The ground and ionic states of cyclohepta-1,3,5-triene and their relationship to norcaradiene states. New ¹H and ¹³C NMR spectra, and analysis of a new experimental photoelectron spectrum by ab initio methods

The strong inter-relationship between cyclohepta-1,3,5-triene (CHT) and norcaradiene (NCD) observed in some reactions, has been extended to include the energy surfaces for some low-lying ionic states. Whilst equilibrium structures of ²A^/ symmetry containing the CHT skeleton were routinely found, the structures emerging with ²A^// symmetry were found to be NCD ionic states. Surface studies, by variation of the C₁ to C₆ distance, showed minima for both state symmetries. Curve crossing which occurs in C_S symmetry, is avoided by distortion to C₁ symmetry. The CHT → NCD structural change is attributed to initial conrotatory closure of the singly occupied molecular orbital. A new synchrotron-based study of the photoelectron spectrum (PES) for CHT up to 25 eV shows little vibrational structure. We have assigned the overall PES up to 17 eV in considerable detail, using a variety of theoretical methods. The vertical ionization energy (VIE) sequence in the PES for CHT ions, is predicted to be: 12A/ < 12A// < 22A/ < 22A//. The calculated lowest ionic state, with lowest vibrational frequency 87 cm^-1, leads to a high density of vibrational states. The Franck-Condon (FC) envelopes of the two lowest PES bands have been analysed. The identity of the PES spectrum, as derived from CHT rather than NCD, was demonstrated by ¹H and ¹³C nuclear magnetic resonance (NMR) spectra of the sample; this agrees with the predicted PES spectra of CHT and NCD. The NCD predicted spectrum shows significant differences from CHT.