Imperial College London

Henry Rzepa

My [previous
posts](http://www.ch.imperial.ac.uk/rzepa/blog/?p=13506 "How many water molecules does it take to ionise HCN/HNC? An NCI exploration.")
have covered the ionization by a small number of discrete water
molecules of the series of halogen acids, ranging from HI (the
strongest, pKa -10) *via* HF (weaker, pKa 3.1) to the pseudo-halogen HCN
(the weakest, pKa 9.2). Here I try out some even stronger acids to see
what the least number of water molecule needed to ionize these might be.

Firstly what must surely be the ultimate acid H(CHB~11~Cl~11~),
discovered by Christopher Reed\[cite\]10.1021/ja058581l\[/cite\] in
2006. This is so strong that it appears that it can even largely ionize
itself; the form on the right (below) is the cationic acid, the form on
the left is its anionic base. The proton itself is
bridged\[cite\]10.5517/CCNBRWL\[/cite\] between the two in a manner
similar to the structure of one form of HCl.4H~2~O [reported in the
earlier
post](http://www.ch.imperial.ac.uk/rzepa/blog/?p=13394 "How many water molecules does it take to ionise HCl?"){target="_blank"}
on the topic.

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![Click for
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Click for 3D
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So it comes as no surprise^‡^ to find\[cite\]10.14469/ch/191134\[/cite\]
that just one water molecule can also ionize H(CHB~11~Cl~11~) to the
anionic form (CHB~11~Cl~11~)^--^.

::: {#attachment_13631 .wp-caption .aligncenter style="width: 410px"}
![Click for
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srcset="https://www.ch.ic.ac.uk/rzepa/blog/wp-content/uploads/2015/03/LEHZOO-H2O.jpg 496w, https://www.ch.ic.ac.uk/rzepa/blog/wp-content/uploads/2015/03/LEHZOO-H2O-300x247.jpg 300w"
sizes="(max-width: 496px) 100vw, 496px"}

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:::

How about triflic acid, CF~3~SO~2~OH, pKa -16), which is also a fair bit
more acidic than HI? Here, only three waters are needed
(ωB97XD/6-311++G(2d,2p) prediction) to ionise to triflate
anion.\[cite\]10.14469/ch/191129\[/cite\]

::: {#attachment_13623 .wp-caption .aligncenter style="width: 410px"}
![Click for
3D](http://www.ch.ic.ac.uk/rzepa/blog/wp-content/uploads/2015/03/HTf.3H2O.jpg){.size-full
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aria-describedby="caption-attachment-13623"
onclick="jmolInitialize('../Jmol/');jmolSetAppletColor('white');jmolApplet([450,450],'load wp-content/uploads/2015/03/HTf3H2O.log;frame 12;spin 3;');"
width="400"
srcset="https://www.ch.ic.ac.uk/rzepa/blog/wp-content/uploads/2015/03/HTf.3H2O.jpg 520w, https://www.ch.ic.ac.uk/rzepa/blog/wp-content/uploads/2015/03/HTf.3H2O-300x290.jpg 300w"
sizes="(max-width: 520px) 100vw, 520px"}

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So, if there a system which is ionised by precisely two water molecules
I will record it here.

------------------------------------------------------------------------

^‡^Perhaps also no surprise is that one H~2~S molecule can also perform
this ionisation.\[cite\]10.14469/ch/191135\[/cite\] This leads us into
another exploration, using molecules other than water to perform these
ionisations.

::: {.wp_orcid_field}
[https://orcid.org/0000-0002-8635-8390](https://orcid.org/0000-0002-8635-8390){target="_blank"
rel="author"}
:::


My previous posts have covered the ionization by a small number of discrete water molecules of the series of halogen acids, ranging from HI (the strongest, pKa -10)
<em>
 via
</em>
HF (weaker, pKa 3.1) to the pseudo-halogen HCN (the weakest, pKa 9.2). Here I try out some even stronger acids to see what the least number of water molecule needed to ionize these might be.  Firstly what must surely be the ultimate acid

Messaggi di Rogue Scholar

Ionizing ultra-strong acids with water molecules.