Loose ion pair

The reaction exothermicities ( —AG°) for forward and back ET in polar media were approximately estimated to be 1.39 and 2.18 eV, respectively [120], Since the back ET is highly exothermic, the relatively small kb-1 values for the compartmentalized system may be ascribed to the combined effect of the inverted region [97-99] and the loose ion-pair state. 

The transition states are composed of loose ion pairs in so far as they involve a charge-delocalized anion, thereby enhancing polarity compared with the ground states (in which the ion pairs are tighter), because of an increase in anionic dissociation as the more bulky product anion is formed. As a consequence, specific micro-wave effects, directly connected to polarity enhancement, should depend on the structure of reactive ion pairs in the GS ... 

If tight ion pairs (between two hard ions) are involved in the reaction the microwave-accelerating effect then becomes more important, because of enhancement of ionic dissociation during the course of the reaction as tight ion pairs (GS) are transformed into more polar loose ion pairs (TS). 

If, on the other hand, loose ion pairs (between soft ions) are involved, microwave acceleration is limited, because ionic interactions are only slightly modified from GS to TS. 

The TS for anionic SN2 reactions involves loose ion pairs as in a charge delocalized (soft) anion. On the another hand, the GS could involve a neutral electrophile and either tight or loose ion pairs depending on the anion structure (hard or soft) (Eq. 32). 

Weak or nonexistent microwave effects are expected for these reactions as the GS and TS exhibit rather similar polarities since they both involve loose ion pairs. 

This effect could be predicted when considering the weak evolution of polarity between the GS and TS as the reactive species consist of loose ion pairs (involving a soft anion). 

The absence or weakness of the microwave effect was assumed to be related to loose ion pairs involving the soft naphthoxide anion in the GS and a small change in polarity in an early TS. When the TS occurred later along the reaction coordinates (e.g. for n-octylation requiring a higher temperature), more polarity is developed and, consequently, the microwave effect could appear (Eq. (42) and Tab. 3.17 limited here to the lithiated base). 

The reactive species is the acylium ion resulting from abstraction of a chloride anion from benzoyl chloride (Eq. 50). This reagent comprises an ion pair formed between two large (soft) ions which are therefore associated as loose ion pairs. According to these assumptions, the absence of a microwave effect should be expected as the polarity evolution is very weak between the GS and TS (two loose ion pairs of similar polarities). 

In the radical anions of the norbornane-linked naphthalenes [37] mentioned earlier (Gerson et al, 1990) no counterion effects were detected for [37a], which has a small spatial separation, but the esr/ENDOR spectra of [37b]- and [37c]- indicate that the electron-spin transfer between the naphthalene moieties is determined by the rate of synchronous counterion migration (Gerson et al., 1990). For tight ion pairs the electron is localized, while for loose ion-pair conditions, e.g. by using solvents of high cation-solvating power, the transfer becomes fast on the hyperfine timescale (k > 107 Hz). 

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,... 

LINEAR STRAIN Long-lived luminescence, PHOSPHORESCENCE JABLONSKI DIAGRAM LONG-RANGE LOOPS LOOSE ION PAIR ION PAIR... 

There are two approaches to the separation of pp into the individual kp and kp values. One approach involves the experimental determination of the individual concentrations of free ions and ion pairs by a combination of conductivity with short-stop experiments or UV-visible spectroscopy. Conductivity directly yields the concentration of free ions that is, only free ions conduct. Short-stop experiments yield the total of the ion-pair and free-ion concentrations. UV-visible spectroscopy for those monomers (mostly aromatic) where it is applicable is also used to obtain the total of the free-ion and ion-pair concentrations. It is usually assumed that ion pairs show the same UV-visible absorption as free ions since the ion pairs in cationic systems are loose ion pairs (due to the large size of the negative counterions see Sec. 5-1). This approach is limited by the assumptions and/or experimental difficulties inherent in the various measurements. Conductivity measurements on systems containing low concentrations of ions are difficult to perform, and impurities can easily lead to erroneous results. The short-stop experiments do not distinguish between ion pairs and free ions, and the assumption of the equivalence of free ions and ion pairs in the spectroscopic method is not firmly established. The second approach involves determination of the... 

It is generally accepted that there is little effect of counterion on reactivity of ion pairs since the ion pairs in cationic polymerization are loose ion pairs. However, there is essentially no experimental data to unequivocally prove this point. There is no study where polymerizations of a monomer using different counterions have been performed under reaction conditions in which the identities and concentrations of propagating species are well established. (Contrary to the situation in cationic polymerization, such experiments have been performed in anionic polymerization and an effect of counterion on propagation is observed see Sec. 5-3e-2.)... 

Epoxide polymerizations taking place in the presence of protonic substances such as water or alcohol involve the presence of exchange reactions. Examples of such polymerizations are those initiated by metal alkoxides and hydroxides that require the presence of water or alcohol to produce a homogeneous system by solubilizing the initiator. Such substances increase the polymerization rate not only by solubilizing the initiator but probably also by increasing the concentration of free ions and loose ion pairs. In the presence of alcohol the exchange... 

Mohammed OF, Adamczyk K, Banerji N, Dreyer J, Lang B, Nibbering ETJ, Vauthey E (2008) Direct femtosecond observation of tight and loose ion pairs upon photoinduced bimolecular electron transfer. Angew Chem Int Ed 47 9044... 

The bare negative ions discussed in Problem 2.23 have a greatly enhanced reactivity. The small amounts of salts that dissolve in nonpolar or weakly polar solvents exist mainly as ion-pairs or ion-clusters, where the oppositely charged ions are close to each other and move about as units. Tight ion-pairs have no solvent molecules between the ions loose ion-pairs are separated by a small number of solvent molecules. 

The effect of dielectric constant. D, on the formation constant of a loose ion pair complex Is probably not large in spite of the increase in the interlonlc ion pair distance. The microscopic rather than the macroscopic dielectric constant determines to a large extent the difference between the coulonb attraction energy of two Ions In a tight and loose ion pair. 

For complex III, the Na+ Is probably as accessible to solva-tlon by solvent molecules as is the Na In the tight Fl-,Na+ Ion pair. Hence, no externally bound solvent molecules need to be removed. This may be different In other systems. For example, the formation constant of a loose Ion pair complex between FI", Na+ and tetraglyme (tetraethylene glycol dimethyl ether) Is nearly four times lower In dloxane than In THF (10). This may be caused by specific solvent effects rather than by the difference In solvent dielectric constant. The flexible glyme ligand wraps Itself around the Na+ Ion, and this may make It more difficult for solvent molecules to remain bound to Na+ In the glyme-separated Ion pair. 

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