The increasing adoption of low-frequency vibrational spectroscopy in laboratories around the world has necessitated the creation of molecular spectral standards for the practical evaluation of spectral bandwidth, resolution and signal-to-noise ratios of data below 200 cm-1. In this work, a series of organic crystals are presented for use as solid-state molecular standards for the testing and calibration of terahertz-IR and terahertz-Raman instruments in the sub-200 cm-1 range (sub-6 THz). These standards include α-lactose monohydrate (C12H22O11 ● H2O, space group = P21, Z = 2), biotin (C10H16N2O3S, space group = P212121, Z = 4), and L-cystine (C6H12N2O4S2, space group = P6122, Z = 6). These were chosen based on several criteria beyond the fact that they exhibit numerous spectral features in the low-frequency region. They are non-hazardous, relatively inexpensive, readily obtainable, and have well-resolved peaks even at room temperature so that they are useful to laboratories that lack cryogenic sample capabilities. Spectra were acquired at 290 K (room) and 78 K (liquid nitrogen) temperatures using a time-domain terahertz-IR spectrometer (10-150 cm-1) based on fiber-coupled InGaAs antennas and a terahertz-Raman instrument (10-250 cm-1) utilizing narrowband volume holographic grating filters with an excitation wavelength of 785 nm. For the presented data, samples were re-crystallized in all cases and the crystal structures verified using powder and single-crystal X-ray diffraction. This ensured that no significant impurities were present, including other polymorphic forms. In addition to being excellent experimental standards, the vibrational spectra serve as computational benchmarks as well. Each standard was fully analyzed using solid-state density functional theory simulations of the crystalline structures and vibrational motions to achieve complete assignments of the observed features. The combination of complementary experiment and theory provides additional confidence in the standard reference quality of these organic crystals.

Crystalline molecular standards for low-frequency vibrational spectroscopies

256th National Meeting (2018)

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