Intermolecular forces instantaneous dipoles

These are the weakest of all intermolecular bonds. They result from the random movement of electrons within an atom or molecule. This movement can result in a separation of charge across the atom or molecule (an instantaneous dipole Fig. 11.7). This small separation of charge (indicated by <5+ and 8 ) will then influence neighboring atoms or molecules, and cause an induced dipole. These van der Waals bonds (sometimes known as London forces) occur between nonpolar molecules or atoms such as I2, 02, H2, N2, Xe, Ne, and between the aliphatic chains of lipids (see below). 

For a complete quantitative description of the solvent effects on the properties of the distinct diastereoisomers of dendrimers 5 (G = 1) and 6 (G = 1), a multiparameter treatment was used. The reason for using such a treatment is the observation that solute/solvent interactions, responsible for the solvent influence on a given process—such as equilibria, interconversion rates, spectroscopic absorptions, etc.—are caused by a multitude of nonspecific (ion/dipole, dipole/dipole, dipole/induced dipole, instantaneous dipole/induced dipole) and specific (hydrogen bonding, electron pair donor/acceptor, and chaige transfer interactions) intermolecular forces between the solute and solvent molecules. It is then possible to develop individual empirical parameters for each of these distinct and independent interaction mechanisms and combine them into a multiparameter equation such as Eq. 2, "... 

London dispersion forces are the weakest of the intermolecular forces and occur between all molecules. These are the only types of intermolecular forces that are possible between nonpolar molecules and are caused by momentary dipoles. Experimental evidence suggests that electrons are not symmetrically distributed about the nucleus at all times. On average, the electrons may be spread out evenly around the nucleus, but there are brief instants when the electron density may be greater on one side of the atom than another. During these periods of time, the atoms develop a temporary or instantaneous polarity. The temporary polarity (which is the cause of the momentary dipole) allows for attraction between particles that are normally nonpolar. London dispersion forces tend to increase as the size and mass of the molecule increase. 

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. 

In terms of intermolecular interactions, the boiling point represents the temperature at which molecules possess enough thermal energy to overcome the various intermolecular attractions binding the molecules into the liquid (e.g. hydrogen bonds, dipole-dipole attraction, instantaneous-dipole induced-dipole attractions). Therefore the boiling point is also an index of the strength of intermolecular attractive forces. 

London force (12) An intermolecular force arising from the formation of instantaneous dipoles between atoms or molecules that are close together. 

In 1930 Fritz London demonstrated that he could account for a weak attractive force between any two molecules, whether polar or nonpolar. He postulated that the electron distribution in molecules is not fixed electrons are in continuous motion, relative to the nucleus. So, for a short time a nonpolar molecule could experience an instantaneous dipole, a short-lived polarity caused by a temporary dislocation of the electron cloud. These temporary dipoles could interact with other temporary dipoles, just as permanent dipoles interact in polar molecules. We now call these intermolecular forces London forces. 

The van der Waals radius determines the shortest distance over which intermolecu-iar forces operate it is aiways larger than the covalent radius. Intermolecular forces are much weaker than bonding (intramolecular) forces. Ion-dipole forces occur between ions and poiar molecules. Dipole-dipole forces occur between oppositely charged poles on polar molecules. Hydrogen bonding, a special type of dipole-dipole force, occurs when H bonded to N, O, or F is attracted to the lone pair of N, O, or F in another molecule. Electron clouds can be distorted (polarized) in an electric field. Dispersion (London) forces are instantaneous dipole-induced dipole forces that occur among all particles and increase with number of electrons (molar mass). Molecular shape determines the extent of contact between molecules and can be a factor in the strength of dispersion 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. 

Professor of Chemistiy at Cambridge. At the atomic bond level, there is no difference between physics and chemistry. Lennaid-Jones, with his colleagues Taylor and Dent, realized that all known intermolecular forces are electromagnetic in nature. They range from the Coulombic forces between ions, to dipole forces between polar molecules, to the weak van der Waals forces which act between all atoms and which are responsible for adhesion. These adhesive forces, sometimes called London, London-van der Waals or dispersion forces, are always attractive because they result from instantaneous dipoles in one atom and their induced dipoles in a neighboring atom. The Lennard-Jones equation can be written... 

London dispersionforces (dispersion forces) Intermolecular forces resulting from the small, instantaneous dipoles (induced dipoles) that occur because of the varying positions of the electrons during their motion about the nuclei. 

Van der Waals Forces Interatomic and intermolecular forces of electrostatic origin. These forces arise due to die small instantaneous dipole moments of the atoms. They are much weaker than valence-bond forces and inversely proportional to the seventh power of the distance between the particles (atoms or molecules). 

Even noble-gas atoms and molecules that are nonpolar experience a weak intermolecular attraction. In any atom or molecule—polar or nonpolar—the electrons are in continuous motion. As a result, at any instant, the electron distribution may be slightly uneven. The momentary, uneven charge creates a positive pole in one part of the atom or molecule and a negative pole in another. This temporary dipole can then induce a dipole in an adjacent atom or molecule. The two are held together for an instant by the weak attraction between the temporary dipoles, as illustrated in Figure 5.13. The intermolecular attractions resulting from the constant motion of electrons and the creation of instantaneous dipoles are called London dispersion forces, after Fritz London, who first proposed their existence in 1930. 

Hydrogen bonding, 463 Instantaneous dipole, 466 Intermolecular forces, 462 Ion-dipole interactions, 467 Lattice point, 472 Lattice strucmre. 472 London dispersion forces, 466 Melting point, 486 Molar heat of fusion (A/ffus), 488... 

This weak intermolecular force is caused by fluctuations in the electron distribution in a molecule that creates temporary, or instantaneous, dipoles. These instantaneous dipoles appear and disappear on a very short time scale. On a longer time scale, there is no fixed dipole and the molecule is nonpolar. However, these instantaneous dipoles exert just enough force on surrounding molecules to hold the molecules in the liquid or solid phase. This force... 

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