Ultrafast Proton Transfer Dynamics on the Repulsive Potential of the Ethanol Dication: Roaming-Mediated Isomerization versus Coulomb Explosion.

Enliang Wang,Thomas Pfeifer,Alexander Dorn,Xu Shan,Xueguang Ren,Lei Chen,Xiangjun Chen

doi:10.1021/acs.jpca.0c02074

Abstract

If a molecular dication is produced on a repulsive potential energy surface (PES), it normally dissociates. Before that, however, ultrafast nuclear dynamics can change the PES and significantly influence the fragmentation pathway. Here, we investigate the electron-impact-induced double ionization and subsequent fragmentation processes of the ethanol molecule using multiparticle coincident momentum spectroscopy and ab initio dynamical simulations. For the electronic ground state of the ethanol dication, we observe several fragmentation channels that cannot be reached by direct Coulomb explosion (CE) but require preceding isomerization. Our simulations show that ultrafast hydrogen or proton transfer (PT) can stabilize the repulsive PES of the dication before the direct CE and form intermediate H2 or H2O. These neutrals stay in the vicinity of the precursor, and roaming mechanisms lead to isomerization and finally PT resulting in emission of H3+ or H3O+. The present findings can help to understand the complex fragmentation dynamics of molecular cations.

Highlights

The removal of two electrons from a neutral molecule is a process of fundamental interest in chemical physics
Of particular interest in the present work are the very fast hydrogen and proton transfer mechanisms that are relevant in various fields of physics, chemistry, and biology.[5−7] In particular, PT is a common and fundamental process in organic chemistry and molecular biology as all organic compounds contain a large fraction of hydrogen atoms.[8]
We demonstrate that the intramolecular PT and roaming dynamics play key roles in stabilizing the ethanol dication and altering significantly the fragmentation pathways

Summary

■ INTRODUCTION

The removal of two electrons from a neutral molecule is a process of fundamental interest in chemical physics. Article performed time-resolved pump-probe experiments using femtosecond laser pulses and ab initio molecular dynamics (AIMD) simulations For different alcohols, they found that, for H3+ production, the intermediate roaming of H2 plays an important role and that the timescale for the fragmentation process is between 100 and 260 fs.[30,32] The calculation of the respective reaction pathways is challenging for conventional transition-state (TS) theory since H2 roaming may explore large regions of the PES and bypass saddle points entirely.[33]. The Mulliken charge analysis shows that the intermediates like H2 and H2O are neutral groups Their long-time motion around the parent molecule can be regarded as a roaming chemical process.[33,34] We demonstrate that the intramolecular PT and roaming dynamics play key roles in stabilizing the ethanol dication and altering significantly the fragmentation pathways

■ RESULTS

■ CONCLUSIONS

■ ACKNOWLEDGMENTS

■ REFERENCES