Hydration and Ion-Pair Formation of NaNO 3 (aq): A Vibrational Spectroscopic and Density Functional Theory Study

Abstract

Qualitative and quantitative Raman and infrared measurements on sodium nitrate (NaNO₃) solutions have been carried out over a wide concentration range (5.56 × 10^–6–7.946 mol/L) in water and heavy water. The Raman spectra were measured from 4000 cm^–1 to low wavenumbers at 45 cm^–1. Band fit analysis on the profile of the 1047 cm^–1 band, ν₁(a $1\text{'}$ ) ${\text{NO}}_3^{-}$ measured at high resolution at 0.90 cm^–1 produced a small contribution at 1027 cm^–1 of the isotopomer N¹⁶O₂¹⁸O(aq). The effect of solute concentration on the Raman and infrared bands has been systematically recorded. Extrapolation of the experimental data resulted in values for all the nitrate bands of the “free”, i.e., fully hydrated ${\text{NO}}_3^{-}$ (aq). However, even in dilute solutions, the vibrational symmetry of the hydrated ${\text{NO}}_3^{-}$ (aq) is broken and the antisymmetric N–O stretch, which is degenerate for the isolated anion, is split by 56 cm^–1. At concentrations >2.5 mol/L, direct contact between Na⁺ and ${\text{NO}}_3^{-}$ was observed and accompanied by large band parameter changes. DFT calculations on ${\text{NO}}_3^{-}$ (H₂O)_n (n = 1–3) led to optimized geometries and vibrational frequencies which reproduced the measured ones within an accuracy of 1%. A hydrated gas phase species Na⁺(H₂O)₁₀ ${\text{NO}}_3^{-}$ was optimized resulting in the geometry and symmetry of the nitrate, which is bound in an antisymmetric bidentate fashion with the nitrate possessing C₁. The ν₁ Na⁺(OH₂) breathing mode in aqueous solution appears at 189 cm⁻¹, whereas in heavy water, ν₁ Na⁺(OD₂) is shifted to 175.6 cm^–1 due to the isotope effect. DFT calculations on hydrated Na⁺(OH₂)_n gas phase clusters provided realistic Na⁺ hydrate structures with n = 4 and 5, which resembled the measured frequency of ν₁ Na⁺ OH₂ mode quite well. Quantitative Raman analysis employing the symmetric stretching band, ν₁(a $1\text{'}$ ) ${\text{NO}}_3^{-}$ , has been carried out down to concentrations as low as 5.56 × 10^–6mol/L. The in-plane deformation mode ν₄(e′) in the Raman scattering at higher concentrations has been used as an indicator band for directly coordinated ${\text{NO}}_3^{-}$ .