Conformational, Concomitant Polymorphs of 4,4‐Diphenyl‐2,5‐cyclohexadienone: Conformation and Lattice Energy Compensation in the Kinetic and Thermodynamic Forms

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Abstract
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 (P21, Z′=1), form B (P , Z′=4), form C (P , 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 CH donors approaching the CO acceptor in CH⋅⋅⋅O chains and in synthons I–III, depending on the molecular conformation. The strength of the CH⋅⋅⋅O interaction in a particular structure correlates with the number of symmetry-independent conformations (Z′) in that polymorph, that is, a short CH⋅⋅⋅O interaction leads to a high Z′ value. Molecular conformation (Econf) and lattice energy (Ulatt) contributions compensate each other in crystal structures A, B, and D resulting in very similar total energies: Etotal 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 Ulatt determined in the crystal structure prediction (Cerius2, COMPASS) by full-body minimization. Re-ranking the calculated frames on consideration of both Econf (Spartan 04) and Ulatt 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 −1 (∼1.3 kJ mol−1).