Intermolecular interactions instantaneous dipole moment

The van der Waals force is ubiquitous in colloidal dispersions and between like materials, always attractive and therefore the most common cause of dispersion destabilization. In its most common form, intermolecular van der Waals attraction originates from the correlation, which arises between the instantaneous dipole moment of any atom and the dipole moment induced in neighbouring atoms. On this macroscopic scale, the interaction becomes a many-body problem where allowed modes of the electromagnetic field are limited to specific frequencies by geometry and the dielectric properties of the system. 

Thus, the physical mechanism of intermolecular bonds that involves the van der Waals attraction is an interaction between electric dipoles of the molecules. Because of the fluctuating quantum mechanical behavior of the electrons in a molecule, all molecules have a fluctuating dipole moment, even though for many of them symmetry consideration requires that it fluctuates about an average value of zero. At a time when a molecule has a certain instantaneous dipole moment, its electric field will induce the dipole moment as a result of the charge redistribution in a nearby molecule. 

There is an important characteristic of polarizability that we have not yet mentioned, namely, its dynamic (time-varying) nature. Because the electrons in a molecule are in constant motion, it is possible that at some particular instant—purely by chance—electrons are concentrated in one region of a molecule. This displacement of electrons causes, for example, a normally nonpolar species to become momentarily polar. An instantaneous dipole is formed. That is, the molecule has an instantaneous dipole moment. After this, electrons in a neighboring molecule may be displaced to produce a dipole— an induced dipole. Taken together, these two events lead to an intermolecular force of attraction (Fig. 12-3). We can call this interaction an instantaneous dipole-induced dipole attraction, but the names more commonly used are dispersion force and London force, the latter in honor of Fritz London who, in 1928, offered a theoretical explanation of these forces. 

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. 

Dispersion forces [0.08 - 8 kj (0.02 - 2 kcal)/mol] are the weakest intermolecular attractive forces. The existence of dispersion forces accounts for the ability to liquefy low-molecular-weight, nonpolar substances, such as hydrogen (H2), neon (Ne), and methane (CHJ. To visualize the origin of dispersion forces, think in terms of instantaneous distributions of electron density rather than average distributions. Consider, for example, neon, a gas at room temperature and 1 atm pressure, which can be liquefied when cooled to — 246°C. The heat from vaporization tells us that the neon-neon attractive interaction in the liquid state is approximately 0.3 kJ (0.07 kcal)/mol. The intermolecular attractive force is accoimted for in the following way. Over time, the distribution of electron density in a neon atom is symmetrical, and there is no dipole moment [Figure 2.23(a)]. However, at any instant, there is a nonzero probability that its electron density will be polarized (shifted) more toward one part of the atom than toward another. This temporary polarization creates a temporary dipole moment, which in turn induces temporary dipole moments in adjacent atoms [Figure 2.23(b)]. 

---