Negative Ion Photoelectron Spectroscopy Confirms the Prediction of the Relative Energies of the Low-Lying Electronic States of 2,7-Naphthoquinone.

Cryogenic negative ion photoelectron (NIPE) spectra of the radical anion of 2,7-naphthoquinone (NQ•-) have been taken at 20 K, using 193, 240, 266, 300, and 355 nm lasers for electron detachment. The electron affinity of the NQ diradical is determined from the first resolved peak in the NIPE spectrum to be 2.880 ± 0.010 eV. CASPT2/aug-cc-pVDZ calculations predict with reasonable accuracy the positions of the 0-0 bands in the three lowest electronic states of NQ. In addition, the Franck-Condon factors calculated from the CASPT2/aug-cc-pVDZ optimized geometries, vibrational frequencies, and normal modes successfully simulate the vibrational structures in these bands. The NIPE spectrum of NQ•- confirms that, as predicted, 3B2 is the ground state, and the 1B2 and 1A1 states are, respectively, 12.7 and 16.4 kcal/mol higher in energy than the triplet ground state. The experimental value of Δ EST = 12.7 kcal/mol in NQ and the finding that 1B2 is the lower energy of the two singlet states confirm the results of the previous calculations on NQ. These calculations predicted an increase in Δ EST on the substitution of both methylene groups in 2,7-naphthoquinodimethane (NQDM) by oxygens in NQ, thus providing a dramatic contrast to the decrease of 17.5 kcal/mol in Δ EST found for substitution of one methylene group by one oxygen on going from trimethylenemethane (TMM) to oxyallyl (OXA).