Ion-pairs stability

From the above statements it follows that it should be possible to derive the growth kinetics and calculate the growth rate of uncontaminated electrolyte crystals when the following parameters are known molecular weight, density, solubility, cation dehydration frequency, ion pair stability coefficient, and the bulk concentration of the solution (or the saturation ratio). If the growth rate is transport controlled, one shall also need the particle size. In table I we have made these calculations for 14 electrolytes of common interest. For the saturation ratio and particle size we have chosen values typical for the range where kinetic experiments have been performed (29,30). The empirical rates are given for comparison. 

The existence of an ion pair stabilized by a solvent molecule in the product-determining step of the reaction has been established by calculations and also supported by the product composition (equation 89). While the formation of the diiodo derivative is characteristic of all the cited solvents, in tetrahydrofuran this iodination takes place with the predominant formation of l-iodomethyl-3-(4-iodobutoxy)adamantane (equation 89). 

In conclusion, it should be mentioned that extraction parameters (the equilibrium constants of exchange reactions and ion-pair stabilities) were introduced into the theory of ion-selective electrodes in [2, 31,33, 34, 35,69]. The theory of ISEs with a liquid membrane and a diffusion potential in the membrane was extended by Buck etal. [11, 13, 14, 73, 74] and Morf [54]. 

Hence, a very important factor of preferential dianion formation is the decrease in electrostatic repulsion between anion-radicals. By changing the ion-pair stability, particularly, by solvent selection, one can manage the equilibrium of liquid-phase electron-transfer reactions. 

Heterolytic cleavage of X-Y => X+ + Y" ion pair, stabilized by resonance or polar solvent. Characteristic of ionic reactions involving nucleophiles and electrophiles. 

The interesting feature of these results is that the more substituted mesylate exhibits lower selectivity than the less substituted mesylate. This is contrary to what one would expect based on ion-pair stabilities. Thus it appears that a concerted SN 2 attack occurs, and... 

It is apparent that debate concerning the reversed-phase ion-pair retention mechanism will continue because of the paucity of pertinent data concerning ion-pair stability constants in hydroorgtuiic solutions. 

Table 3. Degree of dissociation and ion pair stabilization energy for cyanine hexafluorophosphate in solvents of various dielectric constants.

The second strategy involves stabilization of the mono-Cp r/°-non-benzyl dialkyl metal cation with 7t-arene coordination. The arene coordination becomes the primary stabilization factor enabling the detection or isolation of the mono-Cp dimethyl cations. Thus, the reaction of Cp MMe3 (M = Zr, Hf) with 1 equiv. of B(C6Fs)3 in toluene/hexanes (1 10) solutions at ambient temperature affords the cationic arene complexes [(Gp M(Me)2(7]6-PhMe)][MeB(C6Fs)3] 304 as the solvent-separated ion pairs stabilized by the coordination of the aromatic solvent (Equation (22)).244 The crystal stmeture of the hafnium complex [(Cp Hf(Me)2(7]6-PhMe)][MeB(C6Fs)3] confirms the formation of the separated, discrete ion pairs in which the bent-sandwich cation is coordinated to an 7]6-toluene ligand.245... 

The role of metal ions as electrophilic catalysts is often underestimated. The formation of good ion-pair stabilization in the transition state in an anionic nucleophilic attack lowers the energy of the transition state. Even a powerful hydride nucleophile such as LiAlH4 will not add to a carbonyl if the lithium ion is kept from complexing with the carbonyl. 

The stability constants of ion pairs (their log /Cassoc values) have been shown to be proportional to the electrostatic function ZMzJd, where z Z/. are the charge of metal cation and ligand, and d rM + ri, the sum of their crystal radii (cf. Fig. 3.5). Mathematical models for predicting ion pair stabilities generally assume this proportionality and include the simple electrostatic model, the Bjerrum model, and the Fuoss model (cf. Langmuir 1979). Such models can predict stabilities in fair agreement with empirical data for monovalent and divalent cation ion pairs. 

Oxygen and H are not bound at the same sites in hemoglobin. Oxygen binds to the iron atoms of the hemes, whereas H binds to any of several amino acid residues in the protein. A major contribution to the Bohr effect is made by His (His HC3) of the 3 subunits. When protonated, this residue forms one of the ion pairs—to Asp (Asp FGl)—that helps stabilize deoxyhemoglobin in the T state (Fig. 5-9). The ion pair stabilizes the protonated form of His HC3, giving this residue an abnormally high piCa in the T state. The falls to its normal value of 6.0 in the R state because the ion pair cannot form, and this residue is largely unpro-... 

The rate ratio for substitution of phenoxy groups by methoxide ions in (69) and (70) has been shown to be very solvent dependent. Although a high rate ratio (k(69) k(70) = 1.8x10 ) was found in methylene chloride, the ratio was only 14 in methanol. The authors propose an ion-pair stabilized phosphorane-like structure (71) for the transition state or intermediate in methylene chloride, which has a favourable axial-equatorial placement of the five-membered ring, to account for the high ratio in this solvent, and an in-line Sn2 mechanism to apply in methanol, where ion-pairing is unimportant The different stereochemical expectations (retention in methylene chloride and inversion in methanol) could not be verified because phosphites isomerize under the reaction conditions. 

Krishtalik s attempt to include the field of the protein polar part in a spherical model of the protein [16] has not reproduced any catalytic effects. All the intraglobular field in chymotrypsin reported in Ref. [16] is extremely small except for the field from Asp 102, which is a part of the reactant system rather than a source for a field on this system. Hence the resultant large Asp"ImH ion-pair stabilization in a low dielectric medium is an artifact of neglecting the Born energy of transferring the ions from water to the hypothetical low dielectric protein [19]... 

The driving force for electron transfer may lie in the ion-pair stabilization that is gained in the solid matrix. Electron transfer strengthens M-M bonding, and a Coulombic ionic attraction between the product Mj CHgX" species would be present. 

The appearance of a new fluorescence band was also observed in the case of the macrocyclic receptors containing naphthalene and vaUne 50 (Fig. 15), as a consequence of an unusual intramolecular exciplex with the amino groups [102]. Binding of protected amino acids in dichloromethane led to a quenching of the exciplex band (390 nm) and enhancement of the monomer band (340 run). This process was found to be enantioselective and dependent on the size of the macrocycle. Measurement of the ratio between the monomer and exciplex emission allowed one to measure differences in fluorescence with a higher precision [103]. A detailed study based on ESl-MS and NMR allowed these authors to propose a proton transfer mechanism from the acidic moiety of the amino acid to the amino group of the receptor, followed by the formation of an intimate ion pair stabilized by aromatic-aromatic interactions. This process competes with the exciplex formation, thus leading to enhancement of the naphthalene emission [104]. 

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