Ionic pairs

The way to obtain the membranes as well their composition has been optimized. The main operational criterion for the membranes is the solubility of applicable ionic pairs. The solubility should be quite low - else the substance will be outwashed from the membrane. At the same time, the ionic pairs which have very low solubility are not suitable too because of the complicated obtaining homogeneous membranes. 

The obtained results exhibit the efficacy of using the mentioned ionic pairs as electrode-active substances for plasticized ionoselective electrodes. 

The specific adsorption, which in the case of ITIES is usually the adsorption of ionic pairs [8], contributes to the Galvani potential, as well as changes the zero charge of this interface. 

Ionic chain polymerisations refer to chain mechanisms in the course of which the propagation step consists of the insertion of a monomer into an ionic bond. The strength of this ionic bond can vary, depending on the nature of the species, the temperature and the polarity of the solvent, between a closed ionic pair in contact up to free ions (see Figure 23). Final polymer microstructure (configuration,...) and molecular mass distribution depend on the actual nature of the active ionic species. 

The solvated H3N. . . HBr complex was studied by Ruiz-Lopez et al.m using the ellipsoidal cavity in the DFT(B88/P86)/SCRF calculations which demonstrated that the solvent stabilizes the ionic pair structure. The authors found only one conformational minimum of the complex, which electronic structure corresponded to that of anionic pair. 

The reactant R2 can also be considered to be a solvent molecule. The global kinetics become pseudo first order in Rl. For a SNl mechanism, the bond breaking in R1 can be solvent assisted in the sense that the ionic fluctuation state is stabilized by solvent polarization effects and the probability of having an interconversion via heterolytic decomposition is facilitated by the solvent. This is actually found when external and/or reaction field effects are introduced in the quantum chemical calculation of the energy of such species [2]. The kinetics, however, may depend on the process moving the system from the contact ionic-pair to a solvent-separated ionic pair, but the interconversion step takes place inside the contact ion-pair following the quantum mechanical mechanism described in section 4.1. Solvation then should ensure quantum resonance conditions. 

It can be shown that the virial type of activity coefficient equations and the ionic pairing model are equivalent, provided that the ionic pairing is weak. In these cases, it is in general difficult to distinguish between complex formation and activity coefficient variations unless independent experimental evidence for complex formation is available, e.g., from spectroscopic data, as is the case for the weak uranium(VI) chloride complexes. It should be noted that the ion interaction coefficients evaluated and tabulated by Cia-vatta [10] were obtained from experimental mean activity coefficient data without taking into account complex formation. However, it is known that many of the metal ions listed by Ciavatta form weak complexes with chloride and nitrate ions. This fact is reflected by ion interaction coefficients that are smaller than those for the noncomplexing perchlorate ion (see Table 6.3). This review takes chloride and nitrate complex formation into account when these ions are part of the ionic medium and uses the value of the ion interaction coefficient (m +,cio4) for (M +,ci ) (m +,noj)- Io... 

As a rule, if the unpaired electron density in the anion-radical is redistributed, the rotation barrier decreases. Thus, the barrier of the phenyl rotation in the benzaldehyde anion-radical is equal to 92 kJ mol", whereas in the 4-nitrobenzaldehyde anion-radical, the barrier decreases to 35 kJ mor (Branca and Gamba 1983). Ion-pair formation enforces the reflux of the unpaired electron from the carbonyl center to the nitro group. Being enriched with spin density, the nitro group coordinates the alkali metal cation and fixes the unpaired electron to a greater degree. The electron moves away from the rotation center. The rotation barrier decreases. The effect was revealed for the anion-radical of 4-nitrobenzophenone and its ionic pairs with lithium, sodium, potassium, and cesium (Branca and Gamba 1983 Scheme 6.19). 

A cation arriving with a nncleophilic anion is another important factor. The nucleophile can attack the substrate in the form of a free ion or an ionic pair. As a rule, lithium salts are less reactive than sodium and potassium salts. Russell and Mndryk (1982) reported several examples of this. The sodium salt of ethyl acetylacetate reacts with 2-nitro-2-chloropropane in DMF yielding ethyl 2-(wo-propylidene) acetylacetate. Under the same conditions, the lithium salt does not react at all. Potassium diethyl phosphite interacts with l-methyl-l-nitro-l-(4-toluylsulfonyl)propane in THF and gives diethyl 1-methyl-l-nitro-l-phosphite. The lithinm salt of the same reactant does not react with the same substrate in the same solvent. 

Ionic pair extraction 2 x MTBA-HSO4/MTBE extraction centrifugation. Eiltered by 0.1 pm nylon... 

Sea sand homogenization, PEE (water), SPE (OasisWAX 3), elution with 0.1% NH40H in MeOH, ionic-pair extraction (MTBE)... 

Then this adduct yields [W3S4Cl3(dmpe)3]+, HCl, and H2 in the rate-determining step. It should be emphasized that this mechanism and pathway (2), in Scheme 10.8 are different. In fact, the latter requires HX assistance to stabilize the ionic pair M-(H2) - -(XHX) . According to the kinetic data presented in this section, such mechanisms are rather rare pathway (1) in Figure 10.8 can be taken as a general mechanism of proton transfer. 

The equilibrium constant for the reaction of the electron transfer from the anion radical salts of aromatic compounds (with the usual isotope content) to neutral molecules of the same compounds containing heavier isotopes is less than unity (entries 1-10 in Table 2-1). This means that for heavier compounds (enriched with neutrons), the electron affinity is smaller. This difference is conserved at different temperatures and reaction mediums (including those favorable to the destruction of ionic pairs—in HMPA and in THF containing 18-crown-6). 

With respect to ionic pairs, perdeuteration in organic anion radicals alters the zero-point energies of the vibrational modes involving the interaction of a metal cation with an organic counterpart. As shown, the anion radical of perdeuterionaphthalene associates with sodium cations in THF more tightly than the anion radical of perprotionaphthalene under the same conditions (Ch.D. Stevenson, Wagner, Reiter 1993). 

With these anthracene-linked dimeric cinchona-PTCs, the Najera group investigated the counterion effect in asymmetric alkylation of 1 by exchanging the classical chloride or bromide anion with tetrafluoroborate (BF4 ) or hexafluorophosphate (PF6-) anions (Scheme 4.10) [17]. They anticipated that both tetrafluoroborate and hexafluorophosphate could form less-tight ionic pairs than chloride or bromide, thus allowing a more easy and rapid complexation of the chiral ammonium cation with the enolate of 1, and therefore driving to a higher enantioselectivity. However, when... 

As shown in Figure 4 of section 2, the dipeptide molecules are arranged in a parallel / -sheet-like structure which is constructed by ionic pairing of carboxyl and amino groups via a hydrogen bonding network one terminal COO bridged two +NH3... 

It follows from this scheme that the use of a protonic solvent (MeOH) as a test for nucleophilic attack at Cl is not absolutely reliable, since ionic pairs of type 5 may be formed by an alternative path. 

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