The radical cation and lowest Rydberg states of 1,4-diaza[2.2.2]bicyclooctane (DABCO)

The radical cation and the two lowest excited singlet Rydberg states of DAB CO (1,4-diazabicyclo[2.2.2]octane) are studied. Experimentally, the radical cation of DABCO is generated by either laser flash photolysis in solution at room temperature or by gamma-irradiation in a Freon glass at 77 K, and i ts electronic absorption and resonance Raman spectra in these two media are reported. The present resonance Raman spectra differ substantially from pr evious reports given in the Literature, and it is concluded that a number o f bands attributed previously to the DABCO radical cation are due to other species. Theoretically, the absorption and resonance Raman spectra are inte rpreted on the basis of density functional theory (DFT; B3LYP/6-31G(d)) cal culations and wave packet propagation methods. The same DFT calculations ar e used to interpret excitation and multiphoton ionization spectra of the tw o lowest singlet Rydberg states, making use of the close similarity between a Rydberg state and its ionic core. From the combined results it is conclu ded that DFT calculations with a relatively modest basis set provide a valu able framework to predict potential energy surfaces of radical cations and Rydberg states in terms of minima and Hessians.