Imperial College London

Henry Rzepa

The [previous
post](http://www.ch.imperial.ac.uk/rzepa/blog/?p=16601){target="_blank"}
looked at anomeric effects set up on centres such as B, Si or P, and
involving two oxygen groups attached to these atoms. Here I vary the
attached groups to include either one or two nitrogen
atoms.\[cite\]10.14469/hpc/936\[/cite\]

.![aminol-sq](http://www.ch.ic.ac.uk/rzepa/blog/wp-content/uploads/2016/07/aminol-sq-1.jpg){.aligncenter
.size-full .wp-image-16611 decoding="async" width="400"
srcset="https://www.ch.ic.ac.uk/rzepa/blog/wp-content/uploads/2016/07/aminol-sq-1.jpg 931w, https://www.ch.ic.ac.uk/rzepa/blog/wp-content/uploads/2016/07/aminol-sq-1-300x237.jpg 300w, https://www.ch.ic.ac.uk/rzepa/blog/wp-content/uploads/2016/07/aminol-sq-1-768x608.jpg 768w"
sizes="(max-width: 931px) 100vw, 931px"}

The plot below shows aminols, C(NHR)(OR"). A torsion along either the
C-O or C-N bond of \~60° implies that (at two coordinate oxygen or three
coordinated nitrogen) there may be a lone pair with a torsion of 180°,
which would set up an antiperiplanar alignment between that lone pair
and the adjacent C-O or C-N bond (the anomeric effect). The clear
hotspot is at angles of \~80°, which does raise the issue of why it
deviates from 60°. Only a location of the lone pair centroid (using eg
the ELF quantum mechanical technique) would cast light on that. There is
a less distinct region for which the C-N torsion is 60° and the C-O
torsion 180°, and an even less distinct region for the reverse
(C-O torsion is 60° and the C-N torsion 180°). This tends to imply that
a nitrogen lone pair is a better donor into a C-O bond than the reverse.
Electronegativity suggests this should indeed be so, with the N lone
pair less bound by the N nucleus and hence easier to release into a
C-O^σ\*^ orbital which is a better acceptor than then
equivalent C-N^σ\*^ orbital.
![aminol](http://www.ch.ic.ac.uk/rzepa/blog/wp-content/uploads/2016/07/aminol-1-1024x773.jpg){.aligncenter
.size-large .wp-image-16613 loading="lazy" decoding="async" width="450"
height="340"
srcset="https://www.ch.ic.ac.uk/rzepa/blog/wp-content/uploads/2016/07/aminol-1-1024x773.jpg 1024w, https://www.ch.ic.ac.uk/rzepa/blog/wp-content/uploads/2016/07/aminol-1-300x227.jpg 300w, https://www.ch.ic.ac.uk/rzepa/blog/wp-content/uploads/2016/07/aminol-1-768x580.jpg 768w, https://www.ch.ic.ac.uk/rzepa/blog/wp-content/uploads/2016/07/aminol-1.jpg 1666w"
sizes="(max-width: 450px) 100vw, 450px"} This plot is where both
heteroatoms are nitrogen (geminal diamines). There are about twice as
many examples, resulting in more distinct clustering. The anomeric
hotspot is now around 70°  and there are equally populated clusters
where only one torsion is \~70°. There is another cluster for which both
torsions are 180° (no stereoelectronic alignment of lone pairs) and
three small clusters where the torsions are either 180° or 0°. There is
finally an intriguing cluster for which both torsions at \~120° (again
no
stereoelectronics). ![diamine](http://www.ch.ic.ac.uk/rzepa/blog/wp-content/uploads/2016/07/diamine-1-1024x882.jpg){.aligncenter
.size-large .wp-image-16612 loading="lazy" decoding="async" width="450"
height="388"
srcset="https://www.ch.ic.ac.uk/rzepa/blog/wp-content/uploads/2016/07/diamine-1-1024x882.jpg 1024w, https://www.ch.ic.ac.uk/rzepa/blog/wp-content/uploads/2016/07/diamine-1-300x258.jpg 300w, https://www.ch.ic.ac.uk/rzepa/blog/wp-content/uploads/2016/07/diamine-1-768x662.jpg 768w, https://www.ch.ic.ac.uk/rzepa/blog/wp-content/uploads/2016/07/diamine-1.jpg 1848w"
sizes="(max-width: 450px) 100vw, 450px"}

Searches like this seem to be good at creating more questions than they
answer. Clearly, the origins of the various hotspots need to be
investigated, ideally using quantum mechanics to quantify the
stereoelectronic interactions involved. So this sort of (ten minute)
exercise is very good at raising research project investigations.

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


The previous post looked at anomeric effects set up on centres such as B, Si or P, and involving two oxygen groups attached to these atoms.

Messaggi di Rogue Scholar

Anomeric effects at carbon involving lone pairs originating from one or two nitrogens.