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Re: Absolute structure



Thanks to Howard for trying to sort out the complexities of absoluteness
for us.  I confess to being not having thought through the problem as I
well as I should have.  Even though there are many different ways in which
absoluteness can appear in crystals, and for some people it may be useful
to distinguish between them, it appears from Howard's comments that there
is a difference between chirality at a molecular level (which in principle
is preserved after the crystal is dissolved) and the same phenomenon at
the level of a crystal, particularly when the components of the crystal
are not chiral molecules (e.g. quartz).  If I understand Howard correctly,
one talks about the 'absolute configuration' of a molecule, a term defined
by chemists with no relation to crystallography, and the 'absolute
structure' of a crystal (i.e. the particular enantiomer of a crystal in a
non-polar space group).  Either, both or neither of these concepts may be
applicable to any particular crystal.  If my interpretation is correct,
then it would be useful to define these two terms in cif.  Other details,
such as whether the molecule racemises in solution, or whether a
non-chiral molecule is present in a polar space group, are beyond the
scope of cif definitions and can be explained as necessary in the text. 

			David


*****************************************************
Dr.I.David Brown
Brockhouse Institute for Materials Research, 
McMaster University, Hamilton, Ontario, Canada
Tel: 1-(905)-525-9140 ext 24710
Fax: 1-(905)-521-2773
*****************************************************



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