Crystalline Molecular Standards for Low-Frequency Vibrational Spectroscopies
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Crystalline Molecular Standards for Low-Frequency Vibrational Spectroscopies Sara J. Dampf 1
& Timothy M. Korter
1
Received: 2 February 2020 / Accepted: 21 June 2020/ # Springer Science+Business Media, LLC, part of Springer Nature 2020
Abstract
The sub-200 cm−1 (sub-6 THz) vibrations of molecular crystals provide identifying features that are characteristic of each solid sample under study. These distinctive vibrational spectra have driven the development of new techniques and instrumentation in analytical spectroscopy. As terahertz time-domain spectroscopy and low-frequency Raman spectroscopy become increasingly prevalent in non-specialist laboratories, the need for a common set of spectral standards for use across these techniques becomes imperative. To meet this need, α-lactose monohydrate, biotin, and L-cystine are proposed here as molecular standards to evaluate instrument performance with both terahertz and Raman spectroscopies, as well serve as benchmarks for quantum mechanical simulations and analyses of these spectra. These substances all reveal a series of readily discernable peaks across the low-frequency region and also over a range of temperatures (295–50 K) making them even more useful. The often overlooked aspect of detailed spectral interpretation and assignment is directly addressed with rigorous solid-state density functional theory simulations of the three compounds based on a standard computational framework. By investigating these proposed molecular crystal standards with commonly available experimental and theoretical approaches, a set of realistic performance expectations can be achieved for both commercial instrumentation and software being used in low-frequency vibrational spectroscopy. Keywords Spectral standards . Terahertz spectroscopy . Low-frequency Raman spectroscopy . Density functional theory . Vibrational spectroscopy . Far-infrared spectroscopy
Electronic supplementary material The online version of this article (https://doi.org/10.1007/s10762-02000725-y) contains supplementary material, which is available to authorized users.
* Timothy M. Korter [email protected]
1
Department of Chemistry, Syracuse University, 1-014 Center for Science and Technology, Syracuse, NY 13244-4100, USA
Journal of Infrared, Millimeter, and Terahertz Waves
1 Introduction Vibrational spectroscopy has long been used to detect, identify, and characterize chemical samples of all types. These studies generally involve infrared absorption and Raman scattering, with spectroscopic selection rules based on dipole moment and polarizability changes, respectively. The measurements typically cover a spectral range of approximately 400–4000 cm−1, encompassing what is often referred to as the functional group and fingerprint regions. The vibrational frequencies of representative species are tabulated in extensive correlation lists that can be consulted and used to interpret measured vibrational spectra of new or unknown components [1, 2]. Low-frequency (≤ 200 cm−1) vibrational spectroscopies are far less expl
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