Hydration dynamics of cyanoferrate anions examined by ultrafast infrared spectroscopy

In this work, we carried out steady-state IR absorption, transient IR pump-probe, and waiting-time-dependent two-dimensional (2D) IR measurements on ferrocyanide and ferricyanide ions solvated in water and deuterated water. These two anions are highly symmetric and have distributed cyano groups with IR-active stretching modes in the 5 μm wavelength region. The line width of their linear IR spectra and the initial value of the vibrational frequency-frequency correlation function extracted from their 2D IR spectra indicate water molecules in the hydration shell of the ferro-species are more inhomogeneously distributed but more tightly bound to the cyano groups than those of the ferri-species. Different charges and their distributions in the two anions cause different hydrogen bonding strengths with solvent. The frequency correlation relaxes somewhat slower in ferrocyanide, agreeing with stronger solute-solvent hydrogen-bonding interaction in this case. Mechanisms of the solvent isotope effect on the vibrational relaxation dynamics of the cyano stretching mode are discussed. These results also suggest that in the hydration shell the ferro-species breaks more water structure than the ferri-species, which is opposite to the situation of the bulk water region (beyond the hydration shell) reported previously. This work demonstrated that combined IR methods can be very useful for understanding the molecular details of the structure and dynamics of the hydrated ions.