Electric nuclear quadrupole coupling reveals dissociation of HCl with a few water molecules.

Investigating the dissociation of acids in the presence of a limited number of water molecules is crucial for understanding various elementary chemical processes. In our study focusing on HCl(H 2 O) n clusters formed in a cold and isolated jet expansion, we utilized the nuclear quadrupole coupling tensor obtained via rotational spectroscopy to decipher the nature of the H-Cl chemical bond in a micro-aqueous environment. For n = 1 to 4, the H-Cl bond is covalent. At n = 5 and 7, the contact ion pair of H 3 O + Cl - is spontaneously formed within the hydrogen bond networks of "book" and "cube" acid-water clusters, respectively.