Potassium-39, ions, solvation

The solid-state structure of 72 (Figure 38) consists of a polymer of alternating THF-solvated potassium cations and diamidomethylzincate anions. The alternating zincate ions, which are rotated by about 180° with respect to each other, sandwich the potassium ions. Each potassium ion is -coordinated to one methyldiamidozincate and -coordinated to the other. The zinc-methyl and zinc-nitrogen bonds of the zincate ion are 1.954(5) and 1.989(8) A, respectively. 

The use of alkali and alkaline earth group metal ions, especially those of sodium, potassium, magnesium, and calcium, for maintenance of electrolyte balance and for signaling and promotion of enzyme activity and protein function are not discussed in this text. Many of these ions, used for signaling purposes in the exciting area of neuroscience, are of great interest. In ribozymes, RNAs with catalytic activity, solvated magnesium ions stabilize complex secondary and tertiary molecular structure. Telomeres, sequences of DNA at the ends of chromosomes that are implicated in cell death or immortalization, require potassium ions for structural stabilization. 

The effects of DN on the solvation energy of the potassium ion and on the standard potential of the hydrogen electrode, which is linearly related to the solvation energy of the hydrogen ion, are shown in Fig. 2.3. Near-linear relations can be observed in both cases [13]. There is also a linear relationship between AN and the solvation energies of the chloride ion in aprotic solvents, as in Fig. 2.4 [13]. However, the chloride ion in protic solvents like water and alcohols behaves somewhat differently than in aprotic solvents [14], probably because of the influence of hydrogen bonding (see below). 

Sodium or potassium ions can also participate in the phase-transfer process when they are converted to lipophilic cations by complexation or by strong specific solvation. A variety of neutral organic compounds are able to form reasonably stable complexes with K+ or Na + and can act as catalysts in typical phase-transfer processes. Such compounds include monocyclic polyethers, or crown ethers (1), and bicyclic aminopolyethers (cryptates) (2). They can solubilize inorganic salts in nonpolar solvents and are particularly recommended for reactions of naked anions. Applications of these compounds have been studied.12,21-31... 

Solvation in non-aqueous solvents has been described as comparable to that of the potassium ion (ionic radii, 126pm Ag+, 133pm K+).220... 

The alkali metals share many common features, yet differences in size, atomic number, ionization potential, and solvation energy leads to each element maintaining individual chemical characteristics. Among K, Na, and Li compounds, potassium compounds are more ionic and more nucleophilic. Potassium ions form loose or solvent-separated ion pairs with counteranions in polar solvents. Large potassium cations tend to stabilize delocalized (soft) anions in transition states. In contrast, lithium compounds are more covalent, more soluble in nonpolar solvents, usually existing as aggregates (tetramers and hexamers) in the form of tight ion pairs. Small lithium cations stabilize localized (hard) counteranions (see Lithium and lithium compounds). Sodium chemistry is intermediate between that of potassium and lithium (see Sodium and sodium alloys). 

The following example shows how fluoride ion, normally a poor nucleophile in hydroxylic (protic) solvents, can be a good nucleophile in an aprotic solvent. Although KF is not very soluble in acetonitrile, 18-crown-6 solvates the potassium ions, and the poorly solvated (and therefore nucleophilic) fluoride ion follows. 

The importance of crown ethers derives from their extraordinary abil ity to solvate metal cations by sequestering the metal in the center of the polyother cavity. For example, l [Pg.723]

Figure 13.23. Path Through a Channel. A potassium ion entering the potassium channel can pass a distance of 22 A into the membrane while remaining solvated with water (blue). At this point, the pore diameter narrows to 3 A (yellow), and potassium must shed its water and interact with carbonyl groups (red) of the pore amino acids.

The reaction of 2,6-Mes2C6H3Li and BBrs produces the 2,6-Mes2CeH3BBr2 derivative in good yield. Reduction of the latter with KCg affords potassium 9-borafluorenyl salts. The product isolated from reduction with four equivalents of KCg in diethyl ether, crystallizes from THF/ hexane as a centrosymmetric dimer in which two THF molecules solvate each potassium ion and also interact with the 9-H and 9-Me groups of the 9-borafluorenyl rings. The product from the reduction of the arylboron dibromide with excess KCg in benzene also forms a centrosymmetric dimer each potassium ion is 7 -coordinated by benzene in addition to the solvation by 9-H and 9-Me groups from the 9-borafluorenyl rings. 

Hydrochloric acid is secreted by parietal cells via H, K -ATPase pumps (proton pumps), of which there are more than one million per cell. The H, K -ATPase pumps utilize the phosphorylation of ATP to exchange water-solvated protons (protonated water, hydroxonium ion, H30 ) for potassium ions. In conjunction with parallel potassium and chloride ion conductances, this ATPase is responsible for the secretion of hydrochloric acid into the secretory canaliculus of the parietal cell, the enclosed space reaching a pH of near 1.0 (Rabon Reuben 1990). In the resting parietal cell, these pumps reside within the membranes of vesicles in the cell cytoplasm. When activated by histamine and gastrin, the parietal cells alter their shape and the vesicles merge with the outer cell membrane to form secretory canaliculi. 

Singh and coworkers examined the effect of polarization energy on computed relative solvation free energies of ions as well. The calculated solvation free energies with and without polarization and experimental values for the mutations of ammonium ion to tetramethyl ammonium ion, chloride ion to bromide ion and sodium ion to potassium ion were -23.56 kcal/mol, -29.82 kcal/mol, -31.70 kcal/mol, -1.95 kcal/mol, -3.23 kcal/mol, -3.33 kcal/mol, -24.66 kcal/mol, -20.09 kcal/mol, and -17.05 kcal/mol, respectively. Therefore, polarization energy added a constant positive value to the free energy change for all the transformations except for the mutation of sodium ion to potassium ion. 

The slopes of these lines provide partial molar enthalpies of solution of 97.5 4.7 and 89.2 2.6 kJ mol1 for NaBr and Nal, respectively, where the thermodynamic reference state is the solid halide. The solvation enthalpies derived from these values are —265.6 9.9 and —225.0 7.9 kJ mol-1 for bromide and iodide ion, respectively. The salts are considered to dissolve in the metal as the dissociated ions, solvated by liquid metal, and the solutions show large deviations from ideal but small deviations from regular behaviour.16 The solubilities of potassium chloride in liquid potassium and in... 

Non-aqueous Solvation.—Structural radii and electron-cloud radii, together with lattice enthalpies and enthalpies of solvation of ionic crystals, have been reviewed.84 The free energies of transfer, AGtr(K+), of potassium ions from water to 14 non-aqueous solvents have been reported, and they were derived from measurements in an electrochemical cell assumed to have a negligible liquid-junction potential. The essentially electrostatic nature of its solvation allows K+ to be used as a model for non-specific solvent-ion interactions. A... 

Potassium ions are also present in this solution, but because of the high dielectric constant and high solvating capability of liquid ammonia the propagating species formed here is truly a free ion, not more than very loosely associated with the cation. The less polar the solvent used, the greater would be the degree of association between the propagating anion and the cation present. Thus, the polarity of the solvent chosen can affect the rate of formation and sometimes also the stereochemistry of the anionic polymerization product. 

The detailed understanding of such a system also allows the assessment of the positions to be attacked, most likely by electrophiles. Moreover, the degree of the system s diatropicity may also be dependent on ion-solvation equilibrium. In this case it has already been shown that the system is not diatropic even if its total number of electrons may predict diatropicity 85). Mullen studied independently a family of anions including the dianion of acenaphthylene (82 ) and he also concludes that in THF solutions the lithium and potassium salts exist predominantly as contact ion pairs 83b). 

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