Chemistry Reference
In-Depth Information
under ambient atmospheric conditions [
115
], and 1:1 cocrystals of theobromine
with trifluoroacetic acid and theobromine with malonic acid, each of which was
prepared by liquid assisted grinding [
116
].
6.3 Structure Determination of an Early-Generation
Dendrimeric Material
Dendrimers are large, highly-branched molecules composed of a core moiety and
radiating functionality. The highly branched architecture can lead to spatially well-
defined voids within the dendrimer, and this structural feature is crucial for many of
the wide-ranging applications of these materials. Many dendrimeric materials
cannot be grown as suitable single crystals for single-crystal XRD, and structure
determination from powder XRD data may represent the only viable opportunity
for structural characterization. The first structure determination of a dendrimeric
material directly from powder XRD data [
87
] concerned the early-generation
dendrimer tetrakis[(3,5-dimethoxybenzyloxy)methyl]methane (TDMM). The space
group was assigned as Fdd2, and density considerations together with results from
high-resolution solid-state
13
C NMR suggested that the asymmetric unit comprises
half the molecule (with the central carbon atom located on a twofold rotation axis).
In the half-molecule, there are 12 variable torsion angles (Fig.
12a
). Structure
solution was carried out using the direct-space genetic algorithm technique followed
by Rietveld refinement. In the crystal structure (Fig.
12b
), the half-molecule that
represents the asymmetric unit has two independent branches radiating from the core.
Interestingly, the conformational properties of these two branches differ significantly,
in terms of both the conformation of the C-CH
2
-O-CH
2
-Ph chain and the confor-
mation of the two methoxy substituents on the benzyloxy ring.
6.4 Structure Determination of Materials Prepared
by Solid-State Dehydration/Desolvation Processes
Although the chemical properties of benzene-1,2,3-tricarboxylic acid (BTCA)
were first studied over 100 years ago, the crystal structure of BTCA was not
reported until a recent powder XRD study [
117
]. In contrast, the crystal structures
of several solvate phases of BTCA were determined previously, including a
dihydrate structure and solvate structures containing different alcohols and other
solvent molecules. The preparation of a “pure” (nonsolvate) crystalline phase of
BTCA by crystal growth from solution is difficult due to the competitive formation
of solvate phases. In such cases of materials that cannot be prepared as a “pure”
(nonsolvate) phase by conventional crystal growth processes, a possible route to
obtain the “pure” phase is to carry out desolvation of a solvate phase at elevated
