Molecular ions are ubiquitous and play pivotal roles^1,2,3 in many reactions, particularly in the context of atmospheric and interstellar chemistry^4,5,6. 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^9,10,11, 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)^12,13,14, 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.

分子离子无处不在，在许多反应中发挥着关键作用^1,2,3 ，特别是在大气和星际化学领域^4,5,6 。然而，由于实验困难，它们的结构和构象转变^7,8 （特别是在气相中）比中性分子的研究较少。一个典型的例子是卤鎓离子^9,10,11 ，其高反应性和环张力使它们成为短寿命的中间体，甚至很容易受到弱亲核试剂的攻击，因此在进行进一步反应之前很难分离或捕获。在这里，我们展示了兆电子伏超快电子衍射 (MeV-UED) ^12,13,14与共振增强多光子电离结合使用，可以监测 1,3-二溴丙烷 (DBP) 阳离子的形成及其随后的结构动力学形成卤离子。我们发现 DBP ⁺阳离子在适当命名的“暗态”中保持了 3.6 ps 的相当长的持续时间，其结构与 DBP 电子基态没有区别。由表面跳跃模拟¹⁵和从头计算¹⁶支持的结构数据表明，阳离子随后衰变为iso -DBP ⁺ ，这是一种不寻常的中间体，具有包含松散结合的^17,18溴原子的四元环，并最终失去溴原子并形成具有三元环结构的溴离子¹⁹ 。我们预计这里使用的方法也可以应用于检查其他分子离子的结构动力学，从而加深我们对离子化学的理解。