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From: "Helder Marques" <HMARQUES;at;AURUM.CHEM.WITS.AC.ZA>
Organization:  CHEMISTRY -  WITS UNIVERSITY
To: CHEMISTRY&$at$&www.ccl.net
Date:          Tue, 23 Sep 1997 12:27:26 GMT + 2:00
Subject:       Virtual orbital energies
Priority: normal
X-mailer: Pegasus Mail v3.22
Message-ID: <1FBAB8D72F7: at :aurum.chem.wits.ac.za>


I would appreciate your views on the following.

We have been doing some semi-empirical (PM3) and ab initio calculations 
(6-31G* basis set) on a series of ligands (pyridines and imidazoles) to 
try to find correlations between the properties of the ligands themselves 
and the stability constants for their binding to an iron(III) porphyrin.  
Our question is the following.  In addition to being sigma donors, the 
ligands are potential pi-acceptors from, and pi-donors to Fe(III).  Hence 
we are interested in the energies of the frontier orbitals.  Of the 
occupied orbitals, two have pi symmetry and one sigma symmetry.  The two 
virtual orbitals (LUMO and LUMO+1) both have pi symmetry.  Now the 
energies of the filled orbitals presumably have some physical meaning.  
But do the computed energies of the virtual orbitals?  Are we justified in 
using these energies in multivariate correlations with the 
experimental stability constants?

Thanks for your time
Helder Marques






         _________________________________________________
        | Prof. Helder M. Marques, Department of Chemistry|
        | University of the Witwatersrand, P.O. Wits      |
        | 2050 Johannesburg,                              |
        | South Africa                                    |
        | Fax: +27+11+339-7967; Tel: +27+11+716-2303      |
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