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 NH4NO3 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 ν3b, 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.