What is the Mechanism of H<sub>3</sub><sup>+</sup> Formation from Cyclopropane?
Sung Hyun Kwon, Shawn Sandhu, Moaid Shaik, Jacob Stamm, Jesse Sandhu, Rituparna Das, Caitlin V. Hetherington, Benjamin G. Levine, Marcos Dantus
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 H 3 +, a crucial cation in interstellar gas-phase chemistry. Ultrafast strong-field ionization followed by disruptive probing measurements indicates that the formation time of H 3 + 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 H 3 + formation time to be 225 ± 13 fs, a time scale similar to the H 3 + 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 H 2 molecule, which then abstracts a proton to form H 3 + . These results further our understanding of complex interstellar chemical reactions and gas-phase reaction dynamics relevant to electron ionization mass spectrometry.