In the previous post, the molecule F3S-C≡SF3 was found to exhibit a valence bond isomerism, one of the S-C bonds being single, the other triple, and with a large barrier (~31 kcal/mol, ν 284i cm-1) to interconversion of the two valence-bond forms.

Imperial College London

Henry Rzepa

In the [previous
post](http://www.ch.ic.ac.uk/rzepa/blog/?p=1347){target="_blank"}, the
molecule F~3~S-C≡SF~3~ was found to exhibit a valence bond isomerism,
one of the S-C bonds being single, the other triple, and with a large
barrier (\~31 kcal/mol, ν 284*i* cm^-1^) to interconversion of the two
valence-bond forms. So an interesting extension of this phenomenon is
shown below:

::: {#attachment_1425 .wp-caption .aligncenter style="width: 315px"}
![](http://www.ch.ic.ac.uk/rzepa/blog/wp-content/uploads/2010/01/S3C3F6.jpg "S3C3F6"){.size-full
.wp-image-1425 loading="lazy" decoding="async"
aria-describedby="caption-attachment-1425"
onclick="jmolInitialize('../Jmol/');jmolSetAppletColor('black');jmolApplet([450,450],'load wp-content/uploads/2010/01/F6S3C3.log;frame 40;zoom 5;moveto 4 0 2 0 90 90;vectors on;vectors 4;vectors scale 5.0; color vectors green; vibration 6;animation mode loop;');"
width="305" height="107"}

A cyclic form of the SCS Motif. Click for 3D
:::

If the same type of valence bond isomerism were to occur, we would now
have three C≡S triple bonds swapping places with three CS single bonds,
a sort of super version of the notation normally shown for benzene
itself. If the barrier to this swapping is finite, then the
interconversion shown above would be a proper equilibrium (the top
arrows), but if there is no barrier, then the interconversion would be a
proper resonance (the bottom double-headed arrow). Another way of posing
the question is whether the so-called [Kekulé vibrational
mode](http://www.ch.ic.ac.uk/rzepa/blog/?p=485){target="_blank"} (which
in effect represents the motions implied above) has a negative force
constant or a positive one respectively for the two sets of arrows
shown.

A B3LYP/cc-pVTZ calculation (DOI:
[10042/to-3646](http://hdl.handle.net/10042/to-3646)) reveals that the
optimized geometry exhibits six equal SC bonds, all 1.616Å long.
Typically, a single SC bond is around 1.82Å, a double 1.65Å and a triple
is about 1.5Å at the same level of theory, so this C=S bond is clearly
at least a double one. A NICS(0) calculation at the centroid has the
value of -14.6 ppm, which indicates aromaticity. We conclude the
appropriate arrow above is the bottom resonance one, rather than the top
equilibrium one. This is confirmed by finding that the Kekulé
vibrational mode has a strongly positive force constant (ν 1083 cm^-1^,
animated in 3D model above), which contrasts with the negative value (ν
284*i* cm^-1^) found for bond shifting in F~3~S-C≡SF~3~ itself. Again,
comparison indicates that a C≡S triple bond has a frequency of around
1400 cm^-1^ and a double around 1200 cm^-1^ (the degenerate C=S
non**--**Kekulé vibrational mode for this system is indeed calculated at
around 1225 cm^-1^). So to summarise; a single F~3~S-C≡SF~3~ unit
reveals very strong bond alternation, and negative force constant
(transition state) for interconversion of the two bond forms, but a
cyclic form reveals the opposite behaviour, with no alternation and
instead strong aromaticity.

In part this difference in behaviour must be due to the constraints on
the geometry of the cyclic form. F~3~S-C≡SF~3~ interconverts via a
highly twisted geometry with C~2~ symmetry, and this twisting is not
exactly possible if you create a cyclic equivalent. In part it is also
due to the aromatic stabilisation energies. In the resonance above, you
should be able to count a total of 12 electrons involved! Nominally, if
you try to apply the 4n+2 aromaticity rule, it does not fit, until you
realise that in fact you must be dealing with two sets of 6 electrons.
The system in fact is a classic double-aromatic, in which six electrons
circulate in the plane of the molecule (the σ-set) and six above and
below (the π-set; the MOs for the molecule confirm exactly this
interpretation). Notice how this itself contrasts with a similarly
aromatic system, the atom swapping in [three nitrosonium
cations](http://www.ch.ic.ac.uk/rzepa/blog/?p=1158){target="_blank"},
where the Kekulé mode force constant was strongly negative.

::: {#attachment_1438 .wp-caption .aligncenter style="width: 399px"}
![](http://www.ch.ic.ac.uk/rzepa/blog/wp-content/uploads/2010/01/F6S3C3-elf.jpg "F6S3C3-elf"){.size-full
.wp-image-1438 loading="lazy" decoding="async"
aria-describedby="caption-attachment-1438"
onclick="jmolInitialize('../Jmol/');jmolSetAppletColor('black');jmolApplet([450,450],'load wp-content/uploads/2010/01/F6S3C3-elf.mol;;zoom 5;moveto 4 0 2 0 90 90;');"
width="389" height="388"}

ELF Analysis for F6S3C3. Click for 3D
:::

To complete the analysis, the ELF basins (above) reveal the six SC
regions to each contain 2.7 electrons, together with three carbon
carbene monosynaptic basins. For comparison, a system with a high degree
of SC triple character (HCS^+^) has around 3.8 in the SC region. Perhaps
a better model is
[TfOSCH](http://www.ch.ic.ac.uk/rzepa/blog/?p=1243){target="_blank"}
(for which the carbon also has a carbene lone pair), which has 2.6e in
the CS region. The carbene lone "pair" for the present molecule
integrates to 2.6e each, which totals to a nice octet of electrons
around each carbon and to around 7 for each S, confirming that whilst
the S is **hypervalent**, its valence octet is not expanded!). This ELF
picture does rather tend to confirm the original resonance structure
representation shown at the top.

All that is needed is is for someone to make this molecule to confirm
its properties. Perhaps by trimerising F~2~SC, itself formed by
cheletropic elimination? It is worth noting that the iso-electronic P/N
(*e.g.* of S/C) analogues are very well known.

::: {#attachment_1499 .wp-caption .aligncenter style="width: 192px"}
[![](http://www.ch.ic.ac.uk/rzepa/blog/wp-content/uploads/2010/01/phosphonitriles.jpg "phosphonitriles"){.size-full
.wp-image-1499 loading="lazy" decoding="async"
aria-describedby="caption-attachment-1499" width="182"
height="144"}](http://www.ch.ic.ac.uk/rzepa/blog/wp-content/uploads/2010/01/phosphonitriles.jpg)

Phosphonitrilic compounds
:::

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


In the previous post, the molecule F

 3

S-C≡SF

 3

was found to exhibit a valence bond isomerism, one of the S-C bonds being single, the other triple, and with a large barrier (~31 kcal/mol, ν 284

 i

cm

 -1

) to interconversion of the two valence-bond forms. So an interesting extension of this phenomenon is shown below: A cyclic form of the SCS Motif.

Messaggi di Rogue Scholar

Contriving aromaticity from S≡C Triple bonds