TROUBLESOME PERCHLORATES. Rita Gronbaek Hazell and Alan Hazell, Dept of Chemistry, University of Aarhus, DK8000 Århus C, Denmark.
Complexes of transition metals are often crystallized as the perchlorate salts, as these usually gives good crystals. Crystallographers are rarely impressed as experience tells us that these anions are often disordered to some degree. Over the years we have developed and adapted the following routines to facilitate work on disordered groups:
1. Fourier synthesis calculated on a sphere of desired radius around an atom, to get some idea as to how (dis)ordered the group is.
2. A freely rotating group can be treated by giving the chlorine atom a scattering factor calculated as: frot(s) = fCl(s) + 4fO(s)sin(4[[pi]]rs)/(4[[pi]]rs)
3. Disorder between 2 or more distinct orientations is dealt with by constraint refinement according to Pawley (1971) using the symmetric/identical molecule constraints and restricting the occupations to add up to 1.0.
4. Thermal motion is dealt with in the TLS or more usually TLX model to save parameters.
5. Large thermal motion together with geometric constraints requires that the TLS model is expanded to include third cumulants (Prince and Finger, 1973).
6. The case of a group rotating freely about a particular direction can be dealt with by means of a zero order Bessel function (Zachariasen, 1945)
The routines are quite general and care is taken to ensure that standard deviations of derived as well as refined parameters are calculated. The computer programs used at Århus stem from SUNY Buffalo and include modified ORFLS, ORFFE and FORDAP. Locally we have a menu driven system to run standard calculations with a minimum of effort. Work is still going on trying to make in particular the constraint refinement more user-friendly. For special needs the user will however still have to read the manuals. Some of the procedures described here may be adapted into other systems without too much trouble.
G.S.Pawley. Adv. Struct. Res. Diffr. Methods (1971) 1.
E. Prince and L.W. Finger. Acta Cryst. (1973). B29, 179.
W.H. Zachariasen. Theory of X-Ray Diffraction in Crystals. (1945) 222.