Photoelectron spectrum of [14]annulene

Abstract

The He I photoelectron (PE) spectrum of [14]annulene has been measured and interpreted. UHF-MINDO/3 and UHF-MNDO computations of different molecular structures, characterized by the same π-perimeter (43 perimeter) but differing with respect to their π-bond situations (delocalized or localized) and by the geometry (different distortions imposed on the 43 perimeter in order to accommodate the four protons pointing inside), allowed us to select the energetically most favourable ones (bond delocalized D_2^-, C_2h^-, C_i^- and C_s structures). The different distortions imposed upon the 43 perimeter in order to accommodate the four protons pointing inside the ring did not dramatically affect their energies. This result is supported by recent abinitio calculations, including electron correlation energy, which favoured a bond delocalized C_s structure. The interpretation of the He I PE spectrum is accomplished by comparing the observed vertical ionization energies with CNDO/S-CI computed energies of the states of the radical cations computed for the different molecular structures. This comparison confirmed that [14]annulene has its π bonds delocalized in accordance with what was deduced from dynamic studies (NMR at different temperatures) and X-ray data. The assignment of the different ionization energies to the relevant states is further substantiated by a correlation of the four first observed ionization energies of [14]annulene with those measured (and assigned by computation) for 15,16-dimethyl-15,16-dihydropyrene and pyrene.

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