The H3O+(H2O)n Reagent Ion: Calculations of the Structure, Thermodynamic Parameters of Hydration, Equilibrium Composition, and Mobility

Abstract

The H₃O⁺(H₂O)_n reagent ion is widely used in ion mobility spectrometry and atmospheric pressure chemical ionization mass spectrometry. In this work, calculated data on the structure and properties of the H₃O⁺(H₂O)_n (n = 0–4) reagent ion are obtained. They are of interest for the study of the regularities of chemical ionization and the prediction and interpretation of ion mobility spectra and mass spectra. Using quantum chemical methods, we calculated the structures and refined the isomeric composition of the H₃O⁺(H₂O)_n ions. It was found that ions with n = 2, 3, and 4 have 2, 5, and 15 isomers, respectively. The most stable isomers, which possess branched non-cyclic structures with the H₃O⁺ core, were determined. The thermodynamic parameters of hydration reactions of the most stable isomers of H₃O⁺(H₂O)_n (n = 0–3) ions and the equilibrium composition of the reagent ion in a wide range of temperatures and concentrations of water vapor are computed. The obtained results agree with the available experimental and calculated data. Using the computed structural data and data on the composition of the reagent ion, its mobility was calculated by the trajectory method. The error of calculations is close to the experimental one.