Fulvalenes, Fulvenes, and Related Molecules: An ab Initio Study

Ab initio calculations using conventional (HF/6-31G* and MP2/6-31G*) and density functional theory (B-LYP/6-31G*) methods have been used to determine the structures of the [n]fulvene and [n,m]-fulvalene (n, m = 3, 5, 7) series of molecules, with particular emphasis on heptafulvalene (n = m = 7: 12). Calculations have also been performed on the parent cycloalkenes: cyclopropane, cyclopentadiene, and cycloheptatriene (1-3, respectively). All the fulvenes (n = 3, 5, 7: 4-6, respectively) and the smaller fulvalenes (n = 3, m = 3, 5, 7: 7-9, respectively, and n = m = 5: 10) are found to be planar. Pentaheptafulvalene (n = 5, m = 7: 11) adopts a very slightly nonplanar C_s arrangement of the five- and seven-membered rings. Heptafulvalene (12) is predicted to have an anti-folded C_2h structure, in accord with the X-ray crystal structure. We propose that the underlying reason for 11 and 12 adopting nonplanar conformations is the proximity of the H₂ and H_2′ hydrogen atoms which promotes a distortion of the rings away from planarity at the central fulvalenic C=C double bond. In the process, π-overlap is lost but this is partially regained by pyramidalization of the carbon centers in the seven-membered ring(s). The degree of folding is substantially more pronounced in 12 than in 11. Our calculated dipole moments, π-electron distributions, bond alternation parameters, and energy comparisons indicate that the unknown smallest fulvalene, triafulvalene (7), is highly destabilized with localized bonding while triapentafulvalene (8), which is also unknown, is predicted to be stabilized and quite delocalized, consistent with Hückel 4n + 2 considerations.