Glass transition dynamics of anti-inflammatory ketoprofen studied by Raman scattering and terahertz time-domain spectroscopy

Molecular dynamics of amorphous pharmaceutical fenofibrate studied by broadband dielectric spectroscopy

Fenofibrate is mainly used to reduce cholesterol level in patients at risk of cardiovascular disease. Thermal transition study with the help of differential scanning calorimetry (DSC) shows that the aforesaid active pharmaceutical ingredient (API) is a good glass former. Based on our DSC study, the molecular dynamics of this API has been carried out by broadband dielectric spectroscopy (BDS) covering wide temperature and frequency ranges. Dielectric measurements of amorphous fenofibrate were performed after its vitrification by fast cooling from a few degrees above the melting point (T_m=354.11 K) to deep glassy state. The sample does not show any crystallization tendency during cooling and reaches the glassy state. The temperature dependence of the structural relaxation has been fitted by single Vogel–Fulcher–Tamman (VFT) equation. From VFT fit, glass transition temperature (T_g) was estimated as 250.56 K and fragility (m) was determined as 94.02. This drug is classified as a fragile glass former. Deviations of experimental data from Kohlrausch–Williams–Watts (KWW) fits on high-frequency flank of α-peak indicate the presence of an excess wing in fenofibrate. Based on Ngai׳s coupling model, we identified the excess wing as true Johari–Goldstein (JG) process. Below the glass transition temperature one can clearly see a secondary relaxation (γ) with an activation energy of 32.67 kJ/mol.

Low-frequency vibrational properties of crystalline and glassy indomethacin probed by terahertz time-domain spectroscopy and low-frequency Raman scattering

In order to clarify the intermolecular vibrations, the low-frequency modes of the glassy and crystalline states of model pharmaceutical indomethacin have been studied using broadband terahertz time-domain spectroscopy and low-frequency Raman scattering. In the crystalline γ-form, the center of symmetry was suggested by the observation of the exclusion principle of the infrared (IR) and Raman selection rules in the frequency range between 0.2 and 6.5 THz. In addition, a boson peak of the glassy state was observed in both IR and Raman spectra and their frequency showed apparent discrepancy. The intermediate correlation length of the glassy structure was estimated to be about 2.5 nm. The existence of hydrogen bonded cyclic dimers in a glassy state was suggested by the observation of the infrared active intermolecular vibrational mode of the hydrogen bonded cyclic dimers as a broad peak at 3.0 THz in the IR spectrum.