SOLID-STATE C-13 CPMAS NMR AND MOLECULAR MECHANICS STUDY OF CONFORMATIONAL RECOGNITION IN MIXED-CRYSTALS OF 2 PHENYLALKYL KETONES

A detailed structural analysis of a dilute mixed crystal of C-13-Me-labeled 1,2-diphenylpropanone (1) in 2-methyl-1,2-diphenylpropanone (2) has been carried out with the help solid state C-13 cross polarization and magic angle spinning (CPMAS) NMR, molecular mechanics calculations of mixed crystal models, and X-ray diffraction techniques. After characterization of the crystal phases of the pure components, mixed crystals prepared with 1-5 % 1 (99.9 % C-13-Me labeled) were investigated by solid-state NMR with the goal of addressing structural questions at the molecular level. In the mixed crystals, the NMR signals assigned to the host remained unperturbed, while signals of 1 were different from those observed in its pure crystal phases (racemic and enantiomorphous). Two different types of spectra were observed from samples obtained in different mixed crystallization experiments. We postulate that two groups of signals in one type arise from disorder of the guest in the crystal host when guest molecules crystallize in their two lowest energy conformers. In contrast, ordered mixed crystallization in the second type results in one signal assigned to the guest in its lowest energy conformer. These conclusions are supported by molecular mechanics calculations carried out for gas phase and model crystal systems.