A photoelectron-spectroscopic investigation of benzologue tropones

Abstract

Abstract The He(I) photoelectron spectra of seventeen tropones are presented. With the help of correlation diagrams, implemented by simple molecular orbital models, a self-consistent interpretation of the observed data can be derived. 1. (a) The orbital sequence in tropone (1) proposed by Bunzli, Frost and Weiler (J. Amer. Chem. Soc., 96 (1974) 1952) and by Muller, Schweig and Vermeer (Angew. Chem., 86 (1974) 275) is confirmed (assuming Koopmans' approximation). 2. (b) The heuristically useful scheme proposed by the latter authors for the separation of the “inductive” and “conjugative” effects of the carbonyl group is reexamined. It is seen that the quantitative parameters derived through it depend critically on the necessary ad hoc assumptions that have to be made for the hypothetical reference system. 3. (c) In the 2,7-polymethylene-4,5-benzotropones (8(n)), the change in the oxygen lone-pair orbital energy with decreasing size n of the alkyl moiety confirms previous deductions concerning the change in electronic structure with increasing out-of-plane deformation of the tropone system. The drift in π-orbital energies, as a function of n, points to an important homoconjugative through-space interaction between the localized two-centre π-orbitals πa and πc in 8(n) with n ⩽ 9. 4. (d) Both LCBO (Linear Combination of Bond Orbitals) and HMO-type models (including first order perturbations for bond alternation) yield equally respectable parametrizations for the tropones investigated. From this we conclude that photoelectron spectroscopy does not yield information concerning the “aromatic character” (?) of tropones.