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

Abstract

[reaction: see text] 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 C(2)-symmetric transition state and not via the D(3)-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 (1)H NMR chemical shifts of 1a and three other low-lying isomers support Oth's original assignment of observed (1)H NMR peaks to 1a at both low and high temperature.

Structures of Annulenes and Model Annulene Systems in the Ground and Lowest Excited States

“…The five low energy isomers of [12]annulene with the indication of cis/trans configuration around double bond, symmetry group, relative energies (kcal/mol) with respect to the CT CT CT isomer and CC bondlengths (Å) on selected structures. CCSD(T)/cc-pVDZ//BHHLYP/6-311+G * * results [208,209].…”

“…More recently, the CCT CCT or di-trans isomer has been formed as dianion [207]. At the CCSD(T)/cc-pVDZ calculation level on the BHHLYP/6-311+G * * optimized geometries (CCSD(T)/cc-pVDZ//BHHLYP/6-311+G * * ) five minima have been located on the [12]annulene hypersurface within 5-6 kcal/mol above the absolute minimum, which is the CCCCCT or mono-trans isomer [208,209]. The five structures, their symmetries and relative energies are shown in Figure 25.…”

“…Among the conformational processes occurring in the tri-trans isomer those of Figure 26, i.e., (a) the exchange 1a 1a of the three inner with the three outer trans hydrogen atoms and (b) the enantiomeric process of the three inner hydrogens 1a 1a , have been investigated in detail [208].…”

“…Figure 27. Energy profiles of the 1a → 1a and 1a → 1a reaction paths, adapted CCSD(T)/cc-pVDZ//BHHLYP/6-311+G * * results [208].…”

Theoretical investigation of the nature and strength of simultaneous interactions of π–π stacking and halogen bond including NMR, SAPT, AIM and NBO analysis

“…The DFT methods significantly reduced computer cost for the calculation of NMR data. Thus, tremendous progress is made by DFT methods for these calculations [49][50][51][52].…”