Calculations of the relative energies of the 2B1g and 2A2u states of cyclobutanetetraone radical cation and radical anion provide further evidence of a 3B2u ground state for the neutral molecule: a proposed experimental test of the prediction of a triplet ground state for (CO)4.

B3LYP, CCSD(T), and CASPT2 calculations with the 6-311+G(2df) basis set have been performed on the radical anion and radical cation of cyclobutanetetraone (1). The very similar energies computed for the (2)B(1g) and (2)A(2u) states of both 1.(+) and 1.(-) indicate that the singly occupied b(1g) and a(2u) MOs in these two states of the radical cation and anion have nearly the same energies, thus supporting the previously made prediction that neutral 1 has a (3)B(2u) ground state. Reaction of squaric acid with O.(-), followed by negative ion photoelectron spectroscopy (NIPES) on the 1.(-) thus formed, is proposed as an experimental test of the startling prediction that tetraketone (1), a molecule that would be expected to be a closed-shell singlet, actually has a triplet ground state.