Talk:London dispersion force

"In vacuum, London forces are weaker than other intermolecular forces such as ionic interactions, hydrogen bonding, or permanent dipole-dipole interactions." ??
Linus Pauling, Van der Waals Forces. Melting Points and Boililng Points, pg 394, General Chemistry, Dover, 1970 "The theory of van der Waals force of attraction between molecules was developed by the physicist F. London in 1929. It had been suggested that the van der Waals attraction between two HCL molecules (or other molecules with a permanent dipole moment; Section 6.8) was the result of the interaction of the permanent dipole moments. Careful calculations of this energy of attraction for two HCl molecules, however, gave a result only 10% of the observed interaction energy. Moreover the interaction energy of molecules of xenon (boiling point -107 degrees C) is nearly as great as that of molecules of hydrogen chloride (boiling point -84 degrees C), although the molecules of xenon, which are single atoms, have no permanent electric dipole moment."

"van der Waals forces." Encyclopædia Britannica. 2007. Encyclopædia Britannica Online. 25 Nov. 2007 . “These specific interactions, or forces, arising from electron fluctuations in molecules (known as London forces, or dispersion forces) are present even between permanently polar molecules and produce, generally, the largest of the three contributions to intermolecular forces.”

Intermolecular Bonding - Van Der Waals Forces, ChemGuide, Jim Clark http://www.chemguide.co.uk/atoms/bonding/vdw.html “Surprisingly dipole-dipole attractions are fairly minor compared with dispersion forces, and their effect can only really be seen if you compare two molecules with the same number of electrons and the same size. [He then gives examples.]

Ray Eston Smith Jr (talk) 18:28, 15 September 2009 (UTC)

Peter Atkins and Julio de Paula, pg 703, Atkins' Physical Chemistry, Seventh Edition, 2002, Oxford University Press, "The dispersion interaction generally dominates all the interactions between molecules other than hydrogen bonds."

Ray Eston Smith Jr (talk) 18:50, 15 September 2009 (UTC)

Yep - your last quote (Atkins & de Paula) puts it correctly - unfortunately there is a lot of misapprehension about dispersion forces because the idea that they are weak settled into textbook and teaching mainstream long ago. So editing is needed. There is a further problem too - the general textbook description (instantaneous, and variable, induced dipoles - even Atkins) comes from an attempt to interpret London's second order perturbation analysis in real terms, whereas what is happening is that the electron density of neighboring molecules are being polarized towards each other. This is potentially a static effect (in solids) but will obviously vary in gases or liquids. see R.F.W Bader J. Phys. Chem. A 1998, 102, 7314-7323 who refers to Richard Feynman's 1939 article which pointed this out (Phys. Rev. 1939, 56, 340). Unfortunately the instantaneous induced description has got so embedded and appears so simple that the proper explanation is buried!

Ian (talk) 11:08, 17 September 2009 (UTC)


 * Someone should put the above on the main page rather than just a link to the "authoritative work". I found the phrase, "electron density of neighboring molecules are being polarized towards each other" particularly helpful. It's obvious that the "real" explanation is quantum mechanical, but the relevant section was too hard for me to understand.  This is predominantly the content, but frankly, the writing is pretty bad.  Tom3118 (talk) 18:51, 10 August 2011 (UTC)

It seems to me that the section on QM theory of dispersion should explain the current best QM understanding of the effect, rather than detail the mathematical path London traveled. I'm not the best person to write that, but I do think perhaps a qualitative explanation preceeding that more modern mathematical treatment might help non-experts grasp it. Below is my cut at such an introduction. Thoughts?

______________ Consider a single Helium atom: The overall probability density for each of the two electrons is identical, and spherically symmetric. However, under the condition that e1 is considered to be instantaneously located on one side of the nucleus, the probability that e2 will be on that same side is not 50%, but slightly less. While the distribution of e2 is spherically symmetric when averaged over all possible positions of e1, it is not symmetric for any *particular* position. Their probability distributions are thus not statistically independent, but are correlated with each other. This effect is called "dynamical correlation".

Electron correlation leads to a slight reduction in the total energy of the system relative to what it would be if the probability density of each electron were dependent only on the average density of the others. This difference is called "correlation energy", and is the quantum-mechanical basis for the London dispersion force. The fully conditional probability distribution of all electrons is not generally expressable as a separable function of the positions of the individual electrons. As a result, the computational expense of computing it accurately grows exponentially with the number of electrons. So, it is often approximated using London's more tractable form.

Ma-Ma-Max Headroom (talk) 18:02, 12 November 2012 (UTC)

Supposition
I'll modify the statement concerning the existence of the force with supposed like this: This force has been supposed/considered to be the only one exerted between rare gas molecules.--188.26.22.131 (talk) 15:44, 19 July 2013 (UTC)