title: Aggregation-induced dissociation of HCl(H2O)4 below 1 K: the smallest droplet of acid.
abstract: Acid dissociation and the subsequent solvation of the charged fragments at ultracold temperatures in nanoenvironments, as distinct from ambient bulk water, are relevant to atmospheric and interstellar chemistry but remain poorly understood. Here we report the experimental observation of a nanoscopic aqueous droplet of acid formed within a superfluid helium cluster at 0.37 kelvin. High-resolution mass-selective infrared laser spectroscopy reveals that successive aggregation of the acid HCl with water molecules, HCl(H2O)n, readily results in the formation of hydronium at n = 4. Accompanying ab initio simulations show that undissociated clusters assemble by stepwise water molecule addition in electrostatic steering arrangements up to n = 3. Adding a fourth water molecule to the ringlike undissociated HCl(H2O)3 then spontaneously yields the compact dissociated H3O+(H2O)3Cl- ion pair. This aggregation mechanism bypasses deep local energy minima on the n = 4 potential energy surface and offers a general paradigm for reactivity at ultracold temperatures.
authors: Lehrstuhl für Physikalische Chemie II, Ruhr-Universität Bochum, 44780 Bochum, Germany.
authors: Anna
authors: A
authors: Gutberlet
authors: Gerhard
authors: G
authors: Schwaab
authors: Ozgür
authors: O
authors: Birer
authors: Marco
authors: M
authors: Masia
authors: Anna
authors: A
authors: Kaczmarek
authors: Harald
authors: H
authors: Forbert
authors: Martina
authors: M
authors: Havenith
authors: Dominik
authors: D
authors: Marx