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Searching on the internet for a really powerful acid, I came across a species called helium hydride ion which is estimated to have a $\mathrm{p}K_\mathrm{a}$ of $-63,$ making it orders of magnitude more acidic than fluoroantimonic acid, which has a $\mathrm{p}K_\mathrm{a}$ of $-31.$

I heard that teflon is the only way to store fluoroantimonic acid. Due to its strong carbon-fluorine bonds, teflon is capable of tolerating the acidity of this superacid.

Now, teflon may be strong enough to store fluoroantimonic acid, but is teflon capable of storing the helium hydride ion, given that it is capable of protonating $\ce{O_2, H_2O, CO_2, NH_3, SO_2}$ and so on? And how do you determine or predict that (via which process)?.

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    $\begingroup$ You cannot bottle up helium hydride ions, no matter what the bottle would be made out of. $\endgroup$
    – Martin - マーチン ♦
    Oct 16, 2022 at 10:01
  • $\begingroup$ @Martin-マーチン teflon lost the match$?$ $\endgroup$
    – Vanessa
    Oct 16, 2022 at 10:12
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    $\begingroup$ @EveryonesaysFU No, helium hydride ion would destroy itself first. Attempting to form a salt to store it results in proton transfer from the $\ce{HeH^+}$ to the proposed anion, so all you have is helium gas plus the acid from the anion. $\endgroup$
    – Oscar Lanzi
    Oct 16, 2022 at 10:37
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    $\begingroup$ @OscarLanzi: On processing the question I quickly came to conclusion it's not a question of being able to withstand the chemical corrosiveness of the ion; its rather since the ion cannot form a salt it's can you possibly make a container that can withstand the electrostatic force of any great amount of it, for which you will find the answer to be no. $\endgroup$
    – Joshua
    Oct 16, 2022 at 22:05
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    $\begingroup$ The question is entirely hypothetical as there is no way to create a significant amount of helium hydride in bulk. $\endgroup$
    – matt_black
    Oct 18, 2022 at 9:22

2 Answers 2

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Helium hydride has been recently discovered by mass spectrometry at extremely low pressure and rather low temperature. It does not exist in usual conditions. It was also discovered in a planetary nebulae. So this ion cannot "stored" as you believe. Whatever their nature, positive ions alone cannot be stored in any container without the same amount of negative ions. And whatever the wall, for example teflon, $\ce{HeH+}$ ions are immediately decomposed into $\ce{He}$ and a proton when touching it.

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    $\begingroup$ This would be better suited as a comment, rather than an answer. And while this address a key problem with the question, it isn't actually an answer to it. $\endgroup$
    – Martin - マーチン ♦
    Oct 16, 2022 at 10:02
  • $\begingroup$ somewhat related to that: Why was helium hydride (HeH+) the universe's first molecule? $\endgroup$
    – uhoh
    Oct 17, 2022 at 0:16
  • $\begingroup$ Assume that hypothetically we're able to produce $1$ liter of helium hydride$?$ Can teflon withstand it$?$ what do you exactly mean by cannot be stored $?$ $\endgroup$
    – Vanessa
    Oct 17, 2022 at 12:49
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    $\begingroup$ @EveryonesaysFU to store macroscopic amounts of any ion we must respect electroneutrality, which means you need a counterion to form a salt. $\ce{HeH^+}$ would react with the countetion instead (see my comment to the q), and while unchanged $\ce{HeH}$ would not need the counterion this alternative would not be bound in the ground state. $\endgroup$
    – Oscar Lanzi
    Oct 17, 2022 at 18:30
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If we consider the idea of helium as a conjugate base, it has an electron pair so in theory it could act as a Lewis base if a sufficently strong Lewis acid was provided. It is important to undertsand that it is rather easy to form compounds of the heavy noble gases, the lone pairs on radon can bind to silver(I) ions to form complexes. This is the reason why a siliver exchanged zeolite is so good at adsorbing radon from air.

The problem is that the smaller size of helium than radon / xenon together with the higher electronegativity of helium will make it such a weak Lewis base.

We can regard Brosted acids such as acetic acid, hydrogen flouride and sulfuric acid to be Lewis acid/base complexes. The Lewis acid in question is the proton while the conjugate base is the Lewis base.

The great problem I see with cationic helium hydride is that we will be dealing with such a one sided covalent bond that it will not be very stable. Also it will tend to break down with great ease. If we construct the orbitial diagram for a hydrogen molecule we can see that HeH+ is isoelectronic with a hydrogen molecule. I predict that the difference in the energies of the atomic orbitials will be large and the bond energy for the He-H bond will be very low.

I think that while HeH+ is formally a Bronsted acid the synthesis of this acid will be difficult to say the least. I think that while this super acid might be "the super acid of super acids" it will be impossible to make in any large amount.

I think that almost any other atom will be able to displace the helium atom in something similar to a SN2 reaction. I think it is likely to be very unstable in condensed media.

There is a paper about the HeH cation, see Zhuan Liu and Paul B. Davies, J. Chem. Phys., 1997, 107, page 337. I will adding some more to the answer.

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