BENZOCYCLOBUTENE AND BENZO[1,2,4,5]DICYCLOBUTENE - THE POSSIBILITY OF VALENCE ISOMERISM

Ab initio calculations are reported on Kekule forms of the title compounds. For benzocyclobutene, the HF and RMP2 quinoid structure 1b is much higher is energy than the expected benzenoid isomer 1a. CASSCF pi calculations, however, do not support the quinoid structure as a local minimum. In the case of benzodicyclobutene, a C2v form, 3b, is ca. 10 kcal/mol higher in energy than the D2h form, 3a. Both structures are obtained at the GVB level in a small basis set. While isomerism between Kekule forms of 3 may exist, the phenomenon will be difficult to demonstrate experimentally. Heats of formation are computed for all species, including pentalene. The heat of hydrogenation of benzocyclobutene 1a to dihydrobenzocyclobutene 2 is calculated to be 52.6 kcal/mol.