The Crystal Structure of Orthocetamol Solved by 3D Electron Diffraction.

Orthocetamol is a regioisomer of the well-known pain medication paracetamol and a promising analgesic and an anti-arthritic medicament itself. However, orthocetamol cannot be grown as single crystals suitable for X-ray diffraction, so its crystal structure has remained a mystery for more than a century. Here, we report the ab-initio structure determination of orthocetamol obtained by 3D electron diffraction, combining a low-dose acquisition method and a dedicated single-electron detector for recording the diffracted intensities. The structure is monoclinic, with a pseudo-tetragonal cell that favors multiple twinning on a scale of a few tens of nanometers. The successful application of 3D electron diffraction to orthocetamol introduces a new gold standard of total structure solution in all cases where X-ray diffraction and electron-microscope imaging methods fail.