Intermolecular Vibrations in Hydrophobic Amino Acid Crystals: Experiments and Calculations

Intermolecular
vibrations of amino acid crystals occur in the THz,
or far-infrared, region of the electromagnetic spectrum. We have measured
the THz and Raman spectra of dl-leucine as well as two polymorphs
of dl-valine, the spectroscopic properties of which have
not previously been compared. Theoretical modeling of intermolecular
vibrations in hydrophobic amino acids is challenging because the van
der Waals interactions between molecules are not accounted for in
standard density functional theory. Therefore, to calculate the vibrational
modes, we used a recently developed approach that includes these nonlocal
electron correlation forces. We discuss methods for comparing results
from different theoretical models using metrics other than calculated
vibrational frequency and intensity, and we also report a new approach
enabling concise comparison of vibrational modes that involve complicated
mixtures of inter- and intramolecular displacements.