Ab initio Study of Structural and Conformational Properties of Five- to Nine-Membered Cyclic 1,2,3-Trienes

Abstract

The structures and relative energies of fundamental conformations of cyclopenta-1,2,3-triene, cyclohexa-1,2,3-triene, cylohepta-1,2,3-triene, cycloocta-1,2,3-triene, and cyclonona-1,2,3-triene were calculated by the HF/6-31G* and MP2/6-31G*//HF/6-31G* methods. Only a C2v symmetric planar conformation is available to cyclopenta-1,2,3-triene and cyclohexa-1,2,3-triene. The calculated energy barrier for ring inversion of the CS symmetric puckerd conformation of cyclohepta-1,2,3-triene via the planar geometry is 62.2 kJ·mol−1. The C2 symmetric twist conformation of cycloocta-1,2,3-triene was calculated to be the most stable one. Conformational racemization of the twist form takes place via the CS symmetric half-chair geometry, which is by 60.8 kJ·mol−1 less stable than the twist conformer. The CS symmetric chair and unsymmetrical twist-boat conformations of cyclonona-1,2,3-triene were calculated to have similar energies; their interconversion takes place via an unsymmetrical low-energy (18.4 kJ·mol−1) transition state. The twist (C2) and boat (CS) geometries of cyclonona-1,2,3-triene are higher in energy by 13.2 and 33.9 kJ·mol−1, respectively. Ring inversion in chair and twist-boat conformations takes place via a twist form as intermediate and requires 33.6 kJ·mol−1.