Ionized Bicyclo[2.2.2]oct-2-ene: A Twisted Olefinic Radical Cation Showing Vibronic Coupling

The bicyclo[2.2.2]oct‐2‐ene radical cation (1.+) exhibits matrix ESR spectra that have two very different types of γ‐exo hydrogens (those hydrogens formally in a W‐plan with the alkene π bond), a(2 H) about 16.9 G and a(2 H) about 1.9 G, instead of the four equivalent hydrogens as would be the case in an untwisted C2v structure. Moreover, deuterium substitution showed that the vinyl ESR splitting is not resolved (and under about 3.5 G); this is also a result of the twist. Enantiomerization of the C2 structures is rapid on the ESR timescale above 110 K (barrier estimated at 2.0 kcal mol−1). Density functional theory calculations estimate the twist angle at the double bond to be 11–12 degrees and the barrier as 1.2–2.0 kcal mol−1. Single‐configuration restricted Hartree–Fock (RHF) calculations at all levels that were tried give untwisted C2v structures for 1.+, while RHF calculations that include configuration interactions (CI) demonstrate that this system undergoes twisting because of a pseudo Jahn–Teller effect (PJTE). Significantly, twisting does not occur until the σ‐orbital of the predicted symmetry is included in the CI active space. UHF calculations at all levels that include electron correlation (even semiempirical) predict twisting at the alkene π bond because they allow the filled α and the β hole of the SOMO to have different geometries. The 2,3‐dimethylbicyclo[2.2.2]oct‐2‐ene radical cation (2.+) is twisted significantly less than 1.+, but has a similar temperature for maximum line broadening. Neither the 2,3‐dioxabicyclo[2.2.2]octane radical cation (3.+) nor its 2,3‐dimethyl‐2,3‐diaza analogue (5.+) shows any evidence of twisting. Calculations show that the orbital energy gap between the SOMO and PJTE‐active orbitals for 3.+ is too large for significant PJTE stabilization to occur. The pseudo‐Jahn–Teller effect is evidenced in ionized bicyclo[2.2.2]oct‐2‐ene by the experimental detection of a double minimum along the torsional vibrational mode at the olefin bond. ESR studies show that in each of the twisted enantiomers, the hyperfine coupling is largely restricted to the two diagonal exo hydrogens (filled circles in figure) such that the spectrum changes from a triplet to a quintet as the twisting rate increases with temperature. Theoretical calculations confirm the role of vibronic coupling.