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

Abstract

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 (NO 3 − ) 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.

Observation of Slow Eigen-Zundel Interconversion in H+(H2O)6 Clusters upon Isomer-Selective Vibrational Excitation and Buffer Gas Cooling in a Cryogenic Ion Trap

“…This enhancement is due to the presence of vibrationally excited clusters prior to fragmentation or collisional cooling. The broadening of the oscillator strength in the excited clusters is expected given the large anharmonic effects which often appear in systems with very strong H-bonds. ,− This red shifting behavior is also evident in the positive features that appear just below the OH A sym and OH A asym stretching bands (labeled X OH sym and X OH asym ), shown in the expanded 3650–4000 cm –1 region in Figure C.…”

Formation and fragmentation of 2-hydroxyethylhydrazinium nitrate (HEHN) cluster ions: a combined electrospray ionization mass spectrometry, molecular dynamics and reaction potential surface study

“…85 Note that, despite the existence of an isolated stable neutral pair of HEÁHNO 3 in the gas phase, this neutral pair does not exist within large HEHN cluster ion. 33,34 A relaxed PES scan along the center-of-mass distance between HEÁHNO 3 and a second [HE + H] + (Fig. S1 in the ESI, † calculated at the oB97XD/6-31+G(d,p) level of theory using Gaussian 16) has shown that the neutral HEÁHNO 3 starts converting to an ion pair even with the presence of a [HE + H] + at a distance of B30 Å.…”

“…Later, Patrick et al 33 reported collision-induced dissociation (CID) of the protonated [(HE) 2 + H] + and [(HEHN) 2 HE + H] + cluster ions formed in the positive electrospray. Recently, Zeng et al 34 explored the structural features and H-bonding motifs in [(HEHN) n HE + H] + ( n = 1–5) using cryogenic vibrational spectroscopy. On the basis of classical molecular dynamics simulations, Prince et al 17 determined the Eyring–Polanyi and Arrhenius parameters, rate constants and branching ratios for the unimolecular dissociation of [(HEHN) n HE + H] + ( n = 1–3).…”

Similarity scores of vibrational spectra reveal the atomistic structure of pentapeptides in multiple basins

“…Advanced technologies in the gas-phase vibrational spectroscopy have made it feasible to elucidate the structure and interaction of complex molecules and clusters. Various systems have been studied to date; for example, ionic liquid, [1][2][3][4] protonated water clusters, [5][6][7][8][9][10] metal oxide clusters, [11][12][13][14][15] and so on. Conformer-selective vibrational spectra of polypeptides [16][17][18][19][20][21][22][23][24][25][26] have been measured with the usage of infrared (IR) and ultraviolet (UV) double resonance spectroscopy and supersonic jet expansions.…”