Naked ions

For some ions the desolvation can proceed down to the naked ion M2+ while for others a reduction of the charge occurs at a given n = r (r for reduction). The charge reduction may take different forms, the simplest of which is loss of Sl+ ... 

When experiments with electrospray producing H3N(CH2)pNH2+ ions were performed in this laboratory,56 it was found that doubly protonated ions could be obtained from methanol-water solutions only when p > 4. For p < 4 only the singly protonated ions were observed, even though the doubly protonated ions are known to be present in the solution. We attribute the failure to observe the doubly protonated ions with p < 4 to the occurrence of charge reduction by deprotonation. Probably methanol, whose gas-phase basicity is greater than that of water, is involved in the deprotonation. The diprotonated diammines, p > 4, could all be dehydrated down to the naked ion either in CID experiments or at higher temperature. 

Doubly charged ion could be dehydrated down to the naked ion without charge reduction. 

The hydrated sulfate S04(H20)2 could be produced by electrospray in the gas phase,80 but neither the triply charged orthophosphate P04 nor the doubly charged HOPOj" were observed as the naked ion or the hydrate.81 CID of the hydrated sulfate led to simple desolvation down to n = r = 4. The decomposition of the r = 4 hydrate led to charge reduction by intracluster proton transfer ... 

Figure 1 shows a reversed micelle where the bulk solvent is a hydrocarbon and the core is a water pool surrounded by surfactant. These systems possess unique features as the physical properties of the water pools only start to approach those of bulk water at high water content when the pool radii are >150 pools with radii as small as 15 can be constructed (1, 25). These systems have been used to investigate the nature of several inorganic reactions by stopped flow methods (26, 27). They have also been used to produce so-called naked ions, i.e., ions that possess a minimum of aqueous solvation (28). The system strongly promotes many reactions, a fact attributed to the unusual nature of the water in this system. 

On entering the diffusion layer, the ion loses its solvation molecules (all ions in solution are solvated) and approaches the metal surface, where it is adsorbed as a naked ion before the electron transfer process takes place. Obviously the wider is this diffusion layer (5), the longer it will take the ion to diffuse across it and the slower will be the overall process. Anything which can diminish or disrupt this layer (i.e. make it smaller) will improve the speed of the process. 

One of the surprising aspects of this and other studies using naked metal ions as models for electron-transfer catalysis are the many analogies found to known transition metal chemistry, either in the gas phase with naked ions or for complexes under more normal conditions. Clearly, such simple models as the beryllium cation cannot account for transition metal reactivity, but they do have the advantage that, because of their very simplicity, the reasons for their effects are relatively clear. The fact that Be can catalyze a given reaction does not necessarily mean that, for instance, a transition metal does not use d-orbitals to catalyze the same reaction but it does mean that d-orbitals are not a prerequi-... 

Another common method of ionisation is Electrospray Ionisation (ES). In this method, the sample is dissolved in a polar, volatile solvent and pumped through a fine metal nozzle, the tip of which is charged with a high voltage. This produces charged droplets from which the solvent rapidly evaporates to leave naked ions which pass into the mass spectrometer. ES is also a relatively mild form of ionisation and is very suitable for biological samples which are usually quite soluble in polar solvent but which are relatively difficult to vaporise in the solid state. Electrospray ionisation tends to lead to less fragmentation of the molecular ion than EL... 

Since non-bound or non-coordinated nucleophiles are even more reactive, crown-ethers [138] and cryptands (polyaminoethers) [139,140] have been used to chelate the alkali metal cations, notably the potassium ion of K[ F]F. This allows the [ F]fluoride anion to be less tightly paired with the cation and therefore to be more reactive, which has been coined the naked ion effect. In practice, the crown-ether (e.g. 18-crown-6) or better the polyaminoether Kryptofix-222 (4,7,13,16,21,24-hexaoxa-1,10-diazabicyclo[8.8.8]hexacosane) is added to the aqueous K[ F]F/K2C03 solution which is then concentrated to dryness [139,140]. The complex (KP FIF-K ) can be further dried, if needed, by one or more cycles of addition of dry acetonitrile and azeotropic evaporation. 

Figure 14.2 Principle of electrospray ionization, (a) The analyte is dissolved in an appropriate solvent and sprayed via a capillary into an electric field. Here, the liquid filament finally forms charged droplets, (b) The solvent of the charged droplets evaporates, resulting in an increase of the surface charge up to a critical boundary, at which a Coulomb explosion occurs. The newly formed droplets undergo the same process. The final products are the desolvated, naked ions, which are then entering the mass spectrometer.

The upshot is that the Born theory of solvation fails because it regards the solvent as a continuous dielectric, whereas in fact solute ions (especially metal cations with z > 1) often interact in a specific manner with solvent molecules. In any event the molecular dielectric is obviously very lumpy on the scale of the ions themselves. The Born theory and other continuous dielectric models work reasonably well when metal ion solute species are treated as solvent complexes such as Cr(OH2)63+ rather than naked ions such as Cr3+, but the emerging approach to solvation phenomena is to simulate solvation dynamically at the molecular level using computer methods. 

In solvent-separated ion pairs, the solvation shells of the cation and the anion touch each other in solvent-bridged ion pairs, the ions share solvent molecules. In contact ion pairs, the cation and the anion are bound directly to each other and are surrounded by a common solvation shell. In penetrated ion pairs, an empty space between edge groups in one ion of a salt is occupied to a certain degree by a counterion. The two latter types of ion pair may have quite a different electronic distribution than the corresponding naked ions. The following examples show the influence of ion-pair formation. 

The structure of S + was determined from the crystalline compound S8(AsF6)2, isolated under naked ion conditions (see Sec. 11.3.4.5). 

From solid polymer electrolytes [198,199] no co-intercalation is observed either [216], for only naked ions are present in the SPE phase. 

Although the gas-phase calculations satisfactorily reproduce the a-anion stabilization effect of R3Si, it should be noted that the calculations are for naked ions, and no account is taken of solvation or covalent bonding to any metal counterion. 

V. 1. A THERMAL REACTIONS WITH NAKED IONS AND ATOMS... 

V. 1. A. Thermal Reactions with Naked Ions and Atoms V.l.B. Thermal Reactions with Ligated Metal Ions V.l.C. Reactions with Photoexcited Metal Ions V.2. Reactions with Metal Atoms in a Matrix References... 

Ions comprise another class of solutes which have been studied extensively by Wipff and coworkers, who have been particularly interested in lanthanide and uranyl ions and their chloro-complexes [44,142]. Their studies show that the chloro-complexes are stabHized by solvation in the ionic liquids based on imidazolium cations and the [PFe]" anion. The principal interaction of the naked ions is with the anions, but on chlorination the cations move closer to complexes such as [EuQe] and [U02Cl4] . In a further study [45] they showed that in an equimolar mixture of water and ionic liquid, water molecules tend to fill the first solvation shell of naked ions in preference to [PFs]" ions, and tend to solvate the chloro-complexes in preference to the imidazolium cations. 

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