Gas-Phase Electron-Diffraction Investigation and Quantum-Chemical Calculations of the Structure of 1,5-Dimethylsemibullvalene-2,4,6,8-tetracarboxylic Dianhydride

The bridged homotropilidines have been of interest for decades because their molecules offer the potential for homoaromaticity. Although many of these have been shown not to be homoaromatic, the energy differences of the delocalized (homoaromatic) forms and the localized (nonhomoaromatic) ones, and the barriers to the interconversion of the localized forms via a Cope rearrangement, have been found to vary greatly. The title compound is a strong candidate for homoaromaticity, and, since the structures of the possible localized and delocalized forms could differ significantly, we have carried out an electron‐diffraction investigation of it augmented by quantum‐mechanical calculations with different basis sets at several levels of theory. Three models were explored: one representing a localized form of Cs symmetry, one a delocalized form of C2v symmetry, and one a 2 : 1 mixture of the localized/delocalized forms. Although none of the models could be ruled out, the experimental evidence slightly favors the Cs form. These results are consistent with those from the DFT B3PW91 calculations with basis sets ranging from 6‐31G(d) to cc‐pVTZ, which, surprisingly, predict essentially equal thermally corrected free energies for each. The results are discussed.