Acta Crystallographica Section B

Structural Science

Volume 58, Part 3 Number 2 (June 2002)

research papers

Acta Cryst. (2002). B58, 502-511 [ doi:10.1107/S0108768102000630 ]

Two phases of C₉H₁₂O₄: why is the structure at 295 K so complicated?

L. L. Duncan, B. O. Patrick and C. P. Brock

Abstract: Molecules of 4,4'-dimethyl-2-hydroxy-6-oxocyclohexene-1-carboxylic acid, C₉H₁₂O₄, crystallize at 295 K in a modulated superstructure with five half-molecules in the asymmetric unit; each molecule is located on one of the mirror planes of the space group Cmc2₁. Reflections with k [not equal to] 5n are systematically weak; a satisfactory refinement can be obtained in a Cmcm pseudocell having b' = b/5. The important modulation involves small rotations of the molecules around axes perpendicular to the mirror plane; there is also an up/down disorder of the CMe₂ fragment in four of the five molecules (two molecules with occupancy factors ca 4:1; two with occupancy factors ca 3:2). The modulation is a response to packing problems that can be traced to the differences between the thin, electron- and oxygen-rich `head' of the molecule and the thicker, methyl-rich `tail'. At 130 K the length of b is reduced by 2/5 and the Pmnb structure is ordered. Both structures can be described as modulated variants of the Cmcm substructure; the wavevectors are 2b'*/5 for the room-temperature structure and b'*/2 for the low-temperature structure, where b'* is the reciprocal axis of the subcell. The structure at room temperature can also be understood as a hybrid of the fully disordered pseudocell structure and the ordered structure that is found at low temperature.

Keywords: modulated phases; mirror symmetry; disorder/order; Z' > 1.