Chemistry Reference
In-Depth Information
Fig. 19 The tetrameric association of 9-ethylbicyclo[3.3.1]nona-9-ol molecules in the crystal
structure determined from powder XRD data
peaks in the powder XRD pattern are substantially overlapped, and the peak
intensities extracted from the data recorded at any one of these temperatures did
not allow the structure to be solved. To overcome this problem, anisotropic thermal
expansion [
132
,
133
] was exploited as a means of improving the intensity extrac-
tion process. As a consequence of anisotropic thermal expansion, different peaks in
the powder XRD pattern shift to a different extent as temperature is varied, and
hence the nature of the peak overlap changes with temperature. By carrying out a
combined analysis of the data recorded at all temperatures studied, a more reliable
extraction of the integrated peak intensities can be obtained. Using the accurate set
of integrated peak intensities obtained from this multipattern peak extraction
process, the crystal structure of 9-ethylbicyclo[3.3.1]nona-9-ol was solved success-
fully using direct methods (and analysis of difference Fourier maps). As shown in
Fig.
19
, this structure has four independent molecules in the asymmetric unit,
assembled into a tetrameric unit held together by O-H
O hydrogen bonds.
7 Concluding Remarks
This chapter has aimed to demonstrate the feasibility of carrying out complete
crystal structure determination from powder XRD data, particularly in the case of
molecular materials, and has illustrated the scope of the methods currently available
for this purpose, as well as discussing some of their current limitations. Although
significant progress has been made in recent years in the development and
