Strategies for structure solution and refinement of small organic molecules from electron diffraction data and limitations of the simulation approach

In recent years, a series of non-linear optically active bis(benzylidene) k etones have been synthesized and investigated by electron crystallography. In most cases, structure refinement was possible by combining electron diff raction analysis and quantum-mechanical calculations with maximum-entropy m ethods. However, when the torsional angles between the phenyl rings and the C=C double bonds are strongly affected by the crystal field, this method f ails because packing-energy calculations are not sufficiently sensitive. Th is problem can be solved by refining the approximate model with SHELXL, if the data set is sufficiently accurate and the model close to the correct st ructure. Here it is shown that a considerably superior data set can be obta ined at 300 kV with online data acquisition.