Spiro[4.4] nonatetraene and its positive and negative radical ions: Molectronic structure investigations

The electronic structure of spiro[4.4]nonatetraene I as well as that of its radical anion and cation were studied li different spectroscopies. The ele ctron-energy-loss spectrum in the gas phase revealed the lowest triplet sta te at 2.98 eV and a group of three overlapping triplet states in the 4.5-5. 0 eV range. as well as a number of valence and Rydberg singlet excited stat es. Electron-impact excitation functions of pure vibrational and triplet st ates identified various states of the negative ion, in particular the groun d state with an attachment energy tit 0.8 eV. an excited state correspondin g to a temporary electron attachment to the 2b, MO at an attachment energy of 2.7 eV. and a core excited state at 4.0 eV. Electronic-absorption spectr oscopy in cryogenic matrices revealed several states of the positive ion. i n particular a richly structured first band at 1.27 eV, and the first elect ronic transition of the radical anion. Vibrations of the ground state of th e cation were probed by IR spectroscopy in a cryogenic matrix. The results are discussed on the basis of density-functional and CASSCF/CASPT2 quantum- chemical calculations. In their various forms, the calculations successfull y rationalized the triplet and the singlet (valence and Rydberg) excitation energies of the neutral molecule. the excitation energies of the radical c ation. its IR spectrum, the vibrations excited in the first electronic abso rption band, and the energies of the ground and the first excited states of the anion. The difference of the anion excitation energies in the gas and condensed phases was rationalized li a calculation of the Jahn-Teller disto rtion of the anion ground state. Contrary to expectations based on a single -con figuration model for the electronic states of 1. it is found that the gap between the first two excited states is different in the singlet and th e triplet manifold. This finding can be traced to the different importance of configuration interaction in the two multiplicity manifolds.