A theoretical and spectroscopic study of γ-crystalline and amorphous indometacin

Amorphous materials are prevalent in the pharmaceutical setting. Whether they are a help or hindrance, their physico-chemical characteristics must be investigated. However, the amorphous form remains a challenge to characterise with many of its properties poorly understood. In this study, gamma-crystalline and amorphous indometacin are investigated using vibrational spectroscopy and quantum chemical calculations. The structure of the single indometacin molecule and the dimer in the gamma-form were optimised using density functional theory calculations. The optimised structures were similar to the conformations in the crystal form, suggesting that conformation of the molecules in the crystal may be close to the average molecular structure in less-ordered states. Infrared and Raman spectra were calculated from the optimised structures. Many modes in the calculated spectra could be matched with the experimental spectra of the gamma-crystalline and amorphous forms, and a description of the matched modes has been provided. By analysis of the theoretical vibrational modes it was confirmed that the amorphous form of indometacin produced by quench cooling the melt consists predominantly of dimers, similar in structure to in the gamma-crystalline form. In addition, differences in intermolecular bonding between the two forms were identified. Quantum mechanical calculations allow improved understanding of amorphous materials and their vibrational spectra.