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Spot the hydrogen atoms

14 January 2017 - Image of the week

paracetamolbyPEDT.PNG Does the phrase "hydrogen atoms omitted for clarification" sound familiar to you? It is the default text accompanying every crystal structure depiction. The reason? it may be that the presence of the hydrogen atoms is obvious so that there is no need to show them. The actual truth: the accuracy of the position of the hydrogen atoms in a crystal structure is very inaccurate and any depiction of them will hopelessly clutter the picture. Problem is, hydrogen has just one electron, reducing scattering power. But thank science we now have precession electron diffraction tomography (PEDT), a brand new technique pioneered by Czech/French team headed by L. Palatimus as first author who practised it on straightforward paracetamol (DOI). To paint the picture old-fashioned X-ray crystallography has been superseded by electron crystallography which has been superseded by precession electron diffraction and then by its tomography child. Technology moves fast. The instrument is especially suited for working with micro crystals. Challenge: any crystal submitted to PEDT suffers from the radiation and will deteriorate. Solution: build up a picture by moving the electron beam over the crystal and build up the image. Results: the measured C-H distances in paracetamol are 1.15 angstroms (textbook value 1.09 angstroms) and the measured angles deviate by 10 degrees.