Dispersion forces Weak dipole bonding that

Most organic species form molecular crystals in which discrete molecules are arranged in fixed positions relative to the lattice points. This of course means that the individual atoms making up the molecules are each arranged at fixed positions relative to each other, the lattice point, and the other molecules. The forces between molecules in molecular crystals are generally weak when compared with the forces within a molecule. The structure of molecular crystals is affected by both the intermolecular forces and the intramolecular forces since the shapes of the individual molecules will affect the way the molecules pack together. In addition, the properties of the individual molecule, such as the polarity, will affect the intermolecular forces. The forces between the molecules in molecular crystals include electrostatic interactions between dipoles, dispersion forces, and hydrogen bond... 

Molecular solids consist of atoms or molecules held together by intermolecular forces (dipole-dipole forces, London dispersion forces, and hydrogen bonds). Because these forces are weak, molecular solids are soft. Furthermore, they normally have relatively low melting points (usually below 200°C). Most substances that are gases or liquids at room temperature form molecular solids at low temperature. Examples include Ar, H2O, and CO2. 

We have now discussed three types of intermolecular forces dispersion forces, dipole forces, and hydrogen bonds. You should bear in mind that all these forces are relatively weak compared with ordinary covalent bonds. Consider, for example, the situation in HzO. The total intermolecular attractive energy in ice is about 50 kj/mol. In contrast, to dissociate one mole of water vapor into atoms requires the absorption of928 kj of energy, that is, 2(OH bond energy). This explains why it is a lot easier to boil water than to decompose it into the elements. Even at a temperature of 1000°C and 1 atm, only about one H20 molecule in a billion decomposes to hydrogen and oxygen atoms. 

Liquid crystal behavior is a genuine supramolecular phenomenon based on the existence of extended weak interactions (dipole-dipole, dispersion forces, hydrogen bonding) between molecules. For the former two to be important enough, it is usually necessary for the molecules to have anisotropic shapes, able to pack efficiently so that these weak interactions can accumulate and co-operate, so as to keep the molecules associated in a preferred orientation, but free enough to move and slide, as they are not connected by rigid bonds. 

The Lewis structure indicates that KrF2 is nonpolar. Thus, it only has very weak London dispersion forces between the molecules. SeF2 is polar and the molecules are attracted by dipole—dipole attractions, which are stronger than London. SnF2 has the highest melting point, because of the presence of strong ionic bonds. 

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. 

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. 

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. 

Liquid-Liquid and Solid-Liquid Solutions Many salts dissolve in water because the strong ion-dipole attractions that water molecules form with the ions are very similar to the strong attractions between the ions themselves and, therefore, can substitute for them. The same salts are insoluble in hexane (CgH ) because the weak ion-induced dipole forces their ions could form with the nonpolar molecules of this solvent cannot substitute for attractions between the ions. Similarly, oil does not dissolve in water because the weak dipole-induced dipole forces between oil and water molecules cannot substitute for the strong H bonds between water molecules. Oil does dissolve in hexane, however, because the dispersion forces in one substitute readily for the dispersion forces in the other. Thus, for a solution to form, like dissolves like means that the forces created between solute and solvent must be comparable in strength to the forces destroyed within both the solute and the solvent. 

The reason Lipkowitz and Darden were able to successfully use rigid bodies in their calculations is because the complexes studied are very weakly bound they are held together by dispersion forces, charge transfer complexation, dipole stacking and limited hydrogen bonding. Indeed, these authors found that root mean squared (RMS) deviations between structures compared before minimization to those after full... 

There are several types of intermolecular forces. Dipole-dipole interactions occur when molecules with dipole moments attract each other. A particularly strong dipole-dipole interaction called hydrogen bonding occurs in molecules that contain hydrogen bonded to a very electronegative element such as N, O, or F. London dispersion forces occur when instantaneous dipoles in atoms or nonpolar molecules lead to relatively weak attractions. 

London dispersion forces are relatively weak forces that arise among noble gas atoms and in nonpolar molecules. London forces arise horn instantaneous dipoles that develop when one atom (or molecule) momentarily distorts the electron cloud of another atom (or molecule). London forces are typicaUy weaker than either permanent dipole-dipole forces or covalent bonds. 

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