Sequential Inversions of Side Blades in [3₆](1,2,3,4,5,6)Cyclophane Studied by Ab Initio Molecular Dynamics and...

The [3₆](1,2,3,4,5,6)cyclophane ([3₆]superphane) is characterized by a conformational phenomenon associated with the possibility of inversion of six trimethylene blades connecting two opposite benzene rings. Detailed characterization of the transition pathway from the most energetically stable C_6h form, having all blades bent in the counterclockwise direction (0), to its equivalent form with all blades bent in the clockwise direction (0') is studied by Ab Initio Molecular Dynamics (AIMD) and Quantum Mechanical (QM) methods. This transition requires four intermediate local minima and five transition states with very similar relative energies in the range 10.2-10.9 kcal/mol (in excellent agreement with the experimental energy barrier of 10.9 kcal/mol) and proceeds via sequential correlated inversions of side blades. Previously unknown structures of the [3₆]superphane are also found. Notably, among them, there is yet another transition state, TS3', which can be considered as an alternative to TS3 being halfway along the 0  ⇌  0' transition pathway. AIMD simulations confirm the transition path realized by sequential blade inversions and support the possibility of TS3' occurrence in the process. Qualitative energy differences between individual stationary points are explained by a model that introduces different types of interactions depending on the mutual arrangements of adjacent lateral superphane blades.