Chemistry Reference
In-Depth Information
Fig. 4 High-resolution solid-
state
13
C NMR spectrum of
the 1:1 cocrystal containing
benzoic acid (BA) and
pentafluorobenzoic acid
(PFBA). The two peaks at ca.
174 ppm represent the
carboxylic acid group of
PFBA and the two peaks at
ca. 167 ppm represent the
carboxylic acid group of BA
(with two molecules of BA and two molecules of PFBA in the asymmetric unit),
rather than C2/c (with one molecule of BA and one molecule of PFBA in the
asymmetric unit).
An example of high-resolution solid-state
13
C NMR yielding direct evidence
that a molecule resides on a special position was encountered in structure determi-
nation of an early-generation dendrimeric material [
87
], with the solid-state
13
C
NMR data supporting the assignment that the molecule is located on a crystallo-
graphic twofold rotation axis. Structure determination of a new polymorph
(
phase) of the latent pigment DPP-Boc [
88
] from powder XRD data also relied
on evidence from high-resolution solid-state
13
C NMR to confirm that the asym-
metric unit comprises half the DPP-Boc molecule.
b
3.2 Aspects of Validation After Structure Refinement
Aspects of validation after Rietveld refinement are focused on (1) establishing
whether the quality of agreement between experimental and calculated powder
XRD patterns is sufficiently good to give confidence that the refined structure is
correct, (2) assessing whether the refined structure is chemically and structurally
sensible, (3) assessing whether there is evidence for disorder in the structure and (4)
assessing whether the use of powder XRD data alone provides an adequate descrip-
tion of the structure or whether complementary techniques are required to resolve
specific questions or ambiguities.
To assess the quality of the final structure obtained in structure determination, it
is important to scrutinize carefully the difference profile obtained in the Rietveld
refinement calculation (which represents the difference between the experimental
