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

104 R.E. Meléndez · A.D. Hamilton
Fig. 5. Hydrogen bonds formed between an acidic hydrogen atom (D-H) and an acceptor (A)
for those who wish to apply this method to their hydrogen bonding analysis [37]
(see also the article by M.R. Caira in this volume).
Perhaps the best known hydrogen bonding pattern is that of carboxylic acids.
Carboxylic acids are divided into two sets based on the symmetry of their
O-H◊◊◊O hydrogen bonding patterns. They can form hydrogen bonding patterns
that contain a center of inversion (the dimer motif) and also aggregate in acentric
one-dimensional chains (catemers) resulting from the formation of hydrogen
bonds to two or more neighboring acids (Fig. 6).
A set of 139 benzoic acids was analyzed by Frankenbach and Etter, and a relationship
between the presence of inversion in the hydrogen bonding pattern and
inversion symmetry in their crystal structure was observed. Of the 118 crystals
that contained the dimer motif, 98% crystallized in centrosymmetric space
groups. On the other hand, of the 21 crystals that contained the chain motifs,
52% crystallized in acentric space groups. This study suggested that hydrogenbonded
aggregates bias the crystal growth process [38]. Dicarboxylic acids with
an antiparallel relationship between the hydrogen bonding groups can be expected
to form one-dimensional (1-D) strands [16]. For example, terephthalic acid
(1,4-benzene dicarboxylic acid) (5) self-assembles via the dimer motif to form
1-D tapes (Fig. 7). This packing is predictable because of the geometry and
functionality of terephthalic acid. However, the arrangement of the chains with
respect to their adjacent neighbors is less predictable, because there are no
strong directional forces between them. Thus, terephthalic acid has at least two
polymorphs [39].
a b
Fig. 6 a, b. Hydrogen bonding patterns of carboxylic acids: a dimer; b catemer
Fig. 7. Hydrogen bonded tape formed by terephthalic acid (5)