Conformational transformation coupled with the order–disorder phase transition in 2-methyl-1,3-cyclohexanedione crystals

The half-chair conformation of the dynamically disordered molecular ring of 2-methyl-1,3-cyclohexanedione, C(7)H(10)O(2), transforms to a sofa below Tc = 244 K, when the crystal undergoes a continuous phase transition induced by the onset of halting large-amplitude vibrations of methylene groups C(4)H(2) and C(5)H(2). The temperature dependence of the crystal structure has been investigated by X-ray diffraction. The Ibam symmetry of the crystal reduces below Tc to space group Pccn. The mechanism of the phase transition and of the conversion of the ring conformation is discussed.