Lithium salt dissociation

As described in previous sections (Sections VI and VII), macromolecular design of polymer/salt hybrids with a highly dissociable lithium borate unit proved to be a valuable approach for single-ion conductive polymers. To further improve the degree of lithium salt dissociation, we have designed a polymer/salt hybrid bearing the boron-stabilized imidoanion (BSI)38 (Fig. 10). 

Ionic polymers may exist as undissociated, unsolvated ion pairs undissociated ion pairs solvated to some extent solvated ions dissociated to some extent or some combination of these. The propagation rate constant kp and the dissociation equilibrium constant K of the lithium salt of anionic... 

Kuhn s carbanion analogues, [44 ], [45 ] and [46 ], have recently been synthesized, and the precursor hydrocarbons [44]-H, [45]-H and [46J-H dissociate into the respective anions in DMSO to show deep blue colours without any added base (Kinoshita et al., 1994). A fullerene anion, Bu Qb [47 ], has also been obtained as a stable carbanion (Fagan et al., 1992) its lithium salt has been isolated in the form Li [47 ]-4MeCN or Li [47"]-3-4THF. Several stable all-hydrocarbon anions of precursor hydrocarbons with low pKa values ( 7) are listed in Table 2, along with their oxidation potentials, ox-... 

Interaction gives only a 1 1 complex which explodes on grinding, but dissociates uneventfully on heating at atmospheric or reduced pressure. The lithium salt is similar. 

On substitution of allyllithium with methyl groups, the structures are distorted tt complexes becoming more jj -like. The previously described allyllithiums are contact ion pairs (CIP) whose dissociation is too low to permit study of the free carbanion. However, this is not the case for a more delocalized system such as 1,3-diphenylallyl whose lithium salts can exist as solvent separated ion pairs (SSIP) in ethereal solutions for which the organic moiety could be treated essentially as a free carbanion55 Boche and coworkers studied the effect of substitution at C(2) in their 1,3-diphenylallyl lithiums on the rotational barriers... 

For lithium electrolytes, the only variable in salt structure is the anion. In a given nonaqueous solvent system, the dissociation of a lithium salt would be... 

On the basis of their previous experiences with lithium borates coordinated by substituted ligands. Barthel and co-workers modified the chelatophos-phate anion by placing various numbers of fluorines on the aromatic ligands. Table 13 lists these modified salts and their major physical properties. As expected, the introduction of the electron-with-drawing fluorines did promote the salt dissociation and reduce the basicity of phosphate anion, resulting in increased ion conductivity and anodic stability. The phosphate with the perfluorinated aromatic ligands showed an anodic decomposition limit of 4.3 V on Pt in EC/DEC solution. So far. these modified lithium phosphates have attracted only academic interest, and their future in lithium ion cell applications remains to be determined by more detailed studies. 

The tendency of the lithium salts to form complexes with ammonia is likewise shown by the partition of ammonia between chloroform and aq. soln. of the lithium salt.78 Dry salts of lithium also form complexes with ammonia, and a comparison of the observed thermal value of the reaction with that computed from the dissociation press, of Clapeyron s equation has been made by J. Bonnefoi, and indicated in Table XXI. According to F. Ephraim, lithium tetrammino-chloride has a vap. press, of 760 mm. at 12°. 

Table 4. Dissociation and reactivity of lithium salt of living polystyrene in benzene-dimethoxy ethane mixtures at 25...

Lithium salts exhibit general high solubility and a high degree of dissociation in other nnnaqueous solvents than liquid ammonia, such as liquid sulfur dioside and acetic acid. 

For iodides, the order of efficiency is tetrabutyl > tetraethyl > tetramethyl 56 A rise in the catalytic effect of initiator is observed for alkali salts and for ammonium salts with increasing diameter of the cation, hence with growing distance between charge centres. This is in agreement with an increase of the electropositivity of cations and the increasing ability of salts to dissociate 54), Despite of this, the remarkable efficiency of lithium salts in curing of epoxy resins 58) or in copolymerization reactions 411 was confirmed in some papers. 

For a C-C bond to cleave, there must be an acceptor substituent at one end and a donor at the other. The potassium aldolates form highly dissociated ion pairs and cleave easily. They give the thermodynamically controlled aldol products. When using lithium salts, the donor character of the C) function is reduced and the retroaldol process is retarded. Hence, the kinetic products are also observed. 

The addition of lithium salts during carbonyl olefination with reactive ylides causes a lowering of the (Z)-stereoselectivity. This effect is observed mainly in benzene or polar media such as ether (i.e. the more strongly the lithium cation is dissociated from its counterion u>) (Table 1). On the other hand, the presence of lithium salts seems to increase the amount of the (Z)-isomer in the reaction of moderate 28), a-fluorosubstituted 29) or resonance-stabilized ylides 30) (Table 1). 

In ethanol acerands 4 and the reference compound 13 dissociate partially in ethanol to give pale blue solutions of phenolate anions (X max 586 nm for 13). Addition of triethylamine to the solution allows nearly complete dissociation. This blue solution was changed to violet-yellow by addition of crystalline alkali or alkaline earth metal salts. For example, a marked hypsochromic shift (A max 586-536 nm) of the longest wavelength absorption band was observed with lithium salts, LiCl and LiC104 [7]. The phenolate anion 4 (n = 2) is less susceptible than that of 4 (n = 1) for color change with metal salts. The uncrowned azophenol... 

The conductivity of nonaqueous electrolyte solution increases with increasing concentration of lithium salt and then decreases through a maximum, as shown in Fig. 2. Such behavior has been explained by the ionic dissociation process of LiA (molecule in solution) as described by the equation. A high dissociation of... 

The nucleophilic reactivity of the lithium salts changes in the same order as in protic solvents (I > Br > Cl cf. Table 5-15). However, the order is completely reversed for the ammonium salts (Cl > Br > I ), and this latter order is the same as that found in dipolar non-HBD solvents such as A,A-dimethylformamide [278]. The small lithium cation, with its high charge density, has a strong tendency to form ion pairs with anions, whereas the electrostatic interaction between the large tetraalkylammonium ion and anions is comparatively weak. Quaternary ammonium salts, therefore, should be practically fully dissociated in acetone solution. Thus, the reactivity order obtained with these salts corresponds to that of the free, non-associated halide ions. On the other hand, the sequence obtained with the lithium salts also reflects the dissociation equilibria of these salts in acetone solution [279]. 

Examination of the Sn2 reaction between ethyl tosylate and halide ions in hexamethylphosphoric triamide (fir = 29.3) with a variety of counter ions [Li , [n-C4H9)4N ] has shown that the rates obtained with Hthium salts are always higher than those with the corresponding tetra-n-butylammonium salts [341]. This is in contrast to the situation observed in acetone [279]. This means that, in this particular solvent, lithium salts are more dissociated than tetraalkylammonium salts. This has indeed been confirmed by conductivity measurements [341, 342], The lithium cation apparently has specific interactions with strong EPD solvents such as [(CH3)2N]3PO cf. Section 3.3.2). 

Infrared absorption spectra have been measured for NaF and KF isolated in solid argon. The fundamental modes of these species and the B3u, B2u, and Blu modes of (NaF)2 and (KF)2 are assigned assuming a planar rhomboid structure of D2h symmetry for the dimers.171 Surface ionization of lithium and its halides has been studied by a double-filament technique. At relatively high temperatures the temperature dependence of the lithium surface ionization current from all molecules studied was identical with that from lithium. Incomplete dissociation of LiCl can account for Li ionization threshold temperatures well above that for surface ionization of Li atoms. Dissociation energies of 4.8 0.1 eV for LiCl(g) and 4.3 0.1 eV for LiBr(g) were obtained.172 The equilibrium ionic forms in salt vapours have been studied. The concentration of M+ and X ions in the saturated vapours of MX (M = Na or K, X = halogen) at 850 °C is 107—1010 ions cm-3 and the concentration of M2X+ and M2X ions is 109—1011 ions cm-3, so that the vapours exhibit measurable electric conductivity. The equilibrium constants for the gaseous reactions are as follows ... 

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