO^? ions. The indicated basis was supplemented with polarization functions of d-type on the O atom and of p-type on the hydrogen atom as well as with diffusion functions of p-type on the oxygen atom. It has been found that binding energies of the water molecule with Li⁺, Na⁺ appeared to be higher and with K⁺ lower than with HCOO^? · H₂O. Potential curve shapes of K⁺ + H₂O and HCOO^? + H₂O reactions are shown to be similar. The molecular mechanism of K⁺ channel selectivity of an excitable membrane is explained on the basis of the obtained calculations.     

Nonempirical calculations of potential energy surfaces,structures, and stabilities of OF2Li+, SF2Li+, F2Li+, and CIFLi+ ions

Nonempirical calculations were carried out for the potential energy surfaces of OF₂Li⁺, SF₂Li⁺, F₂Li⁺, and ClFLi⁺ using the SCF/TEHD+P approximation. The stability of the bridged structure in comparison with the classical structure is enhanced upon replacement of the oxygen atom by sulfur and replacement of fluorine by chlorine. A bridged structure with Li–F–M and . bonds is the major form for all the fluorides examined. The classical structure LiMF _k ⁺, where Li is bound directly to the M atom, corresponds to an apex of a potential barrier and is either close or greater on an energy scale to the dissociation limit to Li⁺+MF _k . A comparison was made with analogous hydride complexes, which, in contrast to the fluorides, display isomerism.Institute of New Chemical Problems, Academy of Sciences of the USSR. Translated from Zhurnal Strukturnoi Khimii, Vol. 30, No. 6, pp. 19–22, November–December, 1989.     

Molecular potential,cation binding,and hydration properties of the carboxylate anion. Ab initio studies with an extended polarized basis set

The carboxylat anion, involved in the structure of numerous compounds of biological interest, participates in a number of intermolecular interactions involving water, cations, and other cellular constituents. A set of ab initio SCF computations have been carried out with an extended polarized basis set on HCOO^–, its molecular electrostatic potential, and its interaction with Li⁺, Na⁺, K⁺, and H₂O. The results are compared with those of a minimal good quality basis set. An evaluation of the basis set superposition error is made in the two basis as well as that of the contribution of the dispersion energy to the hydration. The analogies and differences in the nucleophilic character of the formate and the phosphate groups are discussed.     

The alkali metal cation effect on the surface-enhanced Raman spectra of phosphate anions adsorbed at silver electrodes

The interaction of adsorbed phosphate anions with alkali metal cations at the Ag|aqueous solution interface has been investigated by surface-enhanced Raman spectroscopy (SERS). Formation of ion pairs at the interface was evident from the cation-induced perturbations in the SER spectra of anions. The frequency of the external vibration, silver–oxygen (Ag---O′), was not sensitive to the nature of cation, while the relative intensity of this mode was cation-dependent and was explored as a sensitive probe for the monitoring of coadsorption of ions at the interface. From the internal phosphate vibrations, both asymmetric modes, δ_as(PO) and ν_as(PO), were found to be the most sensitive to the nature of the cation. At a relatively positive potential (0.00 V vs. Ag | AgCl) the spectral parameters for the Cs⁺ and K⁺ cations were very similar indicating the same bonding type with anions. A more inhomogeneous chemical environment for the phosphate oxygen atoms was detected in the case of Na⁺ and Li⁺ cations. An increase in ν_as(PO) frequency by ca. 10 cm⁻¹ was the characteristic spectral signature for the interaction of phosphates with Li⁺. The formation of water-shared ion pairs at the interface was suggested based on the absence of splitting in the ν_as(PO) mode and the previously observed frequency sensitivity of this band to solvent H₂O substitution by D₂O. At negative potential (−0.80 V), a stabilization effect of Cs⁺ on the phosphate adlayer was detected based on the twofold increase in intensity of the ν(Ag---O′) mode compared with Li⁺. Splitting of the ν_as(PO) mode suggested the contact interaction of anions with specifically adsorbed Cs⁺ cations.     

Association of metal cations with alkanes: Na(CH4)+ versus Cu(CH4)+ as molecular models

Summary Ab initio molecular orbital calculations give small stabilization energies for the various Na(CH₄)⁺ adducts (less than 4 kcal mol^–1), but predict a stronger binding for the copper compounds (about 13 kcal mol^–1). The different behaviour of Na⁺ and Cu⁺, already present at the SCF level, is reinforced by electron correlation. This can be attributed to an important contribution of the dispersion energy to the binding energy of the copper ion: about 40% of the total, including basis set superposition corrections.Dedicated to Mrs A. Pullman     

Vibrational spectroscopic and density functional theory studies on ion solvation and ion association of lithium tetrafluoroborate in N,N-dimethylcarbamoyl chloride-based solvents

Solvation and association interactions in solutions of LiBF₄/DMCC (DMCC for N,N-dimethylcarbamoyl chloride) and LiBF₄/DMCC–DME (DME for 1,2-dimethoxyethane) have been studied as a function of concentration of lithium tetrafluoroborate by infrared and Raman spectroscopy. Strong interactions between Li⁺ and solvent molecules or BF₄⁻ anions are observed. The apparent solvation numbers of Li⁺ in LiBF₄/DMCC solutions were deduced. Band-fitting to the B–F stretching band of BF₄⁻ anion permits detailed assess of the ion pairing. Based on the calculations of density function theory, optimal structures of Li⁺(DMCC)_n (n = 1–3) were suggested. It is found that the lithium ion was preferentially solvated by DME in DMCC–DME binary solvents. This finding is supported by quantum chemistry calculations.     

Analysis of the interaction energy in the Cu+-H2O and Cl−-H2O systems,with CP corrections to the BSSE of the separate terms,and MC simulations of the aqueous systems with and without CP corrections

The interaction energyE of the systems Cu⁺-H₂O and Cl^–-H₂O has been computed over a wide range of distances and orientations with the MINI-1 basis set in the SCF approximation. The interaction energy has been decomposed according to the Kitaura-Morokuma scheme, with and without counterpoise (CP) corrections to the basis set superposition error. The importance of this correction is analysed by its effect upon Monte Carlo calculations of the Cu⁺-water and Cl^–-water systems, using two-body potentials without and with CP corrections. The effect of CP corrections on theE analysis is similar to that found in other systems of analogous composition (of the general type ion plus neutral ligands), but with significant differences in the details. The effect of the CP corrections to the interaction potential, and then on the results of the Monte Carlo simulations, is small for the Cu⁺ ion, but remarkable for the Cl^– ion.     

Nonempirical calculations of models of interstitial defects in an LiF crystal

Models of interstitial Li⁺ and F^– ions in an LiF crystal have been calculated by the nonempirical Hartree-Fock-Roothaan method with the 4-31G+ basis and with the aid of a nonempirical pairwise potential. It has been shown that the energies needed for the adiabatic transfer of Li⁺ and F^– ions from infinity to interstitial positions in Li₄F₄ cubes are equal to +2.3 and +8.1 eV, respectively. The energies required for the transfer of Li⁺ and F^– ions from infinity to interstitial positions in a large cubic cluster of 4000 LiF molecules (a cube with an edge consisting of 20 atoms) are determined to a considerable extent by the long-range interactions of nonelectro-static nature and are equal to –0.25 and +9.9 eV, respectively, without consideration of the relaxation of the lattice.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 10, pp. 2272–2277, October, 1989.We express our thanks to P. Charskii for supplying his version of the HONDO-5 + MP2 program and to V. G. Zakzhevskii for supplying the GAUSSIAN-SM program.     

Anab initio pair potential for the interaction between a water molecule and a formate ion

The potential surface for the interaction between a rigid formate ion and a rigid water molecule has been investigated byab initio methods. An analytical potential expression was derived to fit the 591 calculated SCF energies. The global minimum on the surface is –16.3 kcal/mol and corresponds to a bifurcated bonding situation.     

Electrostatic,sequential bond energies and structures of Li+·(N2)n complexes: computational study

The MP2 and CCSD calculations of the geometries and binding energies of the Li⁺·(N₂)_n (n?=?1–4) complexes are obtained. The potential energy surface showed that these complexes exhibit one minimum state and one transition state. The mono- and di-ligated complexes exhibit linear configurations with a binding energy of 11.1 and 21.2 kcal mol^?1, respectively. Trigonal planar and tetrahedral configurations are obtained for tri- and tetra-ligated complexes, respectively. The computed sequential bond dissociation energies (BDEs) of Li⁺·(N₂)_n (n?=?1–4) complexes are also calculated in which the mono-ligated complex has the largest BDE value. The obtained trend is mainly dependent on the variation in the ion-quadrupole interaction of these ion complexes. These calculations predict that these complexes are of purely electrostatic nature.

     

The minimal basis set MINI-1—powerful tool for calculating intermolecular interactions. II. Ionic complexes

Optimum geometries and stabilization energies are determined for complexes of H₂O, NH₃, CH₄, C₂H₄, CO, and N₂ with metal cations including Li⁺, Na⁺, K⁺, Rb⁺, Be²⁺, Mg²⁺, Ca²⁺, Zn²⁺, and Al³⁺, for the complex (HO)₂PO ₂ ^– ...Mg²⁺ and for the complexes of water with F^–, Cl^–, and Br^– by SCF calculations employing the MINI-1 minimal gaussian basis sets. The Boys-Bernardi method was used to evaluate the superposition error. Comparison with the extended basis set results revealed that the MINI-1 set gives uniformly good results for a broad variety of ionic complexes and therefore should be preferred to other small basis sets.     

Ion-exchange chromatography using vanadyl and vandate salts as mobile phases with indirect detection

Summary Vanadyl sulfate, VOSO₄, was characterized as the mobile phase for the ion exchange separation of Li⁺, Na⁺, NH ₄ ⁺ , and K⁺ using indirect photometric detection at 254 nm. Detection limits ranged from 0.2 ppm for Li⁺ to 1 ppm for K⁺. Indirect electrochemical detection of these separated cations by reduction of VO (II) to V³⁺ was compared to spectrophotometric detection. The potential of the vanadate species, HVO ₄ ^2– , for the separation of F^–, Cl^–, and SO ₄ ^2– , with indirect photometric detection was also demonstrated.     

Enhanced Li+ binding energies of some azines: a molecular orbital study

Summary Hartree-Fock calculations with the 6–31G* basis have been performed to investigate the structure and Li⁺ binding energies of the complexes between Li⁺ and pyridine, diazines, triazines and tetrazines. Structures have been fully optimized at the 3–21G level. As for azole-Li⁺ and methyldiazole-Li⁺ complexes, a topological analysis of the Laplacian of the electronic charge density reveals that the azine-Li⁺ is a typical closed-shell interaction and that the stabilization of the complex is mainly electrostatic. BSSE is quite significant, specially for Li⁺-bridging complexes. The correlation between calculated Li⁺ binding energies and proton affinities follows two different linear relationships, one for those cases where Li⁺ is singly coordinated and a different one for those cases in which an additional three-membered ring is formed. The enhanced stability of these particular conformations explains why while polyazines are less basic than pyridine when the reference acid is a proton; pyridazine and 1,2,4 triazine are more basic than pyridine when the reference acid is Li⁺. The effect on Li⁺ binding energies of systematic nitrogen substitution roughly follows an additive model.     

SCF LCAO MO studies on the hydration of ions: The system F− · 2H2O

The energy surface of the dihydrated fluoride anion (F·2H₂O)^–1 is studied for a number of different geometry points near the equilibrium structure within the SCF LCAO MO framework, using an extended gaussian basis set to approximate the molecular wavefunctions. For the first and second hydration step of the fluoride anion the corresponding hydration energies are calculated to beB ₁ ^scf =24.1 kcal/mole andB ₂ ^SCF =20.8 kcal/mole (experimental measurements: 23.3 kcal/mole and 16.6 kcal/mole, respectively). The hydration energies and equilibrium bond distances obtained for the dihydrated fluoride anion (F·2H₂O)^– are compared with those found for the monohydrate (FHOH)^– and with corresponding results of the dihydrated lithium cation (Li · 2H₂O)⁺. The system (F·2H₂O)^– is taken as a very simple model to discuss some basic features of the hydration process of small ions and to study the influence of a negative ion on an adjacent hydrogen bond.We would like to thank our technical staff for valuable help in carrying out these calculations.     

Raman spectra in the libration region of concentrated aqueous solutions of lithium-6 and lithium-7 halides. Evidence for tetrahedral Li(OH2) 4 +

The Raman spectra of saturated solutions of⁶LiCl and⁷LiCl have been decomposed into Gaussian components, one of which is a polarized band that occurs at 360 cm^–1 when the ion is⁶Li⁺ and shifts to 335 cm^–1 when the ion is⁷Li⁺. Equivalent bands occur in the spectra of saturated solutions of⁶LiBr and⁷LiBr at 343 and 320 cm^–1, respectively. These bands are assigned to solvent-separated ion aggregates. The Raman spectra of 8.0 and 3.5 m solutions of the isotopic lithium chlorides have been decomposed into five Gaussian components, three of which are assigned to water librations. In addition, there is a polarized band at 440 cm^–1 independent of the lithium isotope used, and a depolarized band which occurs at 385 cm^–1 in the⁶LiCl solutions and 360 cm^–1 in the⁷LiCl solutions. We interpret these two additional bands as theA ₁ andF ₂ stretching modes of Li⁺ tetrahedrally solvated by water molecules.     

Effects of alkali-metal cations on nitrate-ion N-O bond force constants in ionic melts

Measured frequencies from pure nitrate liquids have been used to calculate the force constants for the nitrate ion and the M⁺-NO₃ ^– systems (M⁺ = Li⁺, Na⁺, K⁺, Rb⁺, and Cs⁺) by a matrix successive-approximation method based on perturbation theory in dependent coordinates. The valency force constants K_q for the N-O bonds are increased by comparison with free NO ₃ ^– . The N-O bond strengthening is not adequately explained by cation-anion interaction because there is electron-density transfer mainly by the mechanism within the NO ₃ ^– . Correction for the vibrational parameters of the interacting particles confirms the previously demonstrated acid-base mechanism for the cation-anion interaction in liquids containing nitrate.Translated from Teoreticheskaya i Éksperimental'naya Khimiya, Vol. 25, No. 2, pp. 230–234, March–April, 1989.     

Chiral recognition and enantiomer assays of N-(3,5-dinitrobenzoyl)amino acid derivatives using electrospray ionization–mass spectrometry

A pair of pseudoenantiomers, anilide derivatives of N-pivaloylproline were prepared and used as chiral selectors for enantiomer discrimination of amides or esters of N-(3,5-dinitrobenzoyl)amino acids in single-stage electrospray ionization/mass spectrometric experiments. Addition of a chiral analyte to a solution of the two pseudoenantiomeric chiral selectors affords selector–analyte complexes in the electrospray ionization mass spectrum where the ratio of these complexes is dependent on the enantiomeric composition of the analyte. The relationship between the ratio of the selector–analyte complexes in the electrospray ionization mass spectrum and the enantiomeric composition of the analyte can be used to relate the extent of the measured enantioselectivity and for quantitative enantiomeric composition determinations. Effects of the added cationic ions (H⁺, Li⁺, Na⁺ and K⁺) and instrument conditions on the selector–analyte ion intensity and the enantioselectivity (α_MS) were investigated. The percent ratio of the sum of the selector–analyte ion counts and the total ion counts decreases accordingly with the increase of the desolvation temperature for H⁺, Na⁺ and K⁺. The ratio for Li⁺ kept almost constant. The best α_MS was observed at a desolvation temperature of 200 °C with the added H⁺. The cone voltage has little effects on the α_MS values though the intensities of selector–analyte complexes are decreased at higher cone voltages. The observed MS enantioselectivities are comparable to the HPLC enantioselectivities and the sense of chiral recognition by MS is consistent with what is observed chromatographically. Quantitative enantiomeric composition determinations for five different samples of N-(3,5-dinitrobenzoyl)leucinyl butylamide at four different concentrations were performed. The average % difference between the HPLC and MS enantiomer determinations is 6.8% and 3.7% for the calibration lines constructed at a concentration of the analyte of 125 μM and 12.5 μM, respectively.     

Quantum-Chemical Investigation of Ionic Association of Lithium Salts LiXF<Subscript>6</Subscript> (X = As,P)

The possibility of formation of various ion clusters for lithium salts LiXF₆ (X = As, P) is studied. The dynamic matrix of the clusters in a gas phase is calculated by numerical and analytical differentiation of the full energy of clusters in the MO LCAO approximation by the Hartree-Fock-Roothaan (HFR) method with the aid of program package PC GAMESS. Stable ionic clusters are ion pairs Li⁺[XF₆]⁻ with bi- and tridentate cation coordination relative to the octahedral anion, ion triplets [XF₆]⁻Li⁺[XF₆]⁻ and Li⁺[XF₆]⁻Li⁺ with bi- and tridentate coordination, and ion dimers { Li⁺[XF₆]⁻}₂ with bidentate coordination. Trimers {Li⁺[XF₆]⁻}₃ and tetramers {Li⁺[XF₆]⁻}₄ in the form of symmetrical ring structures with monodentate coordination are stable only for [AsF₆]⁻. For stable ion species, densities of vibrational states and IR spectra are calculated.__________Translated from Elektrokhimiya, Vol. 41, No. 5, 2005, pp. 546–555.Original Russian Text Copyright © 2005 by Popov, Nikiforov, Bushkova, Zhukovskii.     

An ab initio study of the potential surface for the reaction H2COH → HCO + H2

The potential surface for the reaction H₂CO⁺H → HCO⁺ + H₂ has been studied by ab initio SCF calculations, using gaussian-type basis functions. A saddle point on the surface has been found, and a reaction path is proposed to explain the observed release of kinetic energy. The energy of activation and ΔE for the reaction have been estimated.