Structure of 2-methyl-1,3-cyclohexanedione crystals

The crystal structure of 2-methyl-1,3-cyclohexanedione, C₇H₁₀O₂, has been determined by X-ray methods. The finalR=0.039 for 513 observed reflections and 74 refined parameters. The molecules are present in the enolized form in the crystal. Hydrogen bonds-OH--O=, in stereochemical conformation [anti-anti]anti, of 2.590(2)Å link the molecules into planar chains along [010]. The molecules have a mirror-plane symmetry. The molecular ring is in a half-chair conformation. Two methylene groups C(4)H₂ and C(5)H₂ are dynamically disordered each in two sites on opposite sides of an otherwise planar molecule. An onset of ordering of the methylene groups would break the symmetry of mirror plane and induce a phase transition changing the space group symmetry of this crystal.