Ab initio study of conformational properties of ( Z, Z)-, ( E, Z)- and ( E, E)-cycloocta-1,3-dienes

Abstract

Ab initio calculations at HF/6-31G ∗ and B3LYP/6-31G ∗ levels of theory for geometry optimization and MP2/6-31G ∗//HF/6-31G ∗ for a single point total energy calculation are reported for the three geometrical isomers of cycloocta-1,3-diene 1– 3. The twist–boat-chair ( 1-TBC) conformation of the ( Z, Z)-isomer 1, with C 2 symmetry, is calculated to be slightly more stable than the twist–boat ( 1-TB, C 1) geometry by 3.2 kJ mol −1. Interconversion between 1-TBC and 1-TB conformations takes place via the C 2 symmetric transition state which is 49.4 kJ mol −1 above 1-TBC form. Degenerate interconversion of 1-TB with itself can take place via C 2 symmetric 1-boat or half-chair ( 1-HC, C s ). The calculated energy barrier for these processes are 30.1 and 51.3 kJ mol −1, respectively. The unsymmetrical boat-chair ( 2-BC) conformation of the ( E, Z)-isomer 2 is calculated to be 19.9 kJ mol −1 above the unsymmetrical twist-chair ( 2-TC) form. The calculated energy barrier for interconversion of 2-BC and 2-TC conformations is 50.0 kJ mol −1, while the barrier for swiveling of the trans double bond through the bridge is 166.8 kJ mol −1. The unsymmetrical 3-twist conformation of ( E, E)-isomer 3 is calculated to be the most stable form. The calculated energy barrier for ring inversion of the 3-twist conformation via C s symmetric 3-chair geometry, is 73.4 kJ mol −1.