Solvated ions, structure study

This distinction is meaningful if the resultant distribution function is of the type shown in Figure 4.7 (Szwarc, 1965). This figure shows that there is a high probability that the cation and anion are either in contact, separated by a solvent molecule or far apart (Szwarc, 1965). Intermediate positions are improbable. The structure of solvated ion-pairs has been studied by Grunwald (1979) using dipole measurements. 

Compared to the structures of Li+-water solvates, the structures of Li+-acetonitrile solvates are in general less studied. The Li+ ion was found to be four coordinate with the use of different techniques, e.g., by NMR where acetonitrile was gradually replaced by water in a 1.6 M solution of LiC104 (130), or based on IR intensities measured for the acetonitrile C-N stretching vibration (131,132). Even mixed coordination of a counter ion and acetonitrile were reported to be four coordinate, viz., in [Li(CH3CN)3Br] for 0.58 M LiBr in CH3CN (133). Extensive... 

Despite the fact that the structure of the interface between a metal and an electrolyte solution has been the subject of numerous experimental and theoretical studies since the early days of physical chemistry," our understanding of this important system is still incomplete. One problem has been the unavailability (until recently) of experimental data that can provide direct structural information at the interface. For example, despite the fact that much is known about the structure of the ion s solvation shell from experimental and theoretical studies in bulk electrolyte solutions, " information about the structure of the adsorbed ion solvation shell has been mainly inferred from the measured capacity of the interface. The interface between a metal and an electrolyte solution is also very complex. One needs to consider simultaneously the electronic structure of the metal and the molecular structure of the water and the solvated ions in the inhomogeneous surface region. The availability of more direct experimental information through methods that are sensitive to the microscopic... 

The gas-phase model would then be tested on condensed phases. In the case of the carbonate ion, the parameters can be used to examine the structure of C02(aq), C032-(aq), and HC03 (aq) as well as the structure of, for example, siderite FeC03 and nahcolite Na(HC03). For the aqueous species, the most instructive comparisons are with the results of ab initio molecular dynamics studies of solvated ions, where the radial distribution functions can be used to check the extent of solvation. Fig. 2, for... 

X-ray and neutron diffraction methods and EXAFS spectroscopy are very useful in getting structural information of solvated ions. These methods, combined with molecular dynamics and Monte Carlo simulations, have been used extensively to study the structures of hydrated ions in water. Detailed results can be found in the review by Ohtaki and Radnai [17]. The structural study of solvated ions in lion-aqueous solvents has not been as extensive, partly because the low solubility of electrolytes in 11011-aqueous solvents limits the use of X-ray and neutron diffraction methods that need electrolyte of -1 M. However, this situation has been improved by EXAFS (applicable at -0.1 M), at least for ions of the elements with large atomic numbers, and the amount of data on ion-coordinating atom distances and solvation numbers for ions in non-aqueous solvents are growing [15 a, 18]. For example, according to the X-ray diffraction method, the lithium ion in for-mamide (FA) has, on average, 5.4 FA molecules as nearest neighbors with an... 

The most important result of the structure studies was undoubtedly the establishment of the fact that the crystalline isopropoxides of all rare earths are not the homoleptic Ln(OPr )3 complexes but oxoalkoxides of Ln50(0Pri)13 composition, where Ln = Sc, Y, Er, Yb (see also Fig. 4.9 a). They appear to be desolvation products of the very unstable [Ln(OPri)3( PrOH)]2 solvates (perfectly soluble and rather reactive) the complex of such composition has been isolated and characterized only for neodymium, but the IR spectroscopic evidence for the existence of such solvates was obtained also for Pr and Er. Desolvation of Ln(OBu )3 2L (Ln = Y, La L = BuOH, THF, Py) leads also to the formation of oxocomplexes the ions corresponding to the fragmentation of the homoleptic species are absent in their mass-spectra (except for [Y3(OBu,)9(tBuOH)2], where the Y3(OR)8+ ion was found along with Y30(0R)6+). The same kind transformations have been observed also for... 

In further studies of ion-pairing, a variety of sec-a-silyl benzylic lithium compounds 39, 40, 41 and 42, were prepared, both externally and internally solvated, the latter by means of a potential ligand attached to the carbanionic moiety. Ion-paired carbanide salts tend to assemble into several arrangements which differ in aggregation, solvation and in the proximity of anion to cation. Many of these species interconvert rapidly relative to the NMR time scale even at quite low temperatures. An internally solvated ion-pair carbanide salt is more likely to assume a single molecular structure, to undergo the latter exchange processes more slowly and thus be more amenable to NMR spectroscopic studies of structure and dynamic behavior. 

Some recent developments in the research of the structure and dynamics of solvated ions are discussed. The solvate structure of lithium ion in dimethyl formamide and preliminary results on the structure of sodium chloride aqueous solutions under high pressures are presented to demonstrate the capabilities of the traditional X-ray diffiraction method at new conditions. Perspectives of solution chemistry studies by combined methods as e.g. diffraction results with reverse Monte Carlo simulations, are also shown. 

Studies on structure and dynamics of liquids have recently been extended to solvate structure of ions in non-aqueous solutions, and to the structure of complexes with relatively complicated ligands. We can also handle special problems like hydrophobic solvation is. Diffraction studies have been performed on new solvents as e.g. trifluoroethanol [23] and tetramethyl urea [26], and on solvent mixtures [27-30]. More recently the preferential solvation of ions has been subjected by an XD investigation in MgCh-water-methMol ternary systems [31], and the solvation structure around the cations proved to undergo the change of solvent molecules proportionally to the relative concentration of the two solvents. 

Only two structural studies have been reported for lithium solvates in non-aqueous solutions. In an XD study of concentrated formamide solution of LiCl [36] the lithium ion was found to be solvated by 5.4 formamide molecules in average and the Li-O distance was reported to be 2.24A, in keeping with the values found in diluted aqueous solutions. More recently, a detailed study has been performed by combination of the ND method with a series of theoretical methods on an 0.6 mol dm LiBr solution in acetonitrile [37]. The lithium ion was found to be tetrahedrally coordinated by three solvent molecules and one bromide ion. The Li-N distance resulted in 2.05 A. 

Structural studies of allylmetal derivatives have also been performed [2]. In allyl compounds in ethereal solvents, the cesium ion is well solvated and an allyl anion is formed. Crystals of cyclopentadienyl rubidium and cesium, which complexed with 18-crown-6, have been isolated and analyzed by X-ray diffraction [3]. 

The proceedings of a conference on metal-ammonia solutions have been published, featuring reviews of the physical properties of dilute and concentrated solutions, electrical, n.m.r., i.r., and Raman spectroscopic studies of diffusion, the solvated electron, kinetics, and solution structure."" Electron spin resonance in metallic Li-NHa systems has been investigated from 12 to 296 K. In the liquid solutions and in the cubic phase of Li(NH3)4 the conduction e.s.r. lineshapes are in agreement with theory. To a good approximation the solvated ions are the only spin scatterers in the liquid state. The paramagnetic susceptibility of liquid Li(NH3)4 indicates that the concentration of localized moments is low and they order antiferromagnetically below 20 K." ... 

There are now several structural studies on cobalt(II) crown ether and cryptand complexes (Table 78), which show the coordination mode to be markedly sensitive to the macrocycle cavity diameter. Both 12-crown-4 and 15-crown-5 (cavity diameters 120-150 pm and 170-220 pm respectively) can include ions to form structures in which every ether oxygen atom is bound to the metal. In contrast Co—O (ether) bonding is destabilized in the larger 18-crown-6 and dicyclohexyl-18-crown 6 polyethers. In the blue complexes obtained from the reaction of these compounds with C0CI2 the role of the cyclic polyether is to solvate discrete [Co(H20)6] cations and [CoC ] " anions and there are no direct Co—O (ether) bonds.A similar effect is seen in the Co" complex of a 27-membered-ring macrocycle where the pentacoordinate structure (267) features only one long Co—O (ether) bond. " ... 

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