Additional ways of determining absolute configuration.

As a follow-up to this request, Timo made these additional points:

Dear Brian,

Thank you very much for your quick reply and the further consideration
of this case by the respective working group.

Personally, I think that the crystallographer's decision to refine a
chiral compound as a particular enantiomer should be described
unequivocally within the .cif, especially when relevant parameters like
Flack x and the corresponding esd's are not (near-)100 % reliable. One
should also think about other methods to determine an absolute
configuration, like chiroptical methods. Professor Rzepa (I have no
connections to him) obviously suggested an attribute co for
_chemical_absolute_configuration to describe that absolute configuration
has been determined by chiro-optical procedures, as can be seen here:

http://www.ch.ic.ac.uk/rzepa/talks/vigo11/3.html

Although I am all but an expert for chiro-optical methods, this sounds
resonable to me. Or _chemical_absolute_configuration could be added with
a property 'other' to allow for a detailed description of the
configurational assignment within a subordinate _details data item.
However, I am not aware of the intended relationships,  dependencies,
and boundaries between _refine_ls_abs_structure_details and
_chemical_absolute_configuration.

Best wishes
Timo


On 28/05/2019 11:58, Brian McMahon wrote:
> 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@chemie.uni-hamburg.de>
> To:     bm@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
>
>
>
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