Solid-state 13C NMR study of indomethacin polymorphism

The purpose of this study was to analyze the difference in the molecular conformation packed in the crystal lattice between the meta-stable α-form and stable γ-form of indomethacin on the basis of solid-state 13C NMR spectral patterns. The chemical shifts of each resonance of the α-form were distinctly different from the γ-form. Each carbon nucleus of the γ-form showed a single signal with no splitting. In contrast, carbon nuclei of the α-form showed a complicated set of resonances for each carbon. For some carbons of the α-form, four signals assigned to one carbon were observed at 203 K. Two of these four signals were merged between the temperature range from 203 to 343 K without a transformation in the crystal structure. It was found that solid-state 13C NMR can be a powerful tool to estimate the number of molecular conformations as well as configurational differences in the packing of molecules in a unit cell.