Phase transition in 9-Hydroxyphenalenone at 255K and crystal structure of the ordered phase at 215K.

9-Hydroxyphenalenone, a molecule with a strong intramolecular 0-H...0 bond, is disordered at room temperature, becoming ordered below the reversible first order phase transition at 255K. Bistable hydrogen atoms in -0-H...0-, -0...H-0 systems have potential as information storage media at the molecular level. The length of the intramolecular asymmetric hydrogen bond at 215K, averaged over the four independent molecules, is 2. 490(5)angstroms with 0-H = 1.12(7)angstroms. One independent molecule undergoes a 35degrees rotation in the molecular plane at the phase transition. Above the transition, this and one other independent molecule become disordered by an additional 120degrees rotation in the molecular plane. The configurational entropy change is calculated as 2.28J mol(-1)K(-1), compared with the experimental entropy change of 2.20J mol(-1)K(-1). The thermal expansion has been followed from 145 to 360K by single crystal X-ray diffractometry. On heating, the a-axis decreases sharply in length at the phase transition by 1.7% the b-axis increases smoothly, the c-axis increases sharply by 1.5% and the angle beta decreases sharply by 2.0%, with the volume expanding abruptly by 0.4%. Corresponding principal linear thermal expansion coefficients at 145K are (alpha)1 =56, (alpha) (2) = 25, (alpha)(3) = 192x10(-6)K(-1) and at 360K are -236,342 and ~283 x 10(-6)K(-1).