Unified view of the hydrogen-bond structure of water in the hydration shell of metal ions (Li+, Mg2+, La3+, Dy3+) as observed in the entire 100–3800 cm−1 regions

Abstract

Hydrogen-bond (H-bond) structure of water in the hydration shell of an anion is fairly well understood, but the corresponding knowledge for cation hydration shell is sparse, mainly because of the overwhelming effect of anion on water that easily masks the subtle effect of cation. Moreover, most of the hydration shell investigations are confined to the OH stretch band of water. Using Raman difference spectroscopy with simultaneous curve fitting (Raman-DS-SCF), we have retrieved the spectrum of water pertaining to the hydration shell of different metal ions (Li+, Mg2+, La3+, Dy3+) in the entire spectral region of 100–3800 cm−1, encompassing the intermolecular H-bond stretch (HOH---OH2; 200 cm−1), librational (300–1000 cm−1), HOH bend (1640 cm−1), bend-librational combination (2120 cm−1) and intramolecular O–H stretch (3000–3700 cm−1) of water. The hydration shell spectra and their polarization dependence (Polarized Raman) are compared for different metal ions and with the hydration shell of Cl− anion and also the high temperature water component (80 °C; extracted by Raman-DS-SCF). These results reveal that the hydration shells of high charge density metal ions have “enhanced” as well as “reduced” H-bond structure. The intermolecular H-bonds become stiffer and the librational freedom of the water gets restricted in the hydration shell. The bend-librational combination band blue-shifts to a large extent and appears toward the tail of the OH stretch, which is expected to facilitate the ultrafast energy dissipation of higher vibrational modes of water.