# Does the MS oxygen peak at m/z = 16 correspond to $\ce{O+}$ or $\ce{O2^{2+}}$?

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.

• I wonder what may be the eq. constant $\ce{O2^2+ <=> 2 O+}$... Aug 30, 2022 at 11:50

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 $$\ce{O^+}$$.

• That looks incorrect - there's no reason to think distance would be so small. Charge would obviously make the bond much longer. Aug 30, 2022 at 13:47
• Well, peroxide is a thing, and IIRC even dihelium dication is metastable, so I think you need evidence to say it doesn't exist. Aug 30, 2022 at 13:58
• What we call peroxide is not so in the gas phase, I have only ever seen it in condensed media intertwined with counterions. Aug 30, 2022 at 14:04
• Yeah, yeah, I even started to look for second affinity for O2, but hey, it's you who decided to answer ;D Aug 30, 2022 at 14:13
• 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 ;) Aug 30, 2022 at 17:39

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.