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

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, NaO2+ fragments to Na+ and O 2. 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 D 0(Na-ONa) = 228 ± 8 kJ mol-1. It also appears that the I.P.(NaO2)≤739 kJ mor-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(O2+) signals leads to an independent measure of - 90≥ΔH f298K(NaO2)≥ - 155 and 194≤D0(Na-O 2)≤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+/O2+ ratios but these appeared to be a factor of 4 smaller than expected for congruency and remained unexplained. By invoking an alternate fragmentation of NaO2+ at higher energies to Na and O2+ (A.P.≈14.6 eV), with a branching ratio of ≥10%, this channel becomes the dominant source of O2+ and the observed Na+/O2+ 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. © 1991 American Institute of Physics.