Imperial College London

Henry Rzepa

About 18 months ago, there was [much
discussion](http://www.ch.imperial.ac.uk/rzepa/blog/?p=11331) on this
blog about a system reported by Bob Pascal and co-workers containing a
short H...H contact of \~1.5Å\[cite\]10.1021/ja407398w\[/cite\]. In this
system, the hydrogens were both attached to Si as Si-H...H-Si and
compressed together by rings. Now a new
report\[cite\]10.1021/jacs.7b01879\[/cite\] and [commented upon by Steve
Bachrach](http://comporgchem.com/blog/?p=4014), claims a similar
distance for hydrogens attached to carbon, *i.e.* C-H...H-C, but without
the ring compression.

![](http://www.ch.ic.ac.uk/rzepa/blog/wp-content/uploads/2017/06/201-1024x987.jpg){.aligncenter
.size-large .wp-image-18559 loading="lazy" decoding="async" width="450"
height="434"}

This new example is the structure of an C~3~-symmetric all-meta
^*t*^Bu-triphenylmethane R-H...H-R dimer determined by neutron
diffraction (DOI:
[10.5517/ccdc.csd.cc1nc1bd](https://dx.doi.org/10.5517/ccdc.csd.cc1nc1bd))
and the close interaction is achieved purely by attractions due to
dispersion forces accumulating in the remainder of the molecules. This
study also reports a diverse set of computed properties for this new
system, but one property reported as part of the [previous
discussion](http://www.ch.imperial.ac.uk/rzepa/blog/?p=11331) was not
presented, the ^1^J~H-H~ coupling constant. I have computed it here in
the hope that it might be possible to measure by some means, perhaps in
the solid state?

The chemical shift of the R~3~CH proton is measured as a singlet^‡^ at
\~7.35 ppm (in deuterated benzene, Figure S6,
SI).^†^ ![](http://www.ch.ic.ac.uk/rzepa/blog/wp-content/uploads/2017/06/NMR.jpg){.aligncenter
.size-full .wp-image-18570 decoding="async" width="300"
srcset="https://www.ch.ic.ac.uk/rzepa/blog/wp-content/uploads/2017/06/NMR.jpg 618w, https://www.ch.ic.ac.uk/rzepa/blog/wp-content/uploads/2017/06/NMR-300x281.jpg 300w"
sizes="(max-width: 618px) 100vw, 618px"}

The value calculated using B3LYP/Def2-TZVPP (gas phase) is 7.39 and 7.69
ppm (averaged to 7.54 for a rapidly exchanging environment). The ^1^J
coupling is calculated as 4.3 Hz at the B3LYP/Def2-TZVPP level, DOI:
[10.14469/hpc/2699](https://doi.org/10.14469/hpc/2699). The
designation ^1^J is normally taken as a 1-bond pathway for the coupling.
In this example, the designation of the H-H region as a "bond" is the
interesting discussion point!

I end by noting here [my
observation](http://www.ch.imperial.ac.uk/rzepa/blog/?p=17579) that
although the neutron diffraction study of ammonium tetraphenylborate
shows the  N-H protons as pointing directly towards the centroid of
phenyl groups, the original
observation\[cite\]10.1107/S0108768199012318\[/cite\] was made that
"e*ven at 20 K the ammonium ion performs large amplitude motions which
allow the N-H vectors to sample the entire face of the aromatic system".
 *The equivalent thermal motion for the triphenylmethane system here
would have the  C-H vectors orbiting around each other in a manner that
increases the H-H separation, but which averages out to them pointing
directly towards one another?  The calculated normal coordinate analysis
of this system is not available from the article SI, so the ease of
 C-C-H bending to achieve such motion is difficult to ascertain. Perhaps
trying to detect the ^1^J coupling might illuminate whether this
happens?

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

**Postscript**. ^†^Prof Schreiner has indicated that that the methine
assignment is 5.79 ppm (**b** below) and not 7.35 as marked with a
diamond in the S6 figure caption (**a** below). This is of course
measured in d6-benzene solution and applies to the monomer, not
presumably the dimer. The calculated value of 7.54 ppm as reported above
applies specifically to the dimer, which suggests a significant shift of
\~2ppm upon dimer formation. It would be interesting to verify this
prediction *via* a solid-state measurement.  
![](http://www.ch.ic.ac.uk/rzepa/blog/wp-content/uploads/2017/06/135-1024x565.jpg){.aligncenter
.size-large .wp-image-18621 loading="lazy" decoding="async" width="450"
height="248"}

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

^‡^[Measuring coupling would require an asymmetric environment to
differentiate the two chemical shifts of the interacting hydrogens.
Although the C~3~ symmetry of the crystal structure could provide such
an environment, it is observed to be fluxional in solution,  which
equalises the two chemical shifts on the NMR time scale. Two
non-equivalent protons exhibiting only mutual couplings manifest as an
AB-type double doublet of peaks in the NMR spectrum. As the difference
in chemical shift between the two nuclei (in units of Hz) approaches in
magnitude the value of the coupling constant between them (also in Hz),
the AB quartet becomes increasingly second-order in appearance. This
means that the intensities of the two outer peaks starts to decrease and
the two inner peak intensities increase. When the chemical shift
difference between them reaches zero, the intensity of the two outer
peaks also becomes zero and the two inner peaks superimpose to become a
single peak. This means that the coupling constant cannot be measured
from the splitting of the peaks (which has vanished). It does **not**
mean of course that the coupling itself has vanished; it merely no
longer manifests in the spectrum.]{.small}

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About 18 months ago, there was much discussion on this blog about a system reported by Bob Pascal and co-workers containing a short H…H contact of ~1.5Å[cite]10.1021/ja407398w[/cite]. In this system, the hydrogens were both attached to Si as Si-H…H-Si and compressed together by rings.

Rogue Scholar Beiträge

Dispersion “bonds”: a new example with an ultra-short H…H distance.