The search for homoaromatic semibullvalenes. 6. X-ray structure and charge density studies of 1,5-dimethyl-2,4,6,8-semibullvalenetetracarboxylic dianhydride in the temperature range 123-15 K

The structure of the bisannelated semibullvalene II, which was originally predicted to be in a symmetrical homoaromatic ground state, was investigated by ultralow-temperature diffraction experiments down to 15 K and by related HF calculations. High-resolution X-ray data taken at 15 K (sin theta/lambda(max) = 1.074 Angstrom(-1)) provided a detailed electron density distribution that allowed a full topological analysis. The experimental and HF electron densities for II are in excellent accord. The electron density regions between C(2,8) clearly reveal a "normal cyclopropyl sigma-bond" and very sparse density between C(4,6) corresponding with little or no interaction between these latter two centers. These results certainly confirm that in the solid state 11 is not homoaromatic. The experimental data at 15 K provide Limiting values of structural parameters for a single tautomer of II. In particular, the C(2,8) and C(4,6) distances are extremely useful base values for a bisannelated semibullvalene with an exceptionally low activation barrier to the Cope rearrangement.