Imperial College London

Henry Rzepa

I am attending a conference. Plenaries at such events can sometimes
provide interesting pointers on things to come (and sometimes they
simply point to things past). At WATOC2014 in Santiago Chile, the first
plenary was by Paul Ayers with the impressive title "*Concepts for
organising chemical knowledge*" which certainly sounds as if it is
pointing forward!

WATOC stands for World Association of Theoretical and cOmputational
Chemists (yes, the acronym does not expand) and so the *organising
concepts* of course relate to quantum mechanics. Ayer's talk however was
more about chemical philosophy, and how we may continue to organise
chemical rules according to sound quantum mechanical principles. He
bases these principles on observables, rather than just mathematical
formulations. The start point is electron distributions (I have noted
elsewhere that these are indeed measurable observables, although the
crystallography required to measure bond-based electron density is only
rarely carried out) in the form of *conceptual DFT*. He strives to find
examples that cannot be explained by *e.g.* the older ideas of molecular
orbitals (as obtained from solution of a single reference determinant)
and in particular to avoid the modern syndrome that as we improve our
ability to achieve numerical accuracy, we tend to lose our insight into
the phenomenon being studied. One example he gives is that of alkynes
such as FC≡CF. If one removes a π-electron from the molecule, this
should result in a decrease in the electron density distribution? Wrong.
It actually **increases** the σ-electron density in the nodal planes of
the lost π-electron. Ayer's point is that **Conceptual DFT** provides a
simple framework for explaining such observations, unlike MO theory.

But the most interesting moment came with discussion of Bader's QTAIM
methods (often used in this blog), as used by Ayers to provide a
formulation of an *electronic stress tensor* approach to molecules.
Expressed as Ehrenfest Forces and the corresponding Hessian, these
provides a handle on the forces on bonds, or in simpler language,
telling us "*how bonds might move*". Think of it as the electronic
equivalent of how nuclei move in molecules (*aka* vibrations). The
reason my ears pricked up is that I have often discussed how organic
chemists tend to conceptualize "*how bonds might move*" by using [curly
arrow
pushing](http://www.ch.imperial.ac.uk/rzepa/blog/?p=11741 "A curly-arrow pushing manual"){target="_blank"}.
This concept, dating back to [ Robinson's original
example](http://www.ch.imperial.ac.uk/rzepa/blog/?p=7234 "The first ever curly arrows."){target="_blank"} is
often dismissed by theoreticians (and it has to be said by some organic
chemists) as having little sound theoretical foundation; it is pure
symbolism, and not to be over-interpreted. So perhaps electronic stress
tensors based on Ehrenfest Forces might indeed tell us whether the
hugely successful curly arrow pushing formalism (itself based on Lewis'
ideas of the shared electron bond formulated in 1916) might yet receive
the theoretical makeover that it badly needs.

But before I end, one more example given by Ayers; keto-enol tautomerism
in say propanone. Here a C-H bond breaks and an OH bond forms.
The Ehrenfest Hessian indeed tells is that this is indeed "*how the
bonds want to move*". At the point the "audience" (in the form of a
question) injected that sense of excitement that a conference
occasionally provides. According to MO theory (!), the \[1,3\]
sigmatropic shift of a hydrogen atom is a [forbidden pericyclic
process](http://www.ch.imperial.ac.uk/rzepa/blog/?p=8246 "Thalidomide. The role of water in the mechanism of its aqueous racemisation.")!
How might the conceptual DFT electronic stress tensor approach tell us
this? I have to confess I am not sure I understood Ayers' answer at this
point. Perhaps more work is needed before MOs are discarded entirely.

Oh, one more comment. Conference talks, presented in the form of slides
which might linger on the projected screen for only a few seconds,
rarely allow the audience to take much more than cursory notes. In
particular,  without some more research,  I cannot here cite any of the
references to Ayer's recent work, since I did not have time to make a
note of them.

**Postscript:** An article by Paul Geerlings, Paul Ayers et al on the
topic of the  Woodward-Hoffmann rules has in fact been
published\[cite\]10.1021/ar200192t\[/cite\].

**Postscript2:** I have computed the density difference isosurface for
FCCF as noted above. It is shown as a thumbnail below (blue is decreased
density on forming the cation, red is the increased density). Click on
this to rotate.

::: {#attachment_12989 .wp-caption .aligncenter style="width: 410px"}
![Click for
3D](http://www.ch.ic.ac.uk/rzepa/blog/wp-content/uploads/2014/10/FCCF.jpg){.size-full
.wp-image-12989 decoding="async"
aria-describedby="caption-attachment-12989"
onclick="jmolInitialize('../Jmol/');jmolSetAppletColor('white');jmolApplet([450,450],'load wp-content/uploads/2014/10/FCCF_sub.cub.xyz;isosurface colour red blue wp-content/uploads/2014/10/FCCF_sub.cub.jvxl translucent;');"
width="400"
srcset="https://www.ch.ic.ac.uk/rzepa/blog/wp-content/uploads/2014/10/FCCF.jpg 502w, https://www.ch.ic.ac.uk/rzepa/blog/wp-content/uploads/2014/10/FCCF-300x172.jpg 300w"
sizes="(max-width: 502px) 100vw, 502px"}

Click for 3D
:::

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


I am attending a conference. Plenaries at such events can sometimes provide interesting pointers on things to come (and sometimes they simply point to things past). At WATOC2014 in Santiago Chile, the first plenary was by Paul Ayers with the impressive title “
<em>
 Concepts for organising chemical knowledge
</em>
” which certainly sounds as if it is pointing forward!

Postagens de Rogue Scholar

WATOC2014 Conference report. Concepts for Organizing Chemical Knowledge