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

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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