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Re: Absolute structure


	Howard points out the importance of relating the absolute
configuration of a molecule to its optical rotation in a crystal.  This is
fine for molecular crystals, but quite often authors report the absolute
configuration of an inorganic crystal that dissolves to form an optically
inactive solution (if it dissolves at all).  I find that in such papers
the authors proudly present the Flack parameter as evidence of having
determined the absolute configuration, but generally fail to relate this
to any other physical property!  For such compounds it is largely a matter
of luck which enantiomer the authors have selected because one would
expect equal numbers of right and left handed crystals to appear in any
preparation.  I would therefore like to add to Howard's definition,
presumably at the end of the first paragraph, the following: 

	'For non-molecular crystals it is recommended that the absolute
configuration determined by diffraction methods be related to some other
physical property, such as morphology, that can be used to distinguish
between crystals of opposite chirality.'

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