Understanding the Terahertz Spectra of Amino Acid Molecular Crystals: Experiment and Theory

Terahertz spectroscopy has gained popularity in the past couple of decades as experimental techniques have matured. Amino acids are important for living organisms, with their strong intermolecular hydrogen bonding key to their biological prevalence. This makes them attractive materials to study with terahertz spectroscopy, which is sensitive to these intermolecular interactions. In this thesis the terahertz spectrum has been explored for amino acids: α-glycine, L-alanine, D-alanine, DL-alanine, β-alanine, L-valine, L-phenylalanine, and L-tyrosine. Many previous studies have explored the terahertz spectra of amino acid samples. However, these studies are generally restricted to room temperature results and powdered pellet samples. The experiments in this thesis provide much greater insight into the terahertz spectra of amino acids. This is done through the use of single crystal samples and spectra utilising polarisation dependence, low temperatures, and temperature dependence. In addition to these experimental advancements, density functional theory (DFT) modelling has also been used to determine the theoretical spectrum based on the molecular structure. Comparing the experimental and theoretical results has provided insight into the molecular motions that result in observed absorptions. Finally, the comparison of DFT calculations and experimental spectra has allowed for the unambiguous terahertz vibrational modes to be characterised. The terahertz spectrum of the amino acids studied can now used for spectral fingerprinting.