I discussed the molecule the molecule CH3F2- a while back.

Imperial College London

Henry Rzepa

I [discussed](http://www.ch.imperial.ac.uk/rzepa/blog/?p=15823) the
molecule the molecule CH~3~F^2-^ a while back. It was a very rare
computed example of a system where the added two electrons populate the
higher valence shells known as Rydberg orbitals as an alternative to
populating the C-F antibonding σ-orbital to produce CH~3~^--^ and F^--^.
The net result was the creation of a weak C-F "hyperbond", in which the
C-F region has an inner conventional bond, with an outer "sheath"
encircling the first bond. But this system very easily dissociates to
CH~3~^--^ and F^--^ and is hardly a viable candidate for experimental
detection.  In an effort to "tune" this effect to see if a better
candidate for such detection might be found, I tried CMe~3~F^2-^. Here
is its story.

The calculation^‡^ is at the ωB97XD/Def2-TZVPPD/SCRF=water level (water
is here used as an approximate model for a condensed environment,
helping to bind the two added electrons).

1.  An NBO (Natural Bond orbital) analysis reveals a total Rydberg
    orbital population of 1.186e and the following bond indices; F
    0.853, C 3.977, C(methyl) [**4.051**]{style="color: #ff0000;"},
    H(\*3) [**1.332**]{style="color: #ff0000;"}. The latter corresponds
    to the three methyl hydrogens aligned antiperiplanar to the C-F
    bond.
2.  To put this value into context, the hydrogen in the FHF^--^ anion
    has an NBO H bond index of 0.724, and the bridging hydrogens in
    diborane only have a value of 0.988. Even the hexa-coordinate
    hydride system \[Co~6~H(CO)~15~\]^--^ [discussed in an earlier
    blog](http://www.ch.imperial.ac.uk/rzepa/blog/?p=10839)  has an H
    bond index of just 0.86. Actually, coordination of six or even
    higher for hydrogen is no longer rare; some 28 crystal structures of
    the type HM~6~ (M=metal) are known (it would be useful to find out
    if any of the other 27 such structures might have a hydrogen bond
    index \>1).
3.  Next, the ELF analysis (Electron localisation function), analysed
    firstly using the excellent MultiWFN
    program.\[cite\]10.1002/jcc.22885\[/cite\]  
    ![](http://www.ch.ic.ac.uk/rzepa/blog/wp-content/uploads/2018/01/MeCF2-ELF-965x1024.jpg){.aligncenter
    .size-large .wp-image-19350 decoding="async" width="400"
    srcset="https://www.ch.ic.ac.uk/rzepa/blog/wp-content/uploads/2018/01/MeCF2-ELF-965x1024.jpg 965w, https://www.ch.ic.ac.uk/rzepa/blog/wp-content/uploads/2018/01/MeCF2-ELF-283x300.jpg 283w, https://www.ch.ic.ac.uk/rzepa/blog/wp-content/uploads/2018/01/MeCF2-ELF-768x815.jpg 768w, https://www.ch.ic.ac.uk/rzepa/blog/wp-content/uploads/2018/01/MeCF2-ELF.jpg 1395w"
    sizes="(max-width: 965px) 100vw, 965px"}  
    This reveals an attractor basin integrating to 1.663e and located
    along the axis of the F-C bond and extended into the region of the
    three antiperiplanar methyl hydrogens. The C-F bond itself only
    supports a basin of 0.729e, typical of the fairly ionic C-F bond.
    The covalent C-Me bonds are also pretty normal, as are the other
    hydrogens.
4.  I also show ELF analysis using the alternative TopMod
    program\[cite\]10.1016/S0097-8485(99)00039-X\[/cite\]; the numerical
    values on this diagram are the calculated bond lengths in Å. The
    basin integrations are very similar to those obtained using
    MultiWFN.  
    ![](http://www.ch.ic.ac.uk/rzepa/blog/wp-content/uploads/2018/01/MeCF2-a-890x1024.jpg){.aligncenter
    .size-large .wp-image-19353 decoding="async" width="400"
    srcset="https://www.ch.ic.ac.uk/rzepa/blog/wp-content/uploads/2018/01/MeCF2-a-890x1024.jpg 890w, https://www.ch.ic.ac.uk/rzepa/blog/wp-content/uploads/2018/01/MeCF2-a-261x300.jpg 261w, https://www.ch.ic.ac.uk/rzepa/blog/wp-content/uploads/2018/01/MeCF2-a-768x884.jpg 768w, https://www.ch.ic.ac.uk/rzepa/blog/wp-content/uploads/2018/01/MeCF2-a.jpg 1198w"
    sizes="(max-width: 890px) 100vw, 890px"}  
    The Wiberg bond orders of the three H...H regions shown connected by
    dashed lines above are 0.154, which contributes to the bond index of
    \>1 at these three hydrogens.
5.  The predicted ^1^H chemical shift of these three "hypervalent"
    hydrogens is +3.0 ppm, whilst the other six methyl hydrogens are at
    -0.87ppm.

So changing CH~3~F^2-^ to CMe~3~F^2-^ has dramatically changed the
bonding picture that emerges, rather than a fine-tuning. The C-F is no
longer a "hyperbond", although the Rydberg occupancy of 1.186e remains
unusually large. Most of the additional electrons have fled the torus
surrounding the C-F bond and relocated to the *exo*-region of that bond
where they now influence the three antiperiplanar methyl hydrogens.
A two-electron-three-centre interaction if you like, but with the
electron basin occupying a tetrahedral vertex rather than the triatom
centroid.

I end with a challenge. Is it possible to find "real" molecules
containing hydrogen where the formal bond index for at least one
hydrogen exceeds 1.0 significantly, thus making it hypervalent? 

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

^‡^The calculations are all collected at FAIR doi;
[10.14469/hpc/3372](https://doi.org/10.14469/hpc/3372).

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I discussed the molecule the molecule CH
<sub>
 3
</sub>
F
<sup>
 2-
</sup>
a while back.

Postagens de Rogue Scholar

Hypervalent hydrogen?