Capturing the generation and structural transformations of molecular ions

• 成果类型: Article
• 署名作者: Heo, Jun; Kim, Doyeong; Segalina, Alekos; Ki, Hosung; Ahn, Doo-Sik; Lee, Seonggon; Kim, Jungmin; Cha, Yongjun; Lee, Kyung Won; Yang, Jie; Nunes, J. Pedro F.; Wang, Xijie; Ihee, Hyotcherl
• 署名单位: Institute for Basic Science - Korea (IBS); Korea Advanced Institute of Science & Technology (KAIST); Stanford University; United States Department of Energy (DOE); SLAC National Accelerator Laboratory; University of Nebraska System; University of Nebraska Lincoln; Samsung Electronics; Samsung; Tsinghua University; Diamond Light Source
• 刊物名称: Nature
• ISSN/ISSBN: 0028-4755
• DOI: 10.1038/s41586-023-06909-5
• 发表日期: 2024-01-25
• 关键词: surface-hopping program; electron-diffraction; newton-x; DYNAMICS; bromonium; chemistry

摘要

Molecular ions are ubiquitous and play pivotal roles1-3 in many reactions, particularly in the context of atmospheric and interstellar chemistry4-6. However, their structures and conformational transitions7,8, particularly in the gas phase, are less explored than those of neutral molecules owing to experimental difficulties. A case in point is the halonium ions9-11, whose highly reactive nature and ring strain make them short-lived intermediates that are readily attacked even by weak nucleophiles and thus challenging to isolate or capture before they undergo further reaction. Here we show that mega-electronvolt ultrafast electron diffraction (MeV-UED)12-14, used in conjunction with resonance-enhanced multiphoton ionization, can monitor the formation of 1,3-dibromopropane (DBP) cations and their subsequent structural dynamics forming a halonium ion. We find that the DBP+ cation remains for a substantial duration of 3.6 ps in aptly named 'dark states' that are structurally indistinguishable from the DBP electronic ground state. The structural data, supported by surface-hopping simulations15 and ab initio calculations16, reveal that the cation subsequently decays to iso-DBP+, an unusual intermediate with a four-membered ring containing a loosely bound17,18 bromine atom, and eventually loses the bromine atom and forms a bromonium ion with a three-membered-ring structure19. We anticipate that the approach used here can also be applied to examine the structural dynamics of other molecular ions and thereby deepen our understanding of ion chemistry. The use of mega-electronvolt ultrafast electron diffraction combined with resonance-enhanced multiphoton ionization yields data that can reveal the formation and subsequent structural relaxation of a molecular ion on an ultrafast timescale.