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Aggregation-induced chemical reactions: acid dissociation in growing water clusters

Forbert, Harald and Masia, Marco and Kaczmarek-Kedziera, Anna and Nail, Nisanth N. and Marx, Dominik (2011) Aggregation-induced chemical reactions: acid dissociation in growing water clusters. Journal of the American Chemical Society, Vol. 133 (11), p. 4062-4072. eISSN 1520-5126. Article.

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DOI: 10.1021/ja1099209

Abstract

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···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)3Cl. 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.

Item Type:Article
ID Code:5766
Status:Published
Refereed:Yes
Uncontrolled Keywords:Chemical reactions, HCl molecule, HENDI spectroscopy, growing water clusters
Subjects:Area 03 - Scienze chimiche > CHIM/02 Chimica fisica
Divisions:001 Università di Sassari > 01 Dipartimenti > Chimica
Publisher:American Chemical Society
eISSN:1520-5126
Deposited On:10 May 2011 16:49

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