ELECTRON-DIFFRACTION STRUCTURE-ANALYSIS OF DIKETOPIPERAZINE - A DIRECT PHASE DETERMINATION

Three-dimensional electron-diffraction intensity data from crystalline textures of diketopiperazine (2,5-piperazinedione) published by B. K. Vainshtein in 1955 [Zh. Fiz. Kim. (1955), 29, 327-344] are found to be suitable for ab initio structure analysis via conventional direct phase determination based on the estimates of three- and four-phase structure invariants. Of the 289 non-zero reflections found in this data set, 196 with \E\ greater-than-or-equal-to 0.5 were used to generate triples and quartets to phase 133 reflections correctly. The resultant [001] and [100] zonal electrostatic potential maps reveal an easily identifiable molecular structure and the atomic coordinates are close to the values found in an X-ray crystal structure.