THE $^{1}L_{b}$ STATE OF 1,6:8,13-ETHANO[14]ANNULENE. VIBRATIONAL STRUCTURE AND THE "A BAND

Author Institution: Department of Chemistry, University of Utah; Department of Chemistry, Los Alamos National Laboratories; Department of Chemistry, University of UtahMeasurements of the single-site selected flourescence excitation spectra of matrix isolated 1,6:8, 13-ethano[14]annulene and its deuterated analog in solid nitrogen are presented. Utilizing molecular mechanics extended to delocalized systems by combining it with the PPP formalism, a ground state force field is constructed. INDO/S calculations of the excitation energy are used to build the harmonic force fields for vibrations in the first excited singlet state assuming the normal modes remain the same. Calculated Franck-Condon intensities for the transition from the ground to the first excited singlet state account for the majority of observed spectra features. The observed long progression in a vibration involving the transannular distance. The previously unassigned ``A band” is proposed to be a part of the fine structure of the $L_{b}$ band. It provides evidence for a pseudo Jahn-Teller coupling of the ground and first excited singlet state through a non-totally symmetric vibration. It can also be interpreted in terms of the two Kekule structures of the perimeter and its origin related to the onset of bond length alternation expected for annulenes