London Forces

Polarizability plays the central role in the most universal intermolecular force. Up to this point, we ve discussed forces that depend on an existing charge, of either an ion or a polar molecule. But what forces cause nonpolar substances like octane, chlorine, and argon to condense and solidify Some force must be acting between the particles, or these substances would be gases under any conditions. The intermolecular force primarily responsible for the condensed states of nonpolar substances is the dispersion force (or London force, named for Fritz London, the physicist who explained the quantum-mechanical basis of the attraction). 

CHAPTER 12 Intermolecular Forces Liquids, Solids, and Phase Changes 

SAMPLE PROBLEM 12.3 Predicting the Types of Intermolecular Force Problem For each pair of substances, identify the key intermolecular force(s) in each substance, and select the substance with the higher boiling point  

Comment Dispersion forces are always present, but in parts (a) and (b), they are much less significant than the other forces involved. 

SAMPLE PROBLEM 12.3 Predicting the Types of Intermolecular Force 

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Van der Waals forces involv ing induced dipoles are of ten called London forces or dispersion forces... 

Boiling Point When describing the effect of alkane structure on boiling point m Sec tion 2 17 we pointed out that van der Waals attractive forces between neutral molecules are of three types The first two involve induced dipoles and are often referred to as dis persion forces or London forces... 

Induced dipole/mduced dipole attraction (Section 2 17) Force of attraction resulting from a mutual and complemen tary polanzation of one molecule by another Also referred to as London forces or dispersion forces Inductive effect (Section 1 15) An electronic effect transmit ted by successive polanzation of the cr bonds within a mol ecule or an ion... 

London force (Section 2 17) See induced dipole induced dipole attraction... 

The term polymer is derived from the Greek words poly and meros, meaning many parts. We noted in the last section that the existence of these parts was acknowledged before the nature of the interaction which held them together was known. Today we realize that ordinary covalent bonds are the intramolecular forces which keep the polymer molecule intact. In addition, the usual type of intermolecular forces—hydrogen bonds, dipole-dipole interactions, and London forces—hold assemblies of these molecules together in the bulk state. The only thing that is remarkable about these molecules is their size, but that feature is remarkable indeed. 

For large deformations or for networks with strong interactions—say, hydrogen bonds instead of London forces—the condition for an ideal elastomer may not be satisfied. There is certainly a heat effect associated with crystallization, so (3H/9L) t. would not apply if stretching induced crystal formation. The compounds and conditions we described in the last section correspond to the kind of system for which ideality is a reasonable approximation. 

Polyethylene. The crystal structure of this polymer is essentially the same as those of linear alkanes containing 20-40 carbon atoms, and the values of Tjj and AHf j are what would be expected on the basis of an extrapolation from data on the alkanes. Since there are no chain substituents or intermolecular forces other than London forces in polyethylene, we shall compare other polymers to it as a reference substance. 

As the bulkiness of the substituents increases, the chains are prevented from coming into intimate contact in the crystal. The intermolecular forces which hold these crystals together are all London forces, and these become weaker as the crystals loosen up owing to substituent bulkiness. Accordingly, the value for the heat of fusion decreases moving down Table 4.2. 

Hydrocarbons without bulky side groups are held together by London forces, the weakest of intermolecular attractions. This means that the free volume tends to be large for these compounds, so a relatively large amount of cooUng is necessary before the free volume collapses. Thus Tg is low for these compounds. 

Until surface contact, the force between molecules is always one of attraction, although this attraction has different origins in different systems. London forces, dipole-dipole attractions, acid-base interactions, and hydrogen bonds are some of the types of attraction we have in mind. In the foregoing list, London forces are universal and also the weakest of the attractions listed. The interactions increase in strength and also in specificity in the order listed. 

Since London forces are universal and nonspecific, we shall eventually emphasize these, realizing that stronger forces may outweigh the London contribution in some systems. We anticipate the best results in nonpolar systems where London forces account for the interactions. 

Factors which adversely influence the separation of veiy fine particle systems are brownian motion and London forces. However, it is possible to counter these forces by the use of dispersants, temperature control, and so on. 

Hamaker [32] first proposed that surface forces could be attributed to London forces, or the dispersion contribution to van der Waals interactions. According to his model, P is proportional to the density of atoms np and s in the particle and substrate, respectively. He then defined a parameter A, subsequently becoming known as the Hamaker constant, such that... 

As previously mentioned, electrodynamic interactions, such as those arising from London forces, can also contribute to the adhesion of particles. These forces are dominated by dipole interactions and are broadly lumped into the classification known as van der Waals interactions. A more detailed description of van der Waals interactions than can be presented in this article is given in books by Israelachvili [95] and by Rimai and Quesnel [96]. 

Van der Waals forces involving induced dipoles are often called London forces, or dispersion forces. 

The dispersion (London) force is a quantum mechanieal phenomenon. At any instant the electronic distribution in molecule 1 may result in an instantaneous dipole moment, even if 1 is a spherieal nonpolar moleeule. This instantaneous dipole induces a moment in 2, which interacts with the moment in 1. For nonpolar spheres the induced dipole-induced dipole dispersion energy function is... 

Reactions of iV -alkylated or arylated azinium compounds with nucleophiles proceed more readily than those of the parent, uncation-ized azines, and the ring tends to open. The iV -substituent may bring into play an accelerative effect from the London forces of attraction. Increased displaceability of the substituent in iV -alkyl-azinium compounds has been noted for 2-halopyridinium (87) 1-haloisoquinolinium, 4-halopyrimidinium, 4-methoxypyrid-inium (88), 4-phenoxy- and 4-acetamido-quinazolinium (89), 3-methylthiopyridazinium, and 2-car boxymethylthiopyrimidi-nium salts (90). The latter was prepared in situ from the iV -alkyl-pyrimidine-2-thione. The activation can be effectively transmitted to... 

London forces acceleration iiVb when the substituent is ortho to Le,... 

The accelerative effect of London forces of attraction between a nucleophile and nearby substituents has been investigated in quinoline and benzene derivatives by Bunnett and co-workers, ii7b, 307 In 2-, 4-, and 6-arylsulfonyl-3-nitrochlorobenzene, Loudon and Shulman found that arylmercaptide ion, presumably through this effect, displaced the arylsulfonyl group while methoxide or ammonia displaced the nitro or chloro group. 

In the first approximation London forces in complex systems may be taken as the sum over all interacting pairs. Higher-order corrections for triple interactions have been given by Axilrod and Teller2,3 and by Jansen and McGinnies.18... 

We shall consider only forces between systems of moderate size which are in their ground electronic states. Dahler and Hirsch-felder8 have discussed the additional complications when excited states are introduced. Yos, Bade, and Jehle42 gave special consideration to London forces between macromolecules but found it difficult to determine the magnitude of the special effects they noted. 

In polyatomic molecules there are separate regions of essentially nonoverlapping electron clouds whose interaction may be treated by London s methods. This was recognized by many authors, but relatively few calculations have emphasized this factor. Indeed sometimes it may have been erroneously dismissed as of negligible magnitude. Bom and Mayer6 Considered London forces in their... 

Since the polarizability is so closely related to the London force, all approximate formulas for the latter are obtained by replacing arrays of unevaluated terms which approximate the polarizability by that quantity itself. We shall examine the several ways in which this has been done with reference to the magnitude of error which may have been introduced. 

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