The world’s smallest droplet of acid has been produced by scientists working at very low temperatures. The droplet of hydrochloric acid comprises just a single hydrogen chloride (HCl) molecule and four water molecules. This seemingly esoteric piece of ultracold chemistry could have significant implications for our understanding of how chlorine-containing compounds such as chlorofluorocarbons (CFCs) damage the ozone layer in the upper atmosphere.
Writing in the 19th June issue of the journal Science, the team based at Ruhr-Universität Bochum, in Germany, explain that how acids dissociate into charged fragments, and are then surrounded by water molecules, or are solvated, at ultracold temperatures in nanoenvironments is very different from their behaviour in bulk volumes of water at room temperature. Such behaviour occurs outside our everyday experience in the upper atmosphere and in interstellar chemistry and is so far poorly understood, the researchers say.
Chemists Anna Gutberlet, Gerhard Schwaab, Özgür Birer, Marco Masia, Anna Kaczmarek, Harald Forbert, Martina Havenith, and Dominik Marx, formed their acid drop in superfluid helium at a fraction of a degree above absolute zero, 0.37 Kelvin.
In order to study this minimalist acid, the team used the powerful analytical technique of high-resolution mass-selective infrared laser spectroscopy. This tool revealed how the successive accumulation of water molecules around the central HCl molecule to form HCl(H2O)n, quickly leads to the formation of the hydronium ion (H3O+) when n = 4. Hydronium is essentially a water molecule with an added hydrogen ion, or proton, and it is the fundamental acidic species in solutions of hydrogen chloride and other acidic compounds.
To augment their studies, the team carried out a simulation of the process from first principles. This ab initio simulation uses the structures of the molecules and the various energies involved and predicts the same step-by-step assembly of undissociated clusters that the team observed. It also points to electrostatic “steering” of the water molecules around the HCl up to n = 3 which form an intermediate ring-shaped system.
The simulation explains how the addition of the fourth water molecule to this undissociated ring HCl(H2O) 3 spontaneously causes the structure to dissociate into an ion pair H3O+ with (H2O)3Cl–. Somehow, the aggregation mechanism bypasses the deep local energy minima that exists when four water molecules attempt to surround the HCl. “This offers a general paradigm for reactivity at ultracold temperatures,” the researchers explain in their Science paper.