Negative Ion Photoelectron Spectroscopy Confirms the Prediction of a Singlet Ground State for the 1,8-Naphthoquinone Diradical

Negative ion photoelectron (NIPE) spectra, with 193, 266, 300, and 355 nm photons, of the radical anion of 1,8-naphthoquinone (1,8-NQ •- ) have been obtained at 20 K. The electron affinity of 1,8-NQ is determined from the first resolved peak in the NIPE spectrum to be 2.965 ± 0.005 eV. Franck-Condon factors (FCFs), calculated from the CASPT2/aug-cc-pVDZ optimized geometries, normal modes, and vibrational frequencies, successfully simulate the intensity and frequencies of the spectral features that are associated with the lowest two electronic states. The NIPE spectra of 1,8-NQ •- and the peak assignments, based on the computed FCFs, confirm the theoretical predictions tha 1 A 1 is the ground state of 1,8-NQ and 3 B 2 is the first excited state. The spectra provide an experimental value of Δ E ST = -0.6 kcal/mol, which is 2 kcal/mol smaller in magnitude than the (12/12)CASPT2/aug-cc-pVTZ calculated value of Δ E ST = -2.6 kcal/mol.