Computational Evaluation of the Evidence for Tri-trans-[12]Annulene

Automerization in tri-trans-[12]annulene (1) was investigated by DFT, MP2, and coupled-cluster methods. Using the highest level of theory employed here, CCSD(T)/cc-pVDZ//BHandHLYP/6-311+G(d,p), we located two low-energy pathways for degenerate conformational change from the lowest-energy conformer of 1 (1a): one with E_a = 4.5 kcal/mol that interconverts the three inner trans hydrogens with the three outer trans hydrogens and one with E_a = 2.7 kcal/mol that interconverts the three inner hydrogens with each other. These results are consistent with the experimental results of Oth and co-workers on [12]annulene 1a (Oth, J. F. M.; Röttele, H.; Schröder, G. Tetrahedron Lett.1970, 61). The conformational exchange of the inner trans hydrogens with the outer ones is predicted to occur via a one-step process involving a C₂-symmetric transition state and not via the D₃-symmetric transition state (1b) that was postulated earlier. Conformer 1b was found to be a shallow minimum 6.7 kcal/mol above 1a with a barrier of 0.4 kcal/mol for conversion to 1a. Finally, GIAO-B3LYP/6-311+G(d,p) and BHandHLYP/6-311+G(d,p) computed ¹H NMR chemical shifts of 1a and three other low-lying isomers support Oth's original assignment of observed ¹H NMR peaks to 1a at both low and high temperature.