Potential-dependent vibrational spectroscopy of solvent molecules at the Pt(111) electrode in a water/acetonitrile mixture studied by sum frequency generation

Sum frequency generation (SFG) vibrational spectra of D(2)O and/or acetonitrile (CH(3)CN) on a Pt(111) single-crystal electrode were obtained as a function of applied potential in a 5 mol % water/acetonitrile mixed solvent with different 0.1 molar MSO(3)CF(3) salts (M = H(+), Li(+), Na(+), K(+), and Cs(+)). The results provide a very specific model for the composition of the inner Helmholtz layer as a function of potential and surface charge. Acetonitrile dominates the inner layer with the CN group directed toward the metal at potentials where the metal has a positive charge. As the surface becomes negatively charged, the acetonitrile orientation flips 180 degrees, with the CH(3) group pointing toward the surface. At even more negative surface charge, D(2)O displaces acetonitrile from the inner layer and is the predominant molecule on the surface. Here water is present as an oriented molecule with the oxygen end pointing toward the metal. The potential (and surface charge) where water is the dominant molecule in the inner Helmholtz layer is determined by the solvation energy of the cation.     

Oligoether-strapped calix[4]pyrrole: an ion-pair receptor displaying cation-dependent chloride anion transport

A ditopic ion-pair receptor (1), which has tunable cation- and anion-binding sites, has been synthesized and characterized. Spectroscopic analyses provide support for the conclusion that receptor 1 binds fluoride and chloride anions strongly and forms stable 1:1 complexes ([1·F](-) and [1·Cl](-)) with appropriately chosen salts of these anions in acetonitrile. When the anion complexes of 1 were treated with alkali metal ions (Li(+), Na(+), K(+), Cs(+), as their perchlorate salts), ion-dependent interactions were observed that were found to depend on both the choice of added cation and the initially complexed anion. In the case of [1·F](-), no appreciable interaction with the K(+) ion was seen. On the other hand, when this complex was treated with Li(+) or Na(+) ions, decomplexation of the bound fluoride anion was observed. In contrast to what was seen with Li(+), Na(+), K(+), treating [1·F](-) with Cs(+) ions gave rise to a stable, host-separated ion-pair complex, [F·1·Cs], which contains the Cs(+) ion bound in the cup-like portion of the calix[4]pyrrole. Different complexation behavior was seen in the case of the chloride complex, [1·Cl](-). Here, no appreciable interaction was observed with Na(+) or K(+). In contrast, treating with Li(+) produces a tight ion-pair complex, [1·Li·Cl], in which the cation is bound to the crown moiety. In analogy to what was seen for [1·F](-), treatment of [1·Cl](-) with Cs(+) ions gives rise to a host-separated ion-pair complex, [Cl·1·Cs], in which the cation is bound to the cup of the calix[4]pyrrole. As inferred from liposomal model membrane transport studies, system 1 can act as an effective carrier for several chloride anion salts of Group 1 cations, operating through both symport (chloride+cation co-transport) and antiport (nitrate-for-chloride exchange) mechanisms. This transport behavior stands in contrast to what is seen for simple octamethylcalix[4]pyrrole, which acts as an effective carrier for cesium chloride but does not operates through a nitrate-for-chloride anion exchange mechanism.     

Structure and energetics of poly(ethylene glycol) cationized by Li(+), Na(+), K(+) and Cs(+): a first-principles study

Density functional theoretical methods, including several basis sets and two functional, were used to collect information on the structure and energetic parameters of poly(ethylene glycol) (PEG), also referred to as poly(ethylene oxide) (PEO), coordinated by alkali metal ions. The oligomer chain is found to form a spiral around the alkali cation, which grows to roughly two helical turns when the oligomer size increases to about the decamer for each alkali ion. Above this size, the additional monomer units do not build the spiral further for Li(+) and Na(+); instead, they form less organized segments outside or next to the initial spiral. The distance of the first layer of co-ordinating O atoms from the alkali cation is 1.9-2.15 ? for Li(+), 2.3-2.5 ? for Na(+), 2.75-3.2 ? for K(+) and 3.5-3.8 ? for Cs(+) complexes. The number of O atoms in the innermost shell is five, six, seven and eleven for Li(+), Na(+), K(+) and Cs(+). The collision cross sections with He increase linearly with the oligomer to a very good approximation. No sign of leaning towards the 2/3 power dependence characterizing spherical particles is observed. The binding energy of the cation to the oligomer increases up to polymerization degree of about 10, where it levels off for each alkali-metal ion, indicating that this is approximately the limit of the oligomer size that can be influenced by the alkali cation. The binding energy-degree of polymerization curves are remarkably parallel for the four cations. The limiting binding energy at large polymerization degrees is about 544 kJ mol(-1), 460 kJ mol(-1), 356 kJ mol(-1) and 314 kJ mol(-1) for Li, Na, K and Cs, respectively. The geometrical features are compared with the X-ray and neutron diffraction data on crystalline and amorphous phases of conducting polymers formed by alkali-metal salts and PEG. The implications of the observations concerning collision cross sections and binding energies to ion mobility spectroscopy and mass spectrometry are discussed.     

Infrared spectroscopy of arginine cation complexes: direct observation of gas-phase zwitterions

The structures of cationized arginine complexes [Arg + M]+, (M = H, Li, Na, K, Rb, Cs, and Ag) and protonated arginine methyl ester [ArgOMe + H]+ have been investigated in the gas phase using calculations and infrared multiple-photon dissociation spectroscopy between 800 and 1900 cm-1 in a Fourier transform ion cyclotron resonance mass spectrometer. The structure of arginine in these complexes depends on the identity of the cation, adopting either a zwitterionic form (in salt-bridge complexes) or a non-zwitterionic form (in charge-solvated complexes). A diagnostic band above 1700 cm-1, assigned to the carbonyl stretch, is observed for [ArgOMe + H]+ and [Arg + M]+, (M = H, Li, and Ag), clearly indicating that Arg in these complexes is non-zwitterionic. In contrast, for the larger alkali-metal cations (K+, Rb+, and Cs+) the measured IR-action spectra indicate that arginine is a zwitterion in these complexes. The measured spectrum for [Arg + Na]+ indicates that it exists predominantly as a salt bridge with zwitterionic Arg; however, a small contribution from a second conformer (most likely a charge-solvated conformer) is also observed. While the silver cation lies between Li+ and Na+ in metal-ligand bond distance, it binds as strongly or even more strongly to oxygen-containing and nitrogen-containing ligands than the smaller Li+. The measured IR-action spectrum of [Arg + Ag]+ clearly indicates only the existence of non-zwitterionic Arg, demonstrating the importance of binding energy in conformational selection. The conformational landscapes of the Arg-cation species have been extensively investigated using a combination of conformational searching and electronic structure theory calculations [MP2/6-311++G(2d,2p)//B3LYP/6-31+G(d,p)]. Computed conformations indicate that Ag+ is di-coordinated to Arg, with the Ag+ chelated by both the N-terminal nitrogen and Neta of the side chain but lacks the strong M+-carbonyl oxygen interaction that is present in the tri-coordinate Li+ and Na+ charge-solvation complexes. Experiment and theory show good agreement; for each ion species investigated, the global-minimum conformer provides a very good match to the measured IR-action spectrum.     

Monovalent ion adsorption at the muscovite (001)-solution interface: relationships among ion coverage and speciation, interfacial water structure, and substrate relaxation

The interfacial structure between the muscovite (001) surface and aqueous solutions containing monovalent cations (3 × 10(-3) m Li(+), Na(+), H(3)O(+), K(+), Rb(+), or Cs(+), or 3 × 10(-2) m Li(+) or Na(+)) was measured using in situ specular X-ray reflectivity. The element-specific distribution of Rb(+) was also obtained with resonant anomalous X-ray reflectivity. The results demonstrate complex interdependencies among adsorbed cation coverage and speciation, interfacial hydration structure, and muscovite surface relaxation. Electron-density profiles of the solution near the surface varied systematically and distinctly with each adsorbed cation. Observations include a broad profile for H(3)O(+), a more structured profile for Li(+) and Na(+), and increasing electron density near the surface because of the inner-sphere adsorption of K(+), Rb(+), and Cs(+) at 1.91 ± 0.12, 1.97 ± 0.01, and 2.26 ± 0.01 ?, respectively. Estimated inner-sphere coverages increased from ~0.6 to 0.78 ± 0.01 to ~0.9 per unit cell area with decreasing cation hydration strength for K(+), Rb(+), and Cs(+), respectively. Between 7 and 12% of the Rb(+) coverage occurred as an outer-sphere species. Systematic trends in the vertical displacement of the muscovite lattice were observed within ~40 ? of the surface. These include a <0.1 ? shift of the interlayer K(+) toward the interface that decays into the crystal and an expansion of the tetrahedral-octahedral-tetrahedral layers except for the top layer in contact with solution. The distortion of the top tetrahedral sheet depends on the adsorbed cation, ranging from an expansion (by ~0.05 ? vertically) in 3 × 10(-3)m H(3)O(+) to a contraction (by ~0.1 ?) in 3 × 10(-3) m Cs(+). The tetrahedral tilting angle in the top sheet increases by 1 to 4° in 3 × 10(-3) m Li(+) or Na(+), which is similar to that in deionized water where the adsorbed cation coverages are insufficient for full charge compensation.     

Cleavage reactions of the complex ions derived from self-complementary deoxydinucleotides and alkali-metal ions using positive ion electrospray ionization with tandem mass spectrometry

The dissociation reactions of the adduct ions derived from the four self-complementary deoxydinucleotides, d(ApT), d(TpA), d(CpG), d(GpC), and alkali-metal ions were studied in detail by positive ion electrospray ionization multiple-stage mass spectrometry (ESI-MS(n)). For the [M + H](+) ions of the four deoxydinucleotides, elimination of 5'-terminus base or loss of both of 5'-terminus base and a deoxyribose were the major dissociation pathway. The ESI-MS(n) spectra showed that Li(+), Na(+), and Cs(+) bind to deoxydinucleotides mainly by substituting the H(+) of phosphate group, and these alkali-metal ions preferred to bind to pyrimidine bases rather than purine bases. For a given deoxydinucleotide, the dissociation pathway of [M + K](+) ions differed clearly from that of [M + Li](+), [M + Na](+), and [M + Cs](+) ions. Some interesting and characteristic cleavage reactions were observed in the product-ion spectra of [M + K](+) ions, including direct elimination of deoxyribose and HPO(3) from molecular ions. The fragmentation behavior of the [M + K](+) and [M + W](+) (W = Li, Na, Cs) adduct ions depend upon the sequence of bases, the interaction between alkali-metal ions and nucleobases, and the steric hindrance caused by bases.     

Effect of cation size and charge on the interaction between silica surfaces in 1:1, 2:1, and 3:1 aqueous electrolytes

Application of two complementary AFM measurements, force vs separation and adhesion force, reveals the combined effects of cation size and charge (valency) on the interaction between silica surfaces in three 1:1, three 2:1, and three 3:1 metal chloride aqueous solutions of different concentrations. The interaction between the silica surfaces in 1:1 and 2:1 salt solutions is fully accounted for by ion-independent van der Waals (vdW) attraction and electric double-layer repulsion modified by cation specific adsorption to the silica surfaces. The deduced ranking of mono- and divalent cation adsorption capacity (adsorbability) to silica, Mg(2+) < Ca(2+) < Na(+) < Sr(2+) < K(+) < Cs(+), follows cation bare size as well as cation solvation energy but does not correlate with hydrated ionic radius or with volume or surface ionic charge density. In the presence of 3:1 salts, the coarse phenomenology of the force between the silica surfaces as a function of salt concentration resembles that in 1:1 and 2:1 electrolytes. Nevertheless, two fundamental differences should be noticed. First, the attraction between the silica surfaces is too large to be attributed solely to vdW force, hence implying an additional attraction mechanism or gross modification of the conventional vdW attraction. Second, neutralization of the silica surfaces occurs at trivalent cation concentrations that are 3 orders of magnitude smaller than those characterizing surface neutralization by mono- and divalent cations. Consequently, when trivalent cations are added to our cation adsorbability series the correlation with bare ion size breaks down abruptly. The strong adsorbability of trivalent cations to silica contrasts straightforward expectations based on ranking of the cationic solvation energies, thus suggesting a different adsorption mechanism which is inoperative or weak for mono- and divalent cations.     

High ionic strength electrokinetics of melamine-formaldehyde latex

The electrokinetic potential of melamine-formaldehyde latex at high ionic strengths was measured by means of two different instruments. The present study confirms that the zeta potentials in 1 M 1-1 electrolyte solutions can be as high as +/-20 mV. The IEP of latex at low ionic strengths was at pH 11. The increase in the electrolyte concentration induced a shift in the IEP to low pH for all studied salts, and this indicates specific adsorption of the anions. The magnitude of the shift depends chiefly on the nature of the anion and increases in the series Cl < NO(3) = Br < I, and the nature of the cation (Li, Na, K, Cs) plays a rather insignificant role.     

Dodecahydroxy-closo-dodecaborate(2-).

The cesium salt of the icosahedral borane anion dodecahydroxy-closo-dodecaborate(2-), Cs(2)[closo-B(12)(OH)(12)], Cs(2)1, was prepared by heating cesium dodecahydro-closo-dodecaborate(2-), Cs(2)[closo-B(12)H(12)], Cs(2)2, with 30% hydrogen peroxide. The other alkali metal salts A(2)1 (A = Li, Na, K, Rb) precipitated upon addition of ACl to warm aqueous solutions of Cs(2)1. The ammonium salt, [NH(4)](2)1, and the (mu-nitrido)bis(triphenylphosphonium) salt, [PPN](2)1, were obtained similarly. The [H(3)O](2)1 salt precipitated upon acidification of aqueous solutions of Cs(2)1 with hydrochloric acid. The solubility of these salts in water was determined by measuring the boron content of saturated aqueous solutions of A(2)1 (A = Li, Na, K, Rb, Cs), [H(3)O](2)1, and [NH(4)](2)1 using ICP-AES. Although these salts are derived from a dianion with twelve pendant hydroxyl groups, the alkali metal salts surprisingly displayed low water solubilities. Water solubility decreases with a decrease in the radius of A(+), except for the lithium salt, which is slightly more soluble than the potassium salt. The [H(3)O](2)1 and the [NH(4)](2)1 salts provide rare examples of water-insoluble hydronium and ammonium salts. The low water solubility of the A(2)1 salts is attributed to the dianion's pendant hydroxyl groups, which appear to function as cross-linking ligands. Four alkali metal salts, A(2)1 (A = Na, K, Rb, Cs), were characterized in the solid state by single-crystal X-ray crystallography. These data revealed intricate networks in which several anions are complexed through their hydroxyl groups to each alkali metal cation. In addition, the anions are engaged in hydrogen bonding with each other and, if present, with water of hydration. This cross-linking results in the precipitation of aggregated salts. Cation coordination numbers decrease with cation radius. Thus, cesium and rubidium are ten-coordinate, whereas potassium is seven-coordinate and sodium is six-coordinate. The geometry of anion 1(2)(-) is independent of cation identity; the B-B and B-O bond lengths of the various A(2)1 salts (A = Na, K, Rb, Cs) are identical.     

Alkali metal ion binding to amino acids versus their methyl esters: affinity trends and structural changes in the gas phase

The relative alkali metal ion (M(+)) affinities (binding energies) between seventeen different amino acids (AA) and the corresponding methyl esters (AAOMe) were determined in the gas phase by the kinetic method based on the dissociation of AA-M(+)-AAOMe heterodimers (M=Li, Na, K, Cs). With the exception of proline, the Li(+), Na(+), and K(+) affinities of the other aliphatic amino acids increase in the order AAAAOMe is already observed for K(+). Proline binds more strongly than its methyl ester to all M(+) except Li(+). Ab initio calculations on the M(+) complexes of alanine, beta-aminoisobutyric acid, proline, glycine methyl ester, alanine methyl ester, and proline methyl ester show that their energetically most favorable complexes result from charge solvation, except for proline which forms salt bridges. The most stable mode of charge solvation depends on the ligand (AA or AAOMe) and, for AA, it gradually changes with metal ion size. Esters chelate all M(+) ions through the amine and carbonyl groups. Amino acids coordinate Li(+) and Na(+) ions through the amine and carbonyl groups as well, but K(+) and Cs(+) ions are coordinated by the O atoms of the carboxyl group. Upon consideration of these differences in favored binding geometries, the theoretically derived relative M(+) affinities between aliphatic AA and AAOMe are in good overall agreement with the above given experimental trends. The majority of side chain functionalized amino acids studied show experimentally the affinity order AAAAOMe. The latter ranking is attributed to salt bridge formation.     

Effect of ions on the vibrational relaxation of liquid water

We study the relaxation of the O-H stretch vibration of water in aqueous salt solutions using femtosecond two-color pump-probe spectroscopy. The vibrational lifetimes are measured for a series of salts consisting of the anions Cl(-), Br(-), and I(-) and the cations Li(+), Na(+), and Mg(2+), for a range of concentrations from 0.5 M up to 6 M (chloride salts), 9 M (bromide salts), and 10 M (iodide salts). In addition to the previously found dependence of the vibrational lifetime on the nature of the anion, the lifetime is found to depend on concentration and is observed to show a small but significant dependence on the nature of the cation. We present a model in which all the effects of ions on the vibrational relaxaton of liquid water are accounted for.     

Synthesis,structures, and conformational characteristics of calixarene monoanions and dianions

The synthesis, complete characterization, and solid state structural and solution conformation determination of calix[n]arenes (n = 4, 6, 8) is reported. A complete series of X-ray structures of the alkali metal salts of calix[4]arene (HC4) illustrate the great influence of the alkali metal ion on the solid state structure of calixanions (e.g., the Li salt of monoanionic HC4 is a monomer; the Na salt of monoanionic HC4 forms a dimer; and the K, Rb, and Cs salts exist in polymeric forms). Solution NMR spectra of alkali metal salts of monoanionic calix[4]arenes indicate that they have the cone conformation in solution. Variable-temperature NMR spectra of salts HC4.M (M = Li, Na, K, Rb, Cs) show that they possess similar coalescence temperatures, all higher than that of HC4. Due to steric hindrance from tert-butyl groups in the para position of p-tert-butylcalix[4]arene (Bu(t)C4), the alkali metal salts of monoanionic Bu(t)C4 exist in monomeric or dimeric form in the solid state. Calix[6]arene (HC6) and p-tert-butylcalix[6]arene (Bu(t)C6) were treated with a 2:1 molar ratio of M(2)CO(3) (M = K, Rb, Cs) or a 1:1 molar ratio of MOC(CH(3))(3) (M = Li, Na) to give calix[6]arene monoanions, but calix[6]arenes react in a 1:1 molar ratio with M(2)CO(3) (M = K, Rb, Cs) to afford calix[6]arene dianions. Calix[8]arene (HC8) and p-tert-butylcalix[8]arene (Bu(t)()C8) have similar reactivity. The alkali metal salts of monoanionic calix[6]arenes are more conformationally flexible than the alkali metal salts of dianionic calix[6]arenes, which has been shown by their solution NMR spectra. X-ray crystal structures of HC6.Li and HC6.Cs indicate that the size of the alkali metal has some influence on the conformation of calixanions; for example, HC6.Li has a cone-like conformation, and HC6.Cs has a 1,2,3-alternate conformation. The calix[6]arene dianions show roughly the same structural architecture, and the salts tend to form polymeric chains. For most calixarene salts cation-pi arene interactions were observed.     

Features of Component Interaction in Concentrated Solutions of Multicomponent Systems

Classical complex formation processes in the multicomponent systems Cd(NO₃)₂-MCl-H₂O (M = Li, Cs) occur at preeutectic concentrations of salts. At posteutectic concentrations of salts the formation of cadmium chloride complexes is of structurally enforced nature and depends on the type of the cybotactic group dominating in the solution. The example of the system Cd(NO₃)₂-LiCl-H₂O was used to show that the law of mass action does not apply to systems in which structurally enforced processes occur. Differences in the solubility of MCl (M = Li, Cs, Na) in concentrated solutions of cadmium nitrate are proposed to explain in terms of differences in interactions of cybotactic groups formed by cadmium nitrate and the corresponding chloride.     

Single-crystal X-ray studies of five alkali metal salts of luminol

Luminol salts of five alkali metals, Li, Na, K, Rb, and Cs, have been prepared and structurally characterized by single-crystal X-ray diffraction. Luminol is deprotonated at the same site whereas each ionic salt has a unique composition and a different number of water molecules. The cation/luminol ion pair to water molecule ratio in the lattices varies as follows: 1 : 0 for K, 1 : 1 for Li, 1 : 2 for Rb, 1 : 3 for Cs, and 1 : 6 for Na. The differences in composition among the five compounds lead to different metal coordination environments in the solid state and distinct 3-D molecular arrangements in the lattice.     

Alkali-metal ion catalysis and inhibition in nucleophilic displacement reactions at carbon,phosphorus and sulfur centres. IX. p-Nitrophenyl diphenyl phosphate

We report on the catalytic effects by alkali-metal ions in the ethanolysis of p-nitrophenyl diphenyl phosphate, in continuation of our studies on alkali-metal ion catalysis and inhibition in nucleophilic displacement reactions at carbon, phosphorus and sulfur centres. The following selectivity order of catalytic reactivity was observed for nucleophilic displacement at the phosphorus center with p-nitrophenoxide as leaving group: Li+ > Na+ > K+ > Cs+. A minor reaction pathway with phenoxide leaving was also found. The kobs data have been dissected into reaction pathways by free ions (kEtO) and by ion pairs (kMOEt), with the latter being dominant, in a 4-membered transition-state. Further analysis is given in terms of initial-state and transition-state stabilization by the alkali-metal ions in terms of the Eisenman model (electrostatic interaction vs. desolvation). Results of ab-initio MO calculations are presented based on interaction between M+ and a model bipyramidal phosphorane intermediate and compared with the sulfurane analogue.     

Comparative Analysis of the Solubility of Alkali Metal Chlorides in Cadmium and Zinc Nitrate Solutions at 25°C

Maximum solubilities of salts MCl (M = Li, Na, K, Cs) in cadmium and zinc nitrate solutions of various concentrations were measured. The effect of the structure of the solution on the mutual solubility was demonstrated.     

Effects of cations on the hydrogen bond network of liquid water: new results from X-ray absorption spectroscopy of liquid microjets

Oxygen K-edge X-ray absorption spectra (XAS) of aqueous chloride solutions have been measured for Li(+), Na(+), K(+), NH(4)(+), C(NH(2))(3)(+), Mg(2+), and Ca(2+) at 2 and 4 M cation concentrations. Marked changes in the liquid water XAS are observed upon addition of the various monovalent cation chlorides that are nearly independent of the identity of the cation. This indicates that interactions with the dissolved monovalent cations do not significantly perturb the unoccupied molecular orbitals of water molecules in the vicinity of the cations and that water-chloride interactions are primarily responsible for the observed spectral changes. In contrast, the addition of the divalent cations engenders changes unique from the case of the monovalent cations, as well as from each other. Density functional theory calculations suggest that the ion-specific spectral variations arise primarily from direct electronic perturbation of the unoccupied orbitals due to the presence of the ions, probably as a result of differences in charge transfer from the water molecules onto the divalent cations.     

Ion exchange in reverse micelles

The distribution and dynamics of alkali cations inside Na-AOT reverse micelles have been investigated using Monte Carlo and molecular dynamics simulations. Water is modeled using the extended simple point charge (SPC/E) model. Simulations were carried out for alkali salts of Li+, Na+, K+, and Cs+ placed into the aqueous core of the reverse micelle, for situations corresponding to one and three molecules of added salt. In all cases, we observe that the larger K+ and Cs+ ions exchange with the Na+ counterion; however, the smaller Li+ ion prefers to remains solvated within the core of the reverse micelle. Our study reveals that the oil-water interface of the Na-AOT reverse micelle has the greatest selectivity toward Cs+ followed by K+ and Li+. A model based on enthalpic contributions illustrates that the solvation energies of the different cations in water control the ion-exchange process. The hydration number of the first water shell for Li+ situated in the aqueous core of the reverse micelle with radius R = 14.1 A was similar to that observed at infinite dilution in bulk water.     

Crystallographic distinction between "contact" and "separated" ion pairs: structural effects on electronic/ESR spectra of alkali-metal nitrobenzenides

The classic nitrobenzene anion-radical (NB(-*) or nitrobenzenide) is isolated for the first time as pure crystalline alkali-metal salts. The deliberate use of the supporting ligands 18-crown-6 and [2.2.2]cryptand allows the selective formation of contact ion pairs designated as (crown)M(+)NB(-*), where M(+) = K(+), Rb(+), and Cs(+), as well as the separated ion pair K(cryptand)(+)NB(-*)-both series of which are structurally characterized by precise low-temperature X-ray crystallography, ESR analysis, and UV-vis spectroscopy. The unusually delocalized structure of NB(-*) in the separated ion pair follows from the drastically shortened N-C bond and marked quinonoidal distortion of the benzenoid ring to signify complete (95%) electronic conjugation with the nitro substituent. On the other hand, the formation of contact ion pairs results in the substantial decrease of electronic conjugation in inverse order with cation size (K(+) > Rb(+)) owing to increased localization of negative charge from partial (NO(2)) bonding to the alkali-metal cation. Such a loss in electronic conjugation (or reverse charge transfer) may be counterintuitive, but it is in agreement with the distribution of odd-electron spin electron density from the ESR data and with the hypsochromic shift of the characteristic absorption band in the electronic spectra. Most importantly, this crystallographic study underscores the importance of ion-pair structure on the intrinsic property (and thus reactivity) of the component ions-as focused here on the nitrobenzenide anion.     

Evaluating the surface charge of C18 stationary phases

The surface charge of four C18 stationary phases was investigated by measuring the flow induced streaming potential, a well known electrokinetic property of charged surfaces. Three of the stationary phases (Symmetry, Gemini, and Xterra-MS) had significantly positive streaming potentials at both pH 3 and 4.5. The fourth (Zorbax-SB) appeared to be essentially neutral at pH 3 and became negative at pH 4.5. Apparent zeta potentials ranged from approximately +16 to -4 mV. The retention behavior was also investigated using chloride as model anion and glycinamide (in its protonated form) as model cation. When the retention factor (k) of glycinamide was subtracted from k of chloride anion, the resulting delta k values showed very similar trends as apparent zeta potential values, suggesting that the simple chromatographic method could be used to estimate zeta potential values, or that the zeta potential values could be useful for ranking columns according to ion exchange or exclusion behavior. The anion exchange capacity of the Symmetry and Gemini columns was also estimated, using a published chromatographic procedure, and the results suggest about 2 microEq. capacity per gram of packing.