Subject: nmr & point charges / free-radical visualisation

From: Jeremy R Greenwood <[email-masked]>
Date: Tue, 4 Apr 1995 11:51:44 +1000 (EST)

Dear CCL,

Two unrelated queries:

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Firstly, it occurs to me, that since the chemical shift of a proton or
carbon 13 nucleus depends on electronic shielding (in an nmr experiment),
there should be some (at least loose and qualitative) correlation between
point charges calculated by one or other theoretical method, and
chemical shift (not withstanding solvent effects).

I am attempting to identify the correct structures of some isomeric
heterocycles from their 1H and 13C spectra. Would you expect changes
in point charges to reflect changes in shift between equivalent
centres of the isomers? If so, calculated by which method? I'm using 
semi-empirical (AM1, PM3), and HF calculations. My intuition is that 
ESP-derived charges are more likely to reflect trends in chemical
shift.

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Secondly, I am interested in visualising localisation of free radicals
calculated by MOPAC or G92. Some kind of map of spin density would
seem to be an appropriate method. Can anyone suggest software (preferably
cheap or free!) which can extract and produce this kind of output?
I believe that Spartan has this capability, but is outside our price-range.

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Jeremy Greenwood
Department of Pharmacology
University of Sydney
NSW 2006 Australia