Real-Time and In Situ Monitoring of Mechanochemical Reactions: A New Playground for All Chemists.

We provide a brief overview of the first techniques for direct, real-time observation of mechanochemical reactions by milling. Whereas mechanisms and kinetics of solid-state reactions induced by temperature or pressure have been extensively investigated, transformations of materials under continuous impact in a milling assembly remain largely unexplored and based on ex situ studies. The recent introduction and development of techniques for in situ monitoring of milling reactions by synchrotron X-ray powder diffraction and Raman spectroscopy has enabled the first direct insight into milling mechanochemistry, opening a new area for studies of chemical reactivity. So far, these techniques have revealed rapid, multistep reaction mechanisms and metastable intermediates that are impossible or difficult to observe or isolate in solution and have highlighted shortcomings of ex situ mechanistic studies. These pioneering advances also highlight the low level of mechanistic understanding and future challenges in developing a clear mechanistic picture of physicochemical transformations by milling.