Imperial College London

Henry Rzepa

Charges in chemistry, like the grin on Lewis Carroll's cat, can be
mysterious creatures. Take for example the following structure, reported
by Paul Lickiss and co-workers (DOI:
[10.1039/b513203g](http://dx.doi.org/10.1039/b513203g){target="_blank"}).

::: {#attachment_4968 .wp-caption .aligncenter style="width: 242px"}
[![](http://www.ch.imperial.ac.uk/rzepa/blog/wp-content/uploads/2011/09/Si.jpg "Si"){.size-full
.wp-image-4968 loading="lazy" decoding="async"
aria-describedby="caption-attachment-4968" width="232"
height="127"}](http://www.ch.imperial.ac.uk/rzepa/blog/wp-content/uploads/2011/09/Si.jpg)

A silenium cation.
:::

A student of chemistry might be wondering what is going on, since this
representation seems to "break the rules". Thus there is a clear-cut
pentavalent carbon atom, but even more mysteriously, there is a positive
charge that seems to be floating uncertainly (much like Carroll's
Cheshire cat). Another Lewis famously introduced the concept of the
covalent electron pair bond in 1916, and ever since then we tend to
represent these types of bond with (straight) lines. If this convention
is adhered to rigidly, then the carbon highlighted with the purple dot
would have five lines to it, 10 electrons, and therefore in violation of
the octet rule for main group elements. Nowadays however, a line between
two atoms is not necessarily interpreted as a Lewis structure, but more
simply representing a connection to be used in a *connection table* for
indexing and searching the structure. So what IS the valency of this
carbon, and where IS that charge located?

One starts by converting the above representation into more formally
correct Lewis, or valence-bond structures.

::: {#attachment_4971 .wp-caption .aligncenter style="width: 410px"}
[![](http://www.ch.imperial.ac.uk/rzepa/blog/wp-content/uploads/2011/09/si.jpg "si"){.size-full
.wp-image-4971 loading="lazy" decoding="async"
aria-describedby="caption-attachment-4971" width="400"
height="75"}](http://www.ch.imperial.ac.uk/rzepa/blog/wp-content/uploads/2011/09/si.jpg)

Valence bond structures
:::

These structures (there are more, but they are related by symmetry to
those shown above) are bound, by the rules, to locate the bonds exactly,
and hence allow one to infer where the charge is. The latter two emerge
as different resonance forms of what we call [Wheland
intermediates](http://www.ch.imperial.ac.uk/rzepa/blog/?p=2423){target="_blank"}.
The former is the silicon equivalent of a tertiary carbocation. Quantum
mechanics now tells us which of these (if any) is the most realistic. To
do this, I invoke the ELF (electron localisation function) method, which
identifies so-called synaptic basins in the function, and how much
electron density is contained in each (there are of course many other
ways of partitioning the electrons).

::: {#attachment_4973 .wp-caption .aligncenter style="width: 269px"}
[![](http://www.ch.imperial.ac.uk/rzepa/blog/wp-content/uploads/2011/09/Si-ELF.jpg "Si-ELF"){.size-full
.wp-image-4973 loading="lazy" decoding="async"
aria-describedby="caption-attachment-4973" width="259"
height="259"}](http://www.ch.imperial.ac.uk/rzepa/blog/wp-content/uploads/2011/09/Si-ELF.jpg)

ELF basin integrations.
:::

Basin 1 has 1.51, basin 2 has 2.39, basin 3 has 2.84 and basin 4 has
2.65e. Let us discuss the significance of these.

1.  Basins **1+2** (+ their symmetric equivalent) together contain
    3.02 + 4.78 = 7.8e. Thus this carbon definitely is not hypervalent,
    since its octet is pretty much satisfied! But notice that this
    carbon is not a conventional so-called sp^3^ hybridized carbon,
    which has four equal two-electron bonds. This one has two bonds with
    significantly less than two electrons, and two with significantly
    more! A most unusual 4-coordinate carbon. Bond 1 has a Wiberg bond
    index of 0.48 and bond  2 is 1.29.
2.  By the same process, each Si atom integrates to 7.72e. If either
    were to be a silacation, that would imply only 6 electrons, which is
    clearly not the case for either  Si.
3.  How about the ortho, meta and para carbons of the phenyl ring? These
    are respectively 7.39, 7.64 and 7.44e. These are definitely a bit
    low, and taken together they constitute the equivalent of a
    six-valence electron carbocation. So it looks as if we have found
    our positive charge, which is delocalized on the phenyl ring in the
    manner of a Wheland intermediate (with more of the +ve charge on the
    *o/p* positions).

Well, if this species is really a Wheland intermediate in which the
cyclic conjugation of π-electrons is disrupted, the phenyl ring should
not be aromatic. In fact, it turns out this ring IS recognisably (if not
highly) aromatic. Its NICS(1) index value is -8.3ppm (benzene is \~-11
on the same scale). Exactly the same phenomenon [was
found](http://www.ch.imperial.ac.uk/rzepa/blog/?p=2423){target="_blank"}
for the supposed Wheland intermediate (which in fact turned out to be a
transition state) identified as the mechanism of nitrosation of benzene
using CF~3~COONO. Can all these disparate properties be reconciled?

Yet another way of looking at what is happening in this molecule is
**Natural bond orbital** analysis (NBO). I have previously used this
technique to [probe the structure of
DNA](http://www.ch.imperial.ac.uk/rzepa/blog/?p=4089){target="_blank"},
and for [identifying unexpected
anomeric](http://www.ch.imperial.ac.uk/rzepa/blog/?p=745){target="_blank"}
effects (amongst others). Applied to this system, it reveals **four**
donor-acceptor interaction energies E(2), each of \~ 10.5 kcal/mol,
between the four permutations of *e.g.* bond **1** acting as the donor,
and bond **3** acting as an acceptor, a **σ~Si-C~/π^\*^~C-C~**
interaction. The value of E(2) corresponds to almost a full anomeric
effect (these tend to be \~15 kcal/mol if the donor is a O lone pair and
the acceptor a  C-O bond), and there are four of them after all! This
particular conjugation is the one that makes the phenyl ring retain much
of its aromaticity, *i.e.* having its cake and eating it.

Notice that individually, each of the effects I have described above is
actually borrowed from fairly conventional introductory level organic
chemistry (Lewis structures, the octet rule, cation stability,
aromaticity, stereoelectronic/anomeric effects), and it shows how a
combination of these in a single molecule can result in quite unusual
properties.

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


Charges in chemistry, like the grin on Lewis Carroll’s cat, can be mysterious creatures. Take for example the following structure, reported by Paul Lickiss and co-workers (DOI: 10.1039/b513203g).    A silenium cation.

Messaggi di Rogue Scholar

Hunt the charge: the Cheshire cat of chemistry