Good vibrations: understanding deep eutectic solvents through the lens of vibrational spectroscopy

This review highlights the variety of ways in which vibrational spectroscopy can be of use in understanding why deep eutectic solvents (DESs) behave the way they do. Throughout six themed sub-sections, selected studies are showcased to illustrate the kind of information that can be garnered using different vibrational spectroscopy techniques, warn against common pitfalls, and advise on best practices. More than 150 studies on the subject have been surveyed, encompassing a variety of techniques from those widely available, such as infrared and Raman spectroscopy, to niche tools, such as inelastic neutron scattering and two-dimensional infrared spectroscopy. The merging of computational chemistry and vibrational spectroscopy tools, arguably the most exciting partnership in the field, is discussed in a section of its own. With this polyvalent toolkit of techniques, one may assess hydrogen bond formation among DES constituents, probe their conformational preferences, determine what happens when DESs solidify (phase segregation into pure compounds or novel framework formation?) and investigate their molecular distribution at the nanoscale level (homogenous solution or nanodomain formation?). Hopefully, researchers who are starting to use these techniques will find this review a useful guide while new arrivals will be inspired by their potentialities.