If you are familiar with the various crystal forms of carbamazepine in the CSD, you will know that one particular form of carbamazepine (CSD refcode: CBMZPN03) is in the trigonal space group R-3 and exhibits noticeable channels running down the unique axis. These channels result in void space of roughly 6.6% of the unit cell volume (or 375 Å per unit cell) and appear to either contain trace amounts of solvent or at least are templated by solvent.

I suspect that the space group R-3 is predisposed to forming these kinds of channel crystal structures containing void space. I think this is true in particular for CSD structures where few systems will have the inherent molecular or polymeric symmetry to reside on the actual symmetry element and instead will arrange themselves around the -3 axis.

To test this theory, I've written the attached script which loops over the entries in the CSD in space group R-3 and then determines an average void percentage. The outcome is 5.81%, but how does that match up to the average void percentage across the whole CSD?

I've attached a script which will do this over the whole CSD.  It's running on my desktop at the moment, but I don't expect the results until tomorrow - the void calculation is computationally fairly heavy.

I'll let you know the results when I get them.

Richard

I've attached a table of spacegroup, average void space (as a percentage of the unit cell volume), number of observations from the 673,606 structures of CSD V536 with 3D coordinates.  I'll leave it to the crystallographically adept to extract any meaning there is in the table.

Richard

Here's the slightly modified script, testing for 3D coordinates.

Richard

Thanks Richard.

It's quite amazing to note, for example, that the space group Fm-3m has an average of over 50% void space in the unit cell. This presumably is mostly due to solvent or guest molecules that are so disordered that they couldn't be observed or well modelled - many may have had the PLATON/SQUEEZE method applied to them.

Pete

Just an extra comment to say that we've reviewed the CSD disorder comments field and observed that 103 out of 318 Fm-3m structures explicitly state that they have used PLATON/SQUEEZE. The proportion of structures exhibiting disorder in this space group is also found to be 67.3% - a significant increase over the CSD average of 23.2%.

Pete

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