Imperial College London

Henry Rzepa

A quartet of articles has recently appeared on the topic of
cyclobutadiene.\[cite\]10.1002/chem.201102942\[/cite\],\[cite\]10.1002/chem.201103017\[/cite\],\[cite\]10.1002/chem.201203234\[/cite\],\[cite\]10.1002/chem.201203235\[/cite\].
You will find a great deal discussed there, but I can boil it down to
this essence. Do the following coordinates (obtained from a (disordered)
[previously](http://www.ccdc.cam.ac.uk/services/scripts/web_structure_request/web_structure_request_php/deposit_request.php?sid=ConQuest&coden=SCIEAS&year=2010&spage=299&volume=329&id=doi%3A10.1126%2Fscience.1188002&pid=ccdc%3A764868){target="_blank"}
published\[cite\]10.1126/science.1188002\[/cite\] x-ray refinement)
correspond to a ***van der Waals complex*** of 1,3-dimethyl
cyclobutadiene and carbon dioxide, or do they instead represent a
***covalent interaction*** between these two components resulting in a
compound with the chemical name 2-oxabicyclo\[2.2.0\]hex-5-en-3-one
(*i.e.* not a cyclobutadiene)?

::: {#attachment_9897 .wp-caption .aligncenter style="width: 410px"}
![Click for
3D](http://www.ch.imperial.ac.uk/rzepa/blog/wp-content/uploads/2013/03/cbd.jpg){.size-full
.wp-image-9897 decoding="async"
aria-describedby="caption-attachment-9897"
onclick="jmolInitialize('../Jmol/','JmolAppletSigned.jar');jmolSetAppletColor('white');jmolApplet([450,450],'load wp-content/uploads/2013/03/5.mol2;select atomno=3,atomno=2;color bond translucent goldenrod;select atomno=6,atomno=1;color bond translucent goldenrod;connect (atomno=21) (atomno=1) none;connect (atomno=19) (atomno=1) none;connect (atomno=19) (atomno=6) none;connect (atomno=19) (atomno=5) none;connect (atomno=19) (atomno=4) none;connect (atomno=19) (atomno=3) none;connect (atomno=19) (atomno=2) none;connect (atomno=19) (atomno=21) none;connect (atomno=18) (atomno=20) none;connect (atomno=18) (atomno=6) none;connect (atomno=18) (atomno=3) none;connect (atomno=18) (atomno=4) none;connect (atomno=18) (atomno=5) none;zoom 10;moveto 4 0 2 0 90 95;measure 2 3;measure 1 6;measure 7 2 1;measure 2 3 10;');"
width="400"}

Click for 3D. The unconnected atoms are the result of disordered partial
occupancy.
:::

The two bonds to concentrate on are shown in gold; a O...C pair with a
distance of 1.61Å as obtained from the x-ray refinement and a C...C pair
with a distance of 1.5Å (and if you want to go further, the O=C=O bond
angle). I list below values obtained from the wonderful
[Webelements](http://www.webelements.com/carbon/atom_sizes.html){target="_blank"}
site. Using these values, this makes a van der Waals O...C contact 3.22Å
and a C...C contact 3.40Å and covalent values of respectively 1.38Å and
1.5Å.

|         |                    |                      |
|---------|--------------------|----------------------|
| Element | Covalent radius, Å | van der Waals radius |
| C       | 0.75               | 1.70                 |
| O       | 0.63               | 1.52                 |

According to chemistry convention, we classify the interaction between a
pair of atoms according to which category best fits the observed
distance. So this should allow you to decide if the molecule is a van
der Waals complex of 1,3-dimethyl cyclobutadiene and carbon dioxide or
the covalent system 2-oxabicyclo\[2.2.0\]hex-5-en-3-one. 

Oh, if the observed O...C pair with a distance of 1.61Å does not seem to
perfectly fit either category above, one of the quartet of articles
above\[cite\]10.1002/chem.201102942\[/cite\] offers the explanation of
an unusual π-anomeric effect lengthening the C...O bond in
2-oxabicyclo\[2.2.0\]hex-5-en-3-one slightly beyond the standard
covalent distance. Of course, if the system were to be a van der Waals
complex, that explanation cannot apply.

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


A quartet of articles has recently appeared on the topic of cyclobutadiene.[cite]10.1002/chem.201102942[/cite],[cite]10.1002/chem.201103017[/cite],[cite]10.1002/chem.201203234[/cite],[cite]10.1002/chem.201203235[/cite]. You will find a great deal discussed there, but I can boil it down to this essence.

Messaggi di Rogue Scholar

To be cyclobutadiene, or not to be, that is the question? You decide.