Isolating the spectral signature of H3O+ in the smallest droplet of dissociated HCl acid

Abstract

The centrally important role of acids in aqueous chemistry has stimulated the search for the smallest droplet of hydrochloric acid. Based on several independent quantum calculations, this appears to be the HCl(H₂O)₄ cluster, which dissociates into the so-called solvent ion pair (SIP), H₃O⁺(H₂O)₃Cl⁻. Experimental verification of this prediction via infra-red spectroscopy is a major challenge and despite several recent reports of this SIP, there remains uncertainty about these observations. In this report, we present a calculation of the IR spectrum of the SIP in a fashion that isolates the contribution from the signature hydronium ion, H₃O⁺. The computed spectrum indicates that the vibrational states of H₃O⁺ are highly mixed, resulting in dispersed spectral features between 1300 and 3000 cm⁻¹, with the region between 2100 and 2900 cm⁻¹ being especially rich. These predictions point out the complexity of the SIP spectrum and offer guidelines for experiment. The energies of the HCl stretch fundamentals for three minima of the undissociated HCl(H₂O)₄ cluster are also reported.