Solvated ions, structure

H3O" is strictly the oxonium ion actually, in aqueous solutions of acid this and Other solvated-proton structures exist, but they are conveniently represented as... 

Let us now return to the question of solvolysis and how it relates to the stracture under stable-ion conditions. To relate the structural data to solvolysis conditions, the primary issues that must be considered are the extent of solvent participation in the transition state and the nature of solvation of the cationic intermediate. The extent of solvent participation has been probed by comparison of solvolysis characteristics in trifluoroacetic acid with the solvolysis in acetic acid. The exo endo reactivity ratio in trifluoroacetic acid is 1120 1, compared to 280 1 in acetic acid. Whereas the endo isomer shows solvent sensitivity typical of normal secondary tosylates, the exx> isomer reveals a reduced sensitivity. This indicates that the transition state for solvolysis of the exo isomer possesses a greater degree of charge dispersal, which would be consistent with a bridged structure. This fact, along with the rate enhancement of the exo isomer, indicates that the c participation commences prior to the transition state being attained, so that it can be concluded that bridging is a characteristic of the solvolysis intermediate, as well as of the stable-ion structure. ... 

Chemical models of electrolytes take into account local structures of the solution due to the interactions of ions and solvent molecules. The underlying information stems from spectroscopic, kinetic, and electrochemical experiments, as well as from dielectric relaxation spectroscopy. The postulated structures include ion pairs, higher ion aggregates, and solvated and selectively solvated ions. 

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. 

Grunwald, E. (1979). Structure of solvated ion pairs from electric dipole moments. Journal of Pure and Applied Chemistry, 51, 53-61. 

Solvated ions have a complicated structure. The solvent molecules nearest to the ion form the primary, or nearest, solvation sheath (Fig. 7.2). Owing to the small distances, ion-dipole interaction in this sheath is strong and the sheath is stable. It is unaffected by thermal motion of the ion or solvent molecules, and when an ion moves it carries along its entire primary shell. In the secondary, or farther shells, interactions are weaker one notices an orientation of the solvent molecules under the effect of the ion. The disturbance among the solvent molecules caused by the ions becomes weaker with increasing distance and with increasing temperature. 

Experiments made at higher degrees of aggregation have provided strong evidence192 for ring-like structures for mixed neutral clusters. For example, under a wide variety of experimental conditions, mixed cluster ions display a maximum intensity atm = 2(n + 1) whenn<5 for (NH3)II (M)mH+, andm = n + 2 whenn<4 for (H20)B(M)mH+ M is a proton acceptor such as acetone, pyridine, and trimethy-lamine. These findings reveal that the cluster ions with these compositions have stable solvation shell structures as discussed above. 

Figure 2.2 (a) The structure of the electrode/electrolyte interface, assuming a single layer of solvated ions adjacent to the electrode. The distance of closest approach of the ions to the electrode is a, and the ion sheet forms the outer Helmholtz plane (OHP). (b) The variation of the potential as a function of the distance from the metal surface for the interface shown in (a). 

Equation (2.33) now defines the double layer in the final model of the structure of the electrolyte near the electrode specifically adsorbed ions and solvent in the IHP, solvated ions forming a plane parallel to the electrode in the OHP and a dilfuse layer of ions having an excess of ions charged opposite to that on the electrode. The excess charge density in the latter region decays exponentially with distance away from the OHP. In addition, the Stern model allows some prediction of the relative importance of the diffuse vs. Helmholtz layers as a function of concentration. Table 2.1 shows... 

Two main structural types have been identified for allyl alkali metal species solvated ions in the form of CIPs where a delocalized anion with metal coordination is perpendicular to the ligand plane,130-134 or unsolvated allylic lithium compounds displaying localized ligand systems with NMR spectra closely resembling those of alkenes.135-138... 

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... 

Although our knowledge of the structure of the electric double layer is based on experimental data collected at finite electrolyte concentrations, understanding the structure of the electric double layer at the microscopic level must begin with knowledge of the structure of a single solvated ion at the interface. This information has been obtained in recent years from molecular dynamics computer simulations. 

Figure 4. Structure of RbjAsj Sen (a) showing the solvated ion complex (Rb )sAs/ with only weak additional bonds between neighboring complexes, and the proposed Metal-Supported Valence-Fluctuation (MSVF) (b) for the...

Figure 5. Structure of the P/6 anion (left) and structure of the solvated ion complex Na4Pi4 6en (right), in which large black dots are Na atoms and shaded...

It has been found that the Li quadrupole parameters x( Li) and /]( Li) are sensitive probes of solid state structures of organolithium compounds, for example with respect to aggregate size, solvation, ion pair structure and the X-Li-X structural angle. These results will be discussed in the following sections. 

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... 

Fig. 2.6 Typical model of solvated ions in structured solvents such as water and alcohols.

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... 

Proton NMR spectroscopy can similarly be used to monitor the concentrations of such salts, and has the advantage of being able to elucidate structures simultaneously 24,25), Independent conductimetric measurements can provide immediate information on the state of ionic aggregation in solution, in particular providing quantitative data concerning the proportion of free solvated ions to those existing as ion paired entities (26). 

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