Imperial College London

Henry Rzepa

Steve Bachrach [on his own
blog](http://comporgchem.com/blog/?p=3618){target="_blank"} has
commented on a recent article\[cite\]10.1002/anie.201505934\[/cite\]
discussing the structure of the trimer of fluoroethanol. Rather than the
expected triangular form with three OH---O hydrogen bonds, the lowest
energy form only had two such bonds, but it matched the microwave data
much better. Here I explore this a bit more.

The stability of the lowest energy form, as is evident from the title of
the article, was attributed to *unusual H-Bond topology and bifurcated
H-bonds* as teased out from bond critical points in the QTAIM analysis
of the topology of the electron density. Here I add to this analysis by
displaying the computed NCI
(non-covalent-interaction)\[cite\]10.1021/jp204278k\[/cite\] surfaces,
as you might see in the comment I posted on Steve's blog. In essence,
the QTAIM had revealed *bond paths* connecting an oxygen to a H-C and
also a bifurcation from an F to two H-C atoms, shown with orange lines
in the diagram there. What might an [NCI
analysis](http://www.ch.imperial.ac.uk/rzepa/blog/?p=9056){target="_blank"}
reveal? The analysis\[cite\]10.14469/ch/191558\[/cite\] is shown below,
where I have added orange arrows to indicate the location of these bond
paths. The arrows point to an NCI feature which corresponds to a weak
dispersion-like stabilisation.

::: {#attachment_14720 .wp-caption .aligncenter style="width: 450px"}
![Click for
3D](http://www.ch.ic.ac.uk/rzepa/blog/wp-content/uploads/2015/10/tfe.jpg){.size-full
.wp-image-14720 decoding="async"
aria-describedby="caption-attachment-14720"
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width="440"
srcset="https://www.ch.ic.ac.uk/rzepa/blog/wp-content/uploads/2015/10/tfe.jpg 1279w, https://www.ch.ic.ac.uk/rzepa/blog/wp-content/uploads/2015/10/tfe-300x264.jpg 300w, https://www.ch.ic.ac.uk/rzepa/blog/wp-content/uploads/2015/10/tfe-1024x902.jpg 1024w, https://www.ch.ic.ac.uk/rzepa/blog/wp-content/uploads/2015/10/tfe-900x793.jpg 900w"
sizes="(max-width: 1279px) 100vw, 1279px"}

Click for 3D
:::

However, as you can spot from the diagram above (and inspect in a 3D
sense if you click on the diagram above to load a 3D Jmol model), there
are many more regions where NCI features appear. The most obvious are
the blue-coded ones, which in fact represent the conventional O...HO
hydrogen bonds, but there are plenty of others as well, including a cyan
one which is not part of the published attributions. I will recapitulate
my comment on Steve's blog; *the point I make here is that apart from
the two regions which have been picked out in the article as responsible
for stabilisation of the low energy structure, there are around 4-5
OTHER regions that also may be stabilising but for which there is no
corresponding critical point in the density. So whilst the above origins
are not incorrect, they may well be very incomplete!*.

There is a tendency to only highlight features which can be *named*, and
perhaps to ignore or pay less attention to those which have no name. The
latter may in fact be more common than we imagine, and cumulatively they
can often have a big impact.

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**Postscript:** A structure has recently been
reported\[cite\]10.1002/anie.201403599\[/cite\],\[cite\]10.5517/CC128NYY\[/cite\]
illustrating an exceptionally strong OH...F interaction of 1.52Å. This
is noteworthy because such hydrogen bonds are rarely strong and indeed
even their very existence is controversial. The cyan NCI region
mentioned above is just such an interaction (of length \~2.0Å).

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


Steve Bachrach on his own blog has commented on a recent article[cite]10.1002/anie.201505934[/cite] discussing the structure of the trimer of fluoroethanol. Rather than the expected triangular form with three OH—O hydrogen bonds, the lowest energy form only had two such bonds, but it matched the microwave data much better. Here I explore this a bit more.

Messaggi di Rogue Scholar

Interactions responsible for the lowest energy structure of the trimer of fluoroethanol.