Ionic Liquid Clusters Generated from Electrospray Thrusters: Cold Ion Spectroscopic Signatures of Size-Dependent Acid-Base Interactions.

We determine the intramolecular distortions at play in the 2-hydroxyethylhydrazinium nitrate (HEHN) ionic liquid (IL) propellant, which presents the interesting case that the HEH+ cation has multiple sites (i.e., hydroxy, primary amine, and secondary ammonium groups) available for H-bonding with the nitrate anion. These interactions are quantified by analyzing the vibrational band patterns displayed by cold cationic clusters, (HEH+)n(NO3-)n-1, n = 2-6, which are obtained using IR photodissociation of the cryogenically cooled, mass-selected ions. The strong interaction involving partial proton transfer of the acidic N-H proton in HEH+ cation to the nitrate anion is strongly enhanced in the ternary n = 2 cluster but is suppressed with increasing cluster size. The cluster spectra recover the bands displayed by the bulk liquid by n = 5, thus establishing the minimum domain required to capture this aspect of macroscopic behavior.