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

Crystalline Polymorphism of Organic Compounds 185
Fig. 8. Schematic description of an apparatus for simultaneous visual and DSC observations
using the Mettler Thermosystem FP800 (Reprinted with permission from [108])
different pseudopolymorphs of the same compound. To ensure transparency to
IR radiation, samples are usually prepared by trituration of crystalline material
and suspension in Nujol or by pressing a co-ground mixture of the compound
with KBr into a disk. However, since complete polymorphic transformation can
sometimes be effected even by gentle manual grinding, it is advisable to confirm
the existence of polymorphs by an independent method prior to a detailed IR
analysis. Differences in molecular conformation and crystal packing features for
different polymorphs of an organic compound manifest themselves in IR
spectra chiefly by frequency shifts and splittings in, e.g., N-H, O-H and C=O
absorption peaks (when these functional groups are present) due to differences
in hydrogen bonding arrangements. The goal of IR analysis is the deduction of
polymorphic structural differences from such spectral effects, but detailed
interpretation is often limited and it is somewhat easier to rationalise these
effects retrospectively if single crystal X-ray data can be obtained for the various
forms.
This is illustrated by the following examples from our laboratory. A study of
the literature on the compound sulphamerazine revealed that its polymorphism
was controversial. The crystal structure of the drug recrystallized from acetone
had been shown to be orthorhombic, Pbca with Z=8 [109]. Centrosymmetric
hydrogen bonded (N-H◊◊◊N) dimers exist in the crystal structure as shown