Subject: RE: The pi-pi stacking of hetero-aromatic ring

From: "Shobe, David" <dshobe~at~sud-chemieinc.com>
Date: Thu, 13 May 2004 14:10:23 -0400

But I wonder, would pi-pi stacking (using Per-Ola's definition) be more common in heterocyclic rings where there would be positively and negatively charged atoms in the ring?  I'm sure this has been studied extensively in purines and pyrimidines at least.  Does pyridine dimerize in the manner of benzene, or do the two molecules pi-stack with oppositely directed dipoles?

--David Shobe, Ph.D., M.L.S.
S|d-Chemie, Inc.
phone (502) 634-7409
fax (502) 634-7724

Don't bother flaming me: I'm behind a firewall.

-----Original Message-----
From: Computational Chemistry List [mailto:chemistry-request~at~ccl.net]On
Behalf Of Per-Ola Norrby
Sent: Thursday, May 13, 2004 10:18 AM
To: Sei-Hwan Kim
Cc: CHEMISTRY~at~ccl.net
Subject: CCL:The pi-pi stacking of hetero-aromatic ring

Sei-Hwan Kim wrote:

>Dear CCLers
>
>I am looking for a reference to the  pi-pi stacking of hetero-aromatic ring.
>  ( face to edge, offset, face to face )

	I'd like to comment on a serious misnomer here.  The 
interaction between two aromatic rings is frequently called pi-pi 
stacking, which implies an attractive interaction between the 
pi-clouds.  In the wast majority of experimental cases, there is no 
such attraction, two negative pi-clouds repel each other!  Only in 
the case of strong lowering of a LUMO and/or raising of a HOMO does 
two pi-faces attract each other (like in oxidized donor stacks, or 
possibly in the case of nitrobenzene with aniline).  It was shown a 
few decades ago (by Liljefors and Pettersson) that the attractive 
interactions of two benzene rings with each other (face to edge or 
offset) can be represented by simple electrostatic effects.  The 
hydrogens are slightly positive, the carbons slightly negative (about 
0.15, a C-H dipole of 0.6 D).  In all stable situations, the 
hydrogens of one ring are situated over the face of another ring. 
Inclusion of this effect is the reason why most current molecular 
mechanics programs give good results for aromatic interaction.  See, 
for example, Houk's computational study of the Wilcox torsional 
balance a few years ago.

	Thus, I'd propose reserving the term "pi-pi stacking" for the 
very rare cases where two very different aromatic rings are in a 
perfect face to face arrangement, and use the more correct "CH-pi 
interaction" for the more common cases of benzene-like molecules 
interacting with each other.

	/Per-Ola
-- 
Per-Ola Norrby, Assoc. Professor, http://organisk.kemi.dtu.dk/PON/
Technical University of Denmark, Department of Chemistry
Building 201, Kemitorvet, DK-2800 Kgs. Lyngby, Denmark
Email: pon~at~kemi.dtu.dk  tel +45-45252123,  fax +45-45933968

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