Understanding Infrared and Raman Spectra of Pharmaceutical Polymorphs

摘要

Infrared and Raman spectra of different crystalline forms of the same organic compound can be used to identify a pure crystal form and quantify a mixture of two forms. Many organic compounds have one or more crystalline or polymorphic forms. The observed differences in the spectra of different polymorphs include changes in frequencies, relative intensities, band contours and the number of bands. Causes of the spectral differences can be interpreted in terms of molecule-molecule interactions in the crystal unit cells. Molecular interactions can distort the shape of a molecule at its site in the unit cell. Moreover, vibrations of one or more molecules can couple in such a way as to increase the number of bands and shift their frequencies. Herein, spectral changes are explained using a hypothetical three-atom molecule and a real five-atom molecule. Finally, the differences in spectra of large molecules are demonstrated with polymorphs of three active pharmaceutical ingredients (API).