Dynamic Processes in [16]Annulene: Möbius Bond-Shifting Routes to Configuration Change
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- 作者：Ryan P. Pemberton ; Colleen M. McShane ; Claire Castro ; William L. Karney
- 刊名：Journal of the American Chemical Society
- 出版年：2006
- 出版时间：December 27, 2006
- 年：2006
- 卷：128
- 期：51
- 页码：16692 - 16700
- 全文大小：731K
- 年卷期：v.128,no.51(December 27, 2006)
- ISSN：1520-5126
文摘
Density functional and ab initio methods have been used to study the mechanisms for key dynamicprocesses of the experimentally known S4-symmetric [16]annulene (1a). Using BH&HLYP/6-311+G** andB3LYP/6-311+G**, we located two viable stepwise pathways with computed energy barriers (Ea = 8-10kcal/mol) for conformational automerization of 1a, in agreement with experimental data. The transition statesconnecting these conformational minima have Möbius topology and serve as starting points for non-degenerate 
-bond shifting (configuration change) via Möbius aromatic transition states. The key transitionstate, TS1-2, that connects the two isomers of [16]annulene (CTCTCTCT, 1 
CTCTTCTT, 2) has anenergy, relative to the S4 isomer, that ranged from 6.9 kcal/mol (B3LYP/6-311+G**) to 16.7 kcal/mol(BH&HLYP/6-311+G**), bracketing the experimental barrier. At our best level of theory, CCSD(T)/cc-pVDZ(est), this barrier is 13.7 kcal/mol. Several other Möbius bond-shifting transition states, as well as Möbiustopology conformational minima, were found with BH&HLYP energies within 22 kcal/mol of 1a, indicatingthat many possibilities exist for facile thermal configuration change in [16]annulene. This bond-shiftingmechanism and the corresponding low barriers contrast sharply with those observed for cis/transisomerization in acyclic polyenes, which occurs via singlet diradical transition states. All Möbius bond-shifting transition states located in [16]- and [12]annulene were found to have RHF UHF instabilitieswith the BH&HLYP method but not with B3LYP. This result appears to be an artifact of the BH&HLYPmethod. These findings support the idea that facile thermal configuration change in [4n]annulenes can beaccounted for by mechanisms involving twist-coupled bond shifting.
-bond shifting (configuration change) via Möbius aromatic transition states. The key transitionstate, TS1-2, that connects the two isomers of [16]annulene (CTCTCTCT, 1 CTCTTCTT, 2) has anenergy, relative to the S4 isomer, that ranged from 6.9 kcal/mol (B3LYP/6-311+G**) to 16.7 kcal/mol(BH&HLYP/6-311+G**), bracketing the experimental barrier. At our best level of theory, CCSD(T)/cc-pVDZ(est), this barrier is 13.7 kcal/mol. Several other Möbius bond-shifting transition states, as well as Möbiustopology conformational minima, were found with BH&HLYP energies within 22 kcal/mol of 1a, indicatingthat many possibilities exist for facile thermal configuration change in [16]annulene. This bond-shiftingmechanism and the corresponding low barriers contrast sharply with those observed for cis/transisomerization in acyclic polyenes, which occurs via singlet diradical transition states. All Möbius bond-shifting transition states located in [16]- and [12]annulene were found to have RHF UHF instabilitieswith the BH&HLYP method but not with B3LYP. This result appears to be an artifact of the BH&HLYPmethod. These findings support the idea that facile thermal configuration change in [4n]annulenes can beaccounted for by mechanisms involving twist-coupled bond shifting.