Solution and Surface\nSolvation of Nitrate Anions with\nIron(III) and Aluminum(III) in Aqueous Environments: A Raman and Vibrational\nSum Frequency Generation Study

Hydrated trivalent metal nitrate salts, Fe(NO<sub>3</sub>)<sub>3</sub>·9H<sub>2</sub>O and Al(NO<sub>3</sub>)<sub>3</sub>·9H<sub>2</sub>O, in both solid and aqueous phases are investigated.\nRaman\nand surface-selective vibrational sum frequency generation (SFG) spectroscopy,\nare used to shed light on ion–ion interactions and hydration\nin several spectral regions spanning low frequency (440–550\ncm<sup>–1</sup>) to higher frequency modes of nitrate and water\n(720, 1050, 1250–1450, and 2800–3750 cm<sup>–1</sup>). These frequencies span the metal–water mode, nitrate in-plane\ndeformation, nitrate symmetric and asymmetric modes, and the OH stretch\nof condensed phase water molecules. Comparison to NaNO<sub>3</sub>, and in some cases KNO<sub>3</sub>, is also shown, providing insight.\nSplitting and frequency shifts are observed and discussed for both\nthe solid state and solution phase. The Lewis acidity of Fe<sup>3+</sup> and Al<sup>3+</sup> ions plays a significant role in the observed\nspectra, in particular for the nitrate asymmetric band splitting and\nfrequency shift. The spectral response from water solvation for iron\nand aluminum nitrates is nonlinear as compared to linear for sodium\nnitrate, suggesting significantly different solvation environments\nthat are limited by water hydration capacity at higher concentrations.\nMoreover, a non-hydrogen bonded OH, dangling OH, from hydrating water\nmolecules is observed spectroscopically for Al and Fe nitrate solutions.\nFurthermore, aluminum nitrate perturbs the surface water structure\nmore than iron nitrate despite aluminum being a weaker Lewis acid.\nThe surface water structure is thus found to be unique for the Al(NO<sub>3</sub>)<sub>3</sub> solutions as compared to both Fe(NO<sub>3</sub>)<sub>3</sub> and NaNO<sub>3</sub>, such that surface solvation is\nmore pronounced. This observation exemplifies the nature of the Fe(III)\nand Al(III) ions and their substantial influence on the surface water\nstructure.