Aggregation-Induced Chemical Reactions: Acid Dissociation in Growing Water Clusters
Aggregation-Induced Chemical Reactions: Acid Dissociation in Growing Water Clusters
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DOI：
10.1021/ja1099209
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发表时间：
2011-03-23
影响因子：
15
通讯作者：
Marx, Dominik
中科院分区：
文献类型：
--
作者：
Forbert, Harald;Masia, Marco;Marx, Dominik
了解超冷条件下的化学反应能力，从而能够通过星际和大气过程进行分子合成，是低温化学中的一个关键问题。尤其是在水溶液微溶剂环境中，酸解离和质子转移反应是普遍存在的。在这里，分析了与氦纳米液滴分离(HENDI)光谱相关的HCl分子在低温下被少量水分子逐步溶剂化时的完全解离。研究发现，当HCl与H2O分子连续聚集时，在加入第四个水分子时，初步获得了一系列直到并包括HCl(H2O)(3)的环状杂分子结构，然后观察到解离的前驱状态-HCl(H2O)(3)中心点H2O。后者的部分聚集结构可视为“活性物种”，它容易导致HCl的解离并形成溶剂共享的离子对H3O+(H2O)(3)Cl。总体而言，这一过程在势能方面大多是下坡的，此外，通过使用由于聚集形成氢键而释放的动能来克服剩余的小势垒。相关势垒不是由热平衡决定的，而是由非热非平衡动力学产生的。这些“聚集诱导的化学反应”预计与超低温下的化学具有广泛的相关性，远远超出亨迪光谱的范围。
Understanding chemical reactivity at ultracold conditions, thus enabling molecular syntheses via interstellar and atmospheric processes, is a key issue in cryochemistry. In particular, acid dissociation and proton transfer reactions are ubiquitous in aqueous microsolvation environments. Here, the full dissociation of a HCl molecule upon stepwise solvation by a small number of water molecules at low temperatures, as relevant to helium nanodroplet isolation (HENDI) spectroscopy, is analyzed in mechanistic detail. It is found that upon successive aggregation of HCl with H2O molecules, a series of cyclic heteromolecular structures, up to and including HCl(H2O)(3), are initially obtained before a precursor state for dissociation, HCl(H2O)(3)center dot center dot center dot H2O, is observed upon addition of a fourth water molecule. The latter partially aggregated structure can be viewed as an "activated species", which readily leads to dissociation of HCl and to the formation of a solvent-shared ion pair, H3O+(H2O)(3)Cl. Overall, the process is mostly downhill in potential energy, and, in addition, small remaining barriers are overcome by using kinetic energy released as a result of forming hydrogen bonds due to aggregation. The associated barrier is not ruled by thermal equilibrium but is generated by athermal non-equilibrium dynamics. These "aggregation-induced chemical reactions" are expected to be of broad relevance to chemistry at ultralow temperature much beyond HENDI spectroscopy.