Photoelectron spectra of polynuclear aromatics. III. The effect of nonplanarity in sterically overcrowded aromatic hydrocarbons

The high-resolution photoelectron spectra of 65 polycyclic aromatic hydrocarbons containing up to 11 condensed benzene rings have been obtained in the gas phase at temperatures between 20 and 450°C. Vibrational analysis of the spectra coupled with semiempirical MO calculations permitted positive identification of the π ionizations in the 6–11 eV range. Irrespective of size and symmetry of the hydrocarbons, two-thirds of all π bands are found to precede the onset of σ ionization, which occurs at 10.6±0.2 eV in the molecules beyond naphthalene. The possibility of Jahn-Teller forces operating in the degenerate ionic ground states of D3h and D6h molecules is discussed, and comments are made on the origin of the 1350 cm−1 vibrational progression that prevails in the first band of all planar hydrocarbons. In sterically overcrowded systems, excitation of low-frequency twisting and out-of-plane bending modes results in a blurring of the vibrational structure. In cases where reliable x-ray data are unavailable (e.g., dibiphenylene-ethylene), this observation allows a distinction to be made between planar and nonplanar hydrocarbons. The present data are of sufficiently high quality to permit the question as to the planarity of phenanthrene to be settled with some certainty.