Superficial and Fundamental Correspondences in the Terahertz/IR (6–15 THz) Absorption Spectra of Aspirin and Benzoic Acid

The terahertz absorption
spectra of aspirin and benzoic acid have
been measured in the range 200–500 cm–1 (6–15
THz). Density-functional theory (DFT) modeling has assigned fundamental
vibrational modes to the observed absorption bands. Hydrogen bonds
between the crystalline planes of aspirin resulted in better agreement
between the experimental and modeled spectra than for benzoic acid.
The similar structure of these two molecules suggests a similar absorption
spectrum, which indeed was obtained experimentally. However, the detailed
crystal structure and molecular differences result in some of the
apparently common absorption bands being assigned to different vibrational
modes through the DFT modeling. Thus, our study importantly reveals
that even though crystalline forms of two similar molecules may have
similar experimental terahertz spectra, the resemblance may be superficial
rather than fundamental.