An electron paramagnetic resonance and theoretical study of the radical cationic C8H8+˙ system

Abstract

EPR spectroscopy shows that the radical cation of cyclooctatetraene (1), as formed by γ-radiolysis in solid freon matrices at 77 K, is converted by visible (blue) light (400–600 nm) to the isomeric bicyclo[3.3.0]octa-2,6-diene-4,8-diyl isomer (2). On subsequently irradiating with red light (>570 nm), 2 is converted to a third isomer with couplings (a= 13.5 G) to two protons which are α- to the SOMO, as may be judged from the g and A anisotropy. Restricted HF AM1 calculations establish the structure 11 for this isomer and provide a rationale for the conversion of 2 to 11 in the clear correlation that is found between the two highest occupied orbitals of excited 2* and 11. Other local minima are located by extensive exploration of the C₈H₈⁺˙ potential energy surface, and possible conversions between selected examples of these are calculated to compare with published experimental results.