Structures of Stoichiometric Sodium Oxide Cluster Cations Studied by Ion Mobility Mass Spectrometry

Structures of stable compositions of sodium oxide cluster cations (Na_nO_m⁺，n≤11) have been investigated by ion mobility mass spectrometry. Stoichiometric compositions series, Na(Na₂O)_(n-1)/2⁺ (n=3, 5, 7, 9, and 11), were observed as stable composition series, and NaO(Na₂O)_(n-1)/2⁺ series (n=5, 7, 9, and 11) were observed as secondary stable series in the mass spectra. To assign the structures of these cluster ion series, collision cross sections between the ions and helium buffer gas were determined experimentally from the ion mobility measurements. Theoretical collision cross sections were also calculated for optimized structures of these compositions. Finally, the structures of Na(Na₂O)_(n-1)/2⁺ and NaO(Na₂O)_(n-1)/2⁺ were assigned to those having similar structural frames for each n except for n=9. All bonds in the assigned structures of Na(Na₂O)_(n-1)/2⁺ were between sodium and oxygen. On the other hand, there was one O-O bond in addition to Na-O bonds in NaO(Na₂O)_(n-1)/2⁺. This result indicates that NaO(Na₂O)_(n-1)/2⁺ have a peroxide ion (O₂^2-) as a substitute for an oxide ion (O^2-) of Na(Na₂O)_(n-1)/2⁺. As a result, both stable series, Na(Na₂O)_(n-1)/2⁺ and NaO(Na₂O)_(n-1)/2⁺, are closed-shell compositions. These closed-shell characteristics have a strong influence on the stability of sodium oxide cluster cations.