C6H8O2, M(r) = 112.13, monoclinic, P2(1)/c, a = 6.1549 (6), b = 11.731 (1), c = 8.194 (1) angstrom, beta = 99.386 (8)-degrees, V = 583.7 (1) angstrom 3, Z = 4, D(x) = 1.276 g cm-1, lambda(Cu K-alpha) = 1.54178 angstrom, mu(Cu K-alpha) = 7.05 cm-1, F(000) = 240, T = 303 K, R = 0.084 for 558 reflections with I greater-than-or-equal-to 1.96-sigma-I. 1,3-Cyclohexanedione (enol form) is known to undergo a strong pressure-induced phase transition involving proton transfer in the hydrogen bond; it also forms inclusion compounds with benzene. In this paper we report the temperature dependence of the 1,3-cyclohexanedione structure. The unit-cell dimensions have been measured between 213 and 323 K and the structure has been determined at 213, 273 and 303 K. At T(c) = 287 (1) K, the crystals undergo a strong structural transformation in which the crystal shape is markedly deformed; the transition is similar to that observed at high pressures. At the transition point the disordered methylene C(5)H2 becomes ordered, molecules significantly change their positions in the crystal lattice and the enolic proton changes its donor and acceptor sites. This offers a unique opportunity for an analysis of the factors destabilizing the position of the proton forming a hydrogen bond. Despite a large change of the unit-cell dimensions [at 273 K a = 5.683 (1), b = 11.623 (2), c = 8.724 (2) angstrom, beta = 95.40 (2)-degrees] the crystal preserves its high-temperature space group P2(1)/c, owing to the disappearance of the non-crystallographic mirror plane along (102BAR) when the crystal is cooled below 287 K.
