Structural and Spectral Signatures of Alkali Metal Ions in Water Clusters

Analysis of the results of nonempirical simulations of Met⁺(H₂O)_n clusters with Met = Li, Na, and K, and n = 20–55 carried out at the density functional level with B3LYP exchange-correlation functional and extended double-zeta basis sets revealed the prevailing structure motifs of the aqua complexes of alkali metal ions and their fingerprints in the infrared absorption spectra in the range of 2000–4000 cm^–1. The hydration structures of the ions are found to be predetermined by the balance between the conjugated H-bonded rings and extended ordered H-bonded sequences. The extension of the latter is shown to decrease with an increase in the effective radius of the ion, which is reflected in the stronger localization of the coupling of OH oscillators within the homodromic structural rings and the respective shift in the characteristic frequencies within the OH stretching domain.