Drawing out the structural information of the first layer of hydrated ions: ATR-FTIR spectroscopic studies on aqueous NH₄NO₃, NaNO₃, and Mg(NO₃)₂ solutions

ATR-FTIR technique was used to obtain the difference spectra of aqueous NH₄NO₃ NaNO₃, and Mg(NO₃) ₂ solutions, with NO₃^- concentrations ranging from 0 to 4.00 mol dm^-3. The water monomers weakly hydrogen bonded with NO₃^- ions showed a positive peak near at 3565 cm ^-1 for both Mg(NO₃)₂ and NH₄NO ₃ solutions. The positive peak was shift to ∼3543 cm^-1 for NaNO₃ solutions due to the total contributions of the hydrated NO₃^- (∼3565 cm^-1) and the hydrated Na ⁺ (∼3440 cm^-1). Compared with perchlorate solutions, the positive peak of nitrate solutions has a red shift of about 20 cm ^-1 and the peak area is about half of that of perchlorate solutions with the same concentrations, indicating that the hydrogen bonding between NO₃^- and water monomers is relative stronger than that between ClO₄^- and water monomers, and NO₃ ^- has a strict requirement on the orientation of water molecules when hydrogen bonded with water monomers due to its planar structure. The ab initio calculations were used to understand the splitting of the ν₃ band and hydration effect on the infrared activation of the ν₁. The absorbance of ν_3b, ν₁ and ν₂ bands, dependent on the type of cations, was observed to departed from Beer low with increasing concentrations, which is considered as the results of the interactions between cations and nitrate ions.