Structural study of the stability of the captopril drug regarding the formation of its captopril disulphide dimer

Captopril disulphide is obtained under hydrothermal conditions. The IR and Raman spectra data are in agreement with the X-ray diffraction results. The disappearance of the band at 2566 cm^–1 (ν(SH)) in both spectra of captopril disulphide is consistent with the formation of the S–S bond. The degradation of the captopril drug is investigated by Raman spectroscopy and the results indicate that after 6 weeks of air exposure, a band at 512 cm^–1, assigned as ν(SS), is observed, suggesting the formation of captopril disulphide. DFT calculations in the solid state are performed for captopril and captopril disulphide. The results indicate that captopril disulphide is approximately 30 kcal?mol^–1 more stable than captopril. The analysis of the total density of states (DOS) reveals that the captopril valence band contains a significant contribution from the S atom, whereas for captopril disulphide, the O atom is the most important for the valence band.