Structure Determination of a Complex Organic Solid from X‐Ray Powder Diffraction Data by a Generalized Monte Carlo Method: The Crystal Structure of Red Fluorescein

The lack of suitable crystals has meant that it has not been possible to carry out a conventional single-crystal X-ray structure analysis to determine the structure of red fluorescein. Instead, the application of a Monte Carlo method has allowed the crystal structure to be determined directly from powder X-ray diffraction data. Importantly, fluorescein was treated as a nonrigid fragment in the structure solution calculation, illustrating the generalized application of the Monte Carlo method. The molecules form layers in the crystal (with extensive CO…︁HO hydrogen bonding), and within each layer the molecules are stacked in columns (as shown on the right).