Negative ion photoelectron spectroscopy confirms the prediction that (CO)5 and (CO)6 each has a singlet ground state.

Cyclobutane-1,2,3,4-tetraone has been both predicted and found to have a triplet ground state, in which a b2g σ molecular orbital (MO) and an a2u π MO are each singly occupied. In contrast, (CO)5 and (CO)6 have each been predicted to have a singlet ground state. These predictions have been tested by generating the (CO)5(•-) and (CO)6(•-) radical anions in the gas phase, using electrospray vaporization of solutions of, respectively, the croconate (CO)5(2-) and rhodizonate (CO)6(2-) dianions. The negative ion photoelectron (NIPE) spectrum of the (CO)5(•-) radical anion gives an electron affinity of EA = 3.830 eV for formation of the singlet ground state of (CO)5. The triplet is found to be higher in energy by 0.850 eV (19.6 kcal/mol). The NIPE spectrum of the (CO)6(•-) radical anion gives EA = 3.785 eV for forming the singlet ground state of (CO)6, with the triplet state higher in energy by 0.915 eV (21.1 kcal/mol). (RO)CCSD(T)/aug-cc-pVTZ//(U)B3LYP/6-311+G(2df) calculations give EA values that are only approximately 1 kcal/mol lower than those measured and ΔE(ST) values that are 2-3 kcal/mol higher than those obtained from the NIPE spectra. Calculations of the Franck-Condon factors for transitions from the ground state of each radical anion, (CO)n(•-) to the lowest singlet and triplet states of the n = 4-6 neutrals, nicely reproduce all of the observed vibrational features in the low-binding energy regions of all three NIPE spectra. Thus, the calculations of both the energies and vibrational structures of the two lowest energy bands in each of the NIPE spectra support the interpretation of the spectra in terms of a singlet ground state for (CO)5 and (CO)6 but a triplet ground state for (CO)4.