Discrimination of Chiral Solids: A Terahertz Spectroscopic Investigation of l- and dl-Serine

The terahertz (THz, far-infrared) spectra of enantiomerically pure and racemic crystalline serine were investigated using time-domain THz spectroscopy and solid-state density functional theory (DFT) in the spectral range of 10-90 cm(-1). The experimental THz spectra of L- and DL-serine at 78 K appear quite similar despite the significant differences in arrangement of the molecules in their crystal structures. Structural analyses of the two systems and calculation of the vibrational modes and intensities were performed using DFT with periodic boundary conditions employing the B3LYP and PW91 density functionals with the 6-31G(d,p) and 6-311G(d,p) basis sets. The applied computational methods produced simulations of the THz spectra in good agreement with experiment, with accurate predictions of the subtle differences in the THz spectra of the two chiral solid-state mixtures of serine. The observed spectral features are assigned as primarily external lattice translations and rotations with lesser contributions due to intramolecular torsions of the -NH(3)(+) and -COO(-) groups modified by intermolecular hydrogen bonding.