Imperial College London

Henry Rzepa

In the [previous
post](http://www.ch.imperial.ac.uk/rzepa/blog/?p=17702), I found
intriguing the mechanism by which methane (CH~4~) inverts by transposing
two of its hydrogens. Here I take a look at silane, SiH~4~.

It appears it is a three-stage process! Firstly, silane eliminates
molecular hydrogen to form a molecular complex between H~2 ~and SiH~2~
(DOI: [10.14469/hpc/2290](https://doi.org/10.14469/hpc/2290)). The
barrier (\~60 kcal/mol) is very much lower than with methane.

![](http://www.ch.ic.ac.uk/rzepa/blog/wp-content/uploads/2017/03/SiH4-elim.gif){.aligncenter
.size-full .wp-image-17715 decoding="async"
width="450"}[![](http://www.ch.ic.ac.uk/rzepa/blog/wp-content/uploads/2017/03/gaussian-78_tot_ener.svg){.aligncenter
.size-large .wp-image-17718 decoding="async"
width="450"}](http://www.ch.ic.ac.uk/rzepa/blog/wp-content/uploads/2017/03/gaussian-78_tot_ener.svg)[![](http://www.ch.ic.ac.uk/rzepa/blog/wp-content/uploads/2017/03/gaussian-78_tot_ener.svg){.aligncenter
.size-large .wp-image-17717 loading="lazy" decoding="async" width="1"
height="1"}](http://www.ch.ic.ac.uk/rzepa/blog/wp-content/uploads/2017/03/gaussian-78_tot_ener.svg)

The H~2~ component of this complex then rotates (DOI:
[10.14469/hpc/2289](https://doi.org/10.14469/hpc/2289)) transposing
atoms 1 and 2. The barrier for this process is tiny (\~4 kcal/mol).

![](http://www.ch.ic.ac.uk/rzepa/blog/wp-content/uploads/2017/03/SiH4-rot.gif){.aligncenter
.size-full .wp-image-17711 decoding="async"
width="450"}![](http://www.ch.ic.ac.uk/rzepa/blog/wp-content/uploads/2017/03/gaussian-27_tot_ener.svg){.aligncenter
.size-large .wp-image-17709 decoding="async" width="450"}

Finally, the rotated H~2~/SiH~2~ complex goes back to silane by the
first route, but now with the two hydrogens transposed.

So this inversion is a**[ stepwise process]{style="color: #ff0000;"}**
in contrast to methane which was**[
concerted]{style="color: #ff0000;"}**, albeit with "frustrated"
elimination of hydrogen. Again a little molecule can show us so much
chemistry, in this case also illustrating the [avoidance of a
Woodward-Hoffmann forbidden cheletropic
elimination](http://www.ch.imperial.ac.uk/rzepa/blog/?p=6977) by
desymmetrisation.

::: {.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, I found intriguing the mechanism by which methane (CH

 4

) inverts by transposing two of its hydrogens. Here I take a look at silane, SiH

 4

. It appears it is a three-stage process! Firstly, silane eliminates molecular hydrogen to form a molecular complex between H

 2

and SiH

 2

(DOI: 10.14469/hpc/2290). The barrier (~60 kcal/mol) is very much lower than with methane.

How does silane invert (its configuration)?

This is a spin-off from the table [I constructed
here](http://www.ch.imperial.ac.uk/rzepa/blog/?p=17633#comment-227278){target="_blank"}
for further chemical examples of the classical/non-classical norbornyl
cation conundrum. One possible entry would include the transition state
for inversion of methane *via* a square planar geometry as compared with
*e.g.* NiH~4~ for which the square planar motif is its minimum. So is
square planar methane a true transition state for inversion (of
configuration) of carbon?

The history of this topic is nicely told as far back as
1993\[cite\]10.1021/ja00069a056\[/cite\], when square planar methane was
shown to be a 4th-order saddle point (*i.e.* four negative force
constants) and not the first order one required of a transition state. A
true transition state was located,^‡^ and here I show it as part of an
animated IRC (intrinsic reaction coordinate). Go to
DOI: [10.14469/hpc/2288](https://doi.org/10.14469/hpc/2288) for the
calculation outputs.

![](http://www.ch.ic.ac.uk/rzepa/blog/wp-content/uploads/2017/03/ch4a.gif){.aligncenter
.size-full .wp-image-17705 decoding="async" width="450"}

To convince yourself that the configuration really does invert, focus on
the CIP rule. With atom 1 pointing behind, atoms 2 → 3  → 4 rotate in a
clockwise direction. Now focus on the final point at the end of the IRC,
when 2 → 3  → 4 rotate anti-clockwise. The configuration has inverted!
The barrier as can be seen below is \~118 kcal/mol. At this value the
half-life for the process would be far longer than the age of the
universe.

[![](http://www.ch.ic.ac.uk/rzepa/blog/wp-content/uploads/2017/03/ch4-3_tot_ener.svg){.aligncenter
.size-large .wp-image-17706 decoding="async"
width="450"}](http://www.ch.ic.ac.uk/rzepa/blog/wp-content/uploads/2017/03/ch4-3_tot_ener.svg)

The process can be described as an interesting variation
on pseudorotation, for which the classic example is of course PF~5~.^†^
Alternatively it can be thought of as the partial extrusion of H~2~ to
give carbene, followed by re-addition of the H~2~ to reform methane.
Partial because the extrusion is never fully achieved.

I have to say I did not expect anything quite so interesting to be
associated with methane;  one can learn from the simplest of molecules!

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

^‡^ It was not entirely trivial to recover appropriate coordinates for
recomputing this TS from the article. But it is in fact an easy one to
find from scratch. Hopefully with the files at
[10.14469/hpc/2288](https://doi.org/10.14469/hpc/2288) to help, this
will not be an issue here.

^†^There are many kinds of pseudo-rotations. For others see
here.\[cite\]10.1021/ic0519988\[/cite\] and
\[cite\]10.1021/ic062473y\[/cite\]

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


This is a spin-off from the table I constructed here for further chemical examples of the classical/non-classical norbornyl cation conundrum. One possible entry would include the transition state for inversion of methane

 via

a square planar geometry as compared with

 e.g.

NiH

 4

for which the square planar motif is its minimum.

Rogue Scholar Beiträge

How does silane invert (its configuration)?

How does methane invert (its configuration)?