Permanent dipole force

Permanent-dipole/permanent-dipole forces are weak attractive forces between permanently polar molecules. S+ atoms in one molecule attract S atoms in another molecule. They act in addition to the induced-dipole/induced-dipole forces. 

The boiling temperature of propanone, however, is 56°C, whereas that of butane is 0°C. This difference is explained by the additional presence of permanent-dipole/permanent-dipole forces between propanone molecules. 

We can understand the relative values of a and b in Table 12-5 in terms of molecular properties. Note that a for helium is very small. This is the case for all noble gases and many other nonpolar molecules, because only very weak attractive forces, called dispersion forces, exist between them. Dispersion forces result from short-lived electrical dipoles produced by the attraction of one atom s nucleus for an adjacent atom s electrons. These forces exist for all molecules but are especially important for nonpolar molecules, which would never liquefy if dispersion forces did not exist. Polar molecules such as ammonia, NH3, have permanent charge separations (dipoles), so they exhibit greater forces of attraction for one another. This explains the high value of a for ammonia. Dispersion forces and permanent dipole forces of attraction are discussed in more detail in Chapter 13. 

The solubility parameter system used here assumes that the energy of evaporation—i.e., the total cohesive energy which holds a liquid together, E—can be divided into contributions from three forces (1) dispersion (London) forces, ED, (2) permanent dipole-permanent dipole forces, EP, and (3) hydrogen-bonding forces, EH. Thus,... 

Ed, Ep, Eh contributions of dispersion forces, permanent dipole-permanent dipole forces, and hydrogen bonds. 

This misconception is reinforced by use of the apparently very different terms hydrogen bond and dipole-dipole force. Both are permanent dipole forces. The difference is their relative strength and not their nature. 

Chandrasekhar and Madhusudana have also considered the calculation of the coefficients required in V. The first contribution that these authors examined was the permanent dipole-permanent dipole forces. These were shown to vary as and provided a V dependence to Ui. It was shown however, that this term vanished when the pair potential V12 is averaged over a spherical molecular distribution function. The authors thus discard this term and provide further arguments for its neglect based on the empirical result that permanent dipoles apparently play a minor role in providing the stability of the nematic phase. The second contribution considered was the dispersion forces based on induced dipole-induced dipole interactions and induced dipole-induced quadrupole interactions. As mentioned above, the first of these gives a dependent contribution, while the second provides a contribution depending on The final contribution considered... 

Dipole/induced dipole attraction (Section 4 6) A force of at traction that results when a species with a permanent dipole induces a complementary dipole in a second species... 

As argued above, this result is found to work best for substances in which both the 1,1 and 2,2 forces are either London or dipole-dipole. Even the case of one molecule with a permanent dipole moment interacting with a molecule which has only polarizability and no permanent dipole moment-such species interact by permanent dipole-induced dipole attraction-is not satisfactorily approximated by Eq. (8.46). In this context the like dissolves like rule means like with respect to the origin of intermolecular forces. 

A polar molecule can also induce a dipole on a neighbouring molecule that possesses no permanent dipole. The resultant intermolecular attraction between the permanent and the induced dipole is spoken of as the induction force. Its magnitude is small and independent of temperature. 

The dipoles are shown interacting directly as would be expected. Nevertheless, it must be emphasized that behind the dipole-dipole interactions will be dispersive interactions from the random charge fluctuations that continuously take place on both molecules. In the example given above, the net molecular interaction will be a combination of both dispersive interactions from the fluctuating random charges and polar interactions from forces between the two dipoles. Examples of substances that contain permanent dipoles and can exhibit polar interactions with other molecules are alcohols, esters, ethers, amines, amides, nitriles, etc. 

Two Molecules Interacting and Held Together by Dispersive Forces and Polar Forces from Permanent Dipoles... 

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. 

In the second type of interaction contributing to van der Waals forces, a molecule with a permanent dipole moment polarizes a neighboring non-polar molecule. The two molecules then align with each other. To calculate the van der Waals interaction between the two molecules, let us first assume that the first molecule has a permanent dipole with a moment u and is separated from a polarizable molecule (dielectric constant ) by a distance r and oriented at some angle 0 to the axis of separation. The dipole is also oriented at some angle from the axis defining the separation between the two molecules. Overall, the picture would be very similar to Fig. 6 used for dipole-dipole interaction except that the interaction is induced as opposed to permanent. 

To understand the origins of dispersion forces, let us consider two Bohr atoms, each of which consists of an electron orbiting around a nucleus comprised of a proton, having a radius ao, often referred to as the first Bohr radius . It is obvious that a Bohr atom has no permanent dipole moment. However, the Bohr atom can be considered to have an instantaneous dipole moment given by... 

It is clear from Table 1 that, for a few highly polar molecules such as water, the Keesom effect (i.e. freely rotating permanent dipoles) dominates over either the Debye or London effects. However, even for ammonia, dispersion forces account for almost 57% of the van der Waals interactions, compared to approximately 34% arising from dipole-dipole interactions. The contribution arising from dispersion forces increases to over 86% for hydrogen chloride and rapidly goes to over 90% as the polarity of the molecules decrease. Debye forces generally make up less than about 10% of the total van der Waals interaction. 

The van der Waals forces for these substances are due mainly to dispersion forces, which decrease with decrease in atomic number for atoms of similar structure. London s calculations (F. London, Z. Physik 63, 245 (1930) have shown the interaction of permanent dipoles to contribute only a small amount to the van der Waals forces for a substance such as hydrogen chloride. 

Dispersive forces are more difficult to describe. Although electric in nature, they result from charge fluctuations rather than permanent electrical charges on the molecule. Examples of purely dispersive interactions are the molecular forces that exist between saturated aliphatic hydrocarbon molecules. Saturated aliphatic hydrocarbons are not ionic, have no permanent dipoles and are not polarizable. Yet molecular forces between hydrocarbons are strong and consequently, n-heptane is not a gas, but a liquid that boils at 100°C. This is a result of the collective effect of all the dispersive interactions that hold the molecules together as a liquid. 

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