Cation-anion forces

From these data it is concluded that the cation is vibrating with respect to a solvent cage for DMSO-t/e and, for solvents of lower dielectric constant in which cation-anion forces are stronger, the anion is a member of the cage thereby achieving intimate contact. A distinction is drawn between this type of ion pair and a diatomic species M A in a solvent cavity. 

Ionic crystals can be cleaved in certain directions. Fig. 19.1 shows why the exertion of a force results in cleavage if two parts of a crystal experience a mutual displacement by a shearing force, ions of like charges come to lie side by side and repel each other. The displacement is easiest along planes which have the fewest cation-anion contacts. In... 

The proportion of the /rans-O-alkylated product [101] increases in the order no ligand < 18-crown-6 < [2.2.2]-cryptand. This difference was attributed to the fact that the enolate anion in a crown-ether complex is still capable of interacting with the cation, which stabilizes conformation [96]. For the cryptate, however, cation-anion interactions are less likely and electrostatic repulsion will force the anion to adopt conformation [99], which is the same as that of the free anion in DMSO. This explanation was substantiated by the fact that the anion was found to have structure [96] in the solid state of the potassium acetoacetate complex of 18-crown-6 (Cambillau et al., 1978). Using 23Na NMR, Cornelis et al. (1978) have recently concluded that the active nucleophilic species is the ion pair formed between 18-crown-6 and sodium ethyl acetoacetate, in which Na+ is co-ordinated to both the anion and the ligand. 

To test the generality of their findings, the authors studied the force measurements using polymers of different molecular weights, and different ionic surfactants (cationic, anionic). In all those cases, they observed the same phenomena [74,75]. 

Crystals with the Rutile and the.Fluorite Structures Interionic Distances for Substances of Unsymmetrical Valence Type.—In a crystal of a substance of unsymmetrical valence type, such as fluorite, CaFs (Fig. 13-10), the equilibrium cation-anion interionic distance cannot be expected necessarily to be given by the sum of the crystal radii of the bivalent calcium ion and the univalent fluoride ion. The sum of the univalent radii of calcium and fluoride, 2.54 A, would give the equilibrium interionic distance in a hypothetical crystal with attractive and repulsive forces corresponding to the sodium chloride arrangement. 

The stability of a covalently attached catalyst will be significantly greater than a catalyst bound to the polymer film via electrostatic forces. This is supported by experiments with ion exchange films, where the electrostatistically bound electroactive species can reversible exchange with the cation/anion in the contacting solutions [11-13]. This behavior does not occur with a covalently-linked species. 

Ion pair interactions are also present in natural supramolecular systems (e.g. DNA-peptides, polyamine-nucleic acids and among proteins or enzymes). Such kinds of attractive forces are strongly dependent on the ionic strength (I) of the solution if extrapolated to I = 0 the average value for the free energy of association for one cation-anion pair is about 8 KJ/mol, and... 

Figure 2. Cation-cation and cation-anion radial distribution functions for molten NaCl, obtained from simulation (force field given in Ref. [285]). Charge ordering is clearly visible, as described in the text.

---