Imperial College London

Henry Rzepa

[I
previously](http://www.ch.imperial.ac.uk/rzepa/blog/?p=13962){target="_blank"}
used data mining of crystal structures to explore the directing
influence of substituents on aromatic and heteroaromatic rings. Here I
explore, quite literally, a different angle to the hydrogen bonding
interactions between a benzene ring and OH or NH groups.

![aromatic-pi-query](http://www.ch.ic.ac.uk/rzepa/blog/wp-content/uploads/2016/06/aromatic-pi-query-1-e1466580253270.jpg){.aligncenter
.size-full .wp-image-16575 loading="lazy" decoding="async" height="199"
width="440"}

I start by defining a benzene ring with a centroid. The distance is from
that centroid to the H atom of an OH or NH group and the angle is
C-centroid-H. To limit the search to approach of the OH or NH group more
or less orthogonal to the ring, the absolute value of the torsion
between the centroid-H vector and the ring C-C vector is constrained to
lie between 70-100° (the other constraints being no disorder, no errors,
T \< 140K and R \< 0.05).\[cite\]10.14469/hpc/673\[/cite\]

![aromatic-pi-HN-140](http://www.ch.ic.ac.uk/rzepa/blog/wp-content/uploads/2016/06/aromatic-pi-HN-140-1-e1466580553520.jpg){.aligncenter
.size-full .wp-image-16576 loading="lazy" decoding="async" height="350"
width="440"}

The above shows the results for NH groups interacting with the aromatic
ring. The maximum distance 2.8Å is more or less the van der Waals
contact distance between a hydrogen and a carbon and as you can see the
contacts \"funnel down\" to the centroid at \< 2.1Å. The shortest
distance\[cite\]10.1107/S0108768199012318\[/cite\] is for ammonium
tetraphenylborate, which you can view in *e.g.* spacefill mode
here\[cite\]10.5517/CC4V6TZ\[/cite\]

![390](http://www.ch.ic.ac.uk/rzepa/blog/wp-content/uploads/2016/06/390.jpg){.aligncenter
.size-full .wp-image-16578 decoding="async" width="400"
srcset="https://www.ch.ic.ac.uk/rzepa/blog/wp-content/uploads/2016/06/390.jpg 422w, https://www.ch.ic.ac.uk/rzepa/blog/wp-content/uploads/2016/06/390-300x290.jpg 300w"
sizes="(max-width: 422px) 100vw, 422px"}

The other interesting close contact derives from a protonated
pyridine\[cite\]10.1021/cg100831c\[/cite\], which can in turn be viewed
here.\[cite\]10.5517/CCWHC5W\[/cite\] The main message from the
distribution shown above is that as the distances between the HN and the
centroid get shorter, the \"trajectory\" of approach remains orthogonal
to the ring (the angle defined above remains \~90°) and heads towards
the centroid of the π-cloud. The hotspot itself (red, \~2.6Å) also lies
along this trajectory.

Recollect that when I used such hydrogen bonding to see if crystal
structures discriminate between the ortho or meta positions of a ring
carrying an electron donating substituent, it was the distance from a HO
to the carbon that was measured as the discriminator. So it\'s a faint
surprise to find that with HN, and without the necessary perturbation of
an electron donating substituent, the intrinsic preference seems to be
for the ring centroid and not any specific carbon atom of the ring.

So how about the OH group? There are in fact rather fewer examples, and
so the statistics are a bit less clear-cut. But there is a tantalising
suggestion that this time, the trajectory is not \~90° but rather less,
implying that the destination is no longer the centroid of the π-cloud
but one of the carbon atoms of the ring itself. For those who like to
\"read between the lines\" and spot things that are absent rather than
present, you may have asked yourself why I did not use NH probes in my
[earlier
post](http://www.ch.imperial.ac.uk/rzepa/blog/?p=13962){target="_blank"}.
Well, it appears that the NH group is less effective at *e.g.* *o/p*
discrimination than is an OH group.

![aromatic-pi-OH-140](http://www.ch.ic.ac.uk/rzepa/blog/wp-content/uploads/2016/06/aromatic-pi-OH-140-1-e1466582127469.jpg){.aligncenter
.size-full .wp-image-16580 loading="lazy" decoding="async" height="311"
width="400"}

I can only speculate as to the origins (real or not) of the difference
in behaviour between OH and NH groups towards a phenyl π-face. Perhaps
it is simply bias in the CSD database? Or might there be electronic
origins? Time to end with that phrase \"watch this space\".

 

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


I previously used data mining of crystal structures to explore the directing influence of substituents on aromatic and heteroaromatic rings. Here I explore, quite literally, a different angle to the hydrogen bonding interactions between a benzene ring and OH or NH groups. I start by defining a benzene ring with a centroid.

Messaggi di Rogue Scholar

How does an OH or NH group approach an aromatic ring to hydrogen bond with its π-face?