Fwd: Suggesting CIF data item values

[Brian McMahon]

Dear Core CIF Group members

I am forwarding a query I have received, and would be interested to hear
your thoughts on the proposal.

Regards
Brian


-------- Forwarded Message --------
Subject: 	Suggesting CIF data item values
Date: 	Mon, 27 May 2019 08:39:54 +0200
From: 	Timo Stein <timo.stein at chemie.uni-hamburg.de>
To: 	bm at iucr.org



Dear Mr. McMahon,


I'm writing you to kindly enquire about how to suggest new data values 
that are associated with CIF data items. I found the form 'Suggest a new 
CIF data item', but I'm not suggesting a new CIF data item, but only a 
single property for _chemical_absolute_configuration. The values listed 
within coreCIF 2.4.5 involve:

   * rm - the absolute configuration was assigned on the basis of a
     reference molecule, e.g. a co-crystallized molecule of known
     absolute configuration
   * ad - the absolute configuration was established by anomalous dispersion
   * rmad - rm + ad
   * syn - absolute configuration of the compound in question is based on
     the chiral information of its synthetical precursor and has not been
     altered
   * unk
   * .

I suggest to add the property synad to emphasize that ad has been used 
to assign a particular absolute configuration _and_ the absolute 
configuration is known. As an example, one could think of a scXRD 
experiment using Cu irradiation on a crystal comprised of an 
/N/-functionalized L-amino acid in trigonal space group /P/32. While the 
Flack parameter /x/ should show preference for /P/32 over the 
enantiomorphic space group /P/31, its standard deviation might be too 
high if a crystal of only mediocre quality containing elements with /Z/ 
<= 8 is studied.

However, if the chemist can ensure that the stereoinformation on 
C(alpha) has not been altered during the process of synthesis, 
purification, and crystallization of the /N/-functionalized L-amino 
acid, _chemical_absolute_configuration synad would be the most 
appropriate description on how the absolute configuration was assigned. 
The absolute configuration 'was established by reference to an 
unchanging chiral centre in the synthetic procedure' (from syn) and 
'confirmed by anomalous-dispersion effects in diffraction measurements 
on the crystal' (from rmad).

Thank you for consideration of this suggestion.


Yours sincerely

Timo Stein



-- 
Timo Stein
AK Prof. Dr. Michael Fröba
Department of Chemistry
Institute of Inorganic Chemistry
University of Hamburg
Martin-Luther-King-Platz 6
D-20146 Hamburg
Tel: +49-40-42838-6102
Fax: +49-40-42838-6348