Mechanism of Magic Number NaCl Cluster Formation from Electrosprayed Water Nanodroplets

Abstract

Events taking place during electrospray ionization (ESI) can trigger the self-assembly of various nanoclusters. These products are often dominated by magic number clusters (MNCs) that have highly symmetrical structures. The literature rationalizes the dominance of MNCs by noting their high stability. However, this argument is not necessarily adequate because thermodynamics cannot predict the outcome of kinetically controlled reactions. Thus, the mechanisms responsible for MNC dominance remain poorly understood. Molecular dynamics (MD) simulations can provide atomistic insights into self-assembly reactions, but even this approach has thus far failed to provide pertinent answers. The current work overcomes this limitation. We focused on salt clusters formed from aqueous NaCl solutions during ESI. The corresponding mass spectra are dominated by the Na 14 Cl 13 + MNC. Simulations of ESI droplets showed nonspecific association of Na + and Cl − , culminating in gaseous clusters via solvent evaporation to dryness (charged residue mechanism). These nascent clusters did not show any preference for MNCs. In mass spectrometry experiments, analyte ions undergo in-source activation prior to detection. We emulated in-source activation by heating nascent clusters in our MD runs. Heating triggered structural fluctuations and dissociation events, generating MNC-dominated product distributions. Why are MNCs preferred after in-source activation? Thermally excited clusters frequently adopt structures consisting of a preformed MNC and a stringlike protrusion that contains the surplus ions. Facile separation of these protrusions releases the MNC (Cluster hot → MNC-protrusion → MNC + protrusion). This work marks the first time that MD simulations were able to capture cluster self-assembly with subsequent "molecular pruning", generating MNC-dominated product distributions that agree with experiments.

The role of analyte concentration in accelerated reaction rates in evaporating droplets

“…Simulations of a 5 nm diameter droplet indicates that full evaporation occurs in 18 ns when the temperature was maintained at 40 °C. 50 This fast evaporation rate is consistent with the larger nanodrops in our experiment surviving on the low microsecond timescale.…”

On the Chemistry of Aqueous Ammonium Acetate Droplets during Native Electrospray Ionization Mass Spectrometry

“…Evaporation of this droplet produced a large Na n Cl m ( n – m )+ CRM cluster. Details of the NaCl droplet behavior can be found in ref . Figure strikingly illustrates the difference between a volatile and a nonvolatile salt during ESI.…”

“…MD simulations were conducted using Gromacs 2020, 70 following established methods with some modifications. 16,71,72 Rayleigh-charged ESI droplets with an initial 3 nm radius were kept at 370 K for 50 ns, followed by 25 ns at 500 K. H-bonded ion pairs were given the opportunity to undergo NH 4 + + Ac − → NH 3 + HAc proton transfer in 250 ps intervals (Figure S2 and TOC Figure). All other details can be found in the SI.…”

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

“…Formation of cluster ions [52][53][54] and nanoclusters 55 represents another complication associated with electrospray. As the outcome of kinetics-controlled in-source cluster formation and inand after-source cluster fragmentation may not be accurately predicted on the sole basis of thermodynamics, molecular dynamics simulations are needed to elucidate mechanisms of these events.…”

“…As the outcome of kinetics-controlled in-source cluster formation and inand after-source cluster fragmentation may not be accurately predicted on the sole basis of thermodynamics, molecular dynamics simulations are needed to elucidate mechanisms of these events. 53,54 For the electrospray thrusters operating in the purely ionic regime, formation of undesirable IL cluster ions, 17,33,40,41,43 in addition to single ions, brings about several specific issues: (1) I sp of the propellant is decreased by large cluster ions; (2) cluster ions of the same charge but different masses are accelerated to different final velocities and momenta by the same electric field, and the resulting ionbeam polydispersity limits the propulsion efficiency; 35,39 (3) cluster ions are metastable (lifetime of ms) 40 and undergo inflight dissociation. 56 Cluster fragmentation may occur via various mechanisms in a spacecraft, e.g., the emitted species strike a downstream surface (i.e., liberation of materials in sputtering), 46,47 the backscattering clusters (happening in a bipolar electrospray thruster) impact the surface of the spacecraft, 16 and collisions between the emitted species in ''traffic jams'' within the electrospray plume (because of the differences in their velocities).…”