6
$\begingroup$

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.

$\endgroup$
1
  • $\begingroup$ I wonder what may be the eq. constant $\ce{O2^2+ <=> 2 O+}$... $\endgroup$
    – Poutnik
    Commented Aug 30, 2022 at 11:50

2 Answers 2

8
$\begingroup$

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 $\ce{O^+}$.

$\endgroup$
6
  • $\begingroup$ That looks incorrect - there's no reason to think distance would be so small. Charge would obviously make the bond much longer. $\endgroup$
    – Mithoron
    Commented Aug 30, 2022 at 13:47
  • $\begingroup$ Well, peroxide is a thing, and IIRC even dihelium dication is metastable, so I think you need evidence to say it doesn't exist. $\endgroup$
    – Mithoron
    Commented Aug 30, 2022 at 13:58
  • $\begingroup$ What we call peroxide is not so in the gas phase, I have only ever seen it in condensed media intertwined with counterions. $\endgroup$ Commented Aug 30, 2022 at 14:04
  • $\begingroup$ Yeah, yeah, I even started to look for second affinity for O2, but hey, it's you who decided to answer ;D $\endgroup$
    – Mithoron
    Commented Aug 30, 2022 at 14:13
  • 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 ;) $\endgroup$
    – Mithoron
    Commented Aug 30, 2022 at 17:39
8
$\begingroup$

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.

$\endgroup$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.