Loose/solvent-separated

The kinetics of the polymerization of butadiene by n-butyllithium in the presence of TMEDA was studied by Hay and McCabe 180). They were unable to distinguish between addition of monomeric n-butyllithium and that of the species (n-BuLi TMEDA) to the monomer as the initiation step. The initiation efficiency varied from 50% at a ratio of [TMEDA] [Li] of 0.9 to 99% at a ratio of 3.35 and it was concluded that propagation involves growth from the loose (solvent separated) ion pair of composition (PBLi 2 TMEDA). The presumption that there is no exchange of TMEDA among the complexed species is not in accordance with the observation of time-averaged signals in the 1H-NMR spectrum 181). 

Perhaps the most noteworthy feature of many anionic polymerizations conducted in ethereal solvents is that the propagation reaction is a dual process, i.e., ion pairs and free ions coexist and exhibit distinctly different reactivies 240 24% The free ion concentration is generally limited to 1 % or less of the total active center concentration while the relative reactivities diminish in the sequence free anion S loose (solvent-separated) ion pair > tight (contact) ion pair. 

Why is a loose, solvent-separated pair more reactive than a tight contact pair A contact pair involves a small, bare cation which becomes partially dissociated in the transition state as shown in the following diagram. This hinders the propagation. The partial dissociation may be unnecessary for the separated pair because the cation is large (owing to its solvation). 

The association of bare, non-solvated ions undoubtedly proceeds without any activation energy and without increase of the entropy of activation. Therefore, the respective ka > 10u M-1s 1. Ions surrounded by tight solvation shells form loose, solvent separated ion-pairs on their encounters, and again it is unlikely that their formation requires activation energy or negativ entropy of activation, i.e., most probably the pertinent ks > 10u M-1s-1. Temperature dependence of ka is determined by temperature dependence of viscosity given by a formal activation energy of 2-3 kcal/mol. 

As mentioned earlier, ion pairs can exist in tight (contact) or loose (solvent-separated) forms. Whenever both forms are present in a solution, K is directly determined as follows (subscripts t and l denote tight and loose forms, respectively) ... 

According to a number of authors (28- 1). the Interaction of cations such as Na with a poly-anlon In water can occur under two different modes either a true contact Ion pair Is formed, as dictated by the mass action law and so that the two hydration shells are perturbed or the sodium cation, as the hexaquo species. Is attracted Into the electrostatic potential In the vicinity of the charged counter-Ion It Is like a free Ion In every respect except for being constrjdned not to diffuse outside of a certain volume surrounding the polyelectrolyte. The former Interaction type Is called site binding. The latter, which Is reminiscent of formation of a loose, solvent-separated. Ion pair Is referred to as atmospheric condensation. (Figures 1-2). 

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,... 

The reactivity of the solvent-separated ion pair is hardly affected by the counterion. This can be observed from the kj values in THF (Table 5-11). Most of the observed propagation in THF is due to solvent-separated ion pairs and kj is a good indication of ks. The variation in kj from Li+ to Cs+ is relatively small and is probably due more to differences in the fractions of solvent-separated ion pairs than to differences in ks. The reactivity of contact ion pairs is more sensitive to the counterion. The variation of kj in dioxane is by a factor of 25 between the different counterions. Since the fraction of solvent-separated ion pairs is extremely small in dioxane, kj is indicative of kc. The larger, more loosely held cesium counterion results in a higher reactivity for the contact ion pair. The variation of kc and ks with solvating power of the reaction medium is not established. Some data indicate that ks is insensitive to solvent while kc increases with increasing solvating power, but these results are limited to ether solvents. 

Bimolecular photoinduced electron transfer between an electron donor and an electron acceptor in a polar solvent may result in the formation of free ions (FI). Weller and coworkers [1] have invoked several types of intermediates for describing this process (Fig.la) exciplex or contact ion pair (CIP), loose ion pair (LIP), also called solvent separated ion pair. The knowledge of the structures of these intermediates is fundamental for understanding the details of bimolecular reactions in solution. However, up to now, no spectroscopic technique has been able to differentiate them. The UV-Vis absorption spectra of the ion pairs and the free ions are very similar [2], Furthermore, previous time resolved resonant Raman investigations [3] have shown that these species exhibit essentially the same high frequency vibrational spectrum. 

The question of different types of ion pair, similar to those characterised in anionic polymerisation (14), has not yet arisen. Solvents are generally more polar than those used in polymerisations proceeding by carbanionic intermediates, and it could be that the major ion pair species is a loose or solvent separated entity. As far as conductance measurements (Ka) are concerned it is not possible to make a distinction between loose and contact ion pairs. For such investigations the use of other analytical methods becomes necessary (49). 

The alkali metals share many common features, yet differences in size, atomic number, ionization potential, and solvation energy leads to each element maintaining individual chemical characteristics. Among K, Na, and Li compounds, potassium compounds are more ionic and more nucleophilic. Potassium ions form loose or solvent-separated ion pairs with counteranions in polar solvents. Large potassium cations tend to stabilize delocalized (soft) anions in transition states. In contrast, lithium compounds are more covalent, more soluble in nonpolar solvents, usually existing as aggregates (tetramers and hexamers) in the form of tight ion pairs. Small lithium cations stabilize localized (hard) counteranions (see Lithium and lithium compounds). Sodium chemistry is intermediate between that of potassium and lithium (see Sodium and sodium alloys). 

This behavior can be understood by the assumption that two different types of ion pairs exist in a thermodynamic equilibrium and add the monomer with different rate constants. The less reactive contact ion pair (tight-ion pair) and the more reactive solvent separated ion pair (loose-ion pair), which is more stable at lower temperatures (31). The curved lines show the transition from one ion pair species to the other. Thus, the polymerization mechanism can be described by this scheme ... 

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. 

Apart from free solvated radical ions (FRI), evidence was gathered for two kinds of ion pairs, which are referred to as tight ion pair (or contact ion pair, CIP) and loose ion pair (or solvent separated ion pair, SSIP) (Scheme 1, Eq. (1)) [3]. It is important to point out that CIP and SSIP are not the only species in solution. There are myriads of spatial cation-anion relationships that lie between them [4]. The SSIP is a pair of two ions of opposite sign with intact solvent shells. This is lacking in the CIP, anion and cation are in direct contact, the whole aggregate being surrounded by solvent molecules. 

The electronic spectrum of 1,5-diphenylpentadienyllithium in thf shows the presence of contact (tight) and.solvent-separated (loose) ion pairs. The smaller the cation and the more delocalized the anion, the greater is the tendency to form loose ion pairs. They are also favored as the temperature is lowered (64,65). 13C-NMR studies (66,67) have been interpreted in terms of appreciable covalency, but the present view is that organolithiums are predominantly ionic (68). Schlosser and Stahle have analyzed coupling constants 2J(C—H) and 3J(H—H) in the 13C- and H-NMR spectra of allyl derivatives of Mg, Li, Na, and K and of pentadienyls of Li and K (69). They conclude that considerable pleating of the allyl and pentadienylmetal structures occurs. The ligand is by no means planar, and the metal binds to the electron-rich odd-numbered sites. The lithium is considered to be q3 whereas the potassium is able to reach t]5 coordination with a U-shaped ligand. 

P selectivity. Crich and coworkers proposed that, under preactivation conditions, the oxocarbenium ion is trapped by a triflate anion to form the more stable a-triflate 65. After addition of the acceptor, the a-triflate intermediate can then be displaced in an SN2-like manner to afford a p-mannoside product (68). The formation of a-glycosyl triflates was confirmed by II, 13C, and 19F NMR analyses of the activated mannosyl donor recorded at low temperature [37], The experimentally determined KIE is approximately 1.12, which is consistent with an oxocarbenium-like TS [38], It was hypothesized that the a-triflate converts into the contact ion pair 66 in which the triflate anion remains at the a face or that an exploded TS is formed where the nucleophile is loosely associated with the oxocarbenium ion as the triflate departs [39,40], The a product 69 can be explained by the formation of the solvent-separated ion pair 67 where the counterion is solvated and facial selectivity is lost. 

The ion pair concept, introduced by Bjerrum [14], was critically reviewed by Szwarc [15] and definitions were given based on the mutual geometry of ions and solvent. The existence of loose and tight ion pairs was suggested by Winstein [16] and Sadek [17] and it is now common to speak about free ions (FI) as well as of solvent-separated ion pairs (SSIP) or contact ion pairs (CIP), having in mind the oversimplified picture ... 

The advancement of the theoretical description of ion-pairing was marked by the distinction between internal (or contact or tight) and external (or solvent separated, loose) ion-pairs. Eigen and Tamm [63,64] proposed a stepwise formation of the contact ion-pair while the formation of the solvent separated ion-pair is diffusion-controlled, the elimination of the solvent molecnles to form the contact ion-pair was the slowest stage. Ultrasonic absorption data snpported the so-called Eigen mechanism represented in Figure 2.1. 

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