Halide Coordination Motifs in Deep Eutectic Solvent Building Blocks Characterized by Helium Nanodroplet Infrared Action Spectroscopy

Abstract

Deep eutectic solvents (DESs) are a broad class of multicomponent solvents that allow for tuning of properties through informed selection of constituent molecules. A comprehensive assessment of DES intermolecular interactions and their link to macroscopic attributes facilitates component selection. In this work, we employ infrared (IR) action spectroscopy in helium nanodroplets to characterize isolated halide–molecule clusters relevant to DESs. Complexes of urea, ethylene glycol, glycerol, and choline with chloride or bromide were generated by electrospray ionization, isolated in an ion trap, and captured in helium nanodroplets for spectroscopic characterization in the IR fingerprint region. For urea–chloride clusters, halide–molecule rather than molecule–molecule interactions dominate, resulting in a shallow potential energy surface with multiple orientations of the urea molecules differing primarily in their relative positioning. For clusters with strong competition between halide–molecule and intermolecular hydrogen bonding, as in glycerol–halide clusters, the potential energy surface becomes more complex, featuring multiple low-energy configurations with distinct hydrogen-bonding networks. Choline–chloride clusters exhibit possible two-point (OH + CH) and three-point (3CH) halide coordination motifs. Comparison to condensed-phase DES FT-IR spectra reveals distinctions between isolated clusters and bulk solution, highlighting the importance of assessing both intermolecular and halide-molecule interactions to accurately represent macroscopic DES structure.