Chemistry Reference
In-Depth Information
Another aspect of validation concerns the detection of disorder in the crystal
structure, recalling that solid-state NMR can provide a means of directly detecting
disorder (as well as distinguishing dynamic vs static disorder) in crystal structures.
Cases of severe disorder generally require an appropriate description of the disorder
to be incorporated in the structural model in both the structure solution and structure
refinement stages. However, if the disorder concerns a sufficiently localized part of
the structure, it may be possible to obtain a reasonable Rietveld fit using an ordered
structural model, which can then be improved by incorporating a description of the
disorder within the structural model. The latter situation was encountered in
structure determination of the
b
polymorph of
p
-formyl-
trans
-cinnamic acid from
powder XRD data [
91
] (Fig.
6
). The structure was solved and refined as an ordered
structure, leading to a good quality of fit in the Rietveld refinement (
R
wp
¼
3.27%).
However, the high-resolution solid-state
13
C NMR spectrum showed evidence for
a
b
H
O
0°, 180°
H
C
C
OH
O
H
c
d
Ordered Model
Disordered Model
R
wp
= 2.87 %
R
wp
= 3.27 %
Fig. 6 Results from Rietveld refinement of the disordered crystal structure of the
b
polymorph of
p
-formyl-
trans
-cinnamic acid. The disorder concerns two orientations of the formyl group as
shown in (a). The crystal structure in (b) shows only the disorder component of higher occupancy.
The results from Rietveld refinement shown at the
bottom
are for (c) an ordered model comprising
only the major orientation of the formyl group, and (d) the final disordered model (
right side
).
Apart from the description of the order/disorder of the formyl group, all other aspects of these
refinement calculations were the same. A slight improvement in the quality of the Rietveld fit for
the disordered model is evident
