Reactive Molecular Dynamics Modeling of Collision-Induced Dissociation of 1-Ethyl-3-methylimidazolium Tetrafluoroborate Ionic Liquid Ions.

Ionic liquids (ILs) have been gaining increasing focus in a variety of applications including emerging electric-propulsion concepts. A quantitative understanding of how IL ions fragment during high-energy collisions with background gases is therefore essential for interpreting mass spectra, predicting ion lifetimes in plasma and vacuum environments, and designing IL-based technologies. This work uses molecular dynamics (MD) simulations with a reactive force field to numerically model the collision-induced dissociation (CID) of isolated ions (both positive and negative) and ion clusters (2:1 and 1:2 clusters) of the prototypical ionic liquid 1-ethyl-3-methylimidazolium tetrafluoroborate (EMIM-BF), colliding with a nitrogen (N) molecule, exploring all possible fragmentation channels arising from the breaking of both ionic and covalent bonds at collision energies ranging from 10 electron volts (eV) to 100 electron volts (eV) in the laboratory frame. The molecular dynamics results are compared with the observations from tandem mass spectrometry (MS2) experiments to assess the reliability of the MD results. The MD modeling predicts the dissociation onset collision energy of the [EMIM] ion to be 20 eV (lab frame), while the [BF] ion requires a collision energy of at least 40 eV (lab frame) to undergo dissociation. The primary fragmentation product of the [EMIM] ion is found to be the 3-methylimidazolium cation, [CHN], with the cyanide anion, [CN], being the major fragment ion at higher collision energies (≥60 eV, lab frame). The [BF] ion, on the other hand, dissociates to form the fluoride ion, [F], and the neutral BF molecule, with the [BF] ion being formed at higher collision energies (≥60 eV, lab frame). Both the 2:1 and 1:2 ion clusters are found to fragment at the lowest simulated collision energy of 10 eV (lab frame), with the fluoride ion, [F], being formed with rising abundance as the collision energy is increased. When compared, the mass spectra from MD modeling and experiments demonstrate a reasonable agreement, which suggests that reactive MD can be a reliable surrogate for CID studies of complex IL ions.