Longest C-C Single Bond among Neutral Hydrocarbons with a Bond Length beyond 1.8 Å

On the basis of the intramolecular "core-shell strategy,'' we designed dihydropyracylene with two spiro(dibenzocycloheptatriene) units, for which theoretical calculations predicted a very weak C-C bond with a bond length around 1.8 angstrom. This bond is expanded by the forced adoption of an eclipsed conformation and by angle strain through a "scissor effect.'' The highly strained hydrocarbon was isolated as a thermally stable compound with no signs of diradical contribution because the weak C-C bond (core) is protected by the shape-persistent fusedring structure (shell). A Raman shift corresponding to the C-C stretching vibration (587 cm(-1)) was very different from that for ethane (993 cm(-1)). The bond length determined by X-ray (1.806(2) angstrom) was greater than the shortest nonbonded intramolecular CC contact (1.80(2) angstrom). The assumed limit for a C-C bond (1.803 angstrom) by supposing linear correlation between bond length and bond-dissociation energy for covalent bonding was proven to be invalid.