High-level studies of the singlet and triplet states of quadricyclane, including analysis of a new experimental vacuum ultraviolet absorption spectrum by configuration interaction and density functional calculations

A synchrotron-based vacuum ultraviolet absorption spectrum (VUV) of quadricyclane (QC) is reported with energies up to 10.8 eV. Extensive vibrational structure has been extracted from the broad maxima by fitting short energy ranges of the VUV spectrum to high level polynomial functions and processing the regular residuals. Comparison of these data with our recent high-resolution photoelectron spectral (PES) of QC showed that this structure must be attributed to Rydberg states. Several of these appear before the valence states at higher energies. Both types of states have been calculated by configuration interaction (CI), including symmetry-adapted cluster studies (SAC-CI), and time dependent density functional theoretical methods (TDDFT). There is a close correlation between the SAC-CI vertical excitation energies (VEE) and both B3LYP and especially CAM-B3LYP determined ones. The VEE for several low-lying s-, p-, d- and f-Rydberg states (RS), have been determined by SAC-CI and adiabatic excitation energies (AEE) by TDDFT methods. Searches for equilibrium structures for 1^1,3A₂, and 1¹B₁ states for QC led to rearrangement to a norbornadiene structure. Determination of the experimental 0⁰ band positions, which show very low cross-sections, has been assisted by matching features in the spectra with Franck-Condon fits. Herzberg-Teller vibrational profiles for the Rydberg states are relatively more intense than the FC ones, but only at high energy, and are attributed to up to 10 quanta. The vibrational fine structure of the RS calculated by both FC and HT procedures, gives an easy route into generating HT profiles for ionic states, which usually require non-standard procedures.