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-la beled 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 calculation s of mixed crystal models, and X-ray diffraction techniques. After cha racterization of the crystal phases of the pure components, mixed crys tals 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 fr om those observed in its pure crystal phases (racemic and enantiomorph ous). Two different types of spectra were observed from samples obtain ed in different mixed crystallization experiments. We postulate that t wo groups of signals in one type arise from disorder of the guest in t he crystal host when guest molecules crystallize in their two lowest e nergy conformers. In contrast, ordered mixed crystallization in the se cond type results in one signal assigned to the guest in its lowest en ergy conformer. These conclusions are supported by molecular mechanics calculations carried out for gas phase and model crystal systems.