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

40 J.P. Glusker
7.1
Descriptions of Hydrogen-Bonding Networks
Extended networks of hydrogen bonds are found in many crystal structures
and, as they run continuously through the crystal, they act cooperatively [3] with
lower energy than expected from the sum of the energies of the individual
hydrogen bonds. Hydrogen-bonding schemes in crystal structures are readily
analyzed, provided all the hydrogen atoms have been included in the refined
crystal structure. It is simply a matter of determining the interatomic distances
and angles between hydrogen-bond donors and acceptors, keeping in mind the
periodicity and space-group symmetry of the crystal structure.
Graph theory has been used [30, 84] as a method for describing such hydrogen
bonding. The pattern is described as G, where G describes the type (S for self
or intramolecular, D for noncyclic finite pattern such as a dimer, R for rings, and
C for chains). The sub- and superscripts d and a refer to the number of donors
and acceptors, and r is the size of the pattern. This scheme highlights extended
systems of hydrogen bonds and similarities and differences of patterns of
hydrogen bonding in crystals of related compounds. Some examples are given in
Fig. 32 (see also the article by M.R. Caira in this volume).
8
Crystal Surface Recognition and Chirality
The effects of certain impurities on the habit of a growing crystal provide
another directional aspect of intermolecular recognition. If an impurity is
similar (but not identical) to the molecules on the surface as well as in the bulk
of the crystal, it is adsorbed on a specific crystal face. Presumably the surface has
recognized that part of the impurity molecule that most resembles the molecules
composing the crystal. The impurity, however, presents to the exterior a
side onto which further molecules of host material cannot bind, as diagrammed
in Fig. 33. Therefore the growth of that face will be inhibited, and will be prominent
in the habit of the resulting crystal. For example, benzamide forms platelike
elongated crystals. If, however, some benzoic acid is added to the solution, the
resulting crystals will be elongated along a different direction.
Directional binding of molecules to crystal faces provides an interesting
means of controlling crystal habit and resolving optically active molecules
[85–87]. It provides food for thought with respect to the design of agents with
specific recognition requirements. An example is provided by the crystal structure
of glycine (which lacks an asymmetric carbon atom). This crystallizes as
bipyramids [86] in a monoclinic, centrosymmetric space group. The packing of
molecules and the prochirality of the C-H groups they present on the surface are
different for the 010 and 01 – 0 faces of the crystal. On addition of resolved amino
acids, pyramidal crystals with a basal face are formed. This is because the pro-R
and pro-S-hydrogen atoms of glycine can be distinguished in the crystal structure
of glycine, one lying on the (010) faces and the other on the (01 – 1) faces, as
shown in Fig. 34. Only D-amino acids [(R)-amino acids] can readily replace glycine
at growing (010) faces while only L-amino acids [(S)-amino acids] can grow