Negative ion photoelectron spectroscopy confirms the prediction that D3h carbon trioxide (CO3) has a singlet ground state

Abstract

The CO3 radical anion (CO3-) has been formed by electrospraying carbonate dianion (CO32-) into the gas phase. The negative ion photoelectron (NIPE) spectrum of CO3- shows that, unlike the isoelectronic trimethylenemethane [C(CH2)3], D3h carbon trioxide (CO3) has a singlet ground state. From the NIPE spectrum, the electron affinity of D3h singlet CO3 was, for the first time, directly determined to be EA = 4.06 ± 0.03 eV, and the energy difference between the D3h singlet and the lowest triplet was measured as ΔEST = - 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.