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

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 Cmc2(1). Reflections with k not equal 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.