title: Dynamic processes in [16]annulene: Möbius bond-shifting routes to configuration change.
abstract: Density functional and ab initio methods have been used to study the mechanisms for key dynamic processes of the experimentally known S4-symmetric [16]annulene (1a). Using BH&HLYP/6-311+G** and B3LYP/6-311+G**, we located two viable stepwise pathways with computed energy barriers (Ea = 8-10 kcal/mol) for conformational automerization of 1a, in agreement with experimental data. The transition states connecting these conformational minima have Möbius topology and serve as starting points for non-degenerate pi-bond shifting (configuration change) via Möbius aromatic transition states. The key transition state, TS1-2, that connects the two isomers of [16]annulene (CTCTCTCT, 1 --> CTCTTCTT, 2) has an energy, 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 Möbius topology conformational minima, were found with BH&HLYP energies within 22 kcal/mol of 1a, indicating that many possibilities exist for facile thermal configuration change in [16]annulene. This bond-shifting mechanism and the corresponding low barriers contrast sharply with those observed for cis/trans isomerization 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 instabilities with the BH&HLYP method but not with B3LYP. This result appears to be an artifact of the BH&HLYP method. These findings support the idea that facile thermal configuration change in [4n]annulenes can be accounted for by mechanisms involving twist-coupled bond shifting.
authors.affiliation: Department of Chemistry, University of San Francisco, 2130 Fulton Street, San Francisco, CA 94117-1080, USA.
authors.forename: Ryan P
authors.initials: RP
authors.lastname: Pemberton
authors.forename: Colleen M
authors.initials: CM
authors.lastname: McShane
authors.forename: Claire
authors.initials: C
authors.lastname: Castro
authors.forename: William L
authors.initials: WL
authors.lastname: Karney
mesh_terms: D003198:Computer Simulation; D047028:Macrocyclic Compounds / Q000737:chemistry*; D008956:Models, Chemical; D008958:Models, Molecular; D008968:Molecular Conformation; D011084:Polycyclic Aromatic Hydrocarbons / Q000737:chemistry*
chemical_list: D047028:Macrocyclic Compounds; D011084:Polycyclic Aromatic Hydrocarbons