Solid-state trans–cis isomerization of captopril determined by thermal Fourier transform infrared (FT-IR) microspectroscopy

Thermal Fourier transform infrared (FT-IR) microspectroscopy was used to investigate the conformational isomerization of captopril in the solid state. The result indicates that the IR peak intensity of captopril for original bands decreased dramatically at 102°C, but for new bands it increased with the rise of temperature. The frequency of C=O stretching mode for carboxylic acid and for amide was located at a higher wavenumber of 1747 cm− 1 and at a lower frequency of 1591 cm− 1 as compared with the general compound, suggesting the existence of trans isomer of captopril in the solid state by intramolecular hydrogen bonding. Beyond 102°C, several new bands at 1720, 1645, and 1610 cm− 1 were observed with the rise of temperature, indicating the coexistence of a cis isomer. However, the cis isomer could transform gradually to the trans isomer after cooling. The thermodynamics of equilibrium mixture of cis/trans isomers were also studied. The trans isomer was more stable than the cis isomer, but the cis isomer was favored at the higher temperature. © 2001 Wiley-Liss, Inc. and the American Pharmaceutical Association J Pharm Sci 90:1034–1039, 2001