Dissociation pathways of protic ionic liquid clusters: Alkylammonium nitrates

Abstract

Protic ionic liquids are promising candidates for many applications, including as spacecraft propellants. For both fundamental interest and understanding clustering and dissociation during electrospray‐based propulsion, it is useful to explore the dissociation pathways of protic ionic liquid clusters, as well as the factors affecting the relative contributions of each pathway to the observed MS/MS spectra. With that said, most of the published reports on ionic liquid cluster dissociation have focused on aprotic ionic liquids. The purpose of the current work is to explore the dissociation pathways (eg, loss of amine, nitric acid, or ion pair) of alkylammonium nitrates using energy‐resolved collision‐induced dissociation. Here, it was found that, in general, protic ionic liquids have multiple dissociation pathways—namely, protic ionic liquids can lose their neutralized cation (here, an alkylamine) or neutralized anion (here, nitric acid)—in addition to the ion pair dissociation familiar to aprotic salt and aprotic ionic liquid clusters. In general, increasing the basicity of the cation (here, through increasing the degree of alkylation) decreases the propensity to follow these alternative pathways. Interestingly, increasing the cluster size has a similar effect: as cluster size increases, nitric acid loss decreases. These results will help better model and design protic ionic liquids for electrospray‐based spacecraft propulsion and help provide a better understanding for the general behavior of protic ionic liquids versus aprotic ionic liquids within mass spectrometers.

Electrospray ionization enters the final frontier: Mass spectrometry's role in understanding electrospray thrusters and their plumes

“…Recently, the dissociation of protic ionic liquid clusters has been investigated. Protic ionic liquid clusters have many additional dissociation channels – summarized in Figure – so the primary goal of these first papers was to investigate which pathways were active and with what relative intensities for model systems, namely alkylammonium nitrates, and for the ionic liquid propellant 2‐hydroxyethylhydrazinium nitrate . In general, proton transfer mediated dissociation was indeed observed for both systems.…”

Thermal‐ and collision‐induced dissociation studies of functionalized imidazolium‐based ionic liquid cations

“…Cluster dissociation studies have investigated relative interaction strengths of common cations and anions. 20,[37][38][39] Additionally, the dissociation of protic ionic liquid clusters, specifically comprised of 2-hydroxyethylhydrazinium nitrate 40 or alkylammonium nitrates, 41 has also been investigated and contrasted with the clustering and dissociation of aprotic ionic liquids. Gas-phase cation dissociation has been studied, 42 including with a focus on specific classes such as zwitterions 43 and sulfur-containing cations.…”

Photoinduced Intermolecular Electron Transfer in Gas Phase Ion Pairs of the 1-Ethyl-3-methylimidazolium Cation and the Bis(trifluoromethylsulfonyl)imide Anion

“…Interactions with UV solar photons are likely to be one of the major causes of chemical breakdown in these plumes and, as such, it is imperative to understand the photochemical reactions occurring in the space environment. Plumes resulting from IL thrusters are initially composed of charged IL clusters, which can dissociate into neutral pairs, bare ions, or neutral molecules. − Recent studies by Patrick et al , and Prince et al focused on the decomposition of such charged IL clusters and found that many lose ion pairs as the dominant product. After these plumes are formed, questions regarding the eventual fate of the ion pairs are does UV radiation cause the ion pairs to become radicals via electron transfer, decompose directly into ions, or break chemical bonds in the component ions to produce smaller ions/neutrals?…”