A reevaluation of the vaporization behavior of sodium oxide and the bond strengths of NaO and Na2O: Implications for the mass spectrometric analyses of alkali/oxygen systems

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Abstract
There has been a long standing disagreement between our flame experiments, which predict a very stable NaO2 molecule, and Na2O(c) vaporization/mass spectrometric studies of Hildenbrand et al., which imply a weak bond strength from an inability to detect such a species. We have now reanalyzed the vaporization experiments and have identified a possible explanation for this frustrating controversy. It appears that on becoming ionized, NaO+2 fragments to Na+ and O2. As a result, mass 23 reflects p(Na)+p(NaO2). This and the changes to the thermochemical data for NaO2 modifies the earlier ion intensity/vapor pressure calibration. As a result, the previously accepted thermochemical values for NaO and Na2O need to be reduced by 18 and 11 kJ mol−1, respectively. Recommended values now become ΔHf298K (NaO)=87±4, D0(NaO)=266±4, ΔHf298K(Na2O) =−36.0±8 and D0(Na–ONa)=228±8 kJ mol−1. It also appears that the I.P.(NaO2)≤739 kJ mol−1 (7.66 eV). The reported Clausius–Clapeyron vapor pressure curves are entirely consistent with this suggestion that the congruent vaporization of Na2O(c) is to Na and NaO2 rather than to Na and O2. A reinterpretation utilizing the previous slopes of the I(Na+) or I(O+2) signals leads to an independent measure of −90≥ΔHf298K(NaO2)≥−155 and 194≤D0(Na–O2)≤259 kJ mol−1. These are to be compared with the higher temperature flame determined values of −139±8 and 243±8 kJ mol−1, respectively. Earlier vaporization mass loss measurements of Brewer and Margrave were in approximate agreement with a vaporization to predominantly Na and O2. Analyses now show that the technique is insensitive and congruent vaporization to Na and NaO2 also fits their data. Hildenbrand and Murad measured Na+/O+2 ratios but these appeared to be a factor of 4 smaller than expected for congruency and remained unexplained. By invoking an alternate fragmentation of NaO+2 at higher energies to Na and O+2 (A.P.≊14.6 eV), with a branching ratio of ≥10%, this channel becomes the dominant source of O+2 and the observed Na+/O+2 signal ratios are consistent with congruency. These results have important implications for all mass spectrometric studies of alkali/oxygen mixtures including, for example, also carbonates and nitrates. A reappraisal for these is in order, particularly with reference to derived thermochemical values.