Imperial College London

Henry Rzepa

Semibullvalene is an unsettling molecule. Whilst it has a classical
structure describable by a combination of Lewis-style two electron and
four electron bonds, its NMR behaviour reveals it to be highly
fluxional. This means that even at low temperatures, the position of
these two-electron bonds rapidly shifts in the equilibrium shown below.
Nevertheless, this dynamic behaviour can be frozen out at sufficiently
low temperatures. But the barrier was sufficiently low that a challenge
was set; could one achieve a system in which the barrier was removed
entirely, to freeze out the coordinates of the molecule into a structure
where the transition state (shown at the top) became instead a true
minimum (bottom)? A [similar
challenge](http://www.ch.imperial.ac.uk/rzepa/blog/?p=783){target="_blank"} had
been set for freezing out the transition state for the Sn2 reaction into
a minimum, the topic also of a [more recent
post](http://www.ch.imperial.ac.uk/rzepa/blog/?p=7601){target="_blank"}
here. Here I explore how close we might be to achieving inversion of the
semibullvalene \[3,3\] sigmatropic potential.

![](http://www.ch.imperial.ac.uk/rzepa/blog/wp-content/uploads/2012/09/semibullvalene.svg "semibullvalene"){.aligncenter
.size-full .wp-image-7679 decoding="async"}

Why might such a frozen transition state be interesting? Well, all
transition states for allowed [thermal]{style="color: #00ff00;"}
pericyclic reactions can be described as aromatic. If one were able to
transmogrify such a transition state into a minimum, then it too would
be expected to be aromatic, but a most unusual type of aromatic. The C-C
bonds which represent the breaking and forming bonds in a \[3,3\]
sigmatropic rearrangement would in effect be two-centre 1-electron
bonds, and those electrons would be part of the aromatic sextet. Such
bonds are normally referred to as homoaromatic, examples of which are
[pretty rare](http://dx.doi.org/10.1021/ct8001915){target="_blank"}. In
my [previous
post](http://www.ch.imperial.ac.uk/rzepa/blog/?p=7643){target="_blank"},
I had noted a crystal structure\[cite\]10.1021/ja00186a064\[/cite\] that
apparently sustains two equal C-C bonds of length 1.99Å. However, a
[calculation at this
geometry](http://hdl.handle.net/10042/20319){target="_blank"} reveals it
in fact to be a transition state (above, top), with an imaginary mode of
275*i* cm^-1^. So the challenge (computationally at least) is to find a
system where this imaginary mode is changed to become real rather than
imaginary.

::: {#attachment_7681 .wp-caption .aligncenter style="width: 241px"}
![](http://www.ch.imperial.ac.uk/rzepa/blog/wp-content/uploads/2012/09/CAZFUE1.jpg "CAZFUE"){.wp-image-7681
loading="lazy" decoding="async"
aria-describedby="caption-attachment-7681"
onclick="jmolInitialize('../Jmol/');jmolSetAppletColor('white');jmolApplet([450,450],'load wp-content/uploads/2012/09/CASFUE.log;frame 27;measure 2 8;measure 6 4;vectors on;vectors 4;vectors scale 5.0; color vectors blue; vibration 20;animation mode loop;');"
width="231" height="224"}

CAZFUE. Click for animation of imaginary transition state mode.
:::

My effort to achieve this involved augmenting CAZFUE with a further two
cyano groups. This did indeed [reduce the imaginary
mode](http://hdl.handle.net/10042/20320){target="_blank"} to 74*i*
cm^-1^; we are getting close! 

::: {#attachment_7684 .wp-caption .aligncenter style="width: 305px"}
![](http://www.ch.imperial.ac.uk/rzepa/blog/wp-content/uploads/2012/09/CAZFUE-tetracycano.jpg "CAZFUE-tetracycano"){.wp-image-7684
loading="lazy" decoding="async"
aria-describedby="caption-attachment-7684" width="295" height="262"}

Tetracyano derivative of CAZFUE. Click for animation.
:::

The next step was to read a recent
article\[cite\]10.1021/ja305581f\[/cite\] in which replacing the key C-C
bond with a C-N bond was observed to reduce the barrier for the
rearrangement to \~ 4 kcal/mol. So I immediately [computed the tetra-azo
system](http://hdl.handle.net/10042/20321){target="_blank"}, in which
the two key C-C bonds are now replaced by N-N bonds in order to extend
this effect.

::: {#attachment_7686 .wp-caption .aligncenter style="width: 298px"}
![](http://www.ch.imperial.ac.uk/rzepa/blog/wp-content/uploads/2012/09/tetraaza.jpg "tetraaza"){.wp-image-7686
loading="lazy" decoding="async"
aria-describedby="caption-attachment-7686"
onclick="jmolInitialize('../Jmol/');jmolSetAppletColor('white');jmolApplet([450,450],'load wp-content/uploads/2012/09/tetraaza.log;frame 71;measure 46 43;measure 44 45;vectors on;vectors 4;vectors scale 5.0; color vectors blue; vibration 20;animation mode loop;');"
width="288" height="256"}

Tetra-azo semibullvalene. Click for animation of key frozen mode.
:::

It was gratifying to observe that the \[3,3\] sigmatropic vibration,
imaginary (*i.e.* corresponding to a transition state) in the previous
examples, became **+ve** (+238 cm^-1^) in this system. The two N-N bonds
are however not completely symmetric (2.06 and 2.17Å), but they are in
effect essentially frozen at the half-way stage of the equilibrium.

The final step in this path is to combine the two effects above, by
exploring the [di-cyano-diaza
derivative](http://hdl.handle.net/10042/20322){target="_blank"}.

::: {#attachment_7692 .wp-caption .aligncenter style="width: 284px"}
![](http://www.ch.imperial.ac.uk/rzepa/blog/wp-content/uploads/2012/09/dicyano-diaza.jpg "dicyano-diaza"){.wp-image-7692
loading="lazy" decoding="async"
aria-describedby="caption-attachment-7692"
onclick="jmolInitialize('../Jmol/');jmolSetAppletColor('white');jmolApplet([450,450],'load wp-content/uploads/2012/09/dicn-diaza.log;frame 17;measure 2 21;measure 22 5;vectors on;vectors 4;vectors scale 5.0; color vectors blue; vibration 20;animation mode loop;');"
width="274" height="237"}

Di-cyano, diazo derivative. Click for 3D.
:::

This now has C~2~ (chiral) [exact two-fold
symmetry](http://dx.doi.org/10.6084/m9.figshare.95855){target="_blank"},
with C-N distances of 2.139Å. The \[3,3\] sigmatropic vibrational mode
is again real, with a value of 255 cm^-1^. A real candidate for
synthesis perhaps?

Finally, is it aromatic? The wavefunction for this system is stable
(which means no triplet state lower in energy can be found), so it
stands a good chance of being so. I will report back on this aspect in a
[later
post.](http://www.ch.imperial.ac.uk/rzepa/blog/?p=7678){target="_blank"}

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

**Postscript:** The above calculation for the last system was done at
the B3LYP/6-311G(d,p)/SCRF=thf level. A similar result is obtained at
*e.g.* a  [MP2/6-311G(d,p)/SCRF=thf
level](http://hdl.handle.net/10042/20323){target="_blank"}; the  \[3,3\]
vibrational mode has the real value of 318 cm^-1^.

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


Semibullvalene is an unsettling molecule. Whilst it has a classical structure describable by a combination of Lewis-style two electron and four electron bonds, its NMR behaviour reveals it to be highly fluxional. This means that even at low temperatures, the position of these two-electron bonds rapidly shifts in the equilibrium shown below. Nevertheless, this dynamic behaviour can be frozen out at sufficiently low temperatures.

Rogue Scholar Beiträge

Frozen Semibullvalene: a holy grail (and a bis-homoaromatic molecule).