Why Cyclooctatetraene Is Highly Stabilized: The Importance of “Two- Way” (Double) Hyperconjugation Judy I. Wu, † Israel Ferna ́ ndez, ‡ Yirong Mo, § and Paul von Rague ́ Schleyer* ,† † Department of Chemistry, University of Georgia, Athens, Georgia 30602, United States ‡ Departamento de Química Orga ́ nica, Facultad de Ciencias Químicas, Universidad Complutense Madrid, 28040, Madrid, Spain § Department of Chemistry, Western Michigan University, Kalamazoo, Michigan 49008, United States * S Supporting Information ABSTRACT: Despite its highly nonplanar geometry, the tub-shaped D 2d cyclooctatetraene (COT) minimum is far from being an unconjugated polyene model devoid of important π interactions. The warped skeleton of D 2d 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 D 2d to D 4h 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. ■ INTRODUCTION Cyclooctatetraene (COT) is one of the decisive molecules in the history of chemistry. Its polyolefinic chemical properties 1,2 are quite unlike those of benzene. This discovery by Willstä tter in 1911 1 rang the “death knell” 3 on Johannes Thiele’s partial valence theory of aromaticity, 4 which predicted that all fully conjugated cyclic polyenes (annulenes) should display benzene-like aromatic behavior. The strongly nonplanar tub-shaped (D 2d ) COT ground state 2 is usually interpreted as being largely free from angle (and other) strain 5 and to be decidedly nonaromatic. 6,7 The twisting of the π system (the CCCC dihedral angles are nearly 60°) is widely regarded to effectively preclude conjugation: neither 4n +2 π aromaticity nor 4n π electron antiaromaticity is present. Fowler et al. demonstrated the absence of a paratropic π ring current (which persisted surprisingly close to the minimum) in the (D 2d ) COT ground state. 8 Hence, the energy of D 2d COT should be quite normal, i.e., neither destabilized nor stabilized. Pauling’s rough estimate of only 5 kcal/mol 9 for the “resonance energy” of COT based on early experimental data supported this expectation. In startling contrast, Polizer, et al. found that the isodesmic bond separation energy (BSE) of the D 2d COT minimum (eq 1, Scheme 1) is extremely large (41 kcal/mol). 10 Unexpectedly, tub-shaped D 2d COT is highly stabilized thermochemically! This qualitative conclusion is confirmed by the further comparisons in Scheme 1 with reference compounds that are stabilized by conjugation, hyperconjugation, and even aroma- ticity. The remarkably large stabilization of COT is nearly 2/3 that of benzene (BSE = 64.6 kcal/mol, eq 2a) and is over half that of styrene (77.3 kcal/mol, eq 2b), 11 the aromatic (and conjugated) C 8 H 8 isomer of COT. Although diene systems like COT with highly twisted adjacent π orbitals are not expected to “conjugate” in the usual sense, the -2.3 kcal/mol energy of eq 3a reveals that the stabilization of D 2d COT is nearly the same as that of four f ully conjugated syn-butadienes, and only modestly less than that of four (less appropriate) anti-butadiene models (eq 3b, -17.1 kcal/mol; also see ref 12). Equation 4 reveals that the stabilization of D 2d COT is nearly the same as 4 times the hyperconjugative stabilization of cis-cyclooctene (see ref 13). Politzer attributed the “considerable degree of stabilization” of COT “to limited π delocalization”, 10 but this Received: January 27, 2012 Published: February 28, 2012 Scheme 1. Homodesmotic and Isodesmic Equations for Evaluating the Stabilization Energies of D 2h COT, Benzene, and Styrene a a All data were computed at the B3LYP/6-311+G** level (exper- imental evaluations, based on the heats of formation of molecules at 298 K, are in parentheses). [a]: Based on a 3.7 kcal/mol syn-anti energy difference. Article pubs.acs.org/JCTC © 2012 American Chemical Society 1280 dx.doi.org/10.1021/ct3000553 | J. Chem. Theory Comput. 2012, 8, 1280-1287