Chemistry Reference
In-Depth Information
Fig. 12 (a) The molecular fragment (
thick lines
), comprising half the TDMM molecule, used in
the direct-space genetic algorithm structure solution calculation from powder XRD data. The
arrows
indicate the variable torsion angles. The central carbon atom is located on a twofold
rotation axis (as indicated), and the other half of the molecule (
faint lines
) is generated by this
symmetry operation. (b) The final refined crystal structure of TDMM, with hydrogen atoms
omitted for clarity
temperature and/or reduced pressure. However, such processes are often associated
with loss of crystal integrity, and a single crystal of the parent (solvate) structure
typically yields a polycrystalline product phase following desolvation (Fig.
13
). In
such cases, powder XRD is essential for structure determination of the product
phase. In this regard, the direct-space genetic algorithm technique was used [
117
]to
solve the structure of the “pure” phase of BTCA from powder XRD data.
A microcrystalline powder sample of the “pure” phase of BTCA was obtained
by dehydration of the dihydrate phase of BTCA at elevated temperature. There are
three independent BTCA molecules in the asymmetric unit, and trial structures in
the genetic algorithm structure solution calculation were defined by a total of 27
structural variables (with nine variables required to define the position, orientation
and conformation of each independent molecule). In the final refined structure of
BTCA (Fig.
14
), all carboxylic acid groups are engaged in intermolecular hydrogen
bonding to other carboxylic acid groups via the double O-H
O hydrogen-bonded
motif found in carboxylic acid “dimers”. The three independent molecules have
similar conformations (the “inner” carboxylic acid group is nearly perpendicular to the
ring, whereas the two “outer” carboxylic acid groups lie closer to the plane of the ring).
The structure of the “pure” phase of BTCA differs substantially from that of BTCA
dihydrate, implying that the solid-state dehydration process is associated with
substantial structural reorganization.
Another material that has a strong propensity to form solvate structures in
crystallization experiments is trithiocyanuric acid (TTCA). Consequently, the
“pure” crystalline phase of TTCA is difficult to obtain by crystallization from
solution, but can be obtained instead by desolvation of these solvate phases. In this
