Subject: Allyl pi system vs. format pi system

From: Douglas_Stack/CAS/UNO/[email-masked]
Date: Thu, 26 Aug 1999 08:01:50 -0500

Dear CClers,

I have question about molecular orbitals  (MO) generated by simple Huckel
calculations involving just conjugated p-orbitals and MO generated via
semi-empirical calculations using all of the atomic valence orbitals.
Specifically, the differences in calculating the allyl pi system vs. the formate
(HCO2-) pi system.  Both systems have 3 p orbitals that combine to form the
familiar Huckel orbitals, we'll call them HMO1, HMO2, and HMO3.  Since the allyl
cation system has just two electrons, HMO1 is the HOMO and HMO2 is the LUMO.  If
one does a semi-empirical calculation using all valence obitals on the allyl
cation system (AM1 or PM3), a graphical representation of the HOMO matches the
familiar all bonded  HMO1 and the LUMO matches the HMO2.  However, with the
formate system, a semi-empirical calculation does not produce a HOMO the matches
HMO2 (with four electrons in this pi system, HMO2 is the HOMO).  The HOMO
generated by the semi-empirical method not only has significant coefficients on
both oxygens, but also has a large coefficients on the hydrogen (connected to
the middle carbon).  The HMO2 for formate matches HOMO-1 in the semi-empirical
calculation not HOMO.  The question is, why does the semi-empirical method  for
formate generate a HOMO that is different than that predicted by simple Huckel
theory?  I understand the latter has only 3 atomic orbitals as a basis set while
the former has 13 by including all valence orbitals.  Can this change the order
of MO we are accustom to when we think of  Huckel explanations of electron
delocalization?  Why would the allyl cation system match semi-empirical but not
the formate, is symmetry involved?

Thanks in advance for any insight.

Douglas E. Stack
Assistant Professor
Department of Chemistry
University of Nebraska at Omaha
Omaha, NE 68182-0109
(402) 554-3647
(402) 544-3888 (fax)
[email-masked]