Low lying quartet states in diacetylene, triacetylene and benzene radical cations

The restricted coupled cluster with perturbative triples (RCCSD(T)) method has been used to calculate equilibrium geometries and excitation energies of the hitherto unknown quartet states in diacetylene, triacetylene and benzene radical cations. Spectroscopic data for the doublet states obtained with the same approach are found to be in good agreement with experiment. Whereas the (2)Pi(u) and (2)Pi(g) states in diacetylene and triacetylene cations form linear linear Renner-Teller pairs, strongly bent minima are calculated for the quartet states correlating with the (4)Pi(g) and (4)Pi(u) linear structures. In both ions there are quartets that lie close to the minimum energies of the first electronically excited doublet states. The (4)A(u) state of benzene radical cation lies below (CE1u)-E-2. and could be populated during the ultrafast nonradiative decay of this doublet state.