Subject: C-13 NMR/ Ab Initio Calcs

From: [email-masked] (Ernie McGoran)
Date: Tue, 09 May 1995 10:45:52 -0800

Could anyone point me to references where ab initio 
calculations/ molecular orbital properties might have been
used to account for the different C -13 chemical shifts
of carbon atoms whose ligands have different cis/trans or 
syn/anti dispositions.  Since atomic charge, by itself, is
not a stand alone C-13 chemical shift predictor, it would seem 
reasonable that the properties of molecular orbitals might provide
a more complete model.  Since the compounds I am interested in 
don't have published  C-13 chemical shift parameters for an empirical
estimate, and since the data set of published compounds is limited, all 
I have to buttress a stereochemical assignment is some published C-13 
data on somewhat dissimilar compounds.  The sterochemical assignments,
on these model compounds were based on the anisotropic influence of 
neighboring (C=C) and distorted cuclobutane rings.  

I know that Gaussian 94 has a NMR chemical shift prediction module, so
it seems reasonable that ab initio calculations would lead to a set of useful
predictions.  I have carried out ab initio calcuations on these compounds
at the 3-21G* level (Spartan), and although there are some differences in
the electrostatic charges and the HOMO/density (and other MO) surfaces, 
translating these into a theoretically sound interpretation escapes me.  
Thanks for any help you can send my way.

Ernie McGoran, Ph. D.                        |  Phone:  (509) 359-7931
Dept. of Chem./Biochem. MS 74         |  FAX:    (509) 359-6937
Eastern Washington University        |  email: [email-masked]
Cheney, WA 99004