Negative ion photoelectron spectroscopy confirms the prediction that D(3h) carbon trioxide (CO(3)) has a singlet ground state

The CO(3) radical anion (CO(3)˙(–)) has been formed by electrospraying carbonate dianion (CO(3)(2–)) into the gas phase. The negative ion photoelectron (NIPE) spectrum of CO(3)˙(–) shows that, unlike the isoelectronic trimethylenemethane [C(CH(2))(3)], D(3h) carbon trioxide (CO(3)) has a singlet ground state. From the NIPE spectrum, the electron affinity of D(3h) singlet CO(3) was, for the first time, directly determined to be EA = 4.06 ± 0.03 eV, and the energy difference between the D(3h) singlet and the lowest triplet was measured as ΔE(ST) = – 17.8 ± 0.9 kcal mol(–1). B3LYP, CCSD(T), and CASPT2 calculations all find that the two lowest triplet states of CO(3) are very close in energy, a prediction that is confirmed by the relative intensities of the bands in the NIPE spectrum of CO(3)˙(–). The 560 cm(–1) vibrational progression, seen in the low energy region of the triplet band, enables the identification of the lowest, Jahn–Teller-distorted, triplet state as (3)A(1), in which both unpaired electrons reside in σ MOs, rather than (3)A(2), in which one unpaired electron occupies the b(2) σ MO, and the other occupies the b(1) π MO.