Wang Jiguo, Mi Xiaoyu, Liang Zhou, Dong Bowen, Deng Yongkai, Zhang Ming, Li Zheng, Liu Yunquan
State Key Laboratory for Mesoscopic Physics, School of Physics, Peking University, Beijing 100871, China.
Collaborative Innovation Center of Extreme Optics, Shanxi University, Taiyuan 030006, Shanxi, China.
J Am Chem Soc. 2025 Jun 18;147(24):20302-20309. doi: 10.1021/jacs.4c18084. Epub 2025 Jun 4.
Unlike in the liquid phase, the debate of whether and how hydrogen-bonded structures exist in neutral ammonia dimer (NH) in the gas phase has been ongoing for several decades. Here, we distinguish the structures of neutral ammonia dimers with and without hydrogen bonds by photoionization because the ions inherit initial structures from the neutral dimers and lead to significantly different Coulomb explosion channels in our pump-probe experiment, i.e., the direct dissociation (NH + NH) and indirect dissociation with proton migration (NH + NH). With quantum chemical and molecular dynamics simulation, we showcase that these two different Coulomb explosion channels originate from the ammonia dimer cations with different structures. The dimer cations without hydrogen bonds correlate with the direct Coulomb explosion channel. In contrast, dimer cations with hydrogen bonds are likely to undergo ultrafast proton migration in ∼48 fs, which has no potential barrier and correlate with the indirect dissociation channel in the Coulomb explosion. The 48 fs characteristic time is used to exclude the slower indirect dissociation initiated from non-H-bonded cations. Our work demonstrates a highly sensitive approach to probe weakly bonded and fluxional structures of gas-phase molecular clusters by utilizing both channel and time resolutions of Coulomb explosion.
与液相不同，气相中中性氨二聚体（NH）中是否存在氢键结构以及如何存在氢键结构的争论已经持续了几十年。在这里，我们通过光电离来区分有氢键和无氢键的中性氨二聚体的结构，因为离子从中性二聚体继承初始结构，并在我们的泵浦 - 探测实验中导致显著不同的库仑爆炸通道，即直接解离（NH + NH）和伴随质子迁移的间接解离（NH + NH）。通过量子化学和分子动力学模拟，我们展示了这两种不同的库仑爆炸通道源于具有不同结构的氨二聚体阳离子。无氢键的二聚体阳离子与直接库仑爆炸通道相关。相反，有氢键的二聚体阳离子可能在约48飞秒内经历超快质子迁移，这没有势垒，并且与库仑爆炸中的间接解离通道相关。48飞秒的特征时间用于排除由非氢键阳离子引发的较慢间接解离。我们的工作展示了一种高度灵敏的方法，通过利用库仑爆炸的通道和时间分辨率来探测气相分子簇的弱键合和易变结构。
