"Unknowns" in Chemical Companies Catalogs

Armel Le Bail armel at fluo.univ-lemans.fr
Thu Mar 5 09:09:48 EST 1998


Hi,

The "Unknown" definition is here "not included in the Cambridge
Structure Database (CSD) nor in the Inorganic Crystal Structure
Database (ICSD)" meaning that the X-ray crystal structure has not
been determined.

Most catalogs of chemical companies do not indicate this essential
point for solids : does the crystal structure has been determined
from X-ray single crystal (or powder) diffraction data ? And this
is a pity.

It seems that these companies consider as sufficient that the
X-ray powder pattern fits with one JCPDS-ICDD card (in case
of inorganic products, because powder diffraction is not
normally used for checking organic solids).

The problem is that many JCPDS-ICDD cards even not give cell
indexation (they also don't care with the information : is the
crystal structure known ?). Examples can be found of compounds
bought at a chemical company of which the structure was
determined ab initio from powder diffracton data. Frequently
the formulae are bizarre with an unprecise number of water
molecules (want one example ? see for instance Zr(OH)2(NO3)2
4.7H2O   J. Solid State Chem. 94 (1991) 27-35). Incidently, this
means that sometimes you may buy rather undefined products...:-(

So here is my question. I was searching for unknown compounds,
inorganic and organic stable solids, available in large quantity from
chemical companies, preferably exclusively in powder form. These
companies were unable to produce for me the list of their "unknowns"
compounds (i.e. with undetermined crystal structure). Has somebody
yet compared the CSD and ICSD databanks with catalogs from
chemical companies ?

More should be asked from these companies. We, buyers,
should require that they add one clear mention in their catalog :
"unknown (or known) crystal structure". This is not difficult to check
against the CSD and ICSD databanks. This would be the info that
maybe the true composition is not as accurate as suggested by the
vendor...

If you have identified such "unknowns" in catalogs please contact me.
The target is the organisation of a Round Robin on ab initio structure
determination from powder diffraction data.

Armel Le Bail - Universite du Maine, Laboratoire des Fluorures,
CNRS ESA 6010, Av. O. Messiaen, 72085 Le Mans cedex 9, France
http://fluo.univ-lemans.fr:8001/
http://ww.cybercable.tm.fr/~cristal/welcome.html






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