Chemistry Reference
In-Depth Information
Table 6 Residual density descriptors applied to experimental data for bullvalene trisepoxide to
study the effects of the extinction correction as implemented in SHELXL
Correction for
d
f
(0)
e
gross
r
0,min
r
0,max
Dr
0
[e
˚
3
]
[e
˚
3
]
[e
˚
3
]
extinction
[e]
Off
2.4129
30.26
0.36
0.29
0.65
On
2.5228
11.03
0.16
0.12
0.28
Fig. 11
N
-phenylpyrrole (PP) in its disordered state. The main domain with occupation factor 0.9
is plotted in
black
, the minor domain in
white
. The anisotropic displacement parameters are shown
at the 50% probability level
3.7
to Statistical Disorder
...
Disorder of solvent, molecules, or parts of molecules is of particular importance in
charge-density studies. As has been mentioned earlier, the presence of unmodeled
disorder influences the model parameters and in this way distorts the density model
in a way not easy to control. Conversely, if disorder is taken into account and if it
can be shown that the remaining residual density does not contain any structural
information except for noise (at the given experimental resolution), one has a
stopping criterion for the refinement and a quality control. Of course, this still
does not prove the model to be correct, but the converse is true: features in the
residual density disprove the model to be correct. Therefore, among all models only
those with a statistical residual density are acceptable.
The example considered here is from 100 K Mo-K
a
data of
N
-phenylpyrrole
H
4
C
4
N(C
6
H
6
) crystallizing in the noncentrosymmetric orthorhombic space group
P
2
1
2
1
2
1
with one molecule in the asymmetric unit, which is disordered by a 2-fold
rotation axis (see Fig.
11
). The occupation factors are approximately 0.9 and 0.1.
More details can be found in [
1
,
19
,
20
].
