Formation of H₃⁺ from ethane dication induced by electron impact

Intramolecular hydrogen or proton migration is a fundamental process in physics, chemistry, and biology and can be initiated rapidly by photoabsorption, ion collisions, or electron impact. Hydrogen displacement can trigger structural rearrangements and isomerization, altering molecular function. In ionic organic molecules, ultrafast hydrogen migration can dominate over direct fragmentation, and double hydrogen migration may play a more important role than generally assumed. Special attention has been given to H3+ formation dynamics in such processes because H3+ has unique structural, dynamical, and astronomical significance, acting as a key protonating species in interstellar chemistry. In laboratory studies, H3+ can form from fragmentation of organic dications, but yields are often low and calculations suggest H3+ formation is energetically unfavorable. Two routes have been proposed: a minimum energy path via a transition state (TS), where a loosely bound complex forms (e.g., H2 attached to the dicationic moiety), and a roaming mechanism involving neutral H2 that proceeds off the MEP on longer timescales. Open questions include whether these two mechanisms can be separated experimentally and whether other mechanisms may lead to H3+ formation. This study investigates H3+ formation dynamics from ethane dication produced by 300 eV electron impact, combining two-body coincidence measurements and quantum chemistry (PES, TS, IRC) with ab initio molecular dynamics on the ground-state PES to determine available mechanisms.

Prior work has shown that hydrogen migration can dominate fragmentation pathways in molecular cations and that double hydrogen migration can be significant. H3+ formation has been observed from various organic dications but often with low yields; theory often finds H3+ channels energetically disfavored. Mechanistically, transition-state analyses indicate H3+ formation along a minimum energy path involving a loosely bound complex where neutral-like H2 attaches to a dicationic fragment. Kinetic energy release (KER) measurements, such as those by Kraus et al. on ethane dications in intense laser fields, can probe PES features by comparison to reverse activation energies from TS calculations. Time-resolved pump–probe experiments combined with AIMD simulations (e.g., Ekanayake et al.) identified H2 roaming and measured ejection times of ~100–260 fs, supporting a roaming pathway far from the MEP predicted by conventional transition state theory. Roaming chemistry typically involves long-lived intermediates and large-amplitude motion that circumvent saddle points. These studies motivate examining whether both TS and roaming operate in ethane dications formed by electron impact and whether they can be distinguished via observables like KER and dissociation times.

Experimental: A COLTRIMS setup at Fudan University was used. A pulsed 300 eV electron beam crossed a supersonic ethane jet in high vacuum. Post-collision ionic fragments were extracted by a 60 V/cm electrostatic field into a time-of-flight tube and detected in coincidence with a position-sensitive detector. Three-dimensional ion momenta were reconstructed from TOF and positions. Ion–ion coincidence TOF maps identified fragmentation channels; KER for two-body channels was obtained via standard COLTRIMS momentum conservation analysis. Branching ratios were determined from coincidence counts.

Theoretical: Ab initio quantum chemistry used Gaussian 16. Ground-state PES, transition states (TS), and intrinsic reaction coordinate (IRC) paths were computed with unrestricted DFT (B97XD functional including dispersion) and aug-cc-pVTZ basis; zero-point energy corrections were applied to stationary points. Potential energy scans were performed both rigidly (freezing all degrees of freedom) and with restricted relaxation to account for intrafragment relaxation and orientation effects. Mulliken charge analyses along scans and AIMD geometries assessed charge localization in dissociation limits (CH3+ + CH3+ versus CH2+ + CH4+). AIMD simulations employed the extended Lagrangian, atom-centered density matrix propagation method at the B3LYP/cc-pVDZ level with a fictitious electronic mass of 0.1 amu and a 0.5 fs time step. Initial geometries and velocities for neutral ethane were sampled from ground-state quantum harmonic oscillator distributions (zero-point vibrational sampling) using Newton-X. Vertical double ionization to the dicationic ground state initiated dynamics propagated solely on the ground-state dication PES. A total of 1000 trajectories were computed; 230 resulted in H3+ formation. Trajectories were analyzed for dissociation times (defined by monotonic increase of center-of-mass separation of fragments) and mechanistic features (MEP-like vs roaming).

• The H3+ + C2H3+ two-body coincidence channel is pronounced for ethane dication formed by 300 eV electron impact. Among the Hn+ (n = 1–3) channels from two-body fragmentation, branching ratios are H+: 1.1%, H2+: 26.0%, H3+: 72.9%. Considering all H+ ions from ethane dication, branching ratios are H+: 76.0%, H2+: 15.3%, H3+: 8.7%.
• The experimental KER distribution for the H3+ channel peaks at 4.7 eV, higher than strong-field ionization studies (~4.0–4.3 eV), likely due to laser-field modifications of the PES in prior work. AIMD-simulated KER on the ground-state PES agrees well with the experiment, indicating dominant ground-state dissociation.
• PES analyses: Vertical double ionization yields [CH3–CH3]2+ with nearly equal charge separation leading to CH3+ + CH3+ on an almost repulsive PES (small barrier). An ultrafast hydrogen migration stabilizes the dication to [CH2–CH4]2+ opening a path to H3+.
• TS/IRC details for CH3+ + CH3+: formation of a diborane-like double-bridged H2CH2CH2 (D2) structure at 27.92 eV precedes a TS at 28.84 eV.
• TS/IRC details for H3+ formation: local minimum for [CH2–CH4]2+ at 27.26 eV; a neutral H2 moiety forms and is emitted while the C–C bond rotates; the TS at 28.97 eV corresponds to proton transfer from carbon to H2, leading to H3+ + C2H3+. At the TS, the H2 moiety is ~1.8 Å from the nearest H, carries ~+0.20e, and has a 0.77 Å bond length.
• Dynamics: AIMD reveals two mechanisms on the ground-state PES: (i) a minimum energy path (MEP) consistent with the TS/IRC, with rapid H migration and H2 formation (~20 fs), reaching the TS by ~45 fs and completing H3+ formation within ~20 fs thereafter; typical dissociation times <120 fs. (ii) A roaming mechanism involving long-lived neutral H2 that explores a wide region before abstracting a proton; dissociation times span ~100–500 fs. A rare trajectory with initial H-atom roaming transitions to H2 roaming before H3+ formation.
• KER characteristics: simulated TS-path KER is sharper with highest intensity near 4.5 eV, while roaming yields a flatter distribution around ~4.5 eV (supporting potential experimental separability).
• Overall, H3+ can form efficiently from ethane dication on the ground-state PES via both TS-guided and roaming pathways; AIMD predicts H3+ formation times ranging from ~70 to 500 fs.

The study addresses whether H3+ formation in ethane dication proceeds via distinct mechanisms and whether ground-state pathways are viable. Agreement between experimental and AIMD KER distributions supports ground-state dissociation as the dominant route. TS and IRC analyses define a clear MEP involving rapid H migration to form [CH2–CH4]2+, neutral H2 formation and ejection, and a TS characterized by proton transfer from carbon to H2, culminating in H3+ formation. AIMD reveals, in addition to this MEP, a roaming mechanism where a neutral H2 radical persists and explores large-amplitude motion before proton abstraction, leading to longer dissociation times and broader KER features. These findings suggest that differences in dissociation timescales and KER profiles could allow experimental separation of mechanisms using time-resolved pump–probe techniques. The results deepen understanding of hydrogen migration and roaming chemistry in molecular dications and clarify that multiple pathways on the ground-state PES can efficiently produce H3+ from ethane.

Combining electron-impact COLTRIMS measurements with quantum chemical calculations and AIMD, the work demonstrates efficient H3+ formation from ethane dication with a KER peak at 4.7 eV. Ground-state PES calculations and KER agreement indicate that H3+ formation proceeds on the electronic ground state. TS/IRC analysis reveals a two-step MEP: ultrafast H migration stabilizing the dication to [CH2–CH4]2+, followed by neutral H2 emission and proton transfer at a distinct TS to yield H3+. AIMD uncovers an additional dominant roaming-induced isomerization pathway with longer timescales and larger-amplitude nuclear motion. Distinct dissociation times and KER characteristics between TS and roaming mechanisms suggest that ultrafast pump–probe experiments could separate them. This work elucidates H3+ formation mechanisms in ionic hydrocarbons and advances insight into hydrogen migration, roaming chemistry, and fragmentation dynamics. Future research could perform time-resolved experiments to distinguish pathways and explore the role of excited states and different ionization conditions.

• Dynamics were simulated only on the electronic ground-state PES of the ethane dication; while justified by KER agreement and prior work, excited electronic states and nonadiabatic effects were not explicitly treated and could influence early-time dynamics via internal conversion.
• The experimental setup provides time-integrated KER and branching ratios but does not directly time-resolve dissociation; separation of TS versus roaming contributions is inferred from simulations and suggested KER differences, necessitating ultrafast pump–probe measurements for definitive separation.
• Electron impact energy was fixed at 300 eV; dependence of mechanism branching on impact energy or other ionization conditions was not explored.
• Rigid versus relaxed PES scans can impose constraints; while TS/IRC and AIMD mitigate this, detailed orientation effects and long-range interactions may be sensitive to the chosen computational levels (B97XD/aug-cc-pVTZ for stationary points; B3LYP/cc-pVDZ for dynamics).