Dipole moments London forces

As well as these permanent dipole moments, random motion of electron density in a molecule leads to a tiny, instantaneous dipole, which can also induce an opposing dipole in neighbouring molecules. This leads to weak intermolecu-lar attractions which are known as dispersive forces or London forces, and are present in all molecules, ions and atoms - even those with no permanent dipole moment. Dispersive forces decrease rapidly with distance, and the attractions are in proportion to 1/r6, where r is the distance between attracting species. 

Even if molecules do not possess a permanent dipole moment weak forces can exist between them. The movement of the valence electrons creates transient dipoles , and these in turn induce dipole moments in adjacent molecules. The transient dipole in one molecule can be attracted to the transient dipole in a neighbouring molecule, and the result is a weak, short-range attractive force known as the London dispersion force. 

As argued above, this result is found to work best for substances in which both the 1,1 and 2,2 forces are either London or dipole-dipole. Even the case of one molecule with a permanent dipole moment interacting with a molecule which has only polarizability and no permanent dipole moment-such species interact by permanent dipole-induced dipole attraction-is not satisfactorily approximated by Eq. (8.46). In this context the like dissolves like rule means like with respect to the origin of intermolecular forces. 

The dispersion (London) force is a quantum mechanieal phenomenon. At any instant the electronic distribution in molecule 1 may result in an instantaneous dipole moment, even if 1 is a spherieal nonpolar moleeule. This instantaneous dipole induces a moment in 2, which interacts with the moment in 1. For nonpolar spheres the induced dipole-induced dipole dispersion energy function is... 

In many electron atoms the maximum contributions to the polarizability and to London forces arise from configurations with more than one electron contributing to the net dipole moment of the atom. But in such configurations the electronic repulsion is especially high. The physical meaning to be attributed to the Qkl terms is just the additional electron repulsive energy which these configurations require. 

SOLUTION The data in Fig. 2.12 show that electronegativity differences decrease from 1 HC1 to HI, and so the dipole moments decrease as well. Therefore, dipole-dipole forces decrease, too, a trend suggesting that the boiling points should decrease from HQ to HI. This prediction conflicts with the data so we examine the London forces. The number of electrons in a molecule increases from HQ to HI, and so the strength of the London interaction increases, too. Therefore, the boiling points should increase from HCl to HI, in accord with the data. This analysis suggests that London forces dominate dipole-dipole interactions for these molecules. 

London-van der Waals forces generally are multipole (dipole-dipole or dipole-induced dipole) interactions produced by a correlation between fluctuating induced multipole (principal dipole) moments in two nearly uncharged polar molecules. Even though the time-averaged, induced multipole in each molecule is zero, the correlation between the two induced moments does not average to zero. As a result an attractive interaction between the two is produced at very small molecular distances. 

It is well known that neutral molecules such as alkanes attract one another, mainly through van der Waals forces. Van der Waals forces arise from the rapidly fluctuating dipoles moment (1015 S-1) of a neutral atom, which leads to polarization and consequently to attraction. This is also called the London potential between two atoms in a vacuum, and is given as... 

London dispersion or induced dipoie forces Any atom or molecule can become polarized when placed in an electrical field E, and the induced dipole moment is proportional to its polarizability factor a by... 

Two later sections (1.6.5 and 1.6.6) look at the crystalline structures of covalently bonded species. First, extended covalent arrays are investigated, such as the structure of diamond—one of the forms of elemental carbon—where each atom forms strong covalent bonds to the surrounding atoms, forming an infinite three-dimensional network of localized bonds throughout the crystal. Second, we look at molecular crystals, which are formed from small, individual, covalently-bonded molecules. These molecules are held together in the crystal by weak forces known collectively as van der Waals forces. These forces arise due to interactions between dipole moments in the molecules. Molecules that possess a permanent dipole can interact with one another (dipole-dipole interaction) and with ions (charge-dipole interaction). Molecules that do not possess a dipole also interact with each other because transient dipoles arise due to the movement of electrons, and these in turn induce dipoles in adjacent molecules. The net result is a weak attractive force known as the London dispersion force, which falls off very quickly with distance. 

A second type of force between water molecules and the metal consists of the dispersion forces. Dispersion forces (or London forces) can be seen classically as follows A time-averaged picture of any atom shows spherical symmetry because the charge due to the electrons orbiting around the nucleus is smoothed out in time. An instantaneous picture of, say, a hydrogen atom, would, however, show a proton here and an electron there—two charges separated by a distance. Hence, every atom has an instantaneous dipole moment of course, the time average of all these dipole moments is zero. This instantaneous dipole will induce an instantaneous dipole in a contiguous atom, and an instantaneous dipole-dipole force will arise. When these... 

One of the more profound manifestations of quantum mechanics is that this curve does not accurately describe reality. Instead, because the motions of electrons are correlated (more properly, the electronic wave functions are correlated), the two atoms simultaneously develop electrical moments that are oriented so as to be mutually attractive. The force associated with tills interaction is referred to variously as dispersion , the London force, or the attractive van der Waals force. In the absence of a permanent charge, the strongest such interaction is a dipole-dipole interaction, usually referred to as an induced dipole-induced dipole interaction, since the moments in question are not permanent. Such an interaction has an inverse sixtli power dependence on the distance between the two atoms. Thus, the potential energy becomes increasingly negative as the two noble gas atoms approach one another from infinity. 

If the solute molecule has a dipole moment, it is expected to differ in various electronic energy states because of the differences in charge distribution. If the solvent is nonpolar, then the rough description of the interaction is dipole-induced dipole type. In polar solvents, dipole-dipole interactions also become important. The London forces are always present. For the calculation of dipole-dipole interaction energy, point dipole approximations are made which are poor description for large extended molecules. 

The London force is always attractive. It holds the molecules of hydrocarbons together, so that gasoline is a liquid at normal temperatures. It holds 12 molecules together as a solid at ordinary temperatures and N2 molecules as a solid at very low temperatures. Even noble gas atoms can have instantaneous dipole moments and so condense to a liquid. 

Intermolecular forces, known collectively as van der Waals forces, are the attractions responsible for holding particles together in the liquid and solid phases. There are several kinds of intermolecular forces, all of which arise from electrical attractions Dipole-dipole forces occur between two polar molecules. London dispersion forces are characteristic of all molecules and result from the presence of temporary dipole moments caused by momentarily unsymmetrical electron distributions. A hydrogen bond is the attraction between a positively polarized hydrogen atom bonded to O, N, or F and a lone pair of electrons on an O, N, or F atom of another molecule. In addition, ion-dipole forces occur between an ion and a polar molecule. 

LONDON FORCES (dispersion) attraction due to induced dipole moments increases with a... 

There are two types of solute-solvent interactions which affect absorption and emission spectra. These are universal interaction and specific interaction. The universal interaction is due to the collective influence of the solvent as a dielectric medium and depends on the dielectric constant D and the refractive index n of the solvent. Thus large environmental perturbations may be caused by van der Waals dipolar or ionic fields in solution, liquids and in solids. The van der Waals interactions include (i) London dispersion force, (ii) induced dipole interactions, and (iii) dipole-dipole interactions. These are attractive interactions. The repulsive interactions are primarily derived from exchange forces (non bonded repulsion) as the elctrons of one molecule approach the filled orbitals of the neighbour. If the solute molecule has a dipole moment, it is expected to differ in various electronic energy states because of the differences in charge distribution. In polar solvents dipole-dipole inrteractions are important. 

---