Mass spectrometric determination of Morse parameters for the fifty-four superoxide states dissociating to the lowest limit

Rationale Superoxide is the most significant homonuclear diatomic anion in biochemistry. Theory predicts 12 doublet (X, A–K) and 12 quartet (a–l) electronic states split by spin orbital coupling into 54 states dissociating to the 3P(O) + 2P(O–) limit. Dissociation energies for the 27 bonding states with positive electron affinities have been determined from mass spectrometric data. However, the 27 antibonding states with negative electron affinities have not been experimentally characterized. Methods The electron affinity of the hydrogen atom per electron, the Hylleraas, is the fundamental measure of electron correlation. It has been used to assign and evaluate experimental electron affinities of atoms and diatomic molecules. The 27 negative electron affinities of oxygen are estimated from the 27 positive values and the Hylleraas. These values are used to determine frequencies and internuclear separations by fitting theoretical electron impact distributions to the gas‐phase mass spectrometric atomic oxygen anion distribution peaking at about 6.5 eV. Results The dissociation energies, internuclear distances and frequencies giving the first complete set of Morse potential energy curves for the 54 superoxide states dissociating to the lowest limit are reported from mass spectrometric data. The potentials are compared to theoretical and empirical literature curves. Conclusions The existence of the 27 bonding and 27 antibonding spin orbital coupling superoxide states dissociating to 3P(O) + 2P(O–) is established from mass analyzed thermal, photon, and electron ionization data. There are electron affinities from 0 to 0.15 eV, and onsets and peaks for dissociative electron attachment that cannot be explained by the 54 states. These support the existence of the 36 superoxide spin states dissociating to [1D(O) + 2P(O– )] and [1S(O) + 2P(O– )] predicted by quantum mechanics. Copyright © 2016 John Wiley & Sons, Ltd.