Insights into the Structures, Energetics, and Vibrations of Monovalent Cation-(Water)_1-6 Clusters

This study details the interactions prevailing in aqueous clusters of monovalent alkali metal, ammonium, and hydronium cations. The calculations involve a detailed evaluation of the structures, thermodynamic energies, and IR spectra of several plausible conformers of M⁺·(H ₂O)_1-6 (M = Li, Na, K, Rb, Cs, NH₄, H ₃O) clusters at the second-order Møller-Plesset (MP2) and density functional levels of theory. A detailed decomposition of the interaction energies has been carried out for complexes involving one or two water molecules using symmetry adapted perturbation theory. Some of the salient insights on the structures include the emergence of the second solvent shell even before the realization of the maximal coordination number of the cation. This effect was more pronounced in clusters involving the larger cations. The quantitative estimates of various components of the interaction energy indicate the predominance of electrostatic energies in the binding of these cations to water molecules. Interestingly, for all the hydrated alkali metal cation complexes, the contribution of electrostatic energy is almost the same as the total attractive interaction energy, whereas the sum of the induction and dispersion energies are almost canceled out by exchange-repulsion energy. However, the contribution of dispersion energies slowly starts increasing as the size of the cation increases and is quite substantial in case of the Cs ⁺ complexes. In the organic cations, the dispersion energies become significant, though not comparable to the electrostatic energies. In addition to the evaluation of the harmonic frequencies of -OH stretching mode of all the structures, the anharmonic frequencies were evaluated for the smaller clusters. As the size of the cation and the size of the water cluster increases, the red shifts associated with the -OH stretching mode progressively become larger for the alkali metal cation containing complexes. For the organic cation (NH ₄⁺, H₃O⁺) containing complexes, an opposite trend is observed. Compared to the isolated water monomer, the ratio of the infrared intensities of the asymmetric and symmetric -OH stretching modes is very small. However, this ratio progressively becomes larger as the size of the cation increases.