In a mass spectrum, while annealing with $\ce{O2}$ of a sample in UHV, I observe peaks at $m/z = 16$ and $32 (= 16 \times 2)$. While the latter is $\ce{O2^+}$, I'm unsure what the peak at $m/z = 16$ is. I lean toward $\ce{O2^{2+}}$ because of the molecular orbital diagram.
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$\begingroup$ I wonder what may be the eq. constant $\ce{O2^2+ <=> 2 O+}$... $\endgroup$– PoutnikAug 30, 2022 at 11:50
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2 Answers
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Most likely the signal is predominantly or entirely $\ce{O^+}$.
The question is whether two $\ce{O^+}$ ions will bind to each other forming a diatomic ion in the gas phase. A rough energy balance calculation suggests they will not.
The observed bond length in the singly charged $\ce{O2^+}$ ion is $112.3$ pm. Now suppose two $\ce{O^+}$ ions, each with a charge equivalent to one electron, are brought together to this internuclear distance. The standard Coulombic repulsion calculation gives about $1.24$ MJ/mol.
This might be overcome if the covalent binding energy is sufficiently strong, but the isoelectronic and similarly sized $\ce{N2}$ molecule has a bond energy of only about $0.94$ MJ/mol. So the covalent binding energy expected for the proposed $\ce{O#O}$ triple bond in the gas phase would be insufficient to overcome the repulsion between two positive charges, and we should expect the proposed $\ce{O2^{2+}}$ ion to be less stable than a pair of single $\ce{O^+}$ ions. Therefore the m/z signal is most probably due predominantly or entirely to $\ce{O^+}$.
answered Aug 30, 2022 at 12:19
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$\begingroup$ That looks incorrect - there's no reason to think distance would be so small. Charge would obviously make the bond much longer. $\endgroup$– MithoronAug 30, 2022 at 13:47
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$\begingroup$ Well, peroxide is a thing, and IIRC even dihelium dication is metastable, so I think you need evidence to say it doesn't exist. $\endgroup$– MithoronAug 30, 2022 at 13:58
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$\begingroup$ What we call peroxide is not so in the gas phase, I have only ever seen it in condensed media intertwined with counterions. $\endgroup$ Aug 30, 2022 at 14:04
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$\begingroup$ Yeah, yeah, I even started to look for second affinity for O2, but hey, it's you who decided to answer ;D $\endgroup$– MithoronAug 30, 2022 at 14:13
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1$\begingroup$ There is paywalled sciencedirect.com/science/article/pii/S1387380614000189 but I checked some other articles and the dication is most likely indeed repulsive, and dimerisation unlikely. Oh well, I learnt bit about interesting topic ;) $\endgroup$– MithoronAug 30, 2022 at 17:39
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This is a very tricky molecule. A single m/z value cannot tell you whether this is singly ionized or doubly charged on a low resolution mass spectrometer. One would look at mixed isotope patterns of oxygen-17 and oxygen-18 molecules. A doubly charged molecule would then appear at half-integer values (17+18)/2 =17.5. Without any mixed isotope enrichment experiment, a real mass spectrum will be noisy, so it is hard to tell anything with ordinary oxygen which is dominated by a single isotope.
