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Intermolecular Force: Molecules
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Intermolecular force is created by the balance of forces that come into play as the result of the electrical properties of adjacent molecules or atoms. Such force is at its maximum at a set distance (less than about one-tenth of a nanometer), which is called the Van der Waals' radius. Intermolecular force suddenly diminishes when the separation of these molecules or atoms increases beyond this distance.
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In chemistry , the term Van der Waals force originally referred to all forms of intermolecular force s; ... in modern usage it tends to refer to intermolecular forces that deal with forces due to the polarization of molecules. Forces that deal with fixed or angle averaged dipoles ( Keesom forces ) and free or rotation dipoles ( Debye forces ) as well as shifts in electron cloud distribution ( London Forces ) are named after the Dutch chemist Johannes Diderik van der Waals who first documented these interactions. Van der Waals interactions are observed in noble gases , which are very stable and tend not to interact. This is why it is difficult to condense them into liquid s. However, the larger the atom of the noble gas (the more electrons it has), the easier it is to condense the gas into a liquid. This happens because when the electron cloud surrounding the gas atom gets large, it does not form a perfect sphere around the nucleus . Rather, it's only spherical if averaged over longer times and generally forms an oval , which has a slight negative charge on one side and a slight positive charge on the other. The atom becomes a temporary dipole.
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In the case of polar substances, London forces exist, but in addition, the presence of a permanent dipole moment in the molecules results in a much stronger dipole-dipole intermolecular force. Thus, polar substances will generally have lower vapor pressures than nonpolar substances with the same molar mass. For example, consider the substances butane, C4H10, and acetone, C3H6O, both of which have a molar mass of 58, and hence equally strong London forces. The vapor pressures of butane and acetone at 25C, respectively are: 1735 mm Hg and 200. mm Hg. Why is the vapor pressure of butane so much higher than that of acetone? The answer is that in butane, which contains only hydrogen and carbon, the bonds are nonpolar since hydrogen and carbon have nearly equal electronegativities.
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This area is important for the development of force fields or intermolecular potentials to serve molecular dynamics calculations. There are two major schemes to study intermolecular interactions: the supermolecule approach and the perturbation approach. The latter defines contributions such as electrostatics, induction, dispersion, repulsion, whereas the former yields one single number for the interaction energy. An intermolecular potential, defined within the perturbation approach, contains atomic parameters that are given largely transferable values. However, if one aspires ultimate predictive power it is appropriate to provide the potential with realistic, tailor-made values. Ab initio calculations on monomers are ideal for that purpose, since they can provide values for all components of intermolecular interaction.
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London forces, named after the German physicist Fritz London, are weak intermolecular forces that arise from the attractive force between transient dipoles (or better multipoles) in otherwise nonpolar molecules. London forces are ... called London dispersion forces and sometimes Van der Waals forces. London forces can be exhibited by nonpolar molecules because electron density moves about a molecule probabilistically. There is a high chance that the electron density will not be evenly distributed throughout a nonpolar molecule. When an uneven distribution occurs, a temporary multipole is created. This multipole may interact with other nearby multipoles.
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Intermolecular forces are responsible for many of the bulk properties of matter in all its phases. A realistic description of the relationship between pressure, volume, and temperature of a gas must include the effects of attractive and repulsive forces between molecules. The viscosity, diffusion, and surface tension of liquids are examples of physical properties which depend strongly on intermolecular forces. Intermolecular forces are ... responsible for the ordered arrangement of molecules in solids, and account for their elasticity and properties (such as the velocity of sound in materials).
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