London dispersion forces continued

The electron density changes continually, so induced dipoles never last more than about 10-11 s. Nevertheless, they last sufficiently long for an interaction to form with the induced dipole of another nitrogen molecule nearby. We call this new interaction the London dispersion force after Fritz London, who first postulated their existence in 1930. 

Relatively weak forces of attraction that exist between nonpolar molecules are called van der Waals forces or London dispersion forces. Dispersion forces between molecules are much weaker than the covalent bonds within molecules. Electrons move continuously within bonds and molecules, so at any time one side of the molecule can have more electron density than the other side, which gives rise to a temporary dipole. Because the dipoles in the molecules are induced, the interactions between the molecules are also called induced dipole-induced dipole interactions. 

Also, water is a polar solvent, with its molecules forming hydrogen bonds. When mixed with non-polar iodine molecules, nearly all of the water molecules continue to hydrogen bond with each other. Thus, the resulting iodine-water attractions are extremely weak in comparison to the combined strength of the hydrogen bonds in water and the London (dispersion) forces in iodine. Consequently, iodine is virtually insoluble in water. 

Even noble-gas atoms and molecules that are nonpolar experience a weak intermolecular attraction. In any atom or molecule—polar or nonpolar—the electrons are in continuous motion. As a result, at any instant, the electron distribution may be slightly uneven. The momentary, uneven charge creates a positive pole in one part of the atom or molecule and a negative pole in another. This temporary dipole can then induce a dipole in an adjacent atom or molecule. The two are held together for an instant by the weak attraction between the temporary dipoles, as illustrated in Figure 5.13. The intermolecular attractions resulting from the constant motion of electrons and the creation of instantaneous dipoles are called London dispersion forces, after Fritz London, who first proposed their existence in 1930. 

London dispersion forces, and/or similar interactions is not a simple task. Chemical bonding, whether noncovalent, covalent, ionic, or metallic, covers a broad, continuous spectrum of electronic interactions and energies. Consequently, the classification of a bond or interaction (e.g., double versus triple or covalent versus noncovalent ) is sometimes open to interpretation. As a result, there is no unique criterion or set of criteria that can be used to define weak interactions or noncovalent interactions. In the second volume of this review series, Scheiner already notes this issue and highlighted the difficulties associated with defining the hydrogen bond. Here, matters are even more complicated because other weak interactions are also considered. 

The Hamaker approach of pairwise addition of London dispersion forces is approximate because multi-body intermolecular interactions are neglected. In addition, it is implicitly assumed in the London equation that induced dipole-induced dipole interactions are not retarded by the finite time taken for one dipole to reorient in response to instantaneous fluctuations in the other. Because of these approximations an alternative approach was introduced by Lifshitz. This method assumes that the interacting particles and the dispersion medium are all continuous i.e. it is not a molecular theory. The theory involves quantum mechanical calculations of the dielectric permittivity of the continuous media. These calculations are complex, and are not detailed further here. 

Whereas many scientists shared Mulliken s initial skepticism regarding the practical role of theory in solving problems in chemistry and physics, the work of London (6) on dispersion forces in 1930 and Hbckel s 7t-electron theory in 1931 (7) continued to attract the interest of many, including a young scientist named Frank Westheimer who, drawing on the physics of internal motions as detailed by Pitzer (8), first applied the basic concepts of what is now called molecular mechanics to compute the rates of the racemization of ortho-dibromobiphenyls. The 1946 publication (9) of these results would lay the foundation for Westheimer s own systematic conformational analysis studies (10) as well as for many others, eg, Hendrickson s (11) and Allinger s (12). These scientists would utilize basic Newtonian mechanics coupled with concepts from spectroscopy (13,14) to develop nonquantum mechanical models of structures, energies, and reactivity. 

The attraction forces which act most frequently in physical adsorption are the nonpolar van der Waals forces. Since London (23) described the close connection between their nature and the cause of optical dispersion, they may also be called dispersion forces. The main contribution to the nonpolar van der Waals forces arises from the interaction of continually changing inducing dipoles and induced dipoles. The interaction energy of a pair of atoms due to this contribution is inversely proportional to the sixth power of the distance ... 

Figure 28-9 When two atoms i and j are separated by infinite distance, there are no interactions between them. As two nonbonded atoms approach one another, two forces have to be considered. Attractive dispersion forces (London forces) result from the interaction of instantaneous dipoles on each atom i and j. As the nonbonded atoms continue to approach one another, a repulsive interaction overwhelms the attractive interaction. and the energy curve rises sharply. The two nonbonded atoms can reach an equilibrium position where repulsive and attractive forces balance. Different mathematical relationships have been used in force field calculations to reproduce the nonbonded steric interactions.

If we continue the quantum mechanical calculations which lead to the homopolar primary valence, we arrive, as a second approximaSon, at a further force effect between structures already saturated with respect to primary valence. From their magnitude, the distance between the interacting particles and also from the dependence of potential upon distance, these forces must be ascribed to secondary valences. Since they are related to the scattering of different wave lengths by the molecule, i.e. to the effect of frequency upon polarizability (dispersion), they are frequently called dispersion forces. They are obtained, according to London, and Slater, from the two equations... 

It will become evident in later sections that the nature of the weak noncovalent interactions in a cluster dictate which computational methods will produce accurate results. In particular, it is far more difficult to compute reliable properties for weakly bound clusters in which dispersion is the dominant attractive component of the interaction. For example, Hartree-Fock supermolecule computations are able to provide qualitatively correct data for hydrogen-bonded systems like (Fi20)2 even with very small basis sets, but this approach does not even bind Ne2- What is the origin of this inconsistency Dispersion is the dominant attractive force in rare gas clusters while the electrostatic component tends to be the most important attractive contribution near the equilibrium structure (H20)2- As London s work demonstrated,dispersion interactions are inherently an electron correlation problem and, consequently, cannot be described by Flartree-Fock computations. To this day, dispersion interactions continue to pose a significant challenge in the field of computational chemistry, particularly those involving systems of delocalized n electrons." ... 

Stabilization of colloidal dispersions can be divided into the two basic mechanisms electrostatic and steric (Fig. 4) [57]. With the van der Waals-London attractive forces acting continuously between colloidal particles, it is necessary, in order to maintain stabiUly, to introduce a repulsive force (electrostatic and steric) to outweigh the attractive force. The electrostatic stabilization provides the repulsive forces between similarly charged electrical double layers to the interactive particles [58, 59] (Fig. 4). Thus, the electrical double layer imparts the electrostatic stabilization. The steric stabilization becomes important when there are hydrophilic macromolecules or chains adsorbed or bounded to the particle surface [60]. When the layers of two interacting particles overlap the concentrahon of these macromolecules (chains) increases as weh as free energy. The molecules of good solvent enter the overlap layer and then separate the particles. This phenomenon is accompanied with the increased osmohe pressure. 

The same logic that we used to obtain the Girifalco-Good-Fowkes equation in Section 6.10 suggests that the dispersion component of the surface tension yd may be better to use than 7 itself when additional interactions besides London forces operate between the molecules. Also, it has been suggested that intermolecular spacing should be explicitly considered within the bulk phases, especially when the interaction at d = d0 is evaluated. The Hamaker approach, after all, treats matter as continuous, and at small separations the graininess of matter can make a difference in the attraction. The latter has been incorporated into one model, which results in the expression... 

Colloids in which the continuous phase is water are divided into two major classes hydrophilic colloids and hydrophobic colloids. A hydrophilic colloid is a colloid in which there is a strong attraction between the dispersed phase and the continuous phase (water). Many such colloids consist of macromolecules (very large molecules) dispersed in water. Except for the large size of the dispersed molecules, these colloids are like normal solutions. Protein solutions, such as gelatin in water, are hydrophilic colloids. Gelatin molecules are attracted to water molecules by London forces and hydrogen bonding. 

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