Imperial College London

Henry Rzepa

The autoionization of water involves two molecules transfering a proton
to give hydronium hydroxide, a process for which the free energy of
reaction is [well
known](http://www.ch.imperial.ac.uk/rzepa/blog/?p=16118){target="_blank"}.
Here I ask what might happen with the next element along in the periodic
table, F.

I have been unable to find much about the autoionization of HF in the
literature; the pH of neat HF appears unreported (unlike that of H~2~O,
which of course is 7). Even the dielectric constant of
liquid HF\[cite\]10.1063/1.1680219\[/cite\],\[cite\]10.1063/1.1319172\[/cite\] seems
to vary widely, the [largest
reported](https://www.honeywellprocess.com/library/marketing/tech-specs/Dielectric%20Constant%20Table.pdf){target="_blank"}
being \~84. It is suggested that liquid HF is much less ordered than
*e.g.* water, and this suggests that a single static model is unlikely
to be entirely realistic. Nonetheless, I thought it might be informative
to take the model I previously constructed for water and try applying it
to HF. Here is part of the geometry optimisation
cycle\[cite\]10.14469/ch/192032\[/cite\] from the original edited water
model. I used ωB97XD/Def2-TZVPPD/SCRF=water for the model. Why continuum
water as the solvation treatment? Well, standard parameters for liquid
HF are not available (perhaps given the variation in dielectric) and
since the upper bound might be similar to water, I decided to use that
to see what I got. Clearly however an approximation.

![](http://www.ch.ic.ac.uk/rzepa/blog/wp-content/uploads/2016/04/HF.svg){decoding="async"
width="450"}

The low energy final geometry corresponds to 10 HF molecules and lies
about 16 kcal/mol lower (in total energy) than the cyclic structure
containing H~2~F^+^.F^--^ species connected by two (HF)~3~ bridges and
two further non-bridge HF molecules hydrogen bonding to the H~2~F^+ ^and
the F^--^. In fact the ionic structure turns out to be a **transition
state** for proton shifting along the chain to create (HF)~10~, with a
free energy barrier of 9.2 kcal/mol above the neutral
form.\[cite\]10.14469/ch/192034\[/cite\] This difference between ionic
and non-ionic forms is considerably less than that for
water as [previously
indicated](http://www.ch.imperial.ac.uk/rzepa/blog/?p=16118){target="_blank"}.
Note also how much shorter the hydrogen bonding H...F distances are in
the HF cluster.

![](http://www.ch.ic.ac.uk/rzepa/blog/wp-content/uploads/2016/04/HF.jpg){decoding="async"
onclick="jmolInitialize('../Jmol/','JmolAppletSigned.jar');jmolSetAppletColor('white');jmolApplet([450,450],'load wp-content/uploads/2016/04/HF.log;spin 3;frame 3;vectors on;vectors 4;vectors scale 8.0;color vectors green;vibration 6;');"
width="400"}

![](http://www.ch.ic.ac.uk/rzepa/blog/wp-content/uploads/2016/04/HF-IRC.svg){decoding="async"
width="450"}

So unlike water, where the hydronium hydroxide is a clear minimum in the
potential with a small but distinct barrier (\~3.5
kcal/mol\[cite\]10.14469/ch/192022\[/cite\]) to proton transfer, with
HF at the same level of theory the barrier is zero. Perhaps the
difference might be because whereas hydronium hydroxide can support
three stabilizing (H~2~O)~3~ bridges, only two (HF)~3~ bridges are
possible with H~2~F^+^.F^--^. It might also be higher levels of theory
(or better/larger models of the HF cluster) could well give a barrier
for the process, but this does tend to suggest that the dynamics of
HF liquid may suggest quite different lifetimes for autoionized forms of
HF compared to water. Liquid HF is clearly just as complicated a liquid
as is H~2~O, certainly much less is known about it.

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


The autoionization of water involves two molecules transfering a proton to give hydronium hydroxide, a process for which the free energy of reaction is well known. Here I ask what might happen with the next element along in the periodic table, F.  I have been unable to find much about the autoionization of HF in the literature;

Messaggi di Rogue Scholar

Autoionization of hydrogen fluoride.