Imperial College London

Henry Rzepa

Andy Extance at the [Chemistry World
blog](http://www.rsc.org/chemistryworld/2013/09/recoupled-bond-dyads-fssf3-strange-bonding){target="_blank"}
has picked up on a fascinating article\[cite\]10.1021/jz401578h\[/cite\]
on the dimer of SF~2~. This molecule has three F atoms on one S, and
only one on the other; FSSF~3~. But all four S-F bonds are of different
length. Lindquist and Dunning claim that the minimum energy pathway to
dissociation to two SF~2~ molecules does not involve breaking either the
*longest* or the *weakest* SF bond. This was too much for me to resist
investigating further!

::: {#attachment_11168 .wp-caption .aligncenter style="width: 288px"}
![Click for
3D](http://www.ch.imperial.ac.uk/rzepa/blog/wp-content/uploads/2013/09/S2F4.jpg){.wp-image-11168
loading="lazy" decoding="async"
aria-describedby="caption-attachment-11168"
onclick="jmolInitialize('../Jmol/');jmolSetAppletColor('white');jmolApplet([450,450],'load wp-content/uploads/2013/09/F3SSF-1195.772018.log;frame 4;measure 2 4;measure 2 5;measure 2 1;measure 2 6;measure 1 3;zoom 150;vectors on;vectors 4;vectors scale 5.0; color vectors purple; vibration 10;animation mode loop;');"
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Click for 3D
:::

The computed ωB97XD/aug-cc-pVTZ geometry
above\[cite\]10.6084/m9.figshare.791578\[/cite\] reveals the extent of
the difference in bond lengths. What Lindquist and Dunning showed is
that the two apparently longest (and implied weakest) SF bonds do not
have the lowest bond dissociation energies, and offered an explanation
using their methodology of coupled bond dyads obtained from GVB theory.
But as a "transition state" person myself, I could not help but note
that no such were described in their article. So I set out to look for
one.

Firstly, I tried to find a transition state for \[1,2\] migration of the
longest S-F bond (labelled 1.741Å above, axial at a trigonal bipyramidal
S). Nope.^‡^ The one labelled 1.654Å seemed unlikely, so in desperation
I tried the shortest S-F bond (1.582Å, equatorial at a trigonal
bipyramidal S). Success! The computed IRC
(ωB97XD/6-311G(d,p)\[cite\]10.6084/m9.figshare.797514\[/cite\] shows a
surprise. The *shortest* S-F bond does indeed do a \[1,2\] migration,
but it is accompanied in its travels by the *longest* S-F bond which
also moves more or less in synchrony, although its motion is far less.

![FSSF3](http://www.ch.imperial.ac.uk/rzepa/blog/wp-content/uploads/2013/09/FSSF31.gif){.aligncenter
.size-full .wp-image-11174 loading="lazy" decoding="async"
onclick="jmolInitialize('../Jmol/');jmolSetAppletColor('white');jmolApplet([450,450],'load wp-content/uploads/2013/09/ts.log;frame 13;vectors on;vectors 4;vectors scale 5.0; color vectors green; vibration 10;animation mode loop;');"
width="365" height="283"}

[![FSSF3G](http://www.ch.imperial.ac.uk/rzepa/blog/wp-content/uploads/2013/09/FSSF3G.svg){.aligncenter
.size-full .wp-image-11175 decoding="async"
width="400"}](http://www.ch.imperial.ac.uk/rzepa/blog/wp-content/uploads/2013/09/FSSF3G.svg)  
![FSSF3](http://www.ch.imperial.ac.uk/rzepa/blog/wp-content/uploads/2013/09/FSSF3.svg){.aligncenter
.size-full .wp-image-11176 decoding="async" width="400"}

This transition state, it transpires is not on the pathway to
dissociation, but is a
[double](http://www.ch.imperial.ac.uk/rzepa/blog/?p=1347 "Ménage à deux: Non-classical SC bonds."){target="_blank"}
\[1,2\] migration (in this it is related to the [coarctate
mechanism](http://www.ch.imperial.ac.uk/rzepa/blog/?p=11110 "Coarctate reactions as a third fundamental organic-mechanistic type."){target="_blank"},
which involves two bonds breaking and two forming in a concurrent
manner). It also has the feature associated with dyotropic
rearrangements\[cite\]10.1039/C2CC33676F\[/cite\] that the central S-S
bond is [shorter at the transition
state](http://www.ch.imperial.ac.uk/rzepa/blog/?p=10706 "Mechanistic arrow pushing. A proposed addition to its rules."){target="_blank"}
((1.937Å) than for either reactant or product (2.073Å).

The barrier is \> 60 kcal/mol (reduced only slightly by
solvation\[cite\]10.6084/m9.figshare.798824\[/cite\]) which does rather
tend to suggest that the formation of SF~2~ almost certainly involves
homolytic dissociation and hence radicals, rather than a closed shell
route found above.

Not relevant to the previous topic, but related to the [post on
ClF~3~](http://www.ch.imperial.ac.uk/rzepa/blog/?p=10937 "VSEPR Theory: A closer look at chlorine trifluoride, ClF3."){target="_blank"}
is the ELF analysis; an attempt to identify where the electron basins
(mono and disynaptic) are. The sulfur bearing the three fluorines has
one lone "pair" (1.88e to be precise, ringed below), whilst the other S
has two (2.25e each). The total integration for the 4-coordinate
trigonal-bipyramidal S valence basins however (6.43e) clearly indicates
no "octet expansion". 

![S2F4-bas](http://www.ch.imperial.ac.uk/rzepa/blog/wp-content/uploads/2013/09/S2F4-bas.jpg){.aligncenter
.wp-image-11178 loading="lazy" decoding="async"
onclick="jmolInitialize('../Jmol/');jmolSetAppletColor('white');jmolApplet([450,450],'load wp-content/uploads/2013/09/FSSF3_bas.mol2;');"
width="336" height="259"}

There are lots lots more to enjoy about this molecule, do go read
the Lindquist and Dunning article for a sense of this.

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

^‡^But I did [find it
eventually](http://www.ch.imperial.ac.uk/rzepa/blog/?p=11165&cpage=1#comment-63482){target="_blank"}.

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


Andy Extance at the Chemistry World blog has picked up on a fascinating article[cite]10.1021/jz401578h[/cite] on the dimer of SF
<sub>
 2
</sub>
. This molecule has three F atoms on one S, and only one on the other; FSSF
<sub>
 3
</sub>
. But all four S-F bonds are of different length.

Postagens de Rogue Scholar

The dimer of SF2: small is beautiful (and weird).