Ion pairs complexation

The kinetics of this process is strongly affected by an association phenomenon. It has been known that the active center is the silanolate ion pair, which is in equUibrium with dormant ion pair complexes (99,100). The polymerization of cyclosiloxanes in the presence of potassium silanolate shows the kinetic order 0.5 with respect to the initiator, which suggests the principal role of dimer complexes (101). 

Neutron activation analysis of a polymer suggests that when Py is used as the electron doner (D), the initiation proceeds through the Cl atom, but when D = DMSO, both Cr and DMSO residues are the primary radicals produced from the photoexcited ion-pair complex. The following reaction scheme is proposed ... 

The ion-pair complex formed by the interaction of hydroxobis(8-quinolyloxo) vanadium (V) [VOQ2OH] and /i-butyl amine is also effective in photoinitiation of polymerization of MMA in bulk and in solution [40]. The quantum yield of initiation and polymerization determined are equal to 0.166 and 35.0, respectively. Hydroxyl radical ( OH) is reported to be the initiating radical and the following photoreaction is suggested ... 

Several groups confirmed spectroscopically the existence of an ion-pair complex Fe -HOJ, which may decompose unimolecularly at high [H2O2]/ [Fe(III)] but become transformed to FeO " " at lower [H202]/[Fe(III)]. FeO " is attacked by a second molecule of H2O2 to provide an alternative route for decomposition, viz. 

Catalysis at interfaces between two immiscible liquid media is a rather wide topic extensively studied in various fields such as organic synthesis, bioenergetics, and environmental chemistry. One of the most common catalytic processes discussed in the literature involves the transfer of a reactant from one phase to another assisted by ionic species referred to as phase-transfer catalyst (PTC). It is generally assumed that the reaction process proceeds via formation of an ion-pair complex between the reactant and the catalyst, allowing the former to transfer to the adjacent phase in order to carry out a reaction homogeneously [179]. However, detailed comparisons between interfacial processes taking place at externally biased and open-circuit junctions have produced new insights into the role of PTC [86,180]. 

Several theoretical models, such as the ion-pair model [342,360,361,363,380], the dyneuaic ion-exchange model [342,362,363,375] and the electrostatic model [342,369,381-386] have been proposed to describe retention in reversed-phase IPC. The electrostatic model is the most versatile and enjoys the most support but is mathematically complex euid not very intuitive. The ion-pair model emd dynamic ion-exchange model are easier to manipulate and more instructive but are restricted to a narrow range of experimental conditions for trtilch they might reasonably be applied. The ion-pair model assumes that an ion pair is formed in the mobile phase prior to the sorption of the ion-pair complex into the stationary phase. The solute capacity factor is governed by the equilibrium constants for ion-pair formation in the mobile phase, extraction of the ion-pair complex into the stationary phase, and the dissociation of th p ion-pair complex in the... 

Majcherczyk, A. and Hiittermann, A. Size-exclusion chromatography of lignin as ion-pair complex, /. Chromatogr. A, 764, 183, 1997. 

Lemli and Knockaert [33] described a spectrophotometric method for the determination of miconazole nitrate suspensions and other organic bases in pharmaceutical preparations by the use of cobalt thiocyanate. The drug and the amines (as their anhydrous hydrochlorides in dichloromethane) react with solid cobalt thiocyanate to form an ion-pair complex that contains two molecules of base to one [Co(SCN)4]2. The complex is determined quantitatively by spectrophotometry versus dichloromethane at 625 nm with rectilinear response for up to 400 pg/mL of the base. This method was applied to miconazole nitrate suspensions and the coefficient of variations were generally <2%. 

El-Ashry et al. [36] studied the complex formation between the bromophenol blue, primaquine, and other important aminoquinoline antimalarials. The colorimetric method used was described as simple and rapid and is based on the interaction of the drug base with bromophenol blue to give a stable ion-pair complex. The spectra of the complex show maxima at 415 420 nm with high apparent molar absorptivities. Beer s law was obeyed in the concentration range 1-8,2-10, and 2-12 pg/mL for amodiaquine hydrochloride, primaquine phosphate, and chloroquine phosphate, respectively. The method was applied to the determination of these drugs in certain formulations and the results were favorably comparable to the official methods. 

Sodium valproate has been determined in pharmaceuticals using a valproate selective electrode [13,14]. The electroactive material was a valproate-methyl-tris (tetra-decyl)ammonium ion-pair complex in decanol. Silver-silver chloride electrode was used as the reference electrode. The electrode life span was >1 month. Determination of 90-1500 pg/mL in aqueous solution by direct potentiometry gave an average recovery of 100.0% and a response time of 1 min. 

While there are only a few examples that can be used for direct detection of desired analytes, many simple molecules and ions do not have optical activity under regular conditions, a chemical reaction is needed to generate an optically active species. The reactions can be acid-base, ion pairing, complexation reactions, or quenching of fluorescence by 02, paramagnetic molecules, etc. Optical sensors for a few analyte or group analytes are summarized below. 

Surface complexation models attempt to represent on a molecular level realistic surface complexes e.g., models attempt to distinguish between inner- or outer-sphere surface complexes, i.e., those that lose portions of or retain their primary hydration sheath, respectively, in forming surface complexes. The type of bonding is also used to characterize different types of surface complexes e.g., a distinction between coordinative (sharing of electrons) or ionic bonding is often made. While surface coordination complexes are always inner-sphere, ion-pair complexes can be either inner- or outer-sphere. Representing model analogues to surface complexes has two parts stoichiometry and closeness of approach of metal ion to... 

DASPE-TFPB), respectively. The obtained solid precipitates were brightly emissive whereas that of the native DASPE-I were almost nonemissive (Fig. 7a the photo is taken under normal illumination and UV-light irradiation). This indicates that, in the solid of the ion-pair species between DASPE+ and TPB (or TFPB ), concentration quenching is effectively suppressed, and more importantly, these ion-pair complexes can generate fluorescent... 

There are a limited number of fluorescent sensors for anion recognition. An outstanding example is the diprotonated form of hexadecyltetramethylsapphyrin (A-7) that contains a pentaaza macrocydic core (Figure 10.31) the selectivity for fluoride ion was indeed found to be very high in methanol (stability constant of the complex 105) with respect to chloride and bromide (stability constants < 102). Such selectivity can be explained by the fact that F (ionic radius 1.19 A) can be accommodated within the sapphyrin cavity to form a 1 1 complex with the anion in the plane of the sapphyrin, whereas Cl and Br are too big (ionic radii 1.67 and 1.82 A, respectively) and form out-of-plane ion-paired complexes. A two-fold enhancement of the fluorescent intensity is observed upon addition of fluoride. Such enhancement can be explained by the fact that the presence of F reduces the quenching due to coupling of the inner protons with the solvent. 

For instance, (kH/kD)a was 1.085 0.011 when the sodium thiophenoxide concentration was 0.0086 mol dm-3 and 1.129 0.010 when the concentration of the sodium thiophenoxide was reduced to 0.0040 mol dm-3 in DMF at 20°C. Conductivity and UV studies of sodium thiophenoxide solutions in DMF, in DMSO, in methanol and in diglyme, and reactions done in the presence of the macrocyclic polyether 15-crown-5 (Westaway and Lai, 1988), showed that the change in the secondary a-deuterium KIE was due to a change in the form of the reacting nucleophile from a solvent-separated ion-pair complex at the higher concentration (see (27)) to a free ion at the lower concentration. 

Table 26 The secondary a-deuterium KIEs for the SN2 reactions between butyl chloride and thiophenoxide ion when the nucleophile is a contact ion-pair, a solvent-separated ion-pair complex and a free ion at 200C.0...

These KIEs were measured using lithium thiophenoxide as the nucleophile. The contact ion-pair was converted into the solvent-separated ion-pair complex by adding between 1.0% and 7.5% water to dry diglyme (Fang and Westaway, 1991). 

On the other hand, new solution species are being identified. For example, some polynuclear species and some ion pair complexes are now recognized as being more significant in aqueous solutions than previously thought. There is therefore a need to develop, extend and up-date the data on new species which come to be recognized as significant. 

The nucleophile in the S.v2 reactions between benzyldimethylphenylammonium nitrate and sodium para-substituted thiophenoxides in methanol at 20 °C (equation 42) can exist as a free thiophenoxide ion or as a solvent-separated ion-pair complex (equation 43)62,63. The secondary alpha deuterium and primary leaving group nitrogen kinetic isotope effects for these Sjv2 reactions were determined to learn how a substituent on the nucleophile affects the structure of the S.v2 transition state for the free ion and ion-pair reactions64. 

Free metal ions Inorganic ion pairs complexes Organic complexes chelates Metals species bound to high molecular weight DOM Metals in colloids Metals sorbed onto colloids Precipitates... 

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