Polarity temporary dipole

The existence of an attractive force between non-polar molecules was first recognized by van der Waals, who published his classic work in 1873. The origin of these forces was not understood until 1930 when Fritz London (1900-1954) published his quantum-mechanical discussion of the interaction between fluctuating dipoles. He showed how these temporary dipoles arose from the motions of the outer electrons on the two molecules. 

Dispersion forces result from temporary dipoles caused by polarization of electron clouds... 

Nonpolar molecules such as ethane H(CH2CH2)H and polyethylene (CH2CH2) are attracted to each other by weak London or dispersion forces resulting from induced dipole-dipole interactions. The temporary dipoles in ethane or along the polyethylene chain are due to instantaneous fluctuations in the density of the electron clouds caused by constant motion of electrons about the nucleus with the homogeneity upset by similar electron movement about the other nucleus. The energy of these forces is about 2 kcal per mole of repeating unit in nonpolar and polar polymers alike, and this force is independent of temperature. These dispersion forces are the major forces present between chains in many elastomers and soft plastics. 

Just as the permanent dipole of a polar molecule can induce a dipole in a nonpolar molecule, a temporary dipole can do the same thing. This gives rise to the weakest of the particle-to-particle attractions the induced dipole—induced dipole attraction, illustrated in Figure 7.7. 

The forces involved in the interaction al a good release interface must be as weak as possible. They cannot be the strong primary bonds associated with ionic, covalent, and metallic bonding neither arc they the stronger of the electrostatic and polarization forces that contribute to secondary van der Waals interactions. Rather, they are the weakest of these types of forces, the so-called London or dispersion forces that arise from interactions of temporary dipoles caused by fluctuations in electron density. They are common to all matter. The surfaces that are solid at room temperature and have the lowest dispersion-force interactions are those comprised of aliphatic hydrocarbons and fluorocarbons. 

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. 

As an example, the charged phosphate group on phosphatidylethanol-amine, for example, can interact with the hydrocarbon (CH2) chain of an amino acid—for example, valine—in a peptide. A similar situation would hold in the example to the right for interaction of the hydrocarbon unit in a peptide chain. In both instances the groups with permanent dipole moments can induce a temporary dipole moment in an adjacent molecule. These interactions, however, are very weak and act only at very short distances thus the polarization energies may be of the order of 0.002-0.004 kcal/mol at a distance of 5 A. 

What about substances made of non-polar molecules You learned in Chapter 3 that weak dispersion forces form between non-polar molecules. As temporary dipoles form, they cause molecules to move closer together. However, these attractions are temporary and weak. Thus, most small non-polar molecules do not hold together long enough to maintain their solid or liquid forms. As a result, most small non-polar molecules exist as gases at room temperature. For example, carbon dioxide (C02) is a gas at normal temperatures. 

The moleeules or atoms comprising the dielectric exhibit a dipole movement distanee. An example of this is the stereochemistry of covalent bonds in a water molecule, giving the water molecule a dipole movement. Water is the typical case of non-symmetric molecule. Dipoles may be a natural feature of the dielectric or they may be induced. Distortion of the electron cloud aroimd non-polar molecules or atoms through the presence of an external electric field can induce a temporary dipole movement. This movement generates friction inside the dielectric and the energy is dissipated subsequently as heat[l]. 

Dispersion forces result from the attraction of the positively charged nucleus of one atom for the electron cloud of an atom in nearby molecules. This induces temporary dipoles in neighboring atoms or molecules. As electron clouds become larger and more diffuse, they are attracted less strongly by their own (positively charged) nuclei. Thus, they are more easily distorted, or polarized, by adjacent nuclei. 

There is a somewhat similar phenomenon in which the presence of a dipole within a molecule induces a temporary dipole, either elsewhere in the molecule or in another molecule. The induced dipole is then attracted to the inducing charge or dipole, and another small attractive force comes into play that is not included in the molecular orbital picture at the most simple level of calculation, but is included when larger basis sets are used. Weak dipolar attractions like these, both the static and the induced, are not strong, and so nonpolar molecules are not well solvated by polar molecules the polar solvent molecules would rather solvate each other and the nonpolar molecules are left to their own devices. As it happens they do not repel each other as much as one might expect. 

Molecules that have permanent dipole or quadrupole moments generate an electric field that induces an attractive response in nonpolar molecules by polarizing the nonpolar molecule so that it exhibits a temporary dipole. In fact, polar molecules can also have induced dipoles due to the electric field effect of another polar molecule in close proximity. The simplified expression for these induced intermolecular potential energies is... 

In 1930 Fritz London demonstrated that he could account for a weak attractive force between any two molecules, whether polar or nonpolar. He postulated that the electron distribution in molecules is not fixed electrons are in continuous motion, relative to the nucleus. So, for a short time a nonpolar molecule could experience an instantaneous dipole, a short-lived polarity caused by a temporary dislocation of the electron cloud. These temporary dipoles could interact with other temporary dipoles, just as permanent dipoles interact in polar molecules. We now call these intermolecular forces London forces. 

Hydrocarbons (compounds that contain only carbon and hydrogen) are nonpolar. The favorable ion-dipole and dipole-dipole interactions responsible for the solubility of ionic and polar compounds do not occur for nonpolar compounds, so these compounds tend not to dissolve in water. The interactions between nonpolar molecules and water molecules are weaker than dipolar interactions. The permanent dipole of the water molecule can induce a temporary dipole in the nonpolar molecule by distorting the spatial arrangements of the electrons in its bonds. Electrostatic attraction is possible between the induced dipole of the nonpolar molecule and the permanent dipole of the water molecule (a dipole-induced dipole interaction), but it is not as strong as that between permanent dipoles. Hence, its consequent lowering of energy is less than that produced by the attraction of the water molecules for one another. The association of nonpolar molecules with water is far less likely to occur than the association of water molecules with themselves. 

Ans. Even though molecules are not polar, transient and temporary dipoles form in all molecules. These are called dispersion forces, and though weak, lead to condensation of nonpolar molecules. 

The van der Waals interaction contains contributions from three effects permanent dipole-dipole interactions found for any polar molecule dipole-induced dipole interactions, where one dipole causes a slight charge separation in bonds that have a high polarizability and dispersion forces, which result from temporary polarity arising from an asymmetrical distribution of electrons around the nucleus. Even atoms of the rare gases exhibit dispersion forces. 

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