Subject: Re: CO2
From: rparson@spot.Colorado.EDU (Robert Parson)
Date: Feb 20 1997
Newsgroups: sci.chem

In article <5efn9s$6qj$4@joe.rice.edu>,
Kevin Douglas Ausman <ausman@rice.edu> wrote:

>The particular intermolecular interaction in carbon dioxide is the London 
>force, also called induced-dipole/induced-diple interactions.

 No. CO2 has a permanent quadrupole moment, so the leading term is
 the permanent quadrupole-permanent quadrupole interaction, a simple
 electrostatic force. You can model this fairly well with partial
 negative charges on the O atoms and partial positive charge on the C. 
 Thus the CO2 dimer has a "slipped parallel" structure, with the O atom
 of one molecule nestled against the C of the other. London dispersion
 forces are also present, of course, but are weaker. In something like
 solid Argon, where there are no permanent moments on the atoms, the
 London dispersion force is indeed the leading term.

>I'd 
>suggest you go look that up, but most explanations I've seen in textbooks 
>are notoriously bad (if someone knows of a good one, please let me 
>know... I'd like to use it).

 Dispersion forces are intrinsically quantum-mechanical so all 
 intuitive explanations involve a bit of fudging. The usual picture
 of a nonpolar atom having a fluctuating "instantaneous dipole" with
 that interacts with the fluctuating dipole on another atom is as
 good as any, and does correspond fairly closely to the things that
 you actually calculate. 

 An excellent monograph on Intermolecular Forces is the new book by
 Anthony J. Stone, _The Theory of Intermolecular Forces_, Oxford 1996.
 Unfortunately it is rather expensive; let us hope that Oxford
 comes out with a paperback edition.

 ------
 Robert