The impervious route to the elusive HOOO- anion

The HOOO− anion, the conjugate base of dihydrogen trioxide, HOOOH, is suggested as a key intermediate in ozonation reactions relevant to atmospheric chemistry, water purification, preparative organic chemistry, etc. Whereas recent theoretical calculations characterize HOOO− as a long-lived metastable species, so far the anion proved experimentally elusive, defying attempts at its direct detection even in solution, where it should be stabilized by solvation. We report the preparation and the proof-positive detection ofHOOO− as an isolated species with a lifetime >0.8s. The preparation proved particularly difficult, since the only viable approach is the indirect route involving the preliminary formation of HOOO• upon neutralization of HOOO+, followed by electron attachment to the radical, utilizing neutralization–reionization (+NR−) mass spectrometry. Direct two-electron reduction of theHOOO+ cation to theHOOO− anion by the vertical process typical of charge reversal (CR) experiments proved inefficient. This observation, and application of neutral and ion decomposition difference (NIDD) mass spectrometry, pioneered by Schwarz and coworkers [Int. J. Mass Spectrom. Ion Processes 172 (1998) 181] suggest that the geometries of HOOO+ and HOOO− are significantly different, whereas that of the HOOO• radical is structurally intermediate between them, which accounts for the diversity of the Franck–Condon factors characterizing the mutual vertical transitions of the three species. This inference is consistent with the available optimized geometries of HOOO• and HOOO+, computed at various levels of theory, and with that of the anion recently reported by Plesnic˘ar et al.