Photoelectron Spectroscopy Study of Quinonimides

Structures and energetics of <i>o</i>-, <i>m</i>-, and <i>p</i>-quinonimide anions (OC₆H₄N^–) and quinoniminyl radicals have been investigated by using negative ion photoelectron spectroscopy. Modeling of the photoelectron spectrum of the ortho isomer shows that the ground state of the anion is a triplet, while the quinoniminyl radical has a doublet ground state with a doublet-quartet splitting of 35.5 kcal/mol. The para radical has doublet ground state, but a band for a quartet state is missing from the photoelectron spectrum indicating that the anion has a singlet ground state, in contrast to previously reported calculations. The theoretical modeling is revisited here, and it is shown that accurate predictions for the electronic structure of the <i>para</i>-quinonimide anion require both an accurate account of electron correlation and a sufficiently diffuse basis set. Electron affinities of <i>o</i>- and <i>p</i>-quinoniminyl radicals are measured to be 1.715 ± 0.010 and 1.675 ± 0.010 eV, respectively. The photoelectron spectrum of the <i>m</i>-quinonimide anion shows that the ion undergoes several different rearrangements, including a rearrangement to the energetically favorable para isomer. Such rearrangements preclude a meaningful analysis of the experimental spectrum