Capturing the generation and structural transformations of molecular ions

Molecular ions are ubiquitous and play pivotal roles^1-3 in many reactions, particularly in the context of atmospheric and interstellar chemistry^4-6. However, their structures and conformational transitions^7,8, particularly in the gas phase, are less explored than those of neutral molecules owing to experimental difficulties. A case in point is the halonium ions^9-11, 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 bound^17,18 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.