Jean-Claude Bunzli - Photoelectron spectrum of [14]annulene

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  Baumann,H., Bunzli,J.C. Journal of the Chemical   Society-Faraday Transactions 1998  94 (18) 2695-21

  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 pi-perimeter (43 perimeter) but   differing with respect to their pi-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 ab initio 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 pi 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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