Charge-induced dipole forces

Induction of electric charge occurs when a charge on one object causes a change in the distribution of charge on a nearby object. Rubbing a balloon to make it "stick" to a wall is an example of charging by induction. There are two types of charge-induced dipole forces. 

This charge-induced dipole force is the most fundamental interaction between a charged particle and a neutral species with no permanent dipole moment of its own. It is known as the Langevin interaction. It is a l/r4-dependent force through which neutral and charged species can interact. There is, however, another way in which an interaction of this kind can occur. In the example just given, the polarisation of the neutral species can only be induced, because Hj has no permanent dipole moment. More complex molecules can, however, possess permanent dipole moments, which leads to a different situation. 

The Hydrogen Bond Charge-Induced Dipole Forces Dispersion (London) Forces... 

Ion-induced dipole and dipole-induced dipole forces are the two types of charge-induced dipole forces they are most important in solution, so we ll focus on them in Chapter 13. Nevertheless, polarizability affects all intermolecular forces. 

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. 

In the case of ions, the attractive electrostatic potential almost always dominates vibrational quenching at thermal energies, because of the charge-induced dipole force for nonpolar molecules and the even stronger charge-dipole interaction for polar molecules. The case of the above-mentioned N2(n)-He system is an exception, as = 0.017 eV is so small that the minimum of k falls below room temperature, and thus only the increase due to repulsive forces is observed in experiments at elevated energies. 

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 potential of mean force due to the solvent structure around the reactants and equilibrium electrolyte screening can also be included (Chap. 2). Chapter 9, Sect. 4 details the theory of (dynamic) hydro-dynamic repulsion and its application to dilute electrolyte solutions. Not only can coulomb interactions be considered, but also the multipolar interactions, charge-dipole and charge-induced dipole, but these are reserved until Chap. 6—8, and in Chaps. 6 and 7 the problems of germinate radical or ion pair recombination (of species formed by photolysis or high-energy radiolysis) are considered. 

To show this connection, consider an ion-pair as above (Sect. 2.1). Not only may the ion-pair diffuse and drift in the presence of an electric field arising from the mutual coulomb interaction, but also charge-dipole, charge-induced dipole, potential of mean force and an external electric field may all be included in the potential energy term, U. Both the diffusion coefficient and drift mobility may be position-dependent and a long-range transfer process, Z(r), may lead to recombination of the ion-pair. Equation (141) for the ion-pair density distribution becomes... 

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). 

From the theoretical side, there has been little achieved in quantitative predictions of the effect of impurity molecules on crystal nucleation. One exception is the work of Cole and Sluckin who considered the nucleation of freezing in an atomic liquid by a charged particle through the mechanism of electrostriction, in which the pressure is increased in the vicinity of an ion due to induced dipole forces, leading to a reduction of the barrier to nucleation. It is clear that much remains to be done in this field. 

Induced dipole forces arise when an ion induces a dipole in a nonpolar atom or molecule and is then attracted to the opposite charge induced. 

Van der Waals forces represent important intermolecular interactions between nonelectrolyte substances, and can be categorized into dipole-dipole, dipole-induced-dipole, and induced-dipole-induced-dipole forces. Polar molecules, by definition, will have a permanent dipole moment, and will interact with the oppositely charged portions or other molecules having permanent dipole moments. The dipole-dipole interaction is known as the orientation effect, or as the Keesom force. 

First of all, we must be aware of the fact that all physical forces acting between molecules are essentially electrostatic (Coulomb forces between charges and dipoles induced dipole forces) or quantum mechanical (dispersion, and repulsive forces) in origin apart from a minor contribution due to mass attraction gravitational forces. Gravitational forces... 

The van der Waals radius determines the shortest distance over which intermolecu-iar forces operate it is aiways larger than the covalent radius. Intermolecular forces are much weaker than bonding (intramolecular) forces. Ion-dipole forces occur between ions and poiar molecules. Dipole-dipole forces occur between oppositely charged poles on polar molecules. Hydrogen bonding, a special type of dipole-dipole force, occurs when H bonded to N, O, or F is attracted to the lone pair of N, O, or F in another molecule. Electron clouds can be distorted (polarized) in an electric field. Dispersion (London) forces are instantaneous dipole-induced dipole forces that occur among all particles and increase with number of electrons (molar mass). Molecular shape determines the extent of contact between molecules and can be a factor in the strength of dispersion forces. 

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