Two P21/n monoclinic phases of fluorene-4-carboxylic acid at 296 K.

The structures of two phases of fluorene-4-carboxylic acid, C14H10O2, from room-temperature growths have been determined. In the alpha phase, O-H...O hydrogen bonding occurs via cyclic dimers about a center of symmetry; in the beta phase, cyclic dimers with O-H...O hydrogen bonds are formed between two crystallographically inequivalent molecules and do not involve a center of symmetry, though centers of symmetry are present in the crystal. The present evidence strongly suggests that the beta phase is metastable with respect to the alpha phase at room temperature, but the regions of stability have not been determined.