Solution of an organic crystal structure from X-ray powder diffraction data by a generalized rigid-body Monte Carlo method: crystal structure determination of 1-methylfluorene

Abstract

Many important crystalline materials do not form single crystals of sufficient size and quality for single crystal X-ray diffraction studies, and in such cases it is essential that the crystal structure can be solved from powder diffraction data; however, there are many difficulties associated with solving crystal structures directly from powder diffraction data. In this paper, we report the successful application of a Monte Carlo technique to solve the previously unknown crystal structure of 1-methylfluorene from X-ray powder diffraction data collected on a conventional laboratory diffractometer. The ‘structural fragment’ used in the Monte Carlo calculation comprised the non-hydrogen atoms of the fluorenyl (C₁₃) group, and this rigid structural fragment was subjected to simultaneous translation and rotation within the unit cell. The correct position of the structural fragment was discriminated readily (on the basis of the agreement between experimental and calculated X-ray powder diffractograms) from wrong positions sampled during the Monte Carlo calculation, and the correct position was then used as the initial structural model in Rietveld refinement and difference Fourier calculations. The work reported in this paper represents the first demonstration of the application of the Monte Carlo approach to solve a previously unknown crystal structure from powder diffraction data in the general case requiring simultaneous translation and rotation of a rigid structural fragment, and paves the way for the future application of the Monte Carlo approach to a much wider array of structural problems.