Subject: Experimental or Optimized

From: Jerry C C Chan <[email-masked]>
Date: Mon, 3 Jun 1996 09:57:22 +0800 (HKT)

Dear Netters,

	Two weeks ago I posted a question on the following issues:

1.  Absolute shielding is considered as the best measure of the quality of
calculated shielding tensor components.  Why not chemical shift anisotropy
and eta? 

2.  Since the experimental CSA and eta "contain" the effects of the
environment, should one choose the experimental geometry rather than
trying to optimized the geometry of the species when the absolute 
shielding scale is not available.

	The enlightening responses from Robert and Georg are greatly
appreciated.  Below please find the responses received.

Jerry,
The Chinese University of Hong Kong

*****************

My bet is that most will tell you to use the experimental geometry but I   
would advocate a high level ab initio optimized geometry. X-ray results   
are influenced by crystal packing which distort the geometry to some   
extent. You usually do the NMR shielding calc in vacume so why not have a   
vacume geometry. Thsi should be obtained from a decent search and   
minimization at a theoretical level comparable to that at which you will   
do the NMR shielding calc at.
  Thank-you
  Robert A. Kirby
  BioChem Therapeutiques Inc.
  275 Armand-Frappier Blvd.
  Laval, Quebec,
  H7V 4A7
  [email-masked]

*****************  

> 1.  Absolute shielding is considered as the best measure of the quality of
> calculated shielding tensor components.  Why not chemical shift anisotropy
> and eta? 

Those two are undoubtely also good measures for the quality of your
calculation -- if you have the experimental results. 
Shieldings are often used because there is always
the possibility that some systematic error remains hidden
in relative shift if it happens to cancel out.

> 2.  Since the experimental CSA and eta "contain" the effects of the
> environment, should one choose the experimental geometry rather than
> trying to optimized the geometry of the species when the absolute 
> shielding scale is not available.

If you have a GOOD experimental structure, than it is probably preferable.
I stress the word "good" here since the shielding is extremely sensitive
to small changes in the geometry (this in turn is just one of the reasons
why experimentalists do NMR measurements).
However, in many cases of interest, you don't have such structures, or
not for the whole set of molecules that you are studying. In these cases,
calculated structures are your choice. Some people use exclusively optimized 
structures since this provides a consistent picture.

Yours, Georg

P.S. I am looking forward to your summary.

==============================================================================
Georg Schreckenbach                      Tel: (Canada)-403-220 8204
Department of Chemistry                  FAX: (Canada)-403-289 9488
University of Calgary                    Email: [email-masked]
2500 University Drive N.W.,  Calgary,  Alberta,  Canada,  T2N 1N4
==============================================================================