Mode-resolved low-frequency lattice dynamics of L-amino acid crystals revealed by complementary low-wavenumber Raman–terahertz spectroscopy and periodic DFT

Low-frequency lattice vibrations are sensitive to intermolecular interactions and crystal packing, but reliable interpretation requires more than matching Raman and terahertz (THz) peak positions. Here, low-wavenumber Raman spectroscopy, THz absorption spectroscopy, and periodic density functional theory calculations were combined to investigate L-alanine, L-glutamic acid, and L-glutamine, which share a common α-amino-acid backbone but differ in side-chain chemistry and crystal packing. The three crystals exhibited distinct low-frequency spectral profiles. Periodic PBE-D3 calculations indicated that the lower-frequency modes are dominated by collective rigid-molecule librations, whereas higher-frequency modes increasingly involve coupling between lattice motion and internal or side-chain coordinates. A selection-rule-aware comparison based on frequency proximity, calculated Raman activities and infrared intensities, and atomic displacement patterns distinguished Raman-selective, IR-selective, cross-technique-compatible, and near-coincident but vibrationally non-equivalent features. These findings demonstrate that frequency coincidence alone is insufficient for establishing Raman–THz mode correspondence. The study provides a mode-resolved approach for interpreting complementary low-frequency Raman and THz spectra of molecular crystals.