London dispersion forces discussion

The discussion thus far has focused on the forces between an array of atoms connected together through covalent bonds and their angles. Important interactions occur between atoms not directly bonded together. The theoretical explanation for attractive and repulsive forces for nonbonded atoms i and j is based on electron distributions. The motion of electrons about a nucleus creates instantaneous dipoles. The instantaneous dipoles on atom i induce dipoles of opposite polarity on atom j. The interactions between the instantaneous dipole on atom i with the induced instantaneous dipole on atom j of the two electron clouds of nonbonded atoms are responsible for attractive interactions. The attractive interactions are know as London Dispersion forces,70 which are related to r 6, where r is the distance between nonbonded atoms i and j. As the two electron clouds of nonbonded atoms i and j approach one another, they start to overlap. There is a point where electron-electron and nuclear-nuclear repulsion of like charges overwhelms the London Dispersion forces.33 The repulsive... 

Mahanty, J., and Ninham, B. W., Dispersion Forces, Academic Press, New York, 1976. (An advanced monograph on dispersion forces. Discusses topics such as London and Lifshitz theories.)... 

These are mainly the London dispersion forces, the nature of which we shall not discuss. See F. London, Z. Phyeik 63, 245 (1930) Also Introduction to Quantum Mechanics. 

However, we will later discuss Eq. (9) again for the determination of London dispersive force of the surface dynamics in the gas chromatographic study. 

The overall shape of the protein, long and narrow or globular, is called its tertiary structure and is maintained by several different types of interactions hydrogen bonding, dipole-dipole interactions, ionic bonds, covalent bonds, and London dispersion forces between nonpolar groups. These bonds, which represent all the bonding types discussed in this text, are summarized in Fig. 22.25. 

In the preceding section we discussed the properties of zinc(II) as an ion. These properties are, of course, important in understanding its role in biological catalysis, but it would be too simplistic to believe that reactivity can be understood solely on this basis. Catalysis occurs in cavities whose surfaces are constituted by protein residues. Catalytic zinc is bound to a water molecule, which often is H-bonded to other residues in the cavity and/or to other water molecules. The structure of the water molecules in the cavity cannot be the same as the structure of bulk water. Furthermore, the substrate interacts with the cavity residues through either hydrophilic (H-bonds or electric charges) or hydrophobic (London dispersive forces) interactions. As a result, the overall thermodynamics of the reaction pathway is quite different from that expected in bulk solutions. Examples of the importance of the above interactions will be given in this chapter. 

Debye and Keesom forces together with London Dispersion forces are known coiiec-tively as van der Waals forces. See Lifshitz-van der Waals forces for a further discussion. They play a significant role in the Adsorption theory of adhesion and in surface phenomena such as Contact angles and interfacial tension. 

The phenomenon of stacking, 1. e. the close and planar association of polar ring molecules possessing aromatic properties, has been well established as an Important affinity contribution In the Interaction within or between various blomolecules. The discussion of the origin of this Interaction, however. Is not completed and there Is no general accord which of the known basic physical contributions Is of most Importance. Some people emphasize the hydrophobic origin while others emphasize London dispersion forces or charge transfer Interactions. 

The temperature and pressure dependence of the F chemical shifts in gaseous CF , SiFf, and SF , and in their mixtures with other gases, have been studied. The chemical shifts, which show a linear dependence on density and non-linear dependence on temperature, were discussed in terms of the London dispersion held and a repulsion held. The change from the gas phase to inhnite dilution in solution in a series of non-polar solvents for the compounds CF4, SiF , SF, CeF, p-FC H Me and l,4-CeH4F2 results in a downheld shift of some 3—16 p.p.m., London dispersion forces being apparently the principal cause. For hexafluorobenzene there is a regular increase in the downheld shift as the solvent changes from methylene chloride to chloroform to carbon tetrachloride of 7.75 to 8.67 to 9.09 p.p.m., in line with the increase in molecular polarizability. ... 

The derivation of the potential energy for London dispersion forces is quite involved, and usually such interactions are not quantitatively modeled by equations of the sort we have been presenting here. Typically, the empirically derived Lennard-Jones "6-12" potential discussed in Chapter 2 or a related function is used. To a first approximation, as with the dipole-induced-dipole, the energy of interaction can be considered to drop off with an dependence. 

The origins of these variable densities of adsorbates are with the adsorption potentials which exist with microporosity, as illustrated in Figure 4.2 and which will be discussed further under Section 4.6.2 (London dispersion forces). 

The stability/instability of any agrochemical dispersion is determined by the balance of three main forces (i) Van der Waals attraction that is universal for all disperse systems and which results mainly from the London dispersion forces between the peu--ticles or droplets, (ii) Double layer repulsion that arises when using ionic surfactants or polyelectrolytes, (iii) Steric repulsion that arises when using adsorbed nonionic surfactants or polymers. A description of these three interaction forces is first given and this is followed by a combination of these forces and discussion of the theories of colloid stability. The latter can account for the stability/instability of the various dispersions. 

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