Carbon tetrachloride London dispersion force

In nonpolar molecules such as carbon tetrachloride, the principal attractive force is the London dispersion force, one of the van der Waals forces (Figure 2-24). The London force arises from temporary dipole moments that are induced in a molecule by other nearby molecules. Even though carbon tetrachloride has no permanent dipole moment, the electrons are not always evenly distributed. A small temporary dipole moment is induced when one molecule approaches another molecule in which the electrons are slightly displaced from a symmetrical arrangement. The electrons in the approaching molecule are displaced slightly so that an attractive dipole-dipole interaction results. 

These are the London dispersion forces of attraction, or London forces, proposed in 1928 by Fritz London. Every atom, ion, or molecule can engage in London forces, as long as it has at least one electron. London forces are the only intermolecular forces possible in nonpolar substances. London forces are significant only when molecules are veiy close together, essentially touching. They are the forces that are responsible for carbon tetrachloride being a liquid at room temperature. The magnitude of London forces increases as the molecular masses of molecules increase. 

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. ... 

Let us first deal with the dispersion (London) interaction. This interaction is of a non-polar nature, in a non-polar liquid such as carbon tetrachloride, London s dispersion interaction is the only force present between two molecules. These non-polar molecules do not possess any permanent dipole moment. The interaction is a resultant of Instantaneous dipoles formed between the nuclei and electrons at zero-point motion of the molecule. Dispersion forces are weak. When two non-polar molecules of the same type approach each other closely enou for their electronic orbitals to overlap, the weak attraction changes to repulsion. Thus, non-polar molecules exist in a state of random distribution to give a disordered array. Another non-polar molecule (whether a solute or a solvent) will mix in all proportions since neither kind of the molecule has any attraction between them. From the foregoing, it is easy to understand that a non-pK)lar solute molecule will interact more with the phase which is non-polar this solute molecule will move fast if the non-polar phase is the mobile phase or will be retarded more and move slowly if the non-p>olar phase is the stationary phase. 

These temporary dipoles last only a fraction of a second, and they constantly change yet they are correlated so their net force is attractive. This attractive force depends on close surface contact of two molecules, so it is roughly proportional to the molecular surface area. Carbon tetrachloride has a larger surface area than chloroform (a chlorine atom is much larger than a hydrogen atom), so the intermolecular London dispersion attractions between carbon tetrachloride molecules are stronger than they are between chloroform molecules. 

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