Structure of Ion–Molecular H+(H2O)n (n = 2–6) Complexes and the Thermodynamic Characteristics of Proton Hydration in Gas Atmosphere

The isomers of ion–molecular H⁺(H₂O)_n complexes including up to six water molecules have been studied using quantum chemistry methods. The atomic positions in the isomers corresponding to the global and deepest local minima of potential energy are calculated using the basin-hopping algorithm. The activation energies of some configuration transformations are estimated. The thermodynamic characteristics of the clustering and decomposition of complexes are determined in a harmonic approximation, and they are in good agreement with experimental data. The possibility of simplifying the theoretical investigation of reactions by averaging the thermodynamic characteristics over various channels for energetically close isomers is shown. A weak dependence of the entropy of a reaction on the complex size has been found. A simplified model is proposed to explain the calculation results, and its use for estimating the entropies of complex clustering and decomposition reactions gives good agreement with the experimental results.