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

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 (ZZ)-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 U 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 U 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 (C) 2003 Elsevier Science B.V. All rights reserved.