MAPPING COMMON MOLECULAR FRAGMENTS IN CRYSTAL-STRUCTURES TO EXPLORE CONFORMATION AND CONFIGURATION-SPACE UNDER THE CONDITIONS OF A MOLECULAR ENVIRONMENT

Changes of torsion angles and deformations of valence angles, especial ly in coordination polyhedra, are mainly responsible for the inherent conformational flexibility of molecules. They allow them to adopt vari ous shapes and to adapt to the requirements in a particular molecular environment. The latter aspect makes these systems only tractable by c omputational methods with an enormous effort. Focusing on common molec ular fragments imbedded in various environments in crystal structures allows one to study possible deformations, to elucidate conformational preferences (low-energy conformations), and to trace conformational i nterconversions. Large data sets of crystal structures can be analyzed by means of statistical methods to discover correlations and systemat ics among the molecular dimensions. In this overview several systems a re described which show the scope of the approach. Conformational stud ies of rigid phenyl rotors reveal deviating properties depending on th e actual constitution of the fragments (Ph-X-Ph, Ph-X-Y-Ph, Ph3X). In these and in more complex systems like metal phosphine complexes, the phenyl groups perform various concerted movements related to different gearing motions of interlocked cog-wheels. Conformational properties and low-energy conformers of ring systems are accessible from the eval uation of crystal data. Also larger and unsymmetric systems composed o f cyclic and open-chain portions can be studied by these methods. Chan ges of the valence angle relationships at a particular atomic center e nable molecules to access different areas of conformational space. Pos sible deformations of coordination geometries, observed in crystal str uctures, are discussed for examples with three- up to eight-coordinati on. Interconversion pathways among different coordination geometries c an be mapped which explain putative reaction pathways or the fluxional behavior of coordination complexes that are known to occur under solu tion or gas-phase conditions.