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

Supramolecular Synthons and Pattern Recognition 87
LEZJAB 1 2
Fig. 9. Polymorphs A and B of 2,6-dihydroxybenzoic acid (LEZJAB)
presence of both phenolic and carboxylic OH groups provides alternative
hydrogen bonding motifs of comparable energy and with it a route towards
polymorphism. (A more detailed discussion of this topic is given in the article
by M.R. Caira in this volume).
9
Comparison of Crystal Structures
A B
Crystals are composed of molecules but crystal structures cannot be derived or
anticipated in simple ways from molecular structures. Similar molecules can
have very dissimilar crystal structures, and dissimilar molecules can have very
similar crystal structures. This is because the core constituents of a crystal are
the patterns and topologies of intermolecular interactions that result from complementary
approaches of molecular functional groups and because the exact
patterns formed depend not just on the functional groups present in the molecules
but also on their relative juxtapositioning. The complexity of this issue will
be further appreciated when it is realised that all portions of a molecule are
supramolecular functionalities. Therefore, a detailed understanding of crystal
packing and crystal design depends very substantially on viewing the molecule
as an organic whole. Indeed, the supramolecular paradigm is particularly
appropriate in the crystalline world because the essential structural attributes of
a crystal are supramolecular rather than molecular in nature.
Given such realities, an immediate need in crystal engineering is to be able to
compare crystal structures. Many will appreciate that the structure of, say,