Chemistry Reference
In-Depth Information
systems to form stacking arrangements of alternating aryl and perfluoroaryl rings,
as observed in the prototypical example of the 1:1 cocrystal of benzene and
hexafluorobenzene [
120
]. The structural properties of the 1:1 cocrystal containing
benzoic acid (C
6
H
5
CO
2
H; BA) and pentafluorobenzoic acid (C
6
F
5
CO
2
H; PFBA)
have been investigated, with structure determination carried out [
86
] directly from
powder XRD data using the direct-space genetic algorithm technique for structure
solution followed by Rietveld refinement. As highlighted in Sect.
3.1
, high-resolution
solid-state
13
C NMR provided direct evidence (prior to structure solution from the
powder XRD data) that there are two crystallographically inequivalent molecules of
BA and two crystallographically inequivalent molecules of PFBA in the asymmet-
ric unit of this structure. In space group Cc, the values of
x
and
z
for one molecule
may be fixed arbitrarily, and thus a total of 26 structural variables were required in
the structure solution calculation (with 1 variable torsion angle for each of the 4
independent molecules in the asymmetric unit). As anticipated, the structure of the
cocrystal (Fig.
16
) comprises stacks of alternating BA and PFBA molecules, and
there are two crystallographically independent stacks of this type in the structure.
Molecules in the two types of stack engage in hydrogen bonding, involving the
carboxylic acid group of a BA molecule in one stack and the carboxylic acid group
of a PFBA molecule in the other stack. The two independent BA molecules and the
two independent PFBA molecules differ appreciably in the torsion angle between
the carboxylic acid and aryl units.
6.6 Structure Determination of the Product Phase from
a Solid-State Photopolymerization Reaction
Many crystalline solids can undergo chemical transformations induced, for exam-
ple, by incident radiation or by heat. An important aspect of such solid-state
reactions is to understand the structural properties of the product phase obtained
directly from the reaction, and in particular to rationalize the relationships between
the structural properties of the product and reactant phases. In many cases, how-
ever, the product phase is amorphous, but for cases in which the product phase is
crystalline, it is usually obtained as a microcrystalline powder that does not contain
single crystals of suitable size and quality to allow structure determination by
single-crystal XRD. In such cases, there is a clear opportunity to apply structure
determination from powder XRD data in order to characterize the structural
properties of product phases.
An example is the photopolymerization of 2,5-distyrylpyrazine (DSP) (Fig.
17
),
with polymerization occurring via intermolecular [2+2] photocyclization reactions
at each end of the monomer molecule. Although this reaction is regarded as a
classic solid-state reaction, having been studied extensively around 40 years ago
[
121
,
122
], structure determination of the polymeric product phase was only carried
