The Radical Cation and Lowest Rydberg States of 1,4-Diaza[2.2.2]bicyclooctane (DABCO)

Abstract

The radical cation and the two lowest excited singlet Rydberg states of DABCO (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 γ-irradiation in a Freon glass at 77 K, and its electronic absorption and resonance Raman spectra in these two media are reported. The present resonance Raman spectra differ substantially from previous reports given in the literature, and it is concluded that a number of bands attributed previously to the DABCO radical cation are due to other species. Theoretically, the absorption and resonance Raman spectra are interpreted on the basis of density functional theory (DFT; B3LYP/6-31G(d)) calculations and wave packet propagation methods. The same DFT calculations are used to interpret excitation and multiphoton ionization spectra of the two lowest singlet Rydberg states, making use of the close similarity between a Rydberg state and its ionic core. From the combined results it is concluded that DFT calculations with a relatively modest basis set provide a valuable framework to predict potential energy surfaces of radical cations and Rydberg states in terms of minima and Hessians.