Terahertz-Raman spectroscopy for in situ benchtop monitoring of changes to extended, supramolecular structure in milling mechanochemistry

Abstract

Low-frequency Raman, also known as terahertz-Raman (THz-Raman), spectroscopy offers a laboratory benchtop-based alternative to synchrotron X-ray diffraction for real-time, in situ monitoring of ball-milling mechanochemical reactions. Although direct monitoring of the long-range structure of materials during mechanochemical reactions is generally challenging by conventional Raman spectroscopy, and typically requires synchrotron X-ray diffraction, here we use THz-Raman spectroscopy to monitor mechanosynthesis of cocrystals, stoichiomorphs, and polymorphs, detect multi-step sequences, and discover solid-state phases in systems difficult to differentiate using fingerprint-region Raman spectroscopy—all through real-time observation of changes in lattice vibrational models. The methodology is augmented by periodic density functional theory (DFT), which enables structural interpretation of spectroscopic changes, notably the identification of THz-Raman bands associated with halogen bond transformations. Simultaneous monitoring of mechanochemical processes in both the fingerprint and low-frequency Raman regions enables real-time observation of changes to extended as well as molecular structure during milling, in a single laboratory benchtop experiment, without synchrotron radiation.