The low-temperature phase transition of 9-methylfluoren-9-ol: comparison of the crystal structures at 100 and 200 K

Crystals of 9-methylfluoren-9-ol, C14H12O, undergo a reversible phase transition at 176 (2) K. The structure of the high-temperature alpha form at 200 K is compared with that of the low-temperature beta form at 100 K. Both polymorphs crystallize in space group P (1) over bar with Z = 4 and contain discrete hydrogen-bonded R-4(4) (8) ring tetramers arranged around crystallographic inversion centres. The most obvious changes observed on cooling the crystals to below 176 K are an abrupt increase of ca 0.5 Angstrom in the shortest lattice translation, and a thermal transition with DeltaH = 1 kJ mol(-1).