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 2Πu and 2Πg states in diacetylene and triacetylene cations form linear-linear Renner-Teller pairs, strongly bent minima are calculated for the quartet states correlating with the 4Πg and 4Πu linear structures. In both ions there are quartets that lie close to the minimum energies of the first electronically excited doublet states. The 4Au state of benzene radical cation lies below C2E1u and could be populated during the ultrafast nonradiative decay of this doublet state. [ABSTRACT FROM AUTHOR]