Complementary Spectroscopy Resolves Low-Frequency Vibrations in Amino Acid Crystals

A combined experimental and computational investigation has demonstrated that matching peak positions alone is insufficient to establish vibrational correspondence between low-wavenumber Raman and terahertz spectra of molecular crystals. Low-frequency lattice vibrations are known to provide sensitive markers of intermolecular interactions and crystal packing architectures. However, reliable interpretation of these vibrational modes requires rigorous analysis beyond superficial spectral comparisons. To achieve reliable interpretations of these spectra, researchers integrated low-wavenumber Raman spectroscopy, terahertz absorption spectroscopy, and periodic density functional theory calculations to analyze the low-frequency lattice dynamics of three distinct crystalline amino acids.

The study focused on L-alanine, L-glutamic acid, and L-glutamine, three molecular crystals that share an identical alpha-amino-acid backbone but differ substantially in their side-chain chemistry and crystal packing environments. Through complementary spectroscopic measurements, the researchers observed that each of the three crystalline materials exhibited distinct low-frequency spectral profiles across the measured ranges.

To understand the microscopic origins of the measured spectra, the authors carried out periodic density functional theory calculations utilizing the PBE-D3 dispersion-corrected functional. These calculations revealed a clear frequency-dependent distinction in the physical character of the lattice vibrations across the investigated systems. In the lower-frequency regime, the vibrational modes are dominated by collective rigid-molecule librations.

To establish unambiguous connections between the complementary spectroscopic techniques, the researchers implemented a selection-rule-aware comparison rather than relying strictly on spectral peak matching. This evaluative framework evaluated frequency proximity alongside calculated Raman activities, calculated infrared intensities, and detailed atomic displacement patterns derived from the periodic calculations. Using this comprehensive criteria, the authors successfully distinguished between Raman-selective modes, infrared-selective modes, cross-technique-compatible transitions, and spectral features that appear near-coincident in frequency yet remain vibrationally non-equivalent, preventing misassignments that arise when comparing different spectroscopic probes.

These findings establish that mere frequency coincidence between Raman and terahertz peaks is insufficient for determining true vibrational mode correspondence in molecular crystals. By accounting for selection rules, intensities, and atomic displacements, the study provides a robust, mode-resolved framework for interpreting complementary low-frequency Raman and terahertz absorption data.

Subject of Research: Low-frequency lattice dynamics of amino acid crystals

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

Article References: Ge, M., Wu, L., Xue, D., Wang, Y., & Qu, Y. (2026). Mode-resolved low-frequency lattice dynamics of L-amino acid crystals revealed by complementary low-wavenumber Raman–terahertz spectroscopy and periodic DFT. BMC Chemistry. https://doi.org/10.1186/s13065-026-01954-3

Image Credits: AI Generated

DOI: 10.1186/s13065-026-01954-3

Keywords: low-frequency lattice dynamics, Raman spectroscopy, terahertz spectroscopy, periodic DFT, amino acid crystals, molecular crystals, crystal packing, vibrational modes, Mode-resolved, low-frequency, lattice, dynamics