Hydration and Ion-Pair Formation of NaNO3(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[Formula: see text]) [Formula: see text] 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 [Formula: see text](aq). However, even in dilute solutions, the vibrational symmetry of the hydrated [Formula: see text](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 [Formula: see text] was observed and accompanied by large band parameter changes. DFT calculations on [Formula: see text](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[Formula: see text] 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[Formula: see text]) [Formula: see text], has been carried out down to concentrations as low as 5.56 × 10–6 mol/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 [Formula: see text].