Negative ion photoelectron spectroscopy confirms the prediction that D_3h carbon trioxide (CO₃) has a singlet ground...

The CO₃ radical anion (CO₃˙^-) has been formed by electrospraying carbonate dianion (CO₃^2-) into the gas phase. The negative ion photoelectron (NIPE) spectrum of CO₃˙^- shows that, unlike the isoelectronic trimethylenemethane [C(CH₂)₃], D_3h carbon trioxide (CO₃) has a singlet ground state. From the NIPE spectrum, the electron affinity of D_3h singlet CO₃ 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 CO₃ are very close in energy, a prediction that is confirmed by the relative intensities of the bands in the NIPE spectrum of CO₃˙^-. 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 ³A₁, in which both unpaired electrons reside in σ MOs, rather than ³A₂, in which one unpaired electron occupies the b₂ σ MO, and the other occupies the b₁ π MO.