Solving the Puzzle of Tetratomic, 23-Valence-Electron Molecules

Because of extensive electron correlation, the ground-state structures, infrared spectra, and chemical bonding properties of a number of tetratomic molecules which possess 23 valence electrons are anomalous. Approaches which have recently been helpful in understanding this phenomenon include \textit{ab initio} calculations and spectroscopic observations of these species trapped in solid neon at temperatures near 4 K. Molecules for which infrared spectra have been obtained and which will be discussed include NNO$_{2}^{-}$, NO$_{3}$, O$_{4}^{+}$, and BF$_{3}^{+}$.