Aggregation-Induced Dissociation of HCl(H 2 O) 4 Below 1 K: The Smallest Droplet of Acid

Abstract

Minimally Acidic Acidity is usually construed in the context of a bulk liquid solvent: billions of trillions of molecules such as HCl, added to hundreds of billions of trillions of water molecules. What happens under sparser conditions, for example, in atmospheric or interstellar environments, when a single HCl molecule might interact with just three or four water molecules? Gutberlet et al. (p. 1545 ; see the Perspective by Zwier ) explored this question using theoretical simulations together with vibrational spectroscopy in ultracold helium droplets that effectively isolated small aqueous HCl clusters. HCl remained intact upon solvation by one, two, or three water molecules. Dissociation into an ion pair, as occurs in bulk water, required the approach of a fourth water molecule and was facilitated by the geometry of the existing (H 2 O) 3 cluster.