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

Supramolecular Synthons and Pattern Recognition 91
certain functional group category, no dominant motif is adopted by the molecules.
Such appears to be the case for acetylenic alcohols containing the fragment
63 [93]. Retrieval of data for sub-structure 63 from the CSD produced a list
of 76 compounds. The hydrogen bonding networks of these molecules did not
produce a single recurring pattern that can be associated with this sub-structure
[94]. Rather, four different motifs could be identified (Scheme 13), with none
being predominant. This is in contrast to the behaviour of simple monoalcohols
that crystallise mainly in two type of O-H◊◊◊O aggregates – chains and rings
[95]. There is significant structural interference in these acetylenic alcohols.
Instead of the CC-H and OH groups associating in their isolated networks,
complex patterns are formed wherein CC-H◊◊◊OH 65 and OH◊◊◊CC-H 66
hydrogen bonds are found in addition to the expected CC-H◊◊◊CC-H 67 and
O-H◊◊◊O-H 64 hydrogen bonds. This may be due to the fact that the four different
hydrogen bonding patterns between the two functional groups, O-H◊◊◊O 64,
C-H◊◊◊O 65, O-H◊◊◊p 66, and C-H◊◊◊p 67 have comparable energies, steric requirements
and topologies.
At a certain level, the absence of patterns in groups of crystal structures is a
manifestation of the fact that the molecular and crystal structures need not be
related in obvious ways. After all, the geminal positioning of hydroxy and
ethynyl groups in 63 is only a molecular structural feature. There is no need that
63
64 65 66
Scheme 13. Four common supramolecular synthons 64–67in 76 occurrences of the acetylenic
alcohol fragment 63
67
68 69