Crystalline Molecular Standards for Low-Frequency Vibrational Spectroscopies

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.