Mass spectrum and sublimation pressure of sodium oxide vapor: Stability of the superoxide molecule NaO2

Abstract

The total sublimation pressure of Na2O(c) has been measured in the range 1000–1050 K by the torsion-effusion method, and the mass spectra of vapors over Na2O(c) and Na2O2(c) have been examined in order to resolve several issues regarding the thermochemical stability of the superoxide molecule NaO2. Measured torsion-effusion pressures are completely consistent with the primary sublimation process Na2O(c)=2Na(g)+0.5 O2(g) and set an upper limit of 200 kJ mol−1 for the bond strength D0(Na–O2). The ionization efficiency curves of Na+ and O2+ in the mass spectrum of vapor over Na2O(c) show no evidence for fragmentation contributions from NaO2. In addition, the absence of a detectable Na+/NaO2 fragment ion signal in vapor over Na2O2(c) can be used to derive an even tighter upper bound of 180 kJ mol−1 for D0(Na–O2). These results are in conflict with various kinetic analyses yielding D0(Na–O2) values of 202 to 243 kJ mol−1, but are in accord with rigorous theoretical calculations giving values near 160 kJ mol−1.