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.

Abstract

The strong inter-relationship between cyclohepta-1,3,5-triene (CHT) and norcaradiene (NCD) systems observed in some reactions has been extended to include the energy surfaces for some low-lying ionic states. Equilibrium structures for ionic states of CHT with 2A' symmetry were routinely found; the structures emerging with 2A'' symmetry were NCD ionic states. A detailed analysis of these surfaces as a function of the C1 to C6 distance showed that while minima occurred for both state symmetries, curve crossing occurs in CS symmetry, which 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 photoelectron spectrum (PES) for CHT up to 25 eV shows little vibrational structure. We have assigned the PES up to 17 eV using a variety of theoretical methods. The calculated lowest ionic state, X2A', is predicted to have a very low vibrational frequency of 87 cm-1, leading to a high density of vibrational states. The Franck-Condon envelopes calculated for the two lowest states are almost completely contained within the envelope of the lowest PES band. A comparison of the predicted PES of CHT and NCD showed much closer agreement of the PES with that of CHT. An analysis of the 1H and 13C nuclear magnetic resonance (NMR) spectra of CHT showed no evidence of NCD. The increased chemical shifts arising from the higher frequencies used here lead to significant changes in appearance when compared with earlier NMR spectra.