Ab Initio Study of Conformational Properties of (Z,Z)-, (E,Z)-, and (E,E)-Cyclonona-1,5-dienes

Ab initio calculations at the HF/6-31G* level of theory for geometry optimization and MP2/6-31G*//HF/6-31G* for a single point total energy calculation are reported for the important energy-minimum conformations and transition-state geometries of (Z,Z)-, (E,Z)-, and (E,E)-cyclonona-1,5-dienes. The C2 symmetric chair conformation of (Z,Z)-cyclonona-1,5-diene is calculated to be the most stable form; the calculated energy barrier for ring inversion of the chair conformation via the Cs symmetric boat-chair geometry is 58.3 kJ mol−1. Interconversion between chair and twist-boat-chair (C1) conformations takes place via the twist (C1) as intermediate. The unsymmetrical twist conformation of (E,Z)-cyclonona-1,5-diene is the most stable form. Ring inversion of this conformation takes place via the unsymmetrical chair and boat-chair geometries. The calculated strain energy for this process is 63.5 kJ mol−1. The interconversion between twist and the boat-chair conformations can take place by swiveling of the trans double bond with respect to the cis double bond and requires 115.6 kJ mol−1. The most stable conformation of (E,E)-cyclonona-1,5-diene is the C2 symmetric twist-boat conformation of the crossed family, which is 5.3 kJ mol−1 more stable than the Cs symmetric chair–chair geometry of the parallel family. Interconversion of the crossed and parallel families can take place by swiveling of one of the double bonds and requires 142.0 kJ mol−1.