Force London’s dispersion

London [11] was the first to describe dispersion forces, which were originally termed London s dispersion forces. Subsequently, London s name has been eschewed and replaced by the simpler term dispersion forces. Dispersion forces ensue from charge fluctuations that occur throughout a molecule that arise from electron/nuclei vibrations. They are random in nature and are basically a statistical effect and, because of this, a little difficult to understand. Some years ago Glasstone [12] proffered a simple description of dispersion forces that is as informative now as it was then. He proposed that,... 

Scatchard also assumed the geometric mean rule for the cohesive energy density between the 1-2 molecules, similar to Berthelot and van Laar, in analogy with the result of London s dispersion force treatments for nonpolar molecules... 

Dispersion forces are the most universal intermolecular forces and therefore in non-polar solvents or stationary phase London s dispersion forces are... 

Non-polar dispersion forces used to describe the retention of non-polar solutes on a non-polar liquid stationary phase. London s dispersion forces postulate an intermolecular induced dipole mechanism to account for attraction of a component onto a non-polar stationary phase. 

Partition retention forces intermolecular forces that result in attraction of solute molecules to the stationary phase. Polar retention forces include dipole-dipole attractions, van der Waal s forces and hydrogen bonding. Non-polar retention forces consist of London s dispersion forces arising from induced polarity in non-polar molecules, remember that like attracts like . 

The Van der Waals attractive forces are forces of molecular origin, though at their basis lie electrical interactions. By their nature, these forces are caused by molecular polarization under the influence of fluctuations of charge distribution in the neighboring molecule and vice versa. These forces are also known as London s dispersive forces. The potential energy of molecular interaction (the London attraction energy) is equal to... 

London s dispersion force, which originates from the attraction between molecules instantaneous dipoles. This is generally the most powerful attraction. 

London s dispersion forces are weak (1-2 kcal/mol), nonspecific attractive forces which take place between any pair of neighboring atoms. They result from the instantaneous dipole moment induced by the electron distribution in the different orbitals, which, at any instant, is asymmetric. The resulting attraction may become important when several atoms m a molecule interact with several atoms in another one their spatial arrangement must then be complementary. That conditions enables dispersion forces to lead to highly specific interactions. 

LW) interactions refer to the purely physical London s (dispersion), the Keesom s (polar) and Debye s (induced polar) interactions and correspond to magnitudes ranging from approximately 0.1 to 10 kJ/mol (but in rare cases may be higher). The polar forces in the bulk of condensed phases are believed to be small due to the self-cancellation occurring in the Boltzmann-averaging of the multi-body... 

Let us first deal with the dispersion (London) interaction. This interaction is of a non-polar nature, in a non-polar liquid such as carbon tetrachloride, London s dispersion interaction is the only force present between two molecules. These non-polar molecules do not possess any permanent dipole moment. The interaction is a resultant of Instantaneous dipoles formed between the nuclei and electrons at zero-point motion of the molecule. Dispersion forces are weak. When two non-polar molecules of the same type approach each other closely enou for their electronic orbitals to overlap, the weak attraction changes to repulsion. Thus, non-polar molecules exist in a state of random distribution to give a disordered array. Another non-polar molecule (whether a solute or a solvent) will mix in all proportions since neither kind of the molecule has any attraction between them. From the foregoing, it is easy to understand that a non-pK)lar solute molecule will interact more with the phase which is non-polar this solute molecule will move fast if the non-polar phase is the mobile phase or will be retarded more and move slowly if the non-p>olar phase is the stationary phase. 

To reflect the contribution of the fundamental nature of the long-range interaction forces across the interface, it was suggested (Fowkes 1964) that surface free energies and work of adhesion may be expressed (O Eq. 3.11) by the sum of two terms the first one representative of London s dispersion interactions (superscript D) and the second representative of nondispersion forces (superscript ND), this latter include Debye induction forces, Keesom orientation forces, and acid—base interactions. 

There are probably several factors which contribute to determining the endo exo ratio in any specific case. These include steric effects, dipole-dipole interactions, and London dispersion forces. MO interpretations emphasize secondary orbital interactions between the It orbitals on the dienophile substituent(s) and the developing 7t bond between C-2 and C-3 of the diene. There are quite a few exceptions to the Alder rule, and in most cases the preference for the endo isomer is relatively modest. For example, whereas cyclopentadiene reacts with methyl acrylate in decalin solution to give mainly the endo adduct (75%), the ratio is solvent-sensitive and ranges up to 90% endo in methanol. When a methyl substituent is added to the dienophile (methyl methacrylate), the exo product predominates. ... 

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. 

Kristyan, S., Pulay, P., 1994, Can (Semi)Local Density Functional Theory Account for the London Dispersion Forces , Chem. Phys. Lett., 229, 175. 

The electron density changes continually, so induced dipoles never last more than about 10-11 s. Nevertheless, they last sufficiently long for an interaction to form with the induced dipole of another nitrogen molecule nearby. We call this new interaction the London dispersion force after Fritz London, who first postulated their existence in 1930. 

As shown by Fowkes (1968) the interfacial energy between two phases (whose surface tensions - with respect to vacuum - are y1 and y2) is subject to the resultant force field made up of components arising from attractive forces in the bulk of each phase and the forces, usually the London dispersion forces (cf. Eq. 4.2) operating accross the interface itself. Then the interfacial tension (energy) between two phases y12 s given by... 

D) Dispersion forces Ever since London s pioneer work in 1930 it has been agreed that dispersion forces contribute to all intermolecular... 

Whereas many scientists shared Mulliken s initial skepticism regarding the practical role of theory in solving problems in chemistry and physics, the work of London (6) on dispersion forces in 1930 and Hbckel s 7t-electron theory in 1931 (7) continued to attract the interest of many, including a young scientist named Frank Westheimer who, drawing on the physics of internal motions as detailed by Pitzer (8), first applied the basic concepts of what is now called molecular mechanics to compute the rates of the racemization of ortho-dibromobiphenyls. The 1946 publication (9) of these results would lay the foundation for Westheimer s own systematic conformational analysis studies (10) as well as for many others, eg, Hendrickson s (11) and Allinger s (12). These scientists would utilize basic Newtonian mechanics coupled with concepts from spectroscopy (13,14) to develop nonquantum mechanical models of structures, energies, and reactivity. 

The carbon sheets in graphite are separated by a distance of 335 pm and are held together by only London dispersion forces. Atmospheric gases can be absorbed between the sheets, thus enabling the sheets to easily slide over one another. As a result, graphite has a slippery feel and can be used as a lubricant. Because the sheets are so far apart, it s relatively difficult for an electron to hop from one sheet to the next and the electrical conductivity in the direction perpendicular to the sheets is therefore about 104 times smaller than the conductivity parallel to the sheets. 

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