Imperial College London

Henry Rzepa

[George
Olah](https://en.wikipedia.org/wiki/George_Andrew_Olah){target="_blank"}
passed away on March 8th. He was part of the generation of scientists in
the post-war 1950s who had access to chemical instrumentation that truly
revolutionised chemistry. In particular he showed how the then newly
available NMR spectroscopy illuminated structures of cations in solvents
such "[Magic acid](http://magic%20acid)". The obituaries will probably
mention his famous "feud" with H. C. Brown over the structure of the
norbornyl cation (X=CH~2~^+^), implicated in the mechanism of many a
solvolysis reaction that characterised the golden period of physical
organic chemistry just before and after WWII. 

[![](http://www.ch.ic.ac.uk/rzepa/blog/wp-content/uploads/2017/03/norbornyl.svg){.aligncenter
.size-large .wp-image-17639 decoding="async"
width="200"}](http://www.ch.ic.ac.uk/rzepa/blog/wp-content/uploads/2017/03/norbornyl.svg)

The dispute between Olah and Brown was not played on a pitch using quite
the same goal posts. Olah did much of his work in magic acid and Brown
did his in aqueous solutions. I was involved in a tiny way when the
discussion about the precise character of the norbornyl cation was
reaching its peak in the mid 1970s. At the time, I was working with
Michael Dewar, who was himself not shy in joining in the fun and
sometimes very acrimonious disputes at conferences. We contributed by
calculating the so-called core-electron carbon ESCA
spectrum.\[cite\]10.1021/ja00444a012\[/cite\] History records that we
came down on the wrong side,^‡^ by suggesting that this form of
spectroscopy supported Brown rather than Winstein/Olah on the basis of a
6:1 spectral deconvolution (classical) rather than 5:2 (non-classical).
More recently of course the crystal structure of the parent cation
itself has been shown to be non-classical\[cite\]
10.1126/science.1238849\[/cite\] (there are other crystal
structures which differ in respect to having one or more additional
methyl groups\[cite\]10.1002/hlca.19940770407\[/cite\]). For a 3D model
of norbornyl cation, see
DOI: [10.5517/CCZ21LN](https://doi.org/10.5517/CCZ21LN). This still
leaves the issue (very slightly) open for the structure of the solvated
cation when formed in water! 

When I started to teach a course in molecular modelling, I touched
briefly on how modelling could contribute and whilst updating the notes
in the 1990s, wondered why the boron analogue had never been so
studied (X=BH~2~). Unlike the crystallographically difficult norbornyl
ion-pair, the iso-electronic boron species would be neutral and not need
a counter-ion. Perhaps it might be a more manageable molecule? Checking
the Cambridge structural database, such a species has never been
reported!^†^ So here as my homage to Olah, I report its calculated
structure (b2plypd3/Def2-TZVPP,
DOI: [10.14469/hpc/2236](https://doi.org/10.14469/hpc/2236)).

![](http://www.ch.ic.ac.uk/rzepa/blog/wp-content/uploads/2017/03/147-1024x756.jpg){.aligncenter
.size-large .wp-image-17649 decoding="async" width="400"
srcset="https://www.ch.ic.ac.uk/rzepa/blog/wp-content/uploads/2017/03/147-1024x756.jpg 1024w, https://www.ch.ic.ac.uk/rzepa/blog/wp-content/uploads/2017/03/147-300x222.jpg 300w, https://www.ch.ic.ac.uk/rzepa/blog/wp-content/uploads/2017/03/147-768x567.jpg 768w, https://www.ch.ic.ac.uk/rzepa/blog/wp-content/uploads/2017/03/147.jpg 1644w"
sizes="(max-width: 1024px) 100vw, 1024px"}

The norbornyl cation has symmetrical C-C bridging distances of
\~1.80±0.02Å and a basal C-C distance of \~1.39±0.02Å. The calculated
values for the boron equivalent are 2.16Å and 1.36Å respectively, with
all positive force constants. B-C bonds are normally 1.66-1.72Å,
significantly longer than C-C bonds, which makes the longer B-C lengths
in this example unsurprising. More interestingly, the species has one
vibrational normal mode (ν 203 cm^-1^) which corresponds to the \[1,2\]
shift of the BH~2 ~group across the basal C-C. For a classical species,
this vibrational motion would correspond to a transition state (an
imaginary vibration) but for a non-classical species it is of course
real. In this sense it is analogous to the so-called real [Kekulé
mode](http://www.ch.imperial.ac.uk/rzepa/blog/?p=12560) in non-classical
benzene, which "equilibrates" the two classical Kekulé structures. The
corresponding calculated vibration for the norbornyl cation itself is ν
194 cm^-1^ (DOI: [10.14469/hpc/2238](http://doi.org/10.14469/hpc/2238)).

Of course, the entire controversy over the structure of this species is
littered with comparisons between not quite similar systems, differing
in a methyl group more or less. So morphing a C^+^ to a B might be seen
as quite a large change. But perhaps if it had been crystallised in say
the 1960s, would the subsequent debates have taken a different turn?

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

^‡^ We were also wrong about the symmetry of the Diels-Alder
cyclisation, which is nowadays accepted to be synchronous rather than
asynchronous for simple  Diels-Alder reactions. But that is another
story.

^†^GAXLIA is perhaps the closest
analogue.\[cite\]10.1021/ja00217a053\[/cite\],

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


George Olah passed away on March 8th. He was part of the generation of scientists in the post-war 1950s who had access to chemical instrumentation that truly revolutionised chemistry.

Messaggi di Rogue Scholar

George Olah and the norbornyl cation.