Quantum mechanical designs toward planar delocalized cyclooctatetraene: A new target for synthesis

Ab initio and hybrid density functional quantum mechanical computation are applied to the structure and energetics of a series of annelated cyclooctat etraenes. Tetrakis-cyclobuteno, perfluorocyclobuteno or bicyclo[2.1.1]hexen o annelations result in planar structures with distinct exo and endo valenc e tautomers of the double bonded cycle. The contribution of each basic anne lation to the exo/endo relative energy is estimated. An additivity scheme f or approximating the energy of a mixed system is developed and compared to the quantum mechanical prediction. Bis bicyclic annelation to the a and d p ositions creates "valence tautomeric frustration" and strongly perturbs the molecular structure. This phenomenon leads to a general design for a plana r cyclooctatetraenes where the "delocalized" diradicaloid state is the mini mum energy form. These compounds are seen as excellent targets for chemical synthesis.