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

Supramolecular Synthons and Pattern Recognition 71
Table 1. Comparison of molecular and solid state supramolecular synthesis
Organic synthesis Crystal engineering
1. Molecular targets Targets are networks
2. Connectivity is defined in terms of Hydrogen bonds, chemical and van der
covalent bonds Waals interactions define supramolecular
connectivity
3. Structure descriptors are functional groups Structure descriptors are supramolecular
synthons
4. Reactant Æ Transition state Æ Product Molecule Æ Crystal nucleus Æ Crystal
5. Build molecule in separate and discrete steps Crystallisation occurs in a single step,
intermediates are not isolable
6. Isomers (geometrical, stereo-) are common Polymorphism is uncommon but still a
in organic molecules nuisance in particular systems
7. Goals: structure elucidation, new reactions Goals: Understanding of intermolecular
and reagents, stereo- and enantiocontrol, total interactions and crystal packing,
synthesis of complex molecules connections between molecular and
crystal structure
8. Motivations: aesthetics, utilitarian Motivations: aesthetics, utilitarian
(pharmaceuticals, agrochemicals, (nanostructures, materials and
fine chemicals) functional solids, NLO, molecular
electronics)
9. Chemical Everest: Vitamin B 12, Supramolecular Everest: Anticipate the
Gingkolide, Palytoxin, Taxol crystal structure(s) of a molecule
5
The Cambridge Structural Database (CSD)
Supramolecular synthons or patterns in crystals are the key structural units that
help in focussing supramolecular synthetic effort and also in facilitating comparisons
between crystal structures. It therefore becomes important to review
methods that are available for their identification and analysis. Synthons are
built from hydrogen bonds and non-covalent interactions and contain information
about the directional preferences of intermolecular interactions and the
topology of attached groups. For an effective and predictable utilisation of synthons
in crystal structure design, a better understanding of the constituent
hydrogen bonds and intermolecular interactions is essential. X-ray crystallography
provides precise and unambiguous information on these interactions
and this information is contained in the Cambridge Structural Database (CSD)
(see also the article by J.P. Glusker in this volume).
The CSD (April 1997, version 6.2, 167,797 entries) contains accurate crystal
structure data for organic, inorganic and organometallic compounds [36]. The
retrieval of geometrical data from the CSD and its subsequent analysis by statistical
procedures allows for extremely reliable conclusions pertaining to the
nature of intermolecular interactions. Only conclusions based on examination
of a statistically significant number of crystal structures are chemically mean-