Vibrational Coupling and Hydrogen-Bond Structure of Water in the Extended Hydration Shell of Metal Ions

Intra- and intermolecular vibrational coupling (VC) and hydrogen bonding (H-bonding) of water are sparsely understood in the hydration shell (HS) of a metal ion, though the corresponding knowledge for an anion is quite extensive. This is primarily due to the overwhelming effect of anions on water, which masks the subtle perturbing influence of most of the cations. Using Raman difference spectroscopy with simultaneous curve fitting (Raman-DS-SCF) in combination with isotopic dilution and polarized Raman spectroscopy, we have elucidated the VC and H-bonding of water in the HS of bi- and trivalent metal ions─Mg²⁺, Ca²⁺, La³⁺, Gd³⁺, Dy³⁺. Polarized Raman measurement of the HS water with VC "turned on" and "turned off" (using isotopically diluted water, HOD) reveals that water retains the intra- and intermolecular vibrational coupling in the HS of high-charge-density metal ions, which is in stark contrast to that of an anion. Hydration shell spectroscopy in HOD unambiguously shows that the average H-bonding of water becomes stronger in the HS than that of bulk water. The first HS water strongly donates two H-bonds to the second HS water (ν̅_max ≈ 3200 cm^-1) but weakly accepts a H-bond from the second HS water (ν̅_max ≈ 3590 cm^-1), which makes the HS water heterogeneous in terms of its H-bond structure. The weakly interacting OH (ν̅_max 3585 cm^-1 in HOD) red-shifts by ∼ 15 cm^-1 while the VC is "turned on" (ν̅_max ≈ 3600 cm^-1 in H₂O), revealing the intramolecular coupling of water in the HS of metal ions.