Ion-induced dipole forces

For ion-molecule reactions where the interaction can be attributed to ion-induced dipole forces it has been shown (11) that the rate constant should be independent of ion energy—i.e., the thermal and 10.5 volt cm.-1 rate constants should be the same. The third column in Table II shows that for most of the reactions studied the ratio k (thermal)/ (10.5 volts/ cm.) is in the range 0.7-1.1. Considering the errors involved this is not significantly different from unity, indicating that most of the reactions... 

For each of the substances the possible answers are ionic bonding, covalent bonding, metallic bonding, hydrogen bonding, dipole-dipole force, or London force. Forces, such as ion-dipole forces and ion-induced dipole forces, are not choices because these require the presence of two or more substances. For example, sodium chloride cannot utilize either of these two forces, but sodium chloride in water can. (Sodium chloride in water exhibits ion-dipole forces.)... 

An ion-induced dipole force results when an ion in close proximity to a non-polar molecule distorts the electron density of the non-polar molecule. The molecule then becomes momentarily polarized, and the two species are attracted to each other. This force is active during every moment of your life, in the bonding between non-polar O2 molecules and the Fe " ion in hemoglobin. Ion-induced dipole forces, therefore, are part of the process that transports vital oxygen throughout your body. 

A dipole-induced dipole force is similar to that of an ion-induced dipole force. In this case, however, the charge on a polar molecule is responsible for inducing the charge on the non-polar molecule. Non-polar gases such as oxygen and nitrogen dissolve, sparingly, in water because of dipole-induced dipole forces. 

The insolubility of ionic compounds in nonpolar solvents is a similar phenomenon. The solvation energies are limited to those from ion-induced dipole forces, which are considerably weaker than ion-dipole forces and not large enough to overcome the very strong ion-ion forces of the lattice. 

The similarity in the adsorption behavior of krypton on the three kinds of mica surfaces suggests that the adsorption here is primarily due to dispersion forces, with very little contribution from ion-induced dipole forces. The results of Barrer and Stuart (1) for the adsorption of argon on various ion-exchanged forms of faujasite are similar. They found that while calcium, strontium, and lithium faujasite—i.e., the materials containing cations with greater polarizing power—did show heat effects correlatable with ion-induced dipole interactions, no such effects were observed with sodium, potassium, or barium zeolites. With the latter materials, they also concluded that the adsorbed argon possessed appreciable mobility. 

In contrast, in dipolar aprotic solvents, anion solvation occurs mainly by ion-dipole and ion-induced dipole forces. The latter are important for large, polarizable, soft anions, with low charge density, in soft dipolar aprotic solvents. Therefore, although these solvents tend to be poor anion solvators, they are usually better, the larger and softer the anion. This has the consequence that the reactivity of anions is exceptionally high in dipolar aprotic solvents, and the rate constants of Sn2 reactions can increase by several powers of ten when the solvent is changed from protic to dipolar aprotic cf. Section 5.4.2). 

Fig. 5-4. Schematic one-dimensional enthalpy diagram for the exothermic bimolecular Finkelstein reaction Cl -I- CFI3—Br Cl—CH3 -I- Br in the gas phase and in aqueous solution [469, 474, 476]. Ordinate standard molar enthalpies oi (a) the reactants, (b, d) loose ion-molecule clusters held together by ion-dipole and ion-induced dipole forces, (c) the activated complex, and (e) the products. Abscissa not defined, expresses only the sequence of (a). ..(e) as they occur in the chemical reaction.

Both involve the interaction of an ion with a dipole. In the first case, the dipole is permanent (preexisting), whereas in the second, it is induced by the approach of the ion. Induced dipole forces are weaker than ion-dipole forces. Examples Na with HCl (ion-dipole) Na with CI2 (induced dipole). 

Kuntz and Roach34 carried out a series of classical trajectory calculations on reactions of the type A+ + BC - AB+ + C. The key in this approach is the construction of a realistic potential function representing the reaction. Kuntz and Roach used a modification of a potential energy function previously reported by Polanyi and Kuntz.35 A LEPS function36 with spectroscopic parameters adapted to the isolectronic reaction C1(D2, D)DC1 is added to a potential which contains the long-range ion-induced dipole forces peculiar to the reaction Ar+(D2, D)ArD +... 

Ion-induced dipole forces are one of two types of charge-induced dipole forces, which rely on the polarizability of the components. They result when an ion s charge distorts the electron cloud of a nearby nonpolar molecule. This type of force plays an essential biological role that initiates the binding of the Fe " " ion in hemoglobin and an O2 molecule in the bloodstream. Because an ion increases the magnitude of any nearby dipole, ion-induced dipole forces also contribute to the solubility of salts in less polar solvents, such as LiCl in ethanol. 

Liquid-Liquid and Solid-Liquid Solutions Many salts dissolve in water because the strong ion-dipole attractions that water molecules form with the ions are very similar to the strong attractions between the ions themselves and, therefore, can substitute for them. The same salts are insoluble in hexane (CgH ) because the weak ion-induced dipole forces their ions could form with the nonpolar molecules of this solvent cannot substitute for attractions between the ions. Similarly, oil does not dissolve in water because the weak dipole-induced dipole forces between oil and water molecules cannot substitute for the strong H bonds between water molecules. Oil does dissolve in hexane, however, because the dispersion forces in one substitute readily for the dispersion forces in the other. Thus, for a solution to form, like dissolves like means that the forces created between solute and solvent must be comparable in strength to the forces destroyed within both the solute and the solvent. 

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