Permanent-induced dipole interactions

The term van der Waals forces includes three types of intermolecular forces London (dispersion) forces, permanent dipole-dipole forces (sometimes referred to as Keesom forces) and permanent-induced dipole interactions (Debye forces). In 1910, van der Waals was awarded the Nohel Prize for his work on the equation of state for gases and liquids concerned with the reasons for non-ideal behaviour in real gases. His equation introduced compensatory terms to account for the non-zero size of the particles and the inter-particle forces between them. This broader definition of van der Waals forces runs contrary to the use of the term in many current textbooks, but is consistent with its use in the IB syllabus. 

The induced counter-dipole can act in a similar manner to a permanent dipole and the electric forces between the two dipoles (permanent and induced) result in strong polar interactions. Typically, polarizable compounds are the aromatic hydrocarbons examples of their separation using induced dipole interactions to affect retention and selectivity will be given later. Dipole-induced dipole interaction is depicted in Figure 12. Just as dipole-dipole interactions occur coincidentally with dispersive interactions, so are dipole-induced dipole interactions accompanied by dispersive interactions. It follows that using an n-alkane stationary phase, aromatic... 

There are three types of interactions that contribute to van der Waals forces. These are interactions between freely rotating permanent dipoles (Keesom interactions), dipole-induced dipole interaction (Debye interactions), and instantaneous dip le-induced dipole (London dispersion interactions), with the total van der Waals force arising from the sum. The total van der Waals interaction between materials arise from the sum of all three of these contributions. 

Closely related to the London interaction is the dipole-induced-dipole interaction, in which a polar molecule interacts with a nonpolar molecule (for example, when oxygen dissolves in water). Like the London interaction, the dipole—induced-dipole interaction arises from the ability of one molecule to induce a dipole moment in the other. However, in this case, the molecule that induces the dipole moment has a permanent dipole moment. The potential energy of the interaction is... 

The inducing field responsible for the energy of the induced dipoles, Umd, has contributions from three terms the permanent or static field, Ustat, the induced dipole-induced dipole interaction, Udip, and the polarization energy, Upou... 

Because the dispersion force acts between neutral molecules it is ubiquitous (compare the gravitational force) however, between polar molecules there are also other forces. Thus, there may be permanent dipole-dipole and dipole-induced dipole interactions and, of course, between ionic species there is the Coulomb interaction. The total force between polar and non-polar (but not ionic) molecules is called the van der Waals force. Each component can be described by an equation of the form V = C/rf, where for the dipole-dipole case n = 6 and C is a function of the dipole moments. Clearly, it is easy to give a reasonable distance dependence to an interaction however, the real difficulty arises in determining the value of C. 

Electrostatic bonding is much more common than covalent bonding in drug-receptor interactions. Electrostatic bonds vary from relatively strong linkages between permanently charged ionic molecules to weaker hydrogen bonds and very weak induced dipole interactions such as van der Waals forces and similar phenomena. Electrostatic bonds are weaker than covalent bonds. 

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. 

Almost all interfacial phenomena are influenced to various extents by forces that have their origin in atomic- and molecular-level interactions due to the induced or permanent polarities created in molecules by the electric fields of neighboring molecules or due to the instantaneous dipoles caused by the positions of the electrons around the nuclei. These forces consist of three major categories known as Keesom interactions (permanent dipole/permanent dipole interactions), Debye interactions (permanent dipole/induced dipole interactions), and London interactions (induced dipole/induced dipole interactions). The three are known collectively as the van der Waals interactions and play a major role in determining material properties and behavior important in colloid and surface chemistry. The purpose of the present chapter is to outline the basic ideas and equations behind these forces and to illustrate how they affect some of the material properties of interest to us. 

Permanent dipole/induced dipole interaction (Debye equation)... 

Dispersion forces (instantaneous-dipole - induced-dipole interactions) even in atoms and molecules having no permanent dipole moment, the continuous movement of electrons results, at any instant, in a small dipole moments, which fluctuatingly polarize the electronic system of the neighboring atoms or molecules. This coupling causes the electronic movements to be synchronized in such... 

Figure 6. Number of open channels for the interaction between H3 and HC1 (SACM calculations from Ref. IS PST phase-space theory full curves permanent + induced dipole dashed smoothed curves permanent dipole J total angular momentum of H3-HCI complex).

Dipole-induced dipole interactions are also important. Nonpolar molecules tend to have their electron clouds attracted (or repelled) by a nearby dipolar molecule oriented with its positive (or negative) end toward the nonpolar species. This induced dipole interacts, on the average, with the dipolar molecule as if it were a permanent dipole. This effect is responsible for the solubility of nonpolar gases such as 02, N2, or C02 in water. 

The moderately polar phases, such as those containing 5-50% phenyl or biphenyl with the remainder as methylpolysiloxane, have no permanent dipole but can be temporarily polarized by a solute. This characteristic produces selectivity to polar solutes through dipole-induced dipole interaction with solutes. The thermal stability of these stationary phases is similar to that of the methylpolysiloxanes. 

There are interactions between neutral molecules with zero or small permanent dipole moments, referred to as induced dipole-induced dipole interactions. Even though a molecule has no or a small dipole moment, the electric charge distribution... 

Van der Waals forces are relatively short-range forces between molecules with permanent dipoles or molecules with induced dipoles, i.e. they almost occur between all molecules. These interactions include dipole-dipole interactions, dipole-induced dipole interactions and dispersion energy. 

In condensed media consisting of molecules, the intermolecular forces such as permanent and induced dipole interactions are generally small compared to intramolecular chemical binding forces. Therefore, the molecular identities and properties are conserved to a certain extent. They nevertheless differ significantly from those of an isolated molecule in the gas phase. Therefore, both in linear and non-linear optics the question arises of how to relate molecular to macroscopic properties. More specifically, how do the individual permanent and induced dipole moments of the molecules translate into the macroscopic polarization of the medium The main problem is to determine the local electric field acting on a molecule in a medium which differs from the average macroscopic field E (Maxwell field) in this medium. 

Most of the potential energy surfaces reviewed so far have been based on effective pair potentials. It is assumed that the parameterization is such as to account for nonadditive interactions, but in a nonexplicit way. A simple example is the use of a charge distribution with a dipole moment of 2.ID in the ST2 model. However, it is well known that there are significant non-pairwise additive interactions in liquid water and several attempts have been made to include them explicitly in simulations. Nonadditivity can arise in several ways. We have already discussed induced dipole interactions, which are a consequence of the permanent diple moment and polarizability of the molecules. A second type of nonadditive interaction arises from the deformation of the molecules in a condensed phase. Some contributions from such terms are implicitly included in calculations based on flexible molecule potentials. Other contributions arises from electron correlation, exchange, and similar effects. A good example is the Axilrod-Teller three-body dispersion interaction ... 

An ion or permanent dipole can induce a dipole moment in an otherwise nonpolar molecule. For ion-induced dipole interaction, the energy is ... 

To a first approximation it is independent of temperature. It is to be noted that the dipole-induced dipole interaction depends on r whether randomized or not. This is connected with the fact that turning the permanent dipole by 180° would not change the sign of the energy in this case for this reverses also the direction of the induced dipole. Only the numerical constant changes as a result of averaging. 

The dipole moments just discussed are permanent dipole moments, intrinsic properties of a molecule. A net separation of charge may also be induced in any molecule by application of an external electric field. The induced dipole moment /x(ind) so created is approximately proportional to the strength of the applied field. Thus, for molecule, /x,4(ind) = where E is the applied field strength and a a is the polarizability of A. If the source of the electric field is a pemiaiient dipole in a neighboring molecule B, then the contribution to U from the permanent dipole/induced dipole interaction is ... 

Compounds, such as those containing the aromatic nucleus and thus tt electrons, are polarizable. When such molecules are in close proximity to a molecule with a permanent dipole, the electric field from the dipole induces a counterdipole in the polarizable molecule. This induced dipole acts in the same manner as a permanent dipole and, thus, polar interactions occur between the molecules. Induced-dipole interactions are, as with polar interactions, always accompanied by dispersive interactions. Aromatic hydrocarbons can be... 

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