Conformational, concomitant polymorphs of 4,4-diphenyl-2,5-cyclohexadienone: Conformation and lattice energy compensation in the kinetic and thermodynamic forms

4,4-Diphenyl-2,5-cyclohexadienone (1) crystallized as four conformational polymorphs and a record number of 19 crystallographically independent molecules have been characterized by low-temperature X-ray diffraction: form A (P2(1), Z' = 1), form B (P (1) over bar, Z' = 4), form C (P (1) over bar, Z' = 12), and form D (Pbca, Z'=2). We have now confirmed by variable-temperature powder X-ray diffraction that form A is the thermodynamic polymorph and B is the kinetic form of the enantiotropic system A-D. Differences in the packing of the molecules in these polymorphs result from different acidic CH donors approaching the C=O acceptor in C-H center dot center dot center dot O chains and in synthons I-III, depending on the molecular conformation. The strength of the C-H... 0 interaction in a particular structure correlates with the number of symmetry-independent conformations (7) in that polymorph, that is, a short C-H... 0 interaction leads to a high Z' value. Molecular conformation (E-conf) and lattice energy (U-latt) contributions compensate each other in crystal structures A, B, and D resulting in very similar total energies: E-total of the stable form A= 1.22 kcal mol(-1), the metastable form B = 1.49 kcal mol(-1), and form D = 1.98 kcal mol(-1). Disappeared polymorph C is postulated as a high-Z', high-energy precursor of kinetic form B. Thermodynamic form A matches with the third lowest energy frame based on the value of U-latt determined in the crystal structure prediction (Cerius(2), COMPASS) by full-body minimization. Re-ranking the calculated frames on consideration of both E-conf (Spartan 04) and U-latt energies gives a perfect match of frame #1 with stable structure A. Diphenylquinone 1 is an experimental benchmark used to validate accurate crystal structure energies of the kinetic and thermodynamic polymorphs separated by < 0.3 kcal mol(-1) (similar to 1.3 kJ mol(-1)).