Free ions-ion pairs equilibria

Addition of common-anion salts produced a considerable decrease of the rates of polymerisation, but this phenomenon was not due to the shifting of free ions-ion pairs equilibria, but to the homoconjugation of the trifluoroacetate anions added with the acid and the corresponding decrease in the strength of the initiator. Stable homoconju-gated salts were plated and characterised analytically and by IR and NMR spectrosco-... 

The above scheme does not include any free ion-ion pair equilibria. Too little is known about the state of chain carriers in most of these systems and we must content ourselves with supposing that the active species are an equilibrium mixture of ions in different states of aggregation. 

So far the existence of ion pair equilibria has been excluded from our considerations. In fact, ion association is a common and characteristic phenomenon in nonaqueous solutions. Ionization of a covalent compound may be defined as the process leading to the formation of solvated ions independent of their presence as associated ions or as free entities. 

The model is based on the proposition that all carbocationic polymerizations of olefins, alkyl vinyl ethers, etc., involve a spectrum of species with differing ionicitieS connected by equilibria formally expressed by the Winstein ionicity spectrum 1] the Winstein spectrum starts with a covalent species and progresses through increasingly polarized and ionized species, to fully ionized solvated ( free ) ion pairs ... 

MtXn ED]. .. MtX , which arises upon the addition of an ED to a protic initiating system HX + MtX . As indicated by the straight and curved equilibria signs respectively, this species may form by adding the MtX ED complex to the free ion pair H +MtX +1 or directly from HX + MtX . The MtXn ED arises, most likely instantaneously, upon the addition of an ED to the charge which contains a stoichiometric excess of MtX . 

Ultrasonic relaxation measurements have also been used for the study of stepwise ion association, but in contrast with the DRS method, it detects the ion pairing equilibria rather than the species formed, which must be assumed. NMR and Raman spectroscopy are sensitive only to CIPs and treat SIPs and 2SIPs, with solvent molecules intervening between the partner ions, as if they were free ions. A thorough treatment of the theories relating to ion association of strong electrolytes and the experimental methods used to study it are presented in the review by Marcus and Hefter [29],... 

Use of table 3a in calculations of lanthanide seawater complexation requires that the total inorganic carbon concentration of seawater (Cr) be partitioned into dissolved aqueous carbon dioxide (C02aq), bicarbonate (HCOj) and carbonate (COj ). Since only a small fraction ( 14%) of COf in seawater is in the form of free ions, with the remainder being ion-paired as NaCOj, CaCO and MgCO, use of table 3a constants requires an assessment of carbonate and bicarbonate ion-pairing equilibria. Table 3b shows a more convenient stability constant formulation wherein lanthanide carbonate stability constants are expressed in terms of total (free plus ion-paired) carbonate ion concentrations in seawater ([C03"]t)- These results can be compared with direct observations of europium carbonate complexation in synthetic seawater (Lee and Byrne 1994). The GdCOj and Gd(C03)2 formation constant results of Lee and Byrne (1994), expressed in terms of... 

The reaction (Eq. (5)) in THF yields labile THF adducts which are converted into the more stable HMPA adducts by addition of HMPA. The various equilibria existing between Na2Fe(CO)4 and several donor solvents are described in a detailed paper by Collman in HMPA, the solvent-separated supernucleophilic ion pair [Na+x HMPA x Fe(CO)4 ] is the kinetically dominant species, with no kinetic contribution from free [Fe(CO)J2 . In THF, Na2Fe(CO)4 is much less dissociated, with tight-ion paired [NaFe(CO)4] as the kinetically important species [96],... 

With radical ions, the dimerization equilibrium is strongly influenced by the solvation and association of radical ions with counter ions. It has been shown that the free ions dimerize much more slowly than do the respective contact ion pairs e.g., the quinoline radical anion does not dimerize in the powerfully solvating hexamethylphosphoramide, but it does dimerize rapidly in tetra-hydrofuran (160). Thus, two equilibria should be distinguished (160), viz. 

Winstein Robinson (1958) used this concept to account for the kinetics of the salt effects on solvolysis reactions. They considered that carbonium ions (cations) and carbanions could exist as contact ion-pairs, solvated ion-pairs and as free ions and that all these forms participated in the reactions and were in equilibrium with each other. These equilibria can be represented, thus ... 

Therefore, this chapter provides a summarized data on the preparation of organic ion-radicals as independent particles that can be free or bound with counterions in ion pairs. The chapter considers liquid-phase equilibria in electron transfer reactions and compares electrode and liquid-phase processes for the same organic compounds. Isotope-containing molecules have specific features as ion-radical precursors, therefore, the generation of the corresponding ion-radicals is considered in Section 2.6 of this chapter. This chapter also pays some attention to the peculiarities of ion-radical formation in living organisms. 

It is not an absolute necessity for LCP to have no free ions. If free ions are present, LCP is possible only if there are fast equilibria between free ions, ion pairs, and covalent species. If the equilibrium between free ions and ion pairs is slow, the result is a bimodal distribution. Further, to have any possibility of LCP with free ions present, the concentration and reactivity of the free ions should not be such that the reaction is too fast. 

Another important type of observation from conductance experiments involves the effect of small quantities (1-2% W/W) of additives on dissociative equilibria. Obviously in any study where the equilibrium between two reactive species is relevant the possible disturbance of that equilibrium by traces of other components in the system is very important In kinetic studies of polymerisations it is usual to carry out experiments in which the concentrations of both initiator and monomer are varied systematically. While it is anticipated that the former will effect the position of equilibrium between free ions and ion pair species, it is easily overlooked that the latter may also have a significant influence, particularly if the polarity of the monomer differs substantially from that of the solvent... 

The equilibria considered up to now have all involved inner sphere complexes. There is the possibility that an inner sphere complex may react with free ligands in solution this includes the solvent itself, to give an outer sphere complex where the ligand enters the secondary solvation shell of the inner sphere complex. If the two species involved in this type of interaction are of opposite sign, which is the situation where this type of complex formation is expected to be most effective, the outer sphere complex is called an ion pair. Fuoss65 has derived an expression (equation 38) for the ion pair formation constant, XIP, from electrostatic arguments ... 

At a quantitative level, near criticality the FL theory overestimates dissociation largely, and WS theory deviates even more. The same is true for all versions of the PMSA. In WS theory the high ionicity is a consequence of the increase of the dielectric constant induced by dipolar pairs. The direct DD contribution of the free energy favors pair formation [221]. One can expect that an account for neutral (2,2) quadruples, as predicted by the MC studies, will improve the performance of DH-based theories, because the coupled mass action equilibria reduce dissociation. Moreover, quadrupolar ionic clusters yield no direct contribution to the dielectric constant, so that the increase of and the diminution of the association constant becomes less pronounced than estimated from the WS approach. Such an effect is suggested from dielectric constant data for electrolyte solutions at low T [138, 139], but these arguments may be subject to debate [215]. We note that according to all evidence from theory and MC simulations, charged triple ions [260], often assumed to explain conductance minima, do not seem to play a major role in the ion distribution. 

---