The interpretation of the short range disorder in the Fluorene-TCNE crystal structure

The interpretation of the short-range static and/or dynamic disorder in a crystal structure from X-ray data is often a rather complex problem. Recently, we synthesized and characterized three co-crystalline complexes between fluorene, showing C-2v symmetry, and three electron withdrawing D-2h compounds, employing FTIR and Raman spectroscopy and single crystal X-ray diffraction techniques. The crystal structures of the complexes of fluorene with the three different electron withdrawing molecules are disordered in the solid state and only approximate structures were obtained by refinement of the single crystal data. Indeed, the fluorene moiety presents a very irregular geometry, showing for example C-C bonds ranging from 1.25Angstrom to 1.75Angstrom. Graphical inspection of the solved crystal structures indicates that the fluorene molecules in these co-crystalline complexes can assume two possible positions, both with 50% population. A two-step procedure to improve the disordered models is described. At first, the two possible ordered structures, with the fluorene molecule in only one of the two populated positions, are "separated" by molecular graphic techniques and then their geometry is fully optimized employing the periodic ab initio "CRYSTAL" code to obtain a chemically sensible model with reasonable distances and angles. The main aim of this communication is to demonstrate that ordered models can be obtained, starting from a disordered crystal structure, focusing our attention on the fluorene-TCNE molecular complex.