SYMMETRY-MODIFIED CONFORMATIONAL MAPPING AND CLASSIFICATION OF THE MEDIUM RINGS FROM CRYSTALLOGRAPHIC DATA .3. ENDO-UNSATURATED 7-MEMBERED RINGS

Crystallographic results retrieved from the Cambridge Structural Datab ase (CSD) have been used to perform systematic conformational analyses for cycloheptene, cyclohepta-1,3-diene, cyclohepta-1,4-diene and cycl ohepta-1,3,5-triene. Conformational mappings based on symmetry-adapted deformation coordinates show that, with the exception of cycloheptene (for which a C4 conformation is dominant). all conformations adopted by these unsaturated systems are intermediate between chair-like and b oat-like forms. Conformational classifications, using symmetry-modifie d Jarvis-Patrick clustering, show a qualitative correlation between cl ass populations and relative energy differences reported in published force-field and ab initio studies. Cyclohepta-1,3-diene is an apparent exception: computational methods predict a C(s)-envelope as the most stable form, in agreement with gas-phase structural studies, but the C 2-twist form, some 8-10 kJ mol-1 higher in energy, is ubiquitous in th e crystallographic literature. However, the majority of crystal struct ures show that one or both of the double bonds arise from benzo-fusion . Force-field calculations, carried out as part of this study, show th at the C conformation of the seven-membered ring is preferred in cases of benzo-fusion to relieve unfavourable H...H interactions.