Imperial College London

Henry Rzepa

My previous post on the topic of mauveine left the [outcome
dangling.](http://www.ch.imperial.ac.uk/rzepa/blog/?p=3507){target="_blank"} Put
simply, λ~max~ is measured at about 549nm for mauveine A, but was
calculated at about 440nm using a modern method for predicting colour
(TD-DFT). According to the colour table below, that would make it
orange, not mauve. Can the theoretical prediction be out by 110nm, or
might it be the structure of the molecule itself that has been wrongly
described?  
![colour
table](http://www.ch.imperial.ac.uk/rzepa/blog/wp-content/uploads/2011/03/colour-table.jpg){.aligncenter
decoding="async" width="400"}

A new idea struck me, summarised below. No crystal structure of a
mauveine has ever been reported, and so the position of the N-H groups
is not determined. It is normally drawn as tautomer **1**, but what
about tautomers **2-4**?

::: {#attachment_5249 .wp-caption .aligncenter style="width: 410px"}
[![](http://www.ch.imperial.ac.uk/rzepa/blog/wp-content/uploads/2011/10/mauveine.jpg "mauveine"){.size-full
.wp-image-5249 loading="lazy" decoding="async"
aria-describedby="caption-attachment-5249" width="400"
height="254"}](http://www.ch.imperial.ac.uk/rzepa/blog/wp-content/uploads/2011/10/mauveine.jpg)

Tautomers of mauveine.
:::

Following the principle of
[completeness](http://www.ch.imperial.ac.uk/rzepa/blog/?p=4952){target="_blank"},
it is important to include a counterion. And because the colour mauve is
recorded in methanol solutions (*i.e.* it is unlikely to be due to
aggregation), we will include some explicit solvent (water) as well. To
illustrate the model, I show the geometries calculated for two
counter-ion isomers of tautomer **4** using
ωB97XD/6-311G(d,p)/SCRF=water.

<table data-border="1">
<colgroup>
<col style="width: 50%" />
<col style="width: 50%" />
</colgroup>
<tbody>
<tr class="odd">
<td><div id="attachment_5256" class="wp-caption aligncenter"
style="width: 209px">
<img
src="http://www.ch.imperial.ac.uk/rzepa/blog/wp-content/uploads/2011/10/tauomer4.jpg"
title="tauomer4" class="size-full wp-image-5256" loading="lazy"
decoding="async" aria-describedby="caption-attachment-5256"
onclick="jmolInitialize(&#39;../Jmol/&#39;,true);jmolSetAppletColor(&#39;white&#39;);jmolApplet([600,600],&#39;load wp-content/uploads/2011/10/4.xml;&#39;);"
width="199" height="106" />
<p>Tautomer 4. Click for 3D.</p>
</div></td>
<td><div id="attachment_5257" class="wp-caption aligncenter"
style="width: 209px">
<img
src="http://www.ch.imperial.ac.uk/rzepa/blog/wp-content/uploads/2011/10/tauomer5.jpg"
title="tauomer5" class="size-full wp-image-5257" loading="lazy"
decoding="async" aria-describedby="caption-attachment-5257"
onclick="jmolInitialize(&#39;../Jmol/&#39;,true);jmolSetAppletColor(&#39;white&#39;);jmolApplet([600,600],&#39;load wp-content/uploads/2011/10/5.xml;&#39;);"
width="199" height="107" />
<p>Tautomer 4, with different arrangement of Chloride. Click for 3D.</p>
</div></td>
</tr>
</tbody>
</table>

The predicted UV/Visible spectra are shown below
(ωB97XD/6-311++G(d,p)/SCRF=water model), and  λ~max~ ranges from 440 to
655nm simply by moving one proton around! The spectrum that matches the
measured best corresponds to tautomer **4**. Unfortunately, it is
calculated to be about 15 kcal/mol higher in energy than tautomer **1**,
which is the lowest in energy.

|                                                                                                                                       |                                                                                                                                       |
|---------------------------------------------------------------------------------------------------------------------------------------|---------------------------------------------------------------------------------------------------------------------------------------|
| [λ~max 440~](http://hdl.handle.net/10042/to-9645){target="_blank"}                                                                    | [λ~max 655~](http://hdl.handle.net/10042/to-9648){target="_blank"}                                                                    |
| ![colour table](http://www.ch.imperial.ac.uk/rzepa/blog/wp-content/uploads/2011/10/M1.svg){.aligncenter decoding="async" width="200"} | ![colour table](http://www.ch.imperial.ac.uk/rzepa/blog/wp-content/uploads/2011/10/M2.svg){.aligncenter decoding="async" width="200"} |
| λ~max 555~                                                                                                                            | [λ~max 490~](http://hdl.handle.net/10042/to-9646){target="_blank"}                                                                    |
| ![colour table](http://www.ch.imperial.ac.uk/rzepa/blog/wp-content/uploads/2011/10/M3.svg){.aligncenter decoding="async" width="200"} | ![colour table](http://www.ch.imperial.ac.uk/rzepa/blog/wp-content/uploads/2011/10/M4.svg){.aligncenter decoding="async" width="200"} |
| [λ~max 487~](http://hdl.handle.net/10042/to-9647){target="_blank"}                                                                    |                                                                                                                                       |
| ![colour table](http://www.ch.imperial.ac.uk/rzepa/blog/wp-content/uploads/2011/10/M5.svg){.aligncenter decoding="async" width="400"} |                                                                                                                                       |

So one step forward, and one back. A better colour match can be obtained
by modelling a different tautomer of mauveine, but this now leaves the
energy unexplained. I think perhaps a determined effort to get mauveine
itself to form good crystals and to analyse those to confirm where the
three exchangeable hydrogens reside would be well worth the effort. Even
then, that will not necessarily tell us what is happening in solution.
Such an old, and famous molecule, and still there is a mystery.

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


My previous post on the topic of mauveine left the outcome dangling. Put simply, λ
<sub>
 max
</sub>
is measured at about 549nm for mauveine A, but was calculated at about 440nm using a modern method for predicting colour (TD-DFT). According to the colour table below, that would make it orange, not mauve. Can the theoretical prediction be out by 110nm, or might it be the structure of the molecule itself that has been wrongly described?

Postagens de Rogue Scholar

The colour of purple is … not orange but mauve?