Why Cyclooctatetraene Is Highly Stabilized: The Importance of "Two-Way" (Double) Hyperconjugation

Despite its highly nonplanar geometry, the tub-shaped D2d cyclooctatetraene (COT) minimum is far from being an unconjugated polyene model devoid of important π interactions. The warped skeleton of D2d COT results in the large stabilization (41.1 kcal/mol) revealed by its isodesmic bond separation energy (BSE). This originates largely from the "two-way" hyperconjugation, back and forth across the C-C single bonds, between the CC/CH σ(σ*) and the C═C (π*)π orbitals. These hyperconjugative effects compensate for the substantial, but not complete, loss of π conjugation upon ring puckering. C-C single bond rotation of 1,3-butadiene involves a similar interplay between π conjugation and "two-way" double hyperconjugation and serves as a simple model for the inversion of D2d to D4h COT. The perpendicular rotational transition states of many other systems, e.g., the allyl cation, styrene, biphenyl, and ethene, are stabilized similarly by "two-way" hyperconjugation.