The negative ion photoelectron spectrum of cyclopropane-1,2,3-trione radical anion, (CO)₃^•- -a joint experimental and computational study

Negative ion photoelectron (NIPE) spectra of the radical anion of cyclopropane-1,2,3-trione, (CO)₃^•-, have been obtained at 20 K, using both 355 and 266 nm lasers for electron photodetachment. The spectra show broadened bands, due to the short lifetimes of both the singlet and triplet states of neutral (CO)₃ and, to a lesser extent, to the vibrational progressions that accompany the photodetachment process. The smaller intensity of the band with the lower electron binding energy suggests that the singlet is the ground state of (CO)₃. From the NIPE spectra, the electron affinity (EA) and the singlet-triplet energy gap of (CO) ₃ are estimated to be, respectively, EA = 3.1 ± 0.1 eV and ΔE_ST = -14 ± 3 kcal/mol. High-level, (U)CCSD(T)/aug-cc-pVQZ//(U)CCSD(T)/aug-cc-pVTZ, calculations give EA = 3.04 eV for the ¹A₁′ ground state of (CO)₃ and ΔE_ST = -13.8 kcal/mol for the energy gap between the ¹A₁′ and ³A₂ states, in excellent agreement with values from the NIPE spectra. In addition, simulations of the vibrational structures for formation of these states of (CO)₃ from the ²A₂ state of (CO)₃^•- provide a good fit to the shapes of broad bands in the 266 nm NIPE spectrum. The NIPE spectrum of (CO)₃^•- and the analysis of the spectrum by high-quality electronic structure calculations demonstrate that NIPES can not only access and provide information about transition structures but NIPES can also access and provide information about hilltops on potential energy surfaces.