What is the Mechanism of H₃⁺ Formation from Cyclopropane?

Abstract

We examine the possibility that three hydrogen atoms in one plane of the cyclopropane dication come together in a concerted “ring-closing” mechanism to form H3 +, a crucial cation in interstellar gas-phase chemistry. Ultrafast strong-field ionization followed by disruptive probing measurements indicates that the formation time of H3 + is 249 ± 16 fs. This time scale is not consistent with a concerted mechanism, but rather a process that is preceded by ring opening. Measurements on propene, an isomer of cyclopropane, reveal the H3 + formation time to be 225 ± 13 fs, a time scale similar to the H3 + formation time in cyclopropane. Ab initio molecular dynamics simulations and the fact that both dications share a common potential energy surface support the ring-opening mechanism. The reaction mechanism following double ionization of cyclopropane involves ring opening, then H-migration, and roaming of a neutral H2 molecule, which then abstracts a proton to form H3 +. These results further our understanding of complex interstellar chemical reactions and gas-phase reaction dynamics relevant to electron ionization mass spectrometry.

Tracking Molecular Fragmentation in Electron–Ionization Mass Spectrometry with Ultrafast Time Resolution

“…More importantly, incorporating mechanisms, such as roaming, that have eluded theories until now should help improve present models. Modern AIMD combined with high-level quantum computation methods has been found to be in very good agreement with our experimental findings including dynamics and yield. , In fact, the recently released program Quantum Chemistry Electron Ionization Mass Spectra (QCEIMS) combined with semiempirical methods performs thousands of trajectories and is able to predict the fragmentation pattern of molecules in silico , with a degree of precision that depends on the level of computation, the time of the trajectories, and matching the internal energy of the radical cation. , With ab initio calculations becoming faster and more accurate and with computers becoming faster and better able to run code in parallel, one could conceive that in the future it will not be prohibitively expensive to carry out such calculations. The sobering thought is that molecules solve this problem in less than 1 ns.…”

“…Interestingly, if a ballistic dissociation product is produced with low kinetic energy compared to a long-range barrier, then it may remain trapped by the main ion. In these cases, one observes roaming of the neutral species near the remaining ion, which can last for hundreds of femtoseconds. ,− …”

Quantum simulation of conical intersections

“…Nonetheless, despite its simplicity, the molecule is an important species in astrochemistry, providing a useful benchmark for the study of CIs. 76–83 We compute the first three states of H 3 + with S z = 0. A compact mapping is used to reduce the number of required qubits to three for the first and second excited states (denoted as E 1 and E 2 ) of H 3 + .…”

Ultrafast Imaging of Jahn–Teller Distortion and the Correlated Proton Migration in Photoionized Cyclopropane

“…Advances in laser technology, especially ultrashort intense lasers, provide a powerful tool to image and steer this ultrafast reaction process. In recent decades, ultrafast proton migration and isomerization, occurring in various organic molecules interacting with the strong laser field, have attracted much attention. − Furthermore, the control of proton migration can be realized by adjusting parameters of the laser pulses. − Utilizing the femtosecond laser pulses, the pump–probe experiments have been carried out to obtain dynamic information on the isomerization and proton migration process in real time. − …”