Combining Density-Functional Theory with Low-Temperature, Polarized Terahertz Spectroscopy of Single Crystals Explicates the Fundamental Modes of L-Alanine

While the fundamental modes of amino acids are accessible in principle by terahertz spectroscopy, reports to date have several shortcomings: (a) material of uncertain purity and morphology and diluted in a binder material is employed; (b) consequently, vibrations along all crystal axes are excited simultaneously; (c) data is restricted to room temperature, where resonances are broad and the background dominant; (d) comparison with theory has been unsatisfactory (in part because the theory assumes zero temperature). Here, we overcome all four obstacles, in reporting detailed polarized THz spectra of single-crystal l -alanine, assigning vibrational modes using density functional theory, and comparing the calculated dipole moment vector direction to the electric field polarization of the measured spectra. The direct and detailed comparison of theory with experiment has corrected previous mode assignments for l -alanine, as well as revealed unreported modes, previously obscured by closely-spaced spectral absorptions. The fundamental modes are thereby determined.