Subject: hyperchem for heterocycle mos?

From: [email-masked] (Dawn Friedman)
Date: Fri, 3 Jun 94 13:29:30 -0400

Mel Schriver asks (specifically):
>
>What it boils down to for me is wether
>I can use AM1 or PM3 to calculate the orbitals of a heterocycle that
>contains S and/or P.  
>
  
  Short answer:  yes.             
    
  In terms of orbital energies, you will not get anything like the
absolute energies you get with ab initio methods.  But your relative
energies will be very useful.  We have just calculated HOMO-LUMO energy
gaps for an extensive series of heterocycles and heterocyclic oligomers
containing S, using PM3 in Hyperchem (and in G92) as well as ab initio
calculations at STO-3G* and 3-21G*.  The absolute values of the HOMO-LUMO
gaps are very different at PM3, but the ordering is the same, and the
correlation with an experimental variable is almost identical.
  
  So I would say that relative orbital energies of related compounds at
PM3 (PM3 because of the sulfur parametrization) can be valuable.
  
  As to whether PM3 orbitals look like STO-3G* or 3-21G* orbitals --
in a general sense, yes; that is, trends are reproduced.  But for more
details, well, I'll be looking into it myself...

  -- Dawn  [email-masked]