Formation of H₃⁺ from ethane dication induced by electron impact

Intramolecular hydrogen migration is a crucial process in various fields, often initiated by methods like photoabsorption, ion collisions, and electron impact. Studies using momentum imaging techniques have shown the dominance of hydrogen migration in molecular cation fragmentation, including the more complex double hydrogen migration. The formation and dynamics of H₃⁺, a significant interstellar molecule, has received special attention. H₃⁺ formation typically involves hydrogen migration and is often energetically unfavorable. Two primary routes for H₃⁺ formation have been identified: one via a minimum energy path predicted by transition state (TS) calculations, often involving a loosely bound complex; and another via a roaming mechanism, revealed through time-resolved pump-probe techniques and ab initio molecular dynamics (AIMD) simulations, characterized by longer timescales and larger amplitude nuclear motion. This work aims to further investigate these mechanisms and explore potential new routes using a combined experimental and theoretical study of H₃⁺ formation from ethane (C₂H₆) dications.

Previous research has explored H₃⁺ formation from various hydrocarbon dications using different ionization methods. Kraus et al. measured the kinetic energy release (KER) of H₃⁺ from ethane dications produced by intense femtosecond laser fields, correlating KER with the reaction path obtained from TS calculations. Other studies focusing on hydrocarbon dications suggest H₃⁺ formation proceeds through a TS representing a loosely bound complex. Time-resolved studies employing pump-probe techniques and AIMD simulations have provided direct access to H₃⁺ formation dynamics, revealing a roaming mechanism in alcohol molecules where H₂⁺ ions are formed, distinct from the TS mechanism due to its longer timescale and wider trajectory. However, questions remain regarding the separation of these mechanisms and the possibility of additional pathways. Studies on H₃⁺ formation from ethane have shown differing yields depending on the ionization method, with electron impact studies showing a lower yield compared to intense laser field studies. This research aims to address the existing gaps in our understanding of H₃⁺ formation from ethane.

This study employs a combined experimental and theoretical approach. The experiment utilizes a cold target recoil ion momentum spectroscopy (COLTRIMS) setup. A pulsed 300 eV electron beam intersects a supersonic ethane jet in a high vacuum chamber. The resulting ions are extracted by an electrostatic field, detected by a position-sensitive detector, and their three-dimensional momentum vectors reconstructed using time-of-flight (TOF) and position data. The theoretical investigation consists of ab initio quantum chemistry calculations using the Gaussian 16 program. Potential energy surfaces (PES), transition states (TS), and intrinsic reaction coordinates (IRC) are obtained using unrestricted density functional theory (DFT) calculations with the B97XD functional and aug-cc-pVTZ basis set. Zero-point energies correct all stationary point energy levels. Ab initio molecular dynamics (AIMD) simulations, performed using the extended Lagrangian MD scheme and atom-centered density matrix propagation method with the B3LYP/cc-pVDZ level of theory, model the dynamics of H₃⁺ formation on the ground-state PES of ethane dication. The initial conditions are sampled from a ground-state quantum harmonic oscillator simulation of ethane's zero-point vibrations, utilizing the Newton-X package. A total of 1000 trajectories were computed, with 230 resulting in H₃⁺ formation.

The experimental results show a dominant H₃⁺ + C₂H₃⁺ coincidence channel among the various fragmentation pathways of ethane dication following electron impact. The branching ratios for H⁺, H₂⁺, and H₃⁺ are 1.1%, 26.0%, and 72.9%, respectively. The kinetic energy release (KER) distribution for the H₃⁺ channel peaks at 4.7 eV, larger than previously reported values from strong laser field ionization. This difference is likely due to the intense laser field's modification of the dication PES. The theoretical KER distribution from AIMD simulations closely matches the experimental distribution, indicating the ground-state dissociation's prominent role. Potential energy scans (both rigid and relaxed) of the ethane dication reveal pathways involving C-C bond cleavage and hydrogen migration, leading to CH₃⁺ + CH₃⁺ and CH₂⁺ + CH₄⁺ dissociation limits. The transition state (TS) calculation identifies a diborane-like double-bridged structure as an intermediate in H₃⁺ formation, with a two-step process involving hydrogen migration followed by proton transfer. The AIMD simulations reveal two mechanisms: a faster mechanism following the intrinsic reaction coordinate of the transition state (<120 fs) and a slower roaming mechanism (100-500 fs) involving longer-lived neutral intermediates and wider trajectories. The roaming mechanism can involve either H₂ or H atom roaming.

The findings show that H₃⁺ formation from ethane dication proceeds via at least two distinct mechanisms: a transition state-mediated pathway and a roaming-induced isomerization pathway. The agreement between experimental and simulated KER distributions validates the focus on ground-state dissociation. The two-step mechanism identified by the TS analysis clarifies the role of hydrogen migration in stabilizing the dication and enabling subsequent proton transfer. The roaming mechanism highlights the importance of exploring wider regions of the PES and the influence of internal energy redistribution. The differences in dissociation times and KER distributions suggest the possibility of experimentally separating these mechanisms using ultrafast pump-probe techniques. These findings contribute to the broader understanding of hydrogen migration, roaming chemistry, and molecular fragmentation dynamics, particularly relevant to interstellar chemistry and other fields.

This work provides a comprehensive investigation of H₃⁺ formation dynamics from ethane dication, combining experimental measurements and sophisticated theoretical calculations. The study reveals the coexistence of transition state and roaming mechanisms, offering a detailed understanding of the underlying dynamics. The discrepancies in time scales and KER distributions for these mechanisms suggest avenues for future research employing ultrafast techniques to disentangle their contributions and gain further insight into the complex interplay of hydrogen migration and roaming chemistry.

The study focuses primarily on the ground-state dissociation of the ethane dication. While the good agreement between experimental and theoretical KER distributions supports this focus, the contribution of excited states cannot be entirely ruled out. The AIMD simulations, while comprehensive, are computationally demanding, limiting the number of trajectories and potentially impacting the statistical representation of all possible pathways. Future studies could benefit from incorporating a more extensive exploration of excited states and increasing the number of simulated trajectories.