Solvent-shared ion pair

According to Eigen and Tamm [87,88], ion-pair formation proceeds stepwise, starting from separated solvated ions which form a solvent-separated ion pair [C+SSA ]°, followed by a solvent-shared ion pair [C+SA ]° and finally a contact ion pair, [C+A ]° [Eqs. (4)-(6)]. All these species are solvated. The types of ion pair formed depend on the relative strength of the interaction of the involved species. 

Solvent-separated ion pairs, in which the first solvation shells of both ions remain intact on pairing may be distingnished from solvent-shared ion pairs, where only one solvent molecule separates the cation and the anion, and contact ion pairs, where no solvent separates them (Fig. 2.6). The parameter a reflects the minimum distance by which the oppositely charged ions can approach each other. This eqnals the sum of the radii of the bare cation and anion pins 2, 1, and 0 diameters of the solvent, respectively, for the three categories of ion pairs. Since a appears in Eq. (2.49), and hence, also in Q(b), it affects the value of the equilibrium constant, K s- The other important variable that affects K ss is the product T and, at a given temperature, the value of the relative permittivity, e. The lower it is, the larger b is and, hence, also K s-... 

In solvent-separated ion pairs, the primary solvation shells of the cation and the anion actually remain intact. In solvent-shared ion pairs, a single solvent molecule exists in the space between the... 

An ion pair in which the constituent ions are separated by one or more solvent (or other neutral) molecules. If and Y represent the constituent ions, a loose ion pair is usually symbohzed by X+ Y. The constituent ions of a loose ion pair can readily exchange with other ions in solution this provides an experimental means for distinguishing loose ion pairs from tight ion pairs. In addition, there are at least two types of loose ion pairs solvent-shared and solvent-separated. See Ion Pair Tight Ion Pair Solvent-Shared Ion Pair Solvent-Separated Ion Pair... 

An ion pair in which the constituent ions are not separated by a solvent or other intervening molecule. Tight ion pairs are also referred to as contact ion pairs. If and represent constituent ions, then a tight ion pair would be symbolized by X+Y. An example of a tight ion pair would be the case in which an enzyme stabilizes a carbonium ion with juxtaposed negatively charged side-chain groups. See Loose Ion Pair Ion Pair Solvent-Shared Ion Pair Solvent-Separated Ion Pair. 

TIGHT ION PAIRS LOOSE ION PAIRS SOLVENT-SHARED ION PAIR SOLVENT-SEPARATED ION PAIR Ion pair return,... 

SOLVENT-SEPARATED ION PAIR SOLVENT-SHARED ION PAIR TIGHT ION PAIR LOSSEN REARRANGEMENT LOW-BARRIER HYDROGEN BONDS (Potential Role in Catalysis)... 

SOLVENT-SHARED ION PAIR SOLVENT-SEPARATED ION PAIR TIME... 

In view ofthe widespread notion that ion pairs are less reactive than free anions [7], the very finding that metal-bound alkoxides cleave esters more rapidly than free alkoxides do came as a surprise. It seems likely that transition state stabilization takes place via chelate structures having the form of a four-membered contact ion pair I, or of the kinetically equivalent six-membered solvent shared ion pairs II and III. 

The solvent-shared ion-pair character of the exciplex increases with the increase in solvent dielectric Constant and ionization potential of the ionor and electron affinity of the acceptor. 

Organic ion radicals exist together with counterions and often form ion pairs. Since the pioneering works of Grunwald (1954), Winstein with co-authors (1954) and Fuoss and Sadek (1954), the terms contact, tight, or intimate ion pair and solvent-separated or loose ion pair have become well known in the chemical world. More recently, Marcus (1985) and Boche (1992) introduced other colloquial expressions, the solvent-shared ion pair and the penetrated ion pair. 

It should be mentioned that Marcus [5] uses the terms inner-sphere ion pair and outher-sphere ion pair for CIP and SSIP, respectively. Depending on the degree of penetration of the solvation shells one may further differentiate between SSIP and solvent shared ion pairs . However, a clear experimental assignment has not yet been performed. Therefore, we will use the designation CIP and SSIP... 

Solvent-shared ion pairs ions linked electrostatically, separated by a water molecule. 

When each ion maintains its own primary solvation shell, the new chemical species is a solvent separated ion-pair (SSIP). If a single solvent layer is shared by ion partners, the species is a solvent shared ion-pair (SIP). If the cation and the anion are in contact and no solvent molecules are present between them, the form is contact ion-pair (CIP) or intimate ion-pair. Figure 2.1 illustrates multistep ion-pair formation equilibrium. What sets ion-pairing apart from complex formation is the absence of directional covalent coordinative bonds resulting from a Lewis base-acid interaction and a special geometrical arrangement. 

Fig. 2-14. Schematic representation of the equilibrium between (a) a solvated contact ion pair, (b) a solvent-shared ion pair, (c) a solvent-separated ion pair, and (d) unpaired solvated ions of a 1 1 ionophore in solution, according to reference [241]. Hatched circles represent solvent molecules of the primary solvation shell.

First, immediately after ionization, contact ion pairs are formed, in which no solvent molecules intervene between the two ions that are in close contact. The contact ion pair constitutes an electric dipole having only one common primary solvation shell. The ion pair separated by the thickness of only one solvent molecule is called a solvent-shared ion pair In solvent-shared ion pairs, the two ions already have their own primary solvation shells. These, however, interpenetrate each other. Contact and solvent-shared ion pairs are separated by an energy barrier which corresponds to the necessity of creating a void between the ions that grows to molecular size before a solvent molecule can occupy it. Further dissociation leads to solvent-separated ion pairs Here, the primary solvation shells of the two ions are in contact, so that some overlap of secondary and further solvation shells takes place. Increase in ion-solvating power and relative permittivity of the solvent favours solvent-shared and solvent-separated ion pairs. However, a clear experimental distinction between solvent-shared and solvent-separated ion pairs is not easily obtainable. Therefore, the designations solvent-shared and solvent-separated ion pairs are sometimes interchangeable. Eventually, further dissociation of the two ions leads to free, i.e. unpaired solvated ions with independent primary and secondary solvation shells. The circumstances under which contact, solvent-shared, and solvent-separated ion pairs can exist as thermodynamically distinct species in solution have been reviewed by Swarcz [138] and by Marcus [241],... 

A further conceptual distinction has sometimes been made between two types of loose ion pairs. In solvent-shared ion pairs, the ionic constituents of the pair are separated by only a single solvent molecule,... 

Figure 6 shows the potential of mean force (PMF) between a sodium ion and a chloride ion in water, at infinite dilution of the two ions, obtained from classical atomistic simulations [75]. The first minimum of the potential corresponds to the contact ion pair (CIP) distance, the second minimum corresponds to the solvent-shared ion pair (SIP) distance, and the third minimum to the solvent-separated ion pair (2SIP) distance. Figure 7a shows an example of a SIP in aqueous NaCl [75]. The infinite dilute potential of mean force in Fig. 6 can be used as an effective pah-potential in implicit solvent simulations. The osmotic coefficient (j) (ps) = nilpJc- T (with n the osmotic pressure and ps the salt number density) can be obtained through the virial route. For the case of a binary mixture of components i and j and pairwise additive, density-independent pair potentials, the virial equation can be expressed as... 

Fig. 7 (a) Solvent-shared ion pair (SIP) in aqueous sodium chloride solution [75]. The central water oxygen (red) coordinates the sodium itm (yellow) while this molecule is at the same time forming a hydrogen bond with the chloride ion (blue), (b) SIP in aqueous sodium (blue) acetate solution [70], In this system, changes in the exeess number of SIPs (the observed munber of SIPs minus the number expected at the corresponding distance if all ions are statistically distributed) within the Li, Na, K ion series are responsible for changes in the salt activity coefficient... 

A receptor for solvent-separated ion pairs is receptor 10, a combination of a dibenzo-18-crown-6 and abridging 1,3-phenyldicarboxamide. In the presence of 1 mol equivalent of metal cation, chloride affinities are enhanced in the following order K+ (ninefold enhancement), Na+ (eightfold enhancement), and Cs" " (no enhancement). An X-ray crystal structure shows that the receptor binds sodium chloride as a solvent-shared ion pair (Figure 8). 

With the aid of a model similar to that used by McClelland the shifts in frequency and the changes in dipole strength of the absorption bands induced by the counter ion were calculated. The calculations revealed that the observed phenomena can only be accounted for if one layer of solvent molecules is present between positive and negative ions in these solvent-shared ion pairs. If this solvent layer is missing more drastic changes, such as the appearance of new bands not present in the spectrum of the free anion, are expected 105). 

As the ion concentration increases, it is not possible to disentangle the effects of cations and of anions on the water reorientation dynamics because of the formation of solvent-shared ion pairs. Nevertheless, van der Postand Bakker [121] showed that sodium ions at concentrations up to 6 m slow down the reorientation of water molecules in aqueous NaCl and Nal, compared with CsCl and KI at the same concentration. Small effects are shown as the concentration of Lil increases up to 2 m, but gradual slowing down of is seen in aqueous Cs SO and Mg(C10 )2 and much more so in aqueous Na SO and MgSO, but the effects diminish as the temperature increases from 22 to 70°C as found by Tielrooij et al. [122]. In 6m NaOH solutions, the reorientation time of the OH hydration complex is r =12 2ps, that is, much slowed down relative to bulk water, because it is a large hydrogen-bonded structure that reorients as a whole according to Liu et al. [123]. 

As a generalization, ultrafast infrared spectroscopy in dilute aqueous salt solutions is rather insensitive to the nature of the cation (unless large and hydrophobic) but does respond to the anion by its hydrogen bonding to the 0-D probe. At larger concentrations, solvent-shared ion pairs show a cooperative cation-anion effect on the reorientation rate of the water molecules. 

A further conceptual distinction has sometimes been made between two types of loose ion pairs. In solvent-shared ion pairs , the ionic constituents of the pair are separated by only a single solvent molecule, whereas in solvent-separated ion pairs , more than one solvent molecule intervenes. However, the term solvent-separated ion pair must be used and interpreted with care since it has also widely been used as a less specific term for loose ion pair. 

CIP = contact ion pair LP = lone pair NDIS = neutron diffraction with isotope substitution RDF = radial distribution function SCW = supercritical water SCWO = supercritical water oxidation SPC = simple point charge SPC/E extended simple point charge SPCG = simple point charge gas phase dipole SShIP = solvent-shared ion pair SSIP = solvent-separated ion pair TST = transition state theory. 

In the second part we study the ion speciation in infinitely dilute NaCI aqueous solutions by determining the constant of as.sociation by constraint molecular dynamics via mean-force potential calculations. We determine the temperature and density dependence of the extent of the ion association. In addition we analyze the kinetics of the interconversion between two ion pair configurations, the contact ion pair and the solvent-shared ion pair, by determining the transition state theory (TST) kinetic rates. 

---