INTRODUCTION

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In 1885 Adolf von Baeyer proposed that if carbon prefers to have a tetrahedral geometry with bond angles of 109.5 degrees, then ring sizes other than 5 and 6 may be too strained to exist. The basis for this proposal was that all ring system are planar, clearly this is not the case. Ring systems can adopt many different conformations just a easily as acyclic compounds do.

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The Cambridge Structural Database is a unique resource providing a wealth of information on the preferred conformations of rings. Indeed, the database has been used extensively to map the conformational space of medium ring-sized cyclic and heterocyclic compounds (Acta Cryst., B49, 910, 1993) and of macrocyclic ether and thioether ligands (Acta Cryst., B53, 241, 1997).

•	By examining crystal structures we can and understand why the observed conformations are adopted in terms of the strain present in these systems