11,11-dimethyl-1,6-methano[10]annulene--an annulene with an ultralong CC bond or a fluxional molecule?

Extensive quantum chemical calculations involving more than 20 different methods and including vibrational, temperature, entropic, and environmental corrections suggest that 11,11-dimethyl-1,6-methano[10]annulene (1) is characterized by a broad, asymmetric single well potential minimum in which the molecule can carry out a large-amplitude vibration. This result is obtained by using CASPT2(14,14) and CCSD(T) together with a VTZ basis set. The average R(C1C6) distance of 1 is close to 1.8 Å, in agreement with X-ray diffraction measurements. Lower level methods fail because a reliable account of the electronic structure of bridged annulenes requires a balanced description of nondynamical and dynamical electron correlation effects as well as a correct assessment of bridge-annulene interactions. An independent determination of the distance R using the mean deviation between the calculated and measured (13)C NMR chemical shifts of 1 leads to a value of 1.79 Å. By using electron density, energy density, and the local C1C6 stretching mode, it is demonstrated that the covalent bond ceases to exist at 1.695 Å and that for larger R values through-space homoaromatic interactions lead to some stabilization. The peculiar potential of 1 is shown to be a result of the interaction of the methyl groups with the perimeter CC bonds bisected by the symmetry plane of the molecule. CASPT2(14,14), CASPT2(10,10), CCSD(T), and BD(T) calculations were also used to provide for the first time reliable descriptions of the valence tautomeric potentials for the parent molecule, 1,6-methano[10]annulene (2), and the system 1,3,5-cycloheptatriene-norcardiene (3). In the latter case, calculations confirm a previous kinetic measurement of the free activation energy but correct NMR-based estimates. The methodology described can be applied to other annulenes and fullerenes.