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Dispersion interactions (Read 2107 times)
Gerrit-Jan Linker
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Dispersion interactions
21.07.08 at 22:29:22
 
Dispersion interactions
 
Dispersion interactions are v/d Waals interactions and are sometimes also called London interactions.
 
When we have 2 atoms at a certain distance from eachother the motions of the electrons are correlated. Stated more properly, the electronic wavefunctions are correlated. The two atoms simultaneously develop electrical moments that are oriented so as to be mutally attractive. The force associated with this interaction is referred to as dispersion, the London force or the attractive van der Waals force.
 
In the absence of a permanent charge, the strongest such interaction is a dipole-dipole interaction, usually referred to as an induced dipole-induced dipole interaction since the moments in question are not permanent.
 
Quote:
Dispersion energy is an attractive component of the energy of intermolecular interaction resulting from the interaction between the instantaneous, time-variable dipole of one system and the induced multipole of the second system.

This interaction cannot be interpreted in terms of classical electrostatics and corresponds to the intersystem component of the correlation energy. For two neutral atoms, the dispersion energy is proportional to the sixth power of the reciprocal distance: E=1/R6

 
Quote:
Electron density  in a molecule becomes redistributed in the proximity to another molecule, because the electrons from different molecules start "feeling" and avoiding each other (see quantum mechanical theory of dispersion forces). This is frequently described as formation of "instantaneous dipoles" that attract each other.

 
See also:
v/d Waals interactions
http://www.oraxcel.com/cgi-bin/yabb2/YaBB.pl?num=1274941998/0#0
 
Sources:
Essentials of Computational Chemistry; 2nd Edition. Christopher J. Cramer, 2004.
GLOSSARY OF TERMS USED IN THEORETICAL ORGANIC CHEMISTRY
http://old.iupac.org/publications/pac/1999/71_10_pdf/7110mirkin_1919.pdf
London dispersion force
http://en.wikipedia.org/wiki/London_dispersion_force
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« Last Edit: 16.12.12 at 16:29:13 by Gerrit-Jan Linker »  

Gerrit-Jan Linker
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Gerrit-Jan Linker
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Re: Dispersion
Reply #1 - 29.07.08 at 22:10:17
 
Dispersion-Interaction. Dispersive interactions are one of the three basic interactive mechanisms that allow molecules to exert forces on one another: dispersive, polar and ionic.  
 
Dispersive forces (or London's dispersive forces) are due to transient, random charges, spontaneously generated continuously all over the molecule. They are not due to permanent dipole or induced permanent dipole interactions, or due to interactions between permanent charges on the molecules as in ionic interactions.
 
Defined more losely:
It may be imagined that an instantaneous picture of a molecule would show various arrangements of nuclei and electrons having dipole moments. These rapidly varying dipoles when averaged over a large number of configurations would give a resultant of zero. However, at any instant they would offer electrical interactions with another molecule resulting in interactive forces.
 
Another description of dispersion interaction:
Dispersion interaction in a system that consists of a part A and B is interaction that is due to the instantaneous change in charge density in both A and B.
 
Quote:
Dispersion interaction is always positive

Ref: thesis Alex de Vries
 
Quote:

Three ways molecules interact with eachother:
- Dispersion interaction. Always present.
- Induction interaction
- Electrostatic interaction

The last 2 interactions depend on the presence of permanent electrostatic moments in either of the subsystems.

Ref: thesis Alex de Vries
 
Dispersion is non-local.
 
Dispersion is a pure quantum mechanical effect.
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« Last Edit: 23.08.13 at 14:03:25 by Gerrit-Jan Linker »  

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