Proton transfer and autoionization in HNO3·HCl·(H2O)n particles

Abstract

The structure and spectroscopic properties of clusters of HNO₃·HCl·(H₂O)_n, with n = 1 to 6, have been calculated at the MP2/aug-cc-pVDZ level of theory. Altogether 22 different clusters have been found as stable structures, with minima in their potential energy surfaces. The clusters can be grouped in families with the same number of water molecules, and with close aggregation energies within each family. The addition of each new water molecule increments the aggregation energy of the clusters by a nearly constant value of 76.2 ± 0.1 Hartree. The proton transfer parameter and the coordination number of HNO₃ and HCl in each cluster have been evaluated, and the wavenumber shifts for the X⁻-H⁺ vibration from the corresponding mode in the isolated molecules have also been predicted. These values allow classification of the acidic species in the clusters into three types, characterized by the strength of the hydrogen bond and the degree of ionization. A correspondence is found between the coordination number of HNO₃ and the magnitude of the X⁻-H⁺ vibrational shift.