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

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