Subject: Use of DFT in calculations of NICS

From: "Mills, Nancy S." <[email-masked]>
Date: Thu, 14 Oct 1999 22:57:03 -0500

I am calculating nucleus independent chemical shifts (NICS) for a fluorenyl
cation with a positively charged substituent at the 9-position. I am
examining the effect of rotation around the bond between the substituent and
the fluorenyl ring. Calculations with RHF/6-31g* give reasonable results (
the maximum difference between values for the five-membered ring is about a
factor of two) , but with B3LYP/6-31g*, the NICS values vary between 28 and
390 for the five-membered ring. Note: exactly the same set of geometries is
used for the RHF/6-31G(d) and B3LYP/6-31G(d) calculations of NICS. Also,
when we have compared the calculated NMR shift of these kind of dications
with the experimental values, calculations with B3LYP/6-31g(d) are a bit
better than those with RHF/6-31G(d).
	Is there a reason why calculations with B3LYP to get NICS would vary
so much with the twisting of a substituent?

Thanks.
Nancy Mills	Phone, (210) 999-7317
Professor and Chair	Fax, (210) 999-7317
Department of Chemistry	[email-masked] <mailto:[email-masked]> 
Trinity University
715 Stadium Dr.
San Antonio, TX  78212-7200