Terahertz Spectroscopy and Molecular Modeling of Barbituric Acid

The well-resolved terahertz (THz) absorption spectrum of barbituric acid has been investigated using terahertz time-domain spectroscopy. Four distinct THz spectral features and two shoulder peaks were observed in the range of 10–124 cm–1. A complete analysis was performed with density functional theory, which provided an excellent agreement between solid-state simulation and experiment. The solid-state analysis indicates that the six experimental spectral features observed at low temperature consist of nine infrared-active vibrational modes. Further simulations based on hydrogen-bond isotopologues were performed to study the involvement of hydrogen bonds in the collective modes. A feature at 118.0 cm–1 mainly stems from the collective vibration of dimer hydrogen bonds (m) while features at 102.0 and 109.6 cm–1 primarily come from the collective vibrations of linear hydrogen bonds (n). The results may be useful for monitoring molecular reaction in industrial production according to the state of hydrogen bonds.