Imperial College London

Henry Rzepa

[In this
post](http://www.ch.imperial.ac.uk/rzepa/blog/?p=9606){target="_blank"},
I pondered upon the C=O infra-red spectroscopic properties of esters,
and showed three possible electronic influences:

![s-cis-ester1](http://www.ch.imperial.ac.uk/rzepa/blog/wp-content/uploads/2013/02/s-cis-ester1.svg){.aligncenter
.size-full .wp-image-9607 decoding="async"}

The red (and blue) arrows imply the C-O bond might shorten and the C=O
bond would lengthen; the green the reverse. So time for a search of the
crystal structure database as a reality check. The query is as follows:

![s-cis-ester1](http://www.ch.imperial.ac.uk/rzepa/blog/wp-content/uploads/2016/06/amides-sq.jpg){.aligncenter
.size-full .wp-image-9607 decoding="async" width="400"}

The response shows the bimodal distribution with as expected the *s-cis*
conformation dominating. There is indeed a hint that for the *s-cis*,
the C-O distance is rather shorter than for the *s-trans* conformation.

![s-cis-ester1](http://www.ch.imperial.ac.uk/rzepa/blog/wp-content/uploads/2016/06/amides-all.jpg){.aligncenter
.size-full .wp-image-9607 decoding="async" width="440"}

Repeating the search, but specifying that the temperature of data
acquisition is \< 90K, one gets a much clearer indication of the
difference in bond lengths.

![s-cis-ester1](http://www.ch.imperial.ac.uk/rzepa/blog/wp-content/uploads/2016/06/amides-90.jpg){.aligncenter
.size-full .wp-image-9607 decoding="async" width="440"}

This alternative representation shows the C-O and the C=O distances,
with red indicating *s-trans* and blue indicating *s-cis* conformations
(T \< 140K). The red dots occupy a bottom right cluster for which the
C-O distance is longer and the C=O shorter than the corresponding blue
cluster.

![s-cis-ester1](http://www.ch.imperial.ac.uk/rzepa/blog/wp-content/uploads/2016/06/amides-1vs2-140.jpg){.aligncenter
.size-full .wp-image-9607 decoding="async" width="440"}

Again reducing the temperature of data collection to \< 90K shows a
rather weak inverse correlation between the two distances for eg the
blue dots.

![s-cis-ester1](http://www.ch.imperial.ac.uk/rzepa/blog/wp-content/uploads/2016/06/amides-1vs2-90.jpg){.aligncenter
.size-full .wp-image-9607 decoding="async" width="440"}

A shame however that this database does not hold IR values for the
carbonyl stretches. I am sure correlations must exist, but how to get at
them (other than manual collection of data).

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


In this post, I pondered upon the C=O infra-red spectroscopic properties of esters, and showed three possible electronic influences:     	   	The red (and blue) arrows imply the C-O bond might shorten and the C=O bond would lengthen; the green the reverse. So time for a search of the crystal structure database as a reality check.

Messages de Rogue Scholar

Why is the carbonyl IR stretch in an ester higher than in a ketone: crystal structure data mining.