198 Topics in Current Chemistry Editorial Board: A. de Meijere KN ...

198 M.R. Caira
[153].An important feature of this study was the determination of the activation
energy for rotation of one of the N-methyl groups in the crystal from variabletemperature
NMR data. The X-ray crystal structure was solved from high-resolution
powder XRD data using direct phasing methods and refined by the
Rietveld method. It was possible to reconcile the NMR-derived activation energy
with the crystal packing which revealed that one of the N-methyl groups is
significantly more hindered than the other. This study illustrates the potential of
a combined approach to probing molecular motion in the solid-state. Its application
to polymorphic systems is an obvious and desirable extension.
3.3
Polymorphic Systems and Polymorphic Changes – Case Studies
Four selected case studies are cited here to illustrate some of the difficulties
encountered in research on organic crystal polymorphism, even when relatively
simple molecules are involved. Since space does not permit discussion, précis of
these model studies are presented in the hope that the interested reader will
consult the original, absorbing accounts for details.
The polymorphism of anthranilic acid has been studied for over a century.
Despite this, many properties of crystals of this simple molecule remain obscure
and a recent re-examination of the phase transitions was undertaken using
X-ray and FT-IR techniques [154]. A special feature of this system is that phase
transitions appear to depend subtly on grinding pressure. The main conclusion
drawn from the investigation cited is that polymorph I is transformed into polymorph
III on heating, not into polymorph II as commonly stated in the literature.
The crystallization behaviour of the high explosive 2,4,6-trinitrotoluene
(TNT) has invoked intensive research since the first report on its optical investigation
in 1879 [155]. Despite numerous on-going studies, the conditions for
crystallizing the monoclinic and orthorhombic forms lack definition. A systematic
re-examination using Laue and Weissenberg X-ray methods, DSC, goniometry
and computer simulations was undertaken to establish the role of crystallization
conditions in determining the nature of polymorphism as well as TNT
crystal morphology [156]. Complicating features of this system are extensive
twinning in the monoclinic phase and remarkable similarity in the structures of
the two polymorphs which results in their XRD patterns being almost indistinguishable.
The study established (inter alia) conditions for crystallizing the two
forms, finding that their occurrence was determined by growth solvent only and
was independent of the rate of crystallization. The orthorhombic polymorph
was shown to be metastable relative to the monoclinic polymorph.
Terephthalic acid (TA) is an important industrial chemical used in the production
of polyester. Its purification involves multi-stage recrystallization under
extreme conditions. The phase characterisation and morphologies of Forms I
and II of TA were studied by HSM, thermal analysis, Raman spectroscopy, single
crystal Laue X-ray diffraction, lattice energy and morphological calculations
[157]. Contrary to earlier reports, this study revealed that Form II (not Form I)
is the thermodynamically stable form of TA under ambient conditions. The
apparent stability of Form I was shown to be due to twinning in microcrystals