Electron diffraction to differentiate C from N and locate protons

Thu, 07.05.2020

Electron Diffraction (ED) is gaining world-wide attention in the crystallography community. As part of our efforts to demonstrate the power of ED, we carried out experiments on [2H]-3-amino-1,2,3 triazole (3-AT, C2N4H4).

As is well known from X-rays, that it is a challenge to distinguish C from N atoms. If we solve the structure from ED data, using same parameters as for X-ray data, the first solution results in a structure in which five out of six atom types are correctly assigned.

Further refinement reveals a residual peak on N2 that also forms a bond to a nitrogen atom of the closest neighbour molecule and can thus be designated as a hydrogen atom, in very good agreement with X-ray data available. Therefore, by chemical reasoning, the remaining N atom has to be at position 1. To double-check, we refine the different hypothetically possible atom positions of 3-AT. The refined model indeed provides the lowest R1, wR2 and GooF values. The free refinement of the hydrogen atom on either N2 or N4 turned out to be crucial.

Electron diffraction confirms once more that it is well suited for distinguishing neighbouring atoms in the periodic table. It can also locate the real position of the hydrogen atoms more easily and more accurately, as opposed to X-rays. All of this includes measurement times comparable to synchrotron single-crystal experiments and crystalline samples, having volumes four to five orders of magnitude smaller than typically required for X-ray investigations. This is the massive potential of ED nano-crystallography.

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