Abstract-Low frequency terahertz spectrum of acetylsalicylic acid over a wide temperature range investigated by FTIR spectroscopy

Lucia M.Lepodise,

https://www.sciencedirect.com/science/article/pii/S1386142519303063

The temperature dependence of the terahertz spectrum of acetylsalicylic acid over an extended temperature range as well as the theoretical modeling results are reported. Several absorption bands which were not observed at room temperature emerged as the sample was cooled down to 14.9 K. All the bands shifted to higher energies upon cooling. The bands which exhibited an anomalous behavior in the frequency shift have a rate of change in the range (4–5) × 10−3 cm−1 K−1−1 while bands which displayed an expected behavior have the rate of change in the range (1–3) × 10−2 cm−1 K−1−1. There is a fluctuation in both the intensity and the width of the bands implying composite bands. There is no fundamental correspondence between the frequency shifts, band widths and band intensities hence making analysis of all the three variables necessary to understand the extent to which the bands have been resolved. A good agreement was observed between the experiment and the model of acetylsalicylic acid crystal. The model of an acetylsalicylic acid crystal with PBEsol DFT functional proved to be better than the previously used model of an acetylsalicylic acid dimer using a B3LYP/6-31++G** DFT functional.