A headache of a crystal structure – Aspirin

What does it look like?

Made up of carbon (grey), oxygen (red) and hydrogren (white), Aspirin has given many scientists headaches - Image generated by the Mercury crystal structure visualisation software http://www.ccdc.cam.ac.uk/Solutions/CSDSystem/Pages/Mercury.aspx

Made up of carbon (grey), oxygen (red) and hydrogen (white), Aspirin has given many scientists headaches – Image generated by the Mercury crystal structure visualisation software http://www.ccdc.cam.ac.uk/Solutions/CSDSystem/Pages/Mercury.aspx

What is it?

For many years we understood there only to be one way that molecules of acetylsalicylic acid, or better known to us Aspirin, could pack in a solid.  However, there had been mutterings since the 1960’s of there being another arrangement – a different polymorph…..

What is a polymorph?  Like the fact that carbon can form into a number of different crystalline forms depending on the conditions (i.e. we have already blogged about graphite, lonsdalite and buckyballs which are all made of carbon) many other molecular materials (like TNT and cocoa butter) can do this too.   When something has the same chemical structure, but a different crystal structure it is known as a polymorph.

So for many years scientists believed that there was a different polymorph of Aspirin, and in 2005 it was announced that this ‘Form II’ aspirin had been found.  But closer examination, by a different team, suggest that what had been ‘discovered’ was still the original Form I.  Instead these workers suggested the reason for the years of speculation of there being a different polymorph of aspirin.  They discovered there are subtleties in the way the aspirin molecules stick together, which they name to be an A or B arrangement, which can slightly alter the symmetry of the whole structure.

Where did the structure come from?

The image is of the ‘classical’ crystal structure of aspirin, as determined by Wheatley in 1965.

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