CCL: What keeps a molecule adsorbed?



 Sent to CCL by: Mariusz Sterzel [msterzel^_^buffalo.edu]
 Dear Noel,
 
Following Ruedenberg, the simple answer is electrons kinetic energy. For more details look at K. Ruedenberg, Reviews of Modern Physics, 34(2):326-376, (1962) and M.S. Gordon, J.H.Jensen, Theoretical Chemistry Accounts, 103:248-251, (2000)
 Mariusz
 --
 Mariusz Sterzel
 Email: msterzel/^at^\buffalo.edu
 On Mon, 3 Oct 2005, Noel O Boyle noel.oboyle2|*|mail.dcu.ie wrote:
 
 Sent to CCL by: "Noel O'Boyle" [noel.oboyle2++mail.dcu.ie]
 Dear all,
 I have been trying to explain the results of a DFT study (using G03)
 where a molecule adsorbs onto a gold cluster through its nitrogen atom.
 I created an isosurface of the change in the electron density associated
 with the adsorption (by calculating the electron densities of the
 molecule and the gold cluster separately at the geometry of the adsorbed
 system). This shows that electron density *decreases* in the area
 between the adsorbate and the surface. This is the opposite of what I
 expected.
 A paper by Bilic, Reimers, Hush and Hafner (JCP, 2002, 116, 8981) shows
 the same effect for ammonia on an infinite surface of gold(111). It
 shows the same result, and says that this is evidence that covalent
 bonding effects are not so important (they also have other evidence).
 It seems that I do not understand bonding very well - what keeps the
 molecule stuck to the surface, if it isn't a shared electron or two?
 Regards,
 Noel
 --
 Dr. Noel M. O'Boyle,
 Group of Dr. John Mitchell (http://www-mitchell.ch.cam.ac.uk),
 Unilever Centre for Molecular Science Informatics,
 Dept. of Chemistry,
 University of Cambridge,
 U.K.>